...))
(register-callback event-type
(make-handler event-type ...))
As far as the library is concerned, ‘handler-proc’ is a procedure
with no arguments, and all the library has to do is call it when the
appropriate event occurs. From the application’s point of view, though,
the handler procedure has used closure to capture an environment that
includes all the context that the handler code needs — ‘event-type’ and
‘user-data’ — to handle the event correctly.
3.4.9 Example 4: Object Orientation
-----------------------------------
Closure is the capture of an environment, containing persistent variable
bindings, within the definition of a procedure or a set of related
procedures. This is rather similar to the idea in some object oriented
languages of encapsulating a set of related data variables inside an
“object”, together with a set of “methods” that operate on the
encapsulated data. The following example shows how closure can be used
to emulate the ideas of objects, methods and encapsulation in Scheme.
(define (make-account)
(let ((balance 0))
(define (get-balance)
balance)
(define (deposit amount)
(set! balance (+ balance amount))
balance)
(define (withdraw amount)
(deposit (- amount)))
(lambda args
(apply
(case (car args)
((get-balance) get-balance)
((deposit) deposit)
((withdraw) withdraw)
(else (error "Invalid method!")))
(cdr args)))))
Each call to ‘make-account’ creates and returns a new procedure,
created by the expression in the example code that begins “(lambda
args”.
(define my-account (make-account))
my-account
⇒
#
This procedure acts as an account object with methods ‘get-balance’,
‘deposit’ and ‘withdraw’. To apply one of the methods to the account,
you call the procedure with a symbol indicating the required method as
the first parameter, followed by any other parameters that are required
by that method.
(my-account 'get-balance)
⇒
0
(my-account 'withdraw 5)
⇒
-5
(my-account 'deposit 396)
⇒
391
(my-account 'get-balance)
⇒
391
Note how, in this example, both the current balance and the helper
procedures ‘get-balance’, ‘deposit’ and ‘withdraw’, used to implement
the guts of the account object’s methods, are all stored in variable
bindings within the private local environment captured by the ‘lambda’
expression that creates the account object procedure.
3.5 Further Reading
===================
• The website .
Includes a nice explanation of continuations.
• The complete text of “Structure and Interpretation of Computer
Programs”, the classic introduction to computer science and Scheme
by Hal Abelson, Jerry Sussman and Julie Sussman, is now available
online at . This site also
provides teaching materials related to the book, and all the source
code used in the book, in a form suitable for loading and running.
4 Programming in Scheme
***********************
Guile’s core language is Scheme, and a lot can be achieved simply by
using Guile to write and run Scheme programs — as opposed to having to
dive into C code. In this part of the manual, we explain how to use
Guile in this mode, and describe the tools that Guile provides to help
you with script writing, debugging, and packaging your programs for
distribution.
For detailed reference information on the variables, functions, and
so on that make up Guile’s application programming interface (API), see
*note API Reference::.
4.1 Guile’s Implementation of Scheme
====================================
Guile’s core language is Scheme, which is specified and described in the
series of reports known as “RnRS”. “RnRS” is shorthand for the
“Revised^n Report on the Algorithmic Language Scheme”. Guile complies
fully with R5RS (*note Introduction: (r5rs)Top.), and is largely
compliant with R6RS and R7RS.
Guile also has many extensions that go beyond these reports. Some of
the areas where Guile extends standard Scheme are:
• Guile’s interactive documentation system
• Guile’s support for POSIX-compliant network programming
• GOOPS – Guile’s framework for object oriented programming.
4.2 Invoking Guile
==================
Many features of Guile depend on and can be changed by information that
the user provides either before or when Guile is started. Below is a
description of what information to provide and how to provide it.
4.2.1 Command-line Options
--------------------------
Here we describe Guile’s command-line processing in detail. Guile
processes its arguments from left to right, recognizing the switches
described below. For examples, see *note Scripting Examples::.
‘SCRIPT ARG...’
‘-s SCRIPT ARG...’
By default, Guile will read a file named on the command line as a
script. Any command-line arguments ARG... following SCRIPT become
the script’s arguments; the ‘command-line’ function returns a list
of strings of the form ‘(SCRIPT ARG...)’.
It is possible to name a file using a leading hyphen, for example,
‘-myfile.scm’. In this case, the file name must be preceded by
‘-s’ to tell Guile that a (script) file is being named.
Scripts are read and evaluated as Scheme source code just as the
‘load’ function would. After loading SCRIPT, Guile exits.
‘-c EXPR ARG...’
Evaluate EXPR as Scheme code, and then exit. Any command-line
arguments ARG... following EXPR become command-line arguments; the
‘command-line’ function returns a list of strings of the form
‘(GUILE ARG...)’, where GUILE is the path of the Guile executable.
‘-- ARG...’
Run interactively, prompting the user for expressions and
evaluating them. Any command-line arguments ARG... following the
‘--’ become command-line arguments for the interactive session; the
‘command-line’ function returns a list of strings of the form
‘(GUILE ARG...)’, where GUILE is the path of the Guile executable.
‘-L DIRECTORY’
Add DIRECTORY to the front of Guile’s module load path. The given
directories are searched in the order given on the command line and
before any directories in the ‘GUILE_LOAD_PATH’ environment
variable. Paths added here are _not_ in effect during execution of
the user’s ‘.guile’ file.
‘-C DIRECTORY’
Like ‘-L’, but adjusts the load path for _compiled_ files.
‘-x EXTENSION’
Add EXTENSION to the front of Guile’s load extension list (*note
‘%load-extensions’: Load Paths.). The specified extensions are
tried in the order given on the command line, and before the
default load extensions. Extensions added here are _not_ in effect
during execution of the user’s ‘.guile’ file.
‘-l FILE’
Load Scheme source code from FILE, and continue processing the
command line.
‘-e FUNCTION’
Make FUNCTION the “entry point” of the script. After loading the
script file (with ‘-s’) or evaluating the expression (with ‘-c’),
apply FUNCTION to a list containing the program name and the
command-line arguments—the list provided by the ‘command-line’
function.
A ‘-e’ switch can appear anywhere in the argument list, but Guile
always invokes the FUNCTION as the _last_ action it performs. This
is weird, but because of the way script invocation works under
POSIX, the ‘-s’ option must always come last in the list.
The FUNCTION is most often a simple symbol that names a function
that is defined in the script. It can also be of the form ‘(@
MODULE-NAME SYMBOL)’, and in that case, the symbol is looked up in
the module named MODULE-NAME.
As a shorthand you can use the form ‘(symbol ...)’, that is, a list
of only symbols that doesn’t start with ‘@’. It is equivalent to
‘(@ MODULE-NAME main)’, where MODULE-NAME is ‘(symbol ...)’ form.
*Note Using Guile Modules:: and *note Scripting Examples::.
‘-ds’
Treat a final ‘-s’ option as if it occurred at this point in the
command line; load the script here.
This switch is necessary because, although the POSIX script
invocation mechanism effectively requires the ‘-s’ option to appear
last, the programmer may well want to run the script before other
actions requested on the command line. For examples, see *note
Scripting Examples::.
‘\’
Read more command-line arguments, starting from the second line of
the script file. *Note The Meta Switch::.
‘--use-srfi=LIST’
The option ‘--use-srfi’ expects a comma-separated list of numbers,
each representing a SRFI module to be loaded into the interpreter
before evaluating a script file or starting the REPL. Additionally,
the feature identifier for the loaded SRFIs is recognized by the
procedure ‘cond-expand’ when this option is used.
Here is an example that loads the modules SRFI-8 (’receive’) and
SRFI-13 (’string library’) before the GUILE interpreter is started:
guile --use-srfi=8,13
‘--r6rs’
Adapt Guile’s initial environment to better support R6RS. *Note
R6RS Incompatibilities::, for some caveats.
‘--r7rs’
Adapt Guile’s initial environment to better support R7RS. *Note
R7RS Incompatibilities::, for some caveats.
‘--debug’
Start with the debugging virtual machine (VM) engine. Using the
debugging VM will enable support for VM hooks, which are needed for
tracing, breakpoints, and accurate call counts when profiling. The
debugging VM is slower than the regular VM, though, by about ten
percent. *Note VM Hooks::, for more information.
By default, the debugging VM engine is only used when entering an
interactive session. When executing a script with ‘-s’ or ‘-c’,
the normal, faster VM is used by default.
‘--no-debug’
Do not use the debugging VM engine, even when entering an
interactive session.
Note that, despite the name, Guile running with ‘--no-debug’ _does_
support the usual debugging facilities, such as printing a detailed
backtrace upon error. The only difference with ‘--debug’ is lack
of support for VM hooks and the facilities that build upon it (see
above).
‘-q’
Do not load the initialization file, ‘.guile’. This option only
has an effect when running interactively; running scripts does not
load the ‘.guile’ file. *Note Init File::.
‘--listen[=P]’
While this program runs, listen on a local port or a path for REPL
clients. If P starts with a number, it is assumed to be a local
port on which to listen. If it starts with a forward slash, it is
assumed to be the file name of a UNIX domain socket on which to
listen.
If P is not given, the default is local port 37146. If you look at
it upside down, it almost spells “Guile”. If you have netcat
installed, you should be able to ‘nc localhost 37146’ and get a
Guile prompt. Alternately you can fire up Emacs and connect to the
process; see *note Using Guile in Emacs:: for more details.
Note: Opening a port allows anyone who can connect to that
port to do anything Guile can do, as the user that the Guile
process is running as. Do not use ‘--listen’ on multi-user
machines. Of course, if you do not pass ‘--listen’ to Guile,
no port will be opened.
Guile protects against the “HTTP inter-protocol exploitation
attack”
(https://en.wikipedia.org/wiki/Inter-protocol_exploitation), a
scenario whereby an attacker can, via an HTML page, cause a
web browser to send data to TCP servers listening on a
loopback interface or private network. Nevertheless, you are
advised to use UNIX domain sockets, as in
‘--listen=/some/local/file’, whenever possible.
That said, ‘--listen’ is great for interactive debugging and
development.
‘--auto-compile’
Compile source files automatically (default behavior).
‘--fresh-auto-compile’
Treat the auto-compilation cache as invalid, forcing recompilation.
‘--no-auto-compile’
Disable automatic source file compilation.
‘--language=LANG’
For the remainder of the command line arguments, assume that files
mentioned with ‘-l’ and expressions passed with ‘-c’ are written in
LANG. LANG must be the name of one of the languages supported by
the compiler (*note Compiler Tower::). When run interactively, set
the REPL’s language to LANG (*note Using Guile Interactively::).
The default language is ‘scheme’; other interesting values include
‘elisp’ (for Emacs Lisp), and ‘ecmascript’.
The example below shows the evaluation of expressions in Scheme,
Emacs Lisp, and ECMAScript:
guile -c "(apply + '(1 2))"
guile --language=elisp -c "(= (funcall (symbol-function '+) 1 2) 3)"
guile --language=ecmascript -c '(function (x) { return x * x; })(2);'
To load a file written in Scheme and one written in Emacs Lisp, and
then start a Scheme REPL, type:
guile -l foo.scm --language=elisp -l foo.el --language=scheme
‘-h, --help’
Display help on invoking Guile, and then exit.
‘-v, --version’
Display the current version of Guile, and then exit.
4.2.2 Environment Variables
---------------------------
The “environment” is a feature of the operating system; it consists of a
collection of variables with names and values. Each variable is called
an “environment variable” (or, sometimes, a “shell variable”);
environment variable names are case-sensitive, and it is conventional to
use upper-case letters only. The values are all text strings, even
those that are written as numerals. (Note that here we are referring to
names and values that are defined in the operating system shell from
which Guile is invoked. This is not the same as a Scheme environment
that is defined within a running instance of Guile. For a description
of Scheme environments, *note About Environments::.)
How to set environment variables before starting Guile depends on the
operating system and, especially, the shell that you are using. For
example, here is how to tell Guile to provide detailed warning messages
about deprecated features by setting ‘GUILE_WARN_DEPRECATED’ using Bash:
$ export GUILE_WARN_DEPRECATED="detailed"
$ guile
Or, detailed warnings can be turned on for a single invocation using:
$ env GUILE_WARN_DEPRECATED="detailed" guile
If you wish to retrieve or change the value of the shell environment
variables that affect the run-time behavior of Guile from within a
running instance of Guile, see *note Runtime Environment::.
Here are the environment variables that affect the run-time behavior
of Guile:
‘GUILE_AUTO_COMPILE’
This is a flag that can be used to tell Guile whether or not to
compile Scheme source files automatically. Starting with Guile
2.0, Scheme source files will be compiled automatically, by
default.
If a compiled (‘.go’) file corresponding to a ‘.scm’ file is not
found or is not newer than the ‘.scm’ file, the ‘.scm’ file will be
compiled on the fly, and the resulting ‘.go’ file stored away. An
advisory note will be printed on the console.
Compiled files will be stored in the directory
‘$XDG_CACHE_HOME/guile/ccache’, where ‘XDG_CACHE_HOME’ defaults to
the directory ‘$HOME/.cache’. This directory will be created if it
does not already exist.
Note that this mechanism depends on the timestamp of the ‘.go’ file
being newer than that of the ‘.scm’ file; if the ‘.scm’ or ‘.go’
files are moved after installation, care should be taken to
preserve their original timestamps.
Set ‘GUILE_AUTO_COMPILE’ to zero (0), to prevent Scheme files from
being compiled automatically. Set this variable to “fresh” to tell
Guile to compile Scheme files whether they are newer than the
compiled files or not.
*Note Compilation::.
‘GUILE_HISTORY’
This variable names the file that holds the Guile REPL command
history. You can specify a different history file by setting this
environment variable. By default, the history file is
‘$HOME/.guile_history’.
‘GUILE_INSTALL_LOCALE’
This is a flag that can be used to tell Guile whether or not to
install the current locale at startup, via a call to ‘(setlocale
LC_ALL "")’(1). *Note Locales::, for more information on locales.
You may explicitly indicate that you do not want to install the
locale by setting ‘GUILE_INSTALL_LOCALE’ to ‘0’, or explicitly
enable it by setting the variable to ‘1’.
Usually, installing the current locale is the right thing to do.
It allows Guile to correctly parse and print strings with non-ASCII
characters. Therefore, this option is on by default.
‘GUILE_LOAD_COMPILED_PATH’
This variable may be used to augment the path that is searched for
compiled Scheme files (‘.go’ files) when loading. Its value should
be a colon-separated list of directories. If it contains the
special path component ‘...’ (ellipsis), then the default path is
put in place of the ellipsis, otherwise the default path is placed
at the end. The result is stored in ‘%load-compiled-path’ (*note
Load Paths::).
Here is an example using the Bash shell that adds the current
directory, ‘.’, and the relative directory ‘../my-library’ to
‘%load-compiled-path’:
$ export GUILE_LOAD_COMPILED_PATH=".:../my-library"
$ guile -c '(display %load-compiled-path) (newline)'
(. ../my-library /usr/local/lib/guile/3.0/ccache)
‘GUILE_LOAD_PATH’
This variable may be used to augment the path that is searched for
Scheme files when loading. Its value should be a colon-separated
list of directories. If it contains the special path component
‘...’ (ellipsis), then the default path is put in place of the
ellipsis, otherwise the default path is placed at the end. The
result is stored in ‘%load-path’ (*note Load Paths::).
Here is an example using the Bash shell that prepends the current
directory to ‘%load-path’, and adds the relative directory
‘../srfi’ to the end:
$ env GUILE_LOAD_PATH=".:...:../srfi" \
guile -c '(display %load-path) (newline)'
(. /usr/local/share/guile/3.0 \
/usr/local/share/guile/site/3.0 \
/usr/local/share/guile/site \
/usr/local/share/guile \
../srfi)
(Note: The line breaks, above, are for documentation purposes only,
and not required in the actual example.)
‘GUILE_EXTENSIONS_PATH’
This variable may be used to augment the path that is searched for
foreign libraries via ‘load-extension’, ‘dynamic-link’,
‘load-foreign-library’, or the like. Its value should be a
colon-separated (semicolon on Windows) list of directories. *Note
Foreign Libraries::.
‘GUILE_WARN_DEPRECATED’
As Guile evolves, some features will be eliminated or replaced by
newer features. To help users migrate their code as this evolution
occurs, Guile will issue warning messages about code that uses
features that have been marked for eventual elimination.
‘GUILE_WARN_DEPRECATED’ can be set to “no” to tell Guile not to
display these warning messages, or set to “detailed” to tell Guile
to display more lengthy messages describing the warning. *Note
Deprecation::.
‘HOME’
Guile uses the environment variable ‘HOME’, the name of your home
directory, to locate various files, such as ‘.guile’ or
‘.guile_history’.
‘GUILE_JIT_THRESHOLD’
Guile has a just-in-time (JIT) code generator that makes running
Guile code fast. *Note Just-In-Time Native Code::, for more. The
unit of code generation is the function. Each function has its own
counter that gets incremented when the function is called and at
each loop iteration in the function. When the counter exceeds the
‘GUILE_JIT_THRESHOLD’, the function will get JIT-compiled. Set
‘GUILE_JIT_THRESHOLD’ to ‘-1’ to disable JIT compilation, or ‘0’ to
eagerly JIT-compile each function as it’s first seen.
‘GUILE_JIT_LOG’
Set to ‘1’, ‘2’, or ‘3’ to give increasing amounts of logging for
JIT compilation events. Used for debugging.
‘GUILE_JIT_STOP_AFTER’
Though we have tested the JIT compiler as well as we can, it’s
possible that it has bugs. If you suspect that Guile’s JIT
compiler is causing your program to fail, set
‘GUILE_JIT_STOP_AFTER’ to a positive integer indicating the maximum
number of functions to JIT-compile. By bisecting over the value of
‘GUILE_JIT_STOP_AFTER’, you can pinpoint the precise function that
is being miscompiled.
---------- Footnotes ----------
(1) The ‘GUILE_INSTALL_LOCALE’ environment variable was ignored in
Guile versions prior to 2.0.9.
4.3 Guile Scripting
===================
Like AWK, Perl, or any shell, Guile can interpret script files. A Guile
script is simply a file of Scheme code with some extra information at
the beginning which tells the operating system how to invoke Guile, and
then tells Guile how to handle the Scheme code.
4.3.1 The Top of a Script File
------------------------------
The first line of a Guile script must tell the operating system to use
Guile to evaluate the script, and then tell Guile how to go about doing
that. Here is the simplest case:
• The first two characters of the file must be ‘#!’.
The operating system interprets this to mean that the rest of the
line is the name of an executable that can interpret the script.
Guile, however, interprets these characters as the beginning of a
multi-line comment, terminated by the characters ‘!#’ on a line by
themselves. (This is an extension to the syntax described in R5RS,
added to support shell scripts.)
• Immediately after those two characters must come the full pathname
to the Guile interpreter. On most systems, this would be
‘/usr/local/bin/guile’.
• Then must come a space, followed by a command-line argument to pass
to Guile; this should be ‘-s’. This switch tells Guile to run a
script, instead of soliciting the user for input from the terminal.
There are more elaborate things one can do here; see *note The Meta
Switch::.
• Follow this with a newline.
• The second line of the script should contain only the characters
‘!#’ — just like the top of the file, but reversed. The operating
system never reads this far, but Guile treats this as the end of
the comment begun on the first line by the ‘#!’ characters.
• If this source code file is not ASCII or ISO-8859-1 encoded, a
coding declaration such as ‘coding: utf-8’ should appear in a
comment somewhere in the first five lines of the file: see *note
Character Encoding of Source Files::.
• The rest of the file should be a Scheme program.
Guile reads the program, evaluating expressions in the order that
they appear. Upon reaching the end of the file, Guile exits.
4.3.2 The Meta Switch
---------------------
Guile’s command-line switches allow the programmer to describe
reasonably complicated actions in scripts. Unfortunately, the POSIX
script invocation mechanism only allows one argument to appear on the
‘#!’ line after the path to the Guile executable, and imposes arbitrary
limits on that argument’s length. Suppose you wrote a script starting
like this:
#!/usr/local/bin/guile -e main -s
!#
(define (main args)
(map (lambda (arg) (display arg) (display " "))
(cdr args))
(newline))
The intended meaning is clear: load the file, and then call ‘main’ on
the command-line arguments. However, the system will treat everything
after the Guile path as a single argument — the string ‘"-e main -s"’ —
which is not what we want.
As a workaround, the meta switch ‘\’ allows the Guile programmer to
specify an arbitrary number of options without patching the kernel. If
the first argument to Guile is ‘\’, Guile will open the script file
whose name follows the ‘\’, parse arguments starting from the file’s
second line (according to rules described below), and substitute them
for the ‘\’ switch.
Working in concert with the meta switch, Guile treats the characters
‘#!’ as the beginning of a comment which extends through the next line
containing only the characters ‘!#’. This sort of comment may appear
anywhere in a Guile program, but it is most useful at the top of a file,
meshing magically with the POSIX script invocation mechanism.
Thus, consider a script named ‘/u/jimb/ekko’ which starts like this:
#!/usr/local/bin/guile \
-e main -s
!#
(define (main args)
(map (lambda (arg) (display arg) (display " "))
(cdr args))
(newline))
Suppose a user invokes this script as follows:
$ /u/jimb/ekko a b c
Here’s what happens:
• the operating system recognizes the ‘#!’ token at the top of the
file, and rewrites the command line to:
/usr/local/bin/guile \ /u/jimb/ekko a b c
This is the usual behavior, prescribed by POSIX.
• When Guile sees the first two arguments, ‘\ /u/jimb/ekko’, it opens
‘/u/jimb/ekko’, parses the three arguments ‘-e’, ‘main’, and ‘-s’
from it, and substitutes them for the ‘\’ switch. Thus, Guile’s
command line now reads:
/usr/local/bin/guile -e main -s /u/jimb/ekko a b c
• Guile then processes these switches: it loads ‘/u/jimb/ekko’ as a
file of Scheme code (treating the first three lines as a comment),
and then performs the application ‘(main "/u/jimb/ekko" "a" "b"
"c")’.
When Guile sees the meta switch ‘\’, it parses command-line argument
from the script file according to the following rules:
• Each space character terminates an argument. This means that two
spaces in a row introduce an argument ‘""’.
• The tab character is not permitted (unless you quote it with the
backslash character, as described below), to avoid confusion.
• The newline character terminates the sequence of arguments, and
will also terminate a final non-empty argument. (However, a
newline following a space will not introduce a final empty-string
argument; it only terminates the argument list.)
• The backslash character is the escape character. It escapes
backslash, space, tab, and newline. The ANSI C escape sequences
like ‘\n’ and ‘\t’ are also supported. These produce argument
constituents; the two-character combination ‘\n’ doesn’t act like a
terminating newline. The escape sequence ‘\NNN’ for exactly three
octal digits reads as the character whose ASCII code is NNN. As
above, characters produced this way are argument constituents.
Backslash followed by other characters is not allowed.
4.3.3 Command Line Handling
---------------------------
The ability to accept and handle command line arguments is very
important when writing Guile scripts to solve particular problems, such
as extracting information from text files or interfacing with existing
command line applications. This chapter describes how Guile makes
command line arguments available to a Guile script, and the utilities
that Guile provides to help with the processing of command line
arguments.
When a Guile script is invoked, Guile makes the command line
arguments accessible via the procedure ‘command-line’, which returns the
arguments as a list of strings.
For example, if the script
#! /usr/local/bin/guile -s
!#
(write (command-line))
(newline)
is saved in a file ‘cmdline-test.scm’ and invoked using the command line
‘./cmdline-test.scm bar.txt -o foo -frumple grob’, the output is
("./cmdline-test.scm" "bar.txt" "-o" "foo" "-frumple" "grob")
If the script invocation includes a ‘-e’ option, specifying a
procedure to call after loading the script, Guile will call that
procedure with ‘(command-line)’ as its argument. So a script that uses
‘-e’ doesn’t need to refer explicitly to ‘command-line’ in its code.
For example, the script above would have identical behaviour if it was
written instead like this:
#! /usr/local/bin/guile \
-e main -s
!#
(define (main args)
(write args)
(newline))
(Note the use of the meta switch ‘\’ so that the script invocation
can include more than one Guile option: *Note The Meta Switch::.)
These scripts use the ‘#!’ POSIX convention so that they can be
executed using their own file names directly, as in the example command
line ‘./cmdline-test.scm bar.txt -o foo -frumple grob’. But they can
also be executed by typing out the implied Guile command line in full,
as in:
$ guile -s ./cmdline-test.scm bar.txt -o foo -frumple grob
or
$ guile -e main -s ./cmdline-test2.scm bar.txt -o foo -frumple grob
Even when a script is invoked using this longer form, the arguments
that the script receives are the same as if it had been invoked using
the short form. Guile ensures that the ‘(command-line)’ or ‘-e’
arguments are independent of how the script is invoked, by stripping off
the arguments that Guile itself processes.
A script is free to parse and handle its command line arguments in
any way that it chooses. Where the set of possible options and
arguments is complex, however, it can get tricky to extract all the
options, check the validity of given arguments, and so on. This task
can be greatly simplified by taking advantage of the module ‘(ice-9
getopt-long)’, which is distributed with Guile, *Note getopt-long::.
4.3.4 Scripting Examples
------------------------
To start with, here are some examples of invoking Guile directly:
‘guile -- a b c’
Run Guile interactively; ‘(command-line)’ will return
‘("/usr/local/bin/guile" "a" "b" "c")’.
‘guile -s /u/jimb/ex2 a b c’
Load the file ‘/u/jimb/ex2’; ‘(command-line)’ will return
‘("/u/jimb/ex2" "a" "b" "c")’.
‘guile -c '(write %load-path) (newline)'’
Write the value of the variable ‘%load-path’, print a newline, and
exit.
‘guile -e main -s /u/jimb/ex4 foo’
Load the file ‘/u/jimb/ex4’, and then call the function ‘main’,
passing it the list ‘("/u/jimb/ex4" "foo")’.
‘guile -e '(ex4)' -s /u/jimb/ex4.scm foo’
Load the file ‘/u/jimb/ex4.scm’, and then call the function ‘main’
from the module ’(ex4)’, passing it the list ‘("/u/jimb/ex4"
"foo")’.
‘guile -l first -ds -l last -s script’
Load the files ‘first’, ‘script’, and ‘last’, in that order. The
‘-ds’ switch says when to process the ‘-s’ switch. For a more
motivated example, see the scripts below.
Here is a very simple Guile script:
#!/usr/local/bin/guile -s
!#
(display "Hello, world!")
(newline)
The first line marks the file as a Guile script. When the user
invokes it, the system runs ‘/usr/local/bin/guile’ to interpret the
script, passing ‘-s’, the script’s filename, and any arguments given to
the script as command-line arguments. When Guile sees ‘-s SCRIPT’, it
loads SCRIPT. Thus, running this program produces the output:
Hello, world!
Here is a script which prints the factorial of its argument:
#!/usr/local/bin/guile -s
!#
(define (fact n)
(if (zero? n) 1
(* n (fact (- n 1)))))
(display (fact (string->number (cadr (command-line)))))
(newline)
In action:
$ ./fact 5
120
$
However, suppose we want to use the definition of ‘fact’ in this file
from another script. We can’t simply ‘load’ the script file, and then
use ‘fact’’s definition, because the script will try to compute and
display a factorial when we load it. To avoid this problem, we might
write the script this way:
#!/usr/local/bin/guile \
-e main -s
!#
(define (fact n)
(if (zero? n) 1
(* n (fact (- n 1)))))
(define (main args)
(display (fact (string->number (cadr args))))
(newline))
This version packages the actions the script should perform in a
function, ‘main’. This allows us to load the file purely for its
definitions, without any extraneous computation taking place. Then we
used the meta switch ‘\’ and the entry point switch ‘-e’ to tell Guile
to call ‘main’ after loading the script.
$ ./fact 50
30414093201713378043612608166064768844377641568960512000000000000
Suppose that we now want to write a script which computes the
‘choose’ function: given a set of M distinct objects, ‘(choose N M)’ is
the number of distinct subsets containing N objects each. It’s easy to
write ‘choose’ given ‘fact’, so we might write the script this way:
#!/usr/local/bin/guile \
-l fact -e main -s
!#
(define (choose n m)
(/ (fact m) (* (fact (- m n)) (fact n))))
(define (main args)
(let ((n (string->number (cadr args)))
(m (string->number (caddr args))))
(display (choose n m))
(newline)))
The command-line arguments here tell Guile to first load the file
‘fact’, and then run the script, with ‘main’ as the entry point. In
other words, the ‘choose’ script can use definitions made in the ‘fact’
script. Here are some sample runs:
$ ./choose 0 4
1
$ ./choose 1 4
4
$ ./choose 2 4
6
$ ./choose 3 4
4
$ ./choose 4 4
1
$ ./choose 50 100
100891344545564193334812497256
To call a specific procedure from a given module, we can use the
special form ‘(@ (MODULE) PROCEDURE)’:
#!/usr/local/bin/guile \
-l fact -e (@ (fac) main) -s
!#
(define-module (fac)
#:export (main))
(define (choose n m)
(/ (fact m) (* (fact (- m n)) (fact n))))
(define (main args)
(let ((n (string->number (cadr args)))
(m (string->number (caddr args))))
(display (choose n m))
(newline)))
We can use ‘@@’ to invoke non-exported procedures. For exported
procedures, we can simplify this call with the shorthand ‘(MODULE)’:
#!/usr/local/bin/guile \
-l fact -e (fac) -s
!#
(define-module (fac)
#:export (main))
(define (choose n m)
(/ (fact m) (* (fact (- m n)) (fact n))))
(define (main args)
(let ((n (string->number (cadr args)))
(m (string->number (caddr args))))
(display (choose n m))
(newline)))
For maximum portability, we can instead use the shell to execute
‘guile’ with specified command line arguments. Here we need to take
care to quote the command arguments correctly:
#!/usr/bin/env sh
exec guile -l fact -e '(@ (fac) main)' -s "$0" "$@"
!#
(define-module (fac)
#:export (main))
(define (choose n m)
(/ (fact m) (* (fact (- m n)) (fact n))))
(define (main args)
(let ((n (string->number (cadr args)))
(m (string->number (caddr args))))
(display (choose n m))
(newline)))
Finally, seasoned scripters are probably missing a mention of
subprocesses. In Bash, for example, most shell scripts run other
programs like ‘sed’ or the like to do the actual work.
In Guile it’s often possible get everything done within Guile itself,
so do give that a try first. But if you just need to run a program and
wait for it to finish, use ‘system*’. If you need to run a sub-program
and capture its output, or give it input, use ‘open-pipe’. *Note
Processes::, and *Note Pipes::, for more information.
4.4 Using Guile Interactively
=============================
When you start up Guile by typing just ‘guile’, without a ‘-c’ argument
or the name of a script to execute, you get an interactive interpreter
where you can enter Scheme expressions, and Guile will evaluate them and
print the results for you. Here are some simple examples.
scheme@(guile-user)> (+ 3 4 5)
$1 = 12
scheme@(guile-user)> (display "Hello world!\n")
Hello world!
scheme@(guile-user)> (values 'a 'b)
$2 = a
$3 = b
This mode of use is called a “REPL”, which is short for “Read-Eval-Print
Loop”, because the Guile interpreter first reads the expression that you
have typed, then evaluates it, and then prints the result.
The prompt shows you what language and module you are in. In this
case, the current language is ‘scheme’, and the current module is
‘(guile-user)’. *Note Other Languages::, for more information on
Guile’s support for languages other than Scheme.
4.4.1 The Init File, ‘~/.guile’
-------------------------------
When run interactively, Guile will load a local initialization file from
‘~/.guile’. This file should contain Scheme expressions for evaluation.
This facility lets the user customize their interactive Guile
environment, pulling in extra modules or parameterizing the REPL
implementation.
To run Guile without loading the init file, use the ‘-q’ command-line
option.
4.4.2 Readline
--------------
To make it easier for you to repeat and vary previously entered
expressions, or to edit the expression that you’re typing in, Guile can
use the GNU Readline library. This is not enabled by default because of
licensing reasons, but all you need to activate Readline is the
following pair of lines.
scheme@(guile-user)> (use-modules (ice-9 readline))
scheme@(guile-user)> (activate-readline)
It’s a good idea to put these two lines (without the
‘scheme@(guile-user)>’ prompts) in your ‘.guile’ file. *Note Init
File::, for more on ‘.guile’.
4.4.3 Value History
-------------------
Just as Readline helps you to reuse a previous input line, “value
history” allows you to use the _result_ of a previous evaluation in a
new expression. When value history is enabled, each evaluation result
is automatically assigned to the next in the sequence of variables ‘$1’,
‘$2’, .... You can then use these variables in subsequent expressions.
scheme@(guile-user)> (iota 10)
$1 = (0 1 2 3 4 5 6 7 8 9)
scheme@(guile-user)> (apply * (cdr $1))
$2 = 362880
scheme@(guile-user)> (sqrt $2)
$3 = 602.3952191045344
scheme@(guile-user)> (cons $2 $1)
$4 = (362880 0 1 2 3 4 5 6 7 8 9)
Value history is enabled by default, because Guile’s REPL imports the
‘(ice-9 history)’ module. Value history may be turned off or on within
the repl, using the options interface:
scheme@(guile-user)> ,option value-history #f
scheme@(guile-user)> 'foo
foo
scheme@(guile-user)> ,option value-history #t
scheme@(guile-user)> 'bar
$5 = bar
Note that previously recorded values are still accessible, even if
value history is off. In rare cases, these references to past
computations can cause Guile to use too much memory. One may clear
these values, possibly enabling garbage collection, via the
‘clear-value-history!’ procedure, described below.
The programmatic interface to value history is in a module:
(use-modules (ice-9 history))
-- Scheme Procedure: value-history-enabled?
Return true if value history is enabled, or false otherwise.
-- Scheme Procedure: enable-value-history!
Turn on value history, if it was off.
-- Scheme Procedure: disable-value-history!
Turn off value history, if it was on.
-- Scheme Procedure: clear-value-history!
Clear the value history. If the stored values are not captured by
some other data structure or closure, they may then be reclaimed by
the garbage collector.
4.4.4 REPL Commands
-------------------
The REPL exists to read expressions, evaluate them, and then print their
results. But sometimes one wants to tell the REPL to evaluate an
expression in a different way, or to do something else altogether. A
user can affect the way the REPL works with a “REPL command”.
The previous section had an example of a command, in the form of
‘,option’.
scheme@(guile-user)> ,option value-history #t
Commands are distinguished from expressions by their initial comma
(‘,’). Since a comma cannot begin an expression in most languages, it
is an effective indicator to the REPL that the following text forms a
command, not an expression.
REPL commands are convenient because they are always there. Even if
the current module doesn’t have a binding for ‘pretty-print’, one can
always ‘,pretty-print’.
The following sections document the various commands, grouped
together by functionality. Many of the commands have abbreviations; see
the online help (‘,help’) for more information.
4.4.4.1 Help Commands
.....................
When Guile starts interactively, it notifies the user that help can be
had by typing ‘,help’. Indeed, ‘help’ is a command, and a particularly
useful one, as it allows the user to discover the rest of the commands.
-- REPL Command: help [all | group | [-c] command]
Show help.
With one argument, tries to look up the argument as a group name,
giving help on that group if successful. Otherwise tries to look
up the argument as a command, giving help on the command.
If there is a command whose name is also a group name, use the ‘-c
COMMAND’ form to give help on the command instead of the group.
Without any argument, a list of help commands and command groups
are displayed.
-- REPL Command: show [topic]
Gives information about Guile.
With one argument, tries to show a particular piece of information;
currently supported topics are ‘warranty’ (or ‘w’), ‘copying’ (or
‘c’), and ‘version’ (or ‘v’).
Without any argument, a list of topics is displayed.
-- REPL Command: apropos regexp
Find bindings/modules/packages.
-- REPL Command: describe obj
Show description/documentation.
4.4.4.2 Module Commands
.......................
-- REPL Command: module [module]
Change modules / Show current module.
-- REPL Command: import module ...
Import modules / List those imported.
-- REPL Command: load file
Load a file in the current module.
-- REPL Command: reload [module]
Reload the given module, or the current module if none was given.
-- REPL Command: binding
List current bindings.
-- REPL Command: in module expression
-- REPL Command: in module command arg ...
Evaluate an expression, or alternatively, execute another
meta-command in the context of a module. For example, ‘,in (foo
bar) ,binding’ will show the bindings in the module ‘(foo bar)’.
4.4.4.3 Language Commands
.........................
-- REPL Command: language language
Change languages.
4.4.4.4 Compile Commands
........................
-- REPL Command: compile exp
Generate compiled code.
-- REPL Command: compile-file file
Compile a file.
-- REPL Command: expand exp
Expand any macros in a form.
-- REPL Command: optimize exp
Run the optimizer on a piece of code and print the result.
-- REPL Command: disassemble exp
Disassemble a compiled procedure.
-- REPL Command: disassemble-file file
Disassemble a file.
4.4.4.5 Profile Commands
........................
-- REPL Command: time exp
Time execution.
-- REPL Command: profile exp [#:hz hz=100] [#:count-calls?
count-calls?=#f] [#:display-style display-style=list]
Profile execution of an expression. This command compiled EXP and
then runs it within the statprof profiler, passing all keyword
options to the ‘statprof’ procedure. For more on statprof and on
the the options available to this command, *Note Statprof::.
-- REPL Command: trace exp [#:width w] [#:max-indent i]
Trace execution.
By default, the trace will limit its width to the width of your
terminal, or WIDTH if specified. Nested procedure invocations will
be printed farther to the right, though if the width of the
indentation passes the MAX-INDENT, the indentation is abbreviated.
These REPL commands can also be called as regular functions in scheme
code on including the ‘(ice-9 time)’ module.
4.4.4.6 Debug Commands
......................
These debugging commands are only available within a recursive REPL;
they do not work at the top level.
-- REPL Command: backtrace [count] [#:width w] [#:full? f]
Print a backtrace.
Print a backtrace of all stack frames, or innermost COUNT frames.
If COUNT is negative, the last COUNT frames will be shown.
-- REPL Command: up [count]
Select a calling stack frame.
Select and print stack frames that called this one. An argument
says how many frames up to go.
-- REPL Command: down [count]
Select a called stack frame.
Select and print stack frames called by this one. An argument says
how many frames down to go.
-- REPL Command: frame [idx]
Show a frame.
Show the selected frame. With an argument, select a frame by
index, then show it.
-- REPL Command: locals
Show local variables.
Show locally-bound variables in the selected frame.
-- REPL Command: error-message
-- REPL Command: error
Show error message.
Display the message associated with the error that started the
current debugging REPL.
-- REPL Command: registers
Show the VM registers associated with the current frame.
*Note Stack Layout::, for more information on VM stack frames.
-- REPL Command: width [cols]
Sets the number of display columns in the output of ‘,backtrace’
and ‘,locals’ to COLS. If COLS is not given, the width of the
terminal is used.
The next 3 commands work at any REPL.
-- REPL Command: break proc
Set a breakpoint at PROC.
-- REPL Command: break-at-source file line
Set a breakpoint at the given source location.
-- REPL Command: tracepoint proc
Set a tracepoint on the given procedure. This will cause all calls
to the procedure to print out a tracing message. *Note Tracing
Traps::, for more information.
The rest of the commands in this subsection all apply only when the
stack is “continuable” — in other words when it makes sense for the
program that the stack comes from to continue running. Usually this
means that the program stopped because of a trap or a breakpoint.
-- REPL Command: step
Tell the debugged program to step to the next source location.
-- REPL Command: next
Tell the debugged program to step to the next source location in
the same frame. (See *note Traps:: for the details of how this
works.)
-- REPL Command: finish
Tell the program being debugged to continue running until the
completion of the current stack frame, and at that time to print
the result and reenter the REPL.
4.4.4.7 Inspect Commands
........................
-- REPL Command: inspect exp
Inspect the result(s) of evaluating EXP.
-- REPL Command: pretty-print exp
Pretty-print the result(s) of evaluating EXP.
4.4.4.8 System Commands
.......................
-- REPL Command: gc
Garbage collection.
-- REPL Command: statistics
Display statistics.
-- REPL Command: option [name] [exp]
With no arguments, lists all options. With one argument, shows the
current value of the NAME option. With two arguments, sets the
NAME option to the result of evaluating the Scheme expression EXP.
-- REPL Command: quit
Quit this session.
Current REPL options include:
‘compile-options’
The options used when compiling expressions entered at the REPL.
*Note Compilation::, for more on compilation options.
‘interp’
Whether to interpret or compile expressions given at the REPL, if
such a choice is available. Off by default (indicating
compilation).
‘prompt’
A customized REPL prompt. ‘#f’ by default, indicating the default
prompt.
‘print’
A procedure of two arguments used to print the result of evaluating
each expression. The arguments are the current REPL and the value
to print. By default, ‘#f’, to use the default procedure.
‘value-history’
Whether value history is on or not. *Note Value History::.
‘on-error’
What to do when an error happens. By default, ‘debug’, meaning to
enter the debugger. Other values include ‘backtrace’, to show a
backtrace without entering the debugger, or ‘report’, to simply
show a short error printout.
Default values for REPL options may be set using
‘repl-default-option-set!’ from ‘(system repl common)’:
-- Scheme Procedure: repl-default-option-set! key value
Set the default value of a REPL option. This function is
particularly useful in a user’s init file. *Note Init File::.
4.4.5 Error Handling
--------------------
When code being evaluated from the REPL hits an error, Guile enters a
new prompt, allowing you to inspect the context of the error.
scheme@(guile-user)> (map string-append '("a" "b") '("c" #\d))
ERROR: In procedure string-append:
ERROR: Wrong type (expecting string): #\d
Entering a new prompt. Type `,bt' for a backtrace or `,q' to continue.
scheme@(guile-user) [1]>
The new prompt runs inside the old one, in the dynamic context of the
error. It is a recursive REPL, augmented with a reified representation
of the stack, ready for debugging.
‘,backtrace’ (abbreviated ‘,bt’) displays the Scheme call stack at
the point where the error occurred:
scheme@(guile-user) [1]> ,bt
1 (map # ("a" "b") ("c" #\d))
0 (string-append "b" #\d)
In the above example, the backtrace doesn’t have much source
information, as ‘map’ and ‘string-append’ are both primitives. But in
the general case, the space on the left of the backtrace indicates the
line and column in which a given procedure calls another.
You can exit a recursive REPL in the same way that you exit any REPL:
via ‘(quit)’, ‘,quit’ (abbreviated ‘,q’), or ‘C-d’, among other options.
4.4.6 Interactive Debugging
---------------------------
A recursive debugging REPL exposes a number of other meta-commands that
inspect the state of the computation at the time of the error. These
commands allow you to
• display the Scheme call stack at the point where the error
occurred;
• move up and down the call stack, to see in detail the expression
being evaluated, or the procedure being applied, in each “frame”;
and
• examine the values of variables and expressions in the context of
each frame.
*Note Debug Commands::, for documentation of the individual commands.
This section aims to give more of a walkthrough of a typical debugging
session.
First, we’re going to need a good error. Let’s try to macroexpand
the expression ‘(unquote foo)’, outside of a ‘quasiquote’ form, and see
how the macroexpander reports this error.
scheme@(guile-user)> (macroexpand '(unquote foo))
ERROR: In procedure macroexpand:
ERROR: unquote: expression not valid outside of quasiquote in (unquote foo)
Entering a new prompt. Type `,bt' for a backtrace or `,q' to continue.
scheme@(guile-user) [1]>
The ‘backtrace’ command, which can also be invoked as ‘bt’, displays
the call stack (aka backtrace) at the point where the debugger was
entered:
scheme@(guile-user) [1]> ,bt
In ice-9/psyntax.scm:
1130:21 3 (chi-top (unquote foo) () ((top)) e (eval) (hygiene #))
1071:30 2 (syntax-type (unquote foo) () ((top)) #f #f (# #) #f)
1368:28 1 (chi-macro # ...)
In unknown file:
0 (scm-error syntax-error macroexpand "~a: ~a in ~a" # #f)
A call stack consists of a sequence of stack “frames”, with each
frame describing one procedure which is waiting to do something with the
values returned by another. Here we see that there are four frames on
the stack.
Note that ‘macroexpand’ is not on the stack – it must have made a
tail call to ‘chi-top’, as indeed we would find if we searched
‘ice-9/psyntax.scm’ for its definition.
When you enter the debugger, the innermost frame is selected, which
means that the commands for getting information about the “current”
frame, or for evaluating expressions in the context of the current
frame, will do so by default with respect to the innermost frame. To
select a different frame, so that these operations will apply to it
instead, use the ‘up’, ‘down’ and ‘frame’ commands like this:
scheme@(guile-user) [1]> ,up
In ice-9/psyntax.scm:
1368:28 1 (chi-macro # ...)
scheme@(guile-user) [1]> ,frame 3
In ice-9/psyntax.scm:
1130:21 3 (chi-top (unquote foo) () ((top)) e (eval) (hygiene #))
scheme@(guile-user) [1]> ,down
In ice-9/psyntax.scm:
1071:30 2 (syntax-type (unquote foo) () ((top)) #f #f (# #) #f)
Perhaps we’re interested in what’s going on in frame 2, so we take a
look at its local variables:
scheme@(guile-user) [1]> ,locals
Local variables:
$1 = e = (unquote foo)
$2 = r = ()
$3 = w = ((top))
$4 = s = #f
$5 = rib = #f
$6 = mod = (hygiene guile-user)
$7 = for-car? = #f
$8 = first = unquote
$9 = ftype = macro
$10 = fval = #
$11 = fe = unquote
$12 = fw = ((top))
$13 = fs = #f
$14 = fmod = (hygiene guile-user)
All of the values are accessible by their value-history names (‘$N’):
scheme@(guile-user) [1]> $10
$15 = #
We can even invoke the procedure at the REPL directly:
scheme@(guile-user) [1]> ($10 'not-going-to-work)
ERROR: In procedure macroexpand:
ERROR: source expression failed to match any pattern in not-going-to-work
Entering a new prompt. Type `,bt' for a backtrace or `,q' to continue.
Well at this point we’ve caused an error within an error. Let’s just
quit back to the top level:
scheme@(guile-user) [2]> ,q
scheme@(guile-user) [1]> ,q
scheme@(guile-user)>
Finally, as a word to the wise: hackers close their REPL prompts with
‘C-d’.
4.5 Using Guile in Emacs
========================
Any text editor can edit Scheme, but some are better than others. Emacs
is the best, of course, and not just because it is a fine text editor.
Emacs has good support for Scheme out of the box, with sensible
indentation rules, parenthesis-matching, syntax highlighting, and even a
set of keybindings for structural editing, allowing navigation,
cut-and-paste, and transposition operations that work on balanced
S-expressions.
As good as it is, though, two things will vastly improve your
experience with Emacs and Guile.
The first is Taylor Campbell’s Paredit
(http://www.emacswiki.org/emacs/ParEdit). You should not code in any
dialect of Lisp without Paredit. (They say that unopinionated writing
is boring—hence this tone—but it’s the truth, regardless.) Paredit is
the bee’s knees.
The second is José Antonio Ortega Ruiz’s Geiser
(http://www.nongnu.org/geiser/). Geiser complements Emacs’
‘scheme-mode’ with tight integration to running Guile processes via a
‘comint-mode’ REPL buffer.
Of course there are keybindings to switch to the REPL, and a good
REPL environment, but Geiser goes beyond that, providing:
• Form evaluation in the context of the current file’s module.
• Macro expansion.
• File/module loading and/or compilation.
• Namespace-aware identifier completion (including local bindings,
names visible in the current module, and module names).
• Autodoc: the echo area shows information about the signature of the
procedure/macro around point automatically.
• Jump to definition of identifier at point.
• Access to documentation (including docstrings when the
implementation provides it).
• Listings of identifiers exported by a given module.
• Listings of callers/callees of procedures.
• Rudimentary support for debugging and error navigation.
• Support for multiple, simultaneous REPLs.
See Geiser’s web page at ’ provides declarations for all of
Guile’s functions and constants. You should ‘#include’ it at the head
of any C source file that uses identifiers described in this manual.
Once you’ve compiled your source files, you need to link them against
the Guile object code library, ‘libguile’.
As noted in the previous section, ‘’ is not in the
default search path for headers. The following command lines give
respectively the C compilation and link flags needed to build programs
using Guile 3.0:
pkg-config guile-3.0 --cflags
pkg-config guile-3.0 --libs
5.2.1 Guile Initialization Functions
------------------------------------
To initialize Guile, you can use one of several functions. The first,
‘scm_with_guile’, is the most portable way to initialize Guile. It will
initialize Guile when necessary and then call a function that you can
specify. Multiple threads can call ‘scm_with_guile’ concurrently and it
can also be called more than once in a given thread. The global state
of Guile will survive from one call of ‘scm_with_guile’ to the next.
Your function is called from within ‘scm_with_guile’ since the garbage
collector of Guile needs to know where the stack of each thread is.
A second function, ‘scm_init_guile’, initializes Guile for the
current thread. When it returns, you can use the Guile API in the
current thread. This function employs some non-portable magic to learn
about stack bounds and might thus not be available on all platforms.
One common way to use Guile is to write a set of C functions which
perform some useful task, make them callable from Scheme, and then link
the program with Guile. This yields a Scheme interpreter just like
‘guile’, but augmented with extra functions for some specific
application — a special-purpose scripting language.
In this situation, the application should probably process its
command-line arguments in the same manner as the stock Guile
interpreter. To make that straightforward, Guile provides the
‘scm_boot_guile’ and ‘scm_shell’ function.
For more about these functions, see *note Initialization::.
5.2.2 A Sample Guile Main Program
---------------------------------
Here is ‘simple-guile.c’, source code for a ‘main’ and an ‘inner_main’
function that will produce a complete Guile interpreter.
/* simple-guile.c --- Start Guile from C. */
#include
static void
inner_main (void *closure, int argc, char **argv)
{
/* preparation */
scm_shell (argc, argv);
/* after exit */
}
int
main (int argc, char **argv)
{
scm_boot_guile (argc, argv, inner_main, 0);
return 0; /* never reached, see inner_main */
}
The ‘main’ function calls ‘scm_boot_guile’ to initialize Guile,
passing it ‘inner_main’. Once ‘scm_boot_guile’ is ready, it invokes
‘inner_main’, which calls ‘scm_shell’ to process the command-line
arguments in the usual way.
5.2.3 Building the Example with Make
------------------------------------
Here is a Makefile which you can use to compile the example program. It
uses ‘pkg-config’ to learn about the necessary compiler and linker
flags.
# Use GCC, if you have it installed.
CC=gcc
# Tell the C compiler where to find
CFLAGS=`pkg-config --cflags guile-3.0`
# Tell the linker what libraries to use and where to find them.
LIBS=`pkg-config --libs guile-3.0`
simple-guile: simple-guile.o
${CC} simple-guile.o ${LIBS} -o simple-guile
simple-guile.o: simple-guile.c
${CC} -c ${CFLAGS} simple-guile.c
5.2.4 Building the Example with Autoconf
----------------------------------------
If you are using the GNU Autoconf package to make your application more
portable, Autoconf will settle many of the details in the Makefile
automatically, making it much simpler and more portable; we recommend
using Autoconf with Guile. Here is a ‘configure.ac’ file for
‘simple-guile’ that uses the standard ‘PKG_CHECK_MODULES’ macro to check
for Guile. Autoconf will process this file into a ‘configure’ script.
We recommend invoking Autoconf via the ‘autoreconf’ utility.
AC_INIT(simple-guile.c)
# Find a C compiler.
AC_PROG_CC
# Check for Guile
PKG_CHECK_MODULES([GUILE], [guile-3.0])
# Generate a Makefile, based on the results.
AC_OUTPUT(Makefile)
Run ‘autoreconf -vif’ to generate ‘configure’.
Here is a ‘Makefile.in’ template, from which the ‘configure’ script
produces a Makefile customized for the host system:
# The configure script fills in these values.
CC=@CC@
CFLAGS=@GUILE_CFLAGS@
LIBS=@GUILE_LIBS@
simple-guile: simple-guile.o
${CC} simple-guile.o ${LIBS} -o simple-guile
simple-guile.o: simple-guile.c
${CC} -c ${CFLAGS} simple-guile.c
The developer should use Autoconf to generate the ‘configure’ script
from the ‘configure.ac’ template, and distribute ‘configure’ with the
application. Here’s how a user might go about building the application:
$ ls
Makefile.in configure* configure.ac simple-guile.c
$ ./configure
checking for gcc... ccache gcc
checking whether the C compiler works... yes
checking for C compiler default output file name... a.out
checking for suffix of executables...
checking whether we are cross compiling... no
checking for suffix of object files... o
checking whether we are using the GNU C compiler... yes
checking whether ccache gcc accepts -g... yes
checking for ccache gcc option to accept ISO C89... none needed
checking for pkg-config... /usr/bin/pkg-config
checking pkg-config is at least version 0.9.0... yes
checking for GUILE... yes
configure: creating ./config.status
config.status: creating Makefile
$ make
[...]
$ ./simple-guile
guile> (+ 1 2 3)
6
guile> (getpwnam "jimb")
#("jimb" "83Z7d75W2tyJQ" 4008 10 "Jim Blandy" "/u/jimb"
"/usr/local/bin/bash")
guile> (exit)
$
5.3 Linking Guile with Libraries
================================
The previous section has briefly explained how to write programs that
make use of an embedded Guile interpreter. But sometimes, all you want
to do is make new primitive procedures and data types available to the
Scheme programmer. Writing a new version of ‘guile’ is inconvenient in
this case and it would in fact make the life of the users of your new
features needlessly hard.
For example, suppose that there is a program ‘guile-db’ that is a
version of Guile with additional features for accessing a database.
People who want to write Scheme programs that use these features would
have to use ‘guile-db’ instead of the usual ‘guile’ program. Now
suppose that there is also a program ‘guile-gtk’ that extends Guile with
access to the popular Gtk+ toolkit for graphical user interfaces.
People who want to write GUIs in Scheme would have to use ‘guile-gtk’.
Now, what happens when you want to write a Scheme application that uses
a GUI to let the user access a database? You would have to write a
_third_ program that incorporates both the database stuff and the GUI
stuff. This might not be easy (because ‘guile-gtk’ might be a quite
obscure program, say) and taking this example further makes it easy to
see that this approach can not work in practice.
It would have been much better if both the database features and the
GUI feature had been provided as libraries that can just be linked with
‘guile’. Guile makes it easy to do just this, and we encourage you to
make your extensions to Guile available as libraries whenever possible.
You write the new primitive procedures and data types in the normal
fashion, and link them into a shared library instead of into a
stand-alone program. The shared library can then be loaded dynamically
by Guile.
5.3.1 A Sample Guile Extension
------------------------------
This section explains how to make the Bessel functions of the C library
available to Scheme. First we need to write the appropriate glue code
to convert the arguments and return values of the functions from Scheme
to C and back. Additionally, we need a function that will add them to
the set of Guile primitives. Because this is just an example, we will
only implement this for the ‘j0’ function.
Consider the following file ‘bessel.c’.
#include
#include
SCM
j0_wrapper (SCM x)
{
return scm_from_double (j0 (scm_to_double (x)));
}
void
init_bessel ()
{
scm_c_define_gsubr ("j0", 1, 0, 0, j0_wrapper);
}
This C source file needs to be compiled into a shared library. Here
is how to do it on GNU/Linux:
gcc `pkg-config --cflags guile-3.0` \
-shared -o libguile-bessel.so -fPIC bessel.c
For creating shared libraries portably, we recommend the use of GNU
Libtool (*note Introduction: (libtool)Top.).
A shared library can be loaded into a running Guile process with the
function ‘load-extension’. In addition to the name of the library to
load, this function also expects the name of a function from that
library that will be called to initialize it. For our example, we are
going to call the function ‘init_bessel’ which will make ‘j0_wrapper’
available to Scheme programs with the name ‘j0’. Note that we do not
specify a filename extension such as ‘.so’ when invoking
‘load-extension’. The right extension for the host platform will be
provided automatically.
(load-extension "libguile-bessel" "init_bessel")
(j0 2)
⇒ 0.223890779141236
For this to work, ‘load-extension’ must be able to find
‘libguile-bessel’, of course. It will look in the places that are usual
for your operating system, and it will additionally look into the
directories listed in the ‘LTDL_LIBRARY_PATH’ environment variable.
To see how these Guile extensions via shared libraries relate to the
module system, *Note Putting Extensions into Modules::.
5.4 General concepts for using libguile
=======================================
When you want to embed the Guile Scheme interpreter into your program or
library, you need to link it against the ‘libguile’ library (*note
Linking Programs With Guile::). Once you have done this, your C code
has access to a number of data types and functions that can be used to
invoke the interpreter, or make new functions that you have written in C
available to be called from Scheme code, among other things.
Scheme is different from C in a number of significant ways, and Guile
tries to make the advantages of Scheme available to C as well. Thus, in
addition to a Scheme interpreter, libguile also offers dynamic types,
garbage collection, continuations, arithmetic on arbitrary sized
numbers, and other things.
The two fundamental concepts are dynamic types and garbage
collection. You need to understand how libguile offers them to C
programs in order to use the rest of libguile. Also, the more general
control flow of Scheme caused by continuations needs to be dealt with.
Running asynchronous signal handlers and multi-threading is known to
C code already, but there are of course a few additional rules when
using them together with libguile.
5.4.1 Dynamic Types
-------------------
Scheme is a dynamically-typed language; this means that the system
cannot, in general, determine the type of a given expression at compile
time. Types only become apparent at run time. Variables do not have
fixed types; a variable may hold a pair at one point, an integer at the
next, and a thousand-element vector later. Instead, values, not
variables, have fixed types.
In order to implement standard Scheme functions like ‘pair?’ and
‘string?’ and provide garbage collection, the representation of every
value must contain enough information to accurately determine its type
at run time. Often, Scheme systems also use this information to
determine whether a program has attempted to apply an operation to an
inappropriately typed value (such as taking the ‘car’ of a string).
Because variables, pairs, and vectors may hold values of any type,
Scheme implementations use a uniform representation for values — a
single type large enough to hold either a complete value or a pointer to
a complete value, along with the necessary typing information.
In Guile, this uniform representation of all Scheme values is the C
type ‘SCM’. This is an opaque type and its size is typically equivalent
to that of a pointer to ‘void’. Thus, ‘SCM’ values can be passed around
efficiently and they take up reasonably little storage on their own.
The most important rule is: You never access a ‘SCM’ value directly;
you only pass it to functions or macros defined in libguile.
As an obvious example, although a ‘SCM’ variable can contain
integers, you can of course not compute the sum of two ‘SCM’ values by
adding them with the C ‘+’ operator. You must use the libguile function
‘scm_sum’.
Less obvious and therefore more important to keep in mind is that you
also cannot directly test ‘SCM’ values for trueness. In Scheme, the
value ‘#f’ is considered false and of course a ‘SCM’ variable can
represent that value. But there is no guarantee that the ‘SCM’
representation of ‘#f’ looks false to C code as well. You need to use
‘scm_is_true’ or ‘scm_is_false’ to test a ‘SCM’ value for trueness or
falseness, respectively.
You also can not directly compare two ‘SCM’ values to find out
whether they are identical (that is, whether they are ‘eq?’ in Scheme
terms). You need to use ‘scm_is_eq’ for this.
The one exception is that you can directly assign a ‘SCM’ value to a
‘SCM’ variable by using the C ‘=’ operator.
The following (contrived) example shows how to do it right. It
implements a function of two arguments (A and FLAG) that returns A+1 if
FLAG is true, else it returns A unchanged.
SCM
my_incrementing_function (SCM a, SCM flag)
{
SCM result;
if (scm_is_true (flag))
result = scm_sum (a, scm_from_int (1));
else
result = a;
return result;
}
Often, you need to convert between ‘SCM’ values and appropriate C
values. For example, we needed to convert the integer ‘1’ to its ‘SCM’
representation in order to add it to A. Libguile provides many function
to do these conversions, both from C to ‘SCM’ and from ‘SCM’ to C.
The conversion functions follow a common naming pattern: those that
make a ‘SCM’ value from a C value have names of the form ‘scm_from_TYPE
(...)’ and those that convert a ‘SCM’ value to a C value use the form
‘scm_to_TYPE (...)’.
However, it is best to avoid converting values when you can. When
you must combine C values and ‘SCM’ values in a computation, it is often
better to convert the C values to ‘SCM’ values and do the computation by
using libguile functions than to the other way around (converting ‘SCM’
to C and doing the computation some other way).
As a simple example, consider this version of
‘my_incrementing_function’ from above:
SCM
my_other_incrementing_function (SCM a, SCM flag)
{
int result;
if (scm_is_true (flag))
result = scm_to_int (a) + 1;
else
result = scm_to_int (a);
return scm_from_int (result);
}
This version is much less general than the original one: it will only
work for values A that can fit into a ‘int’. The original function will
work for all values that Guile can represent and that ‘scm_sum’ can
understand, including integers bigger than ‘long long’, floating point
numbers, complex numbers, and new numerical types that have been added
to Guile by third-party libraries.
Also, computing with ‘SCM’ is not necessarily inefficient. Small
integers will be encoded directly in the ‘SCM’ value, for example, and
do not need any additional memory on the heap. See *note Data
Representation:: to find out the details.
Some special ‘SCM’ values are available to C code without needing to
convert them from C values:
Scheme value C representation
#f SCM_BOOL_F
#t SCM_BOOL_T
() SCM_EOL
In addition to ‘SCM’, Guile also defines the related type
‘scm_t_bits’. This is an unsigned integral type of sufficient size to
hold all information that is directly contained in a ‘SCM’ value. The
‘scm_t_bits’ type is used internally by Guile to do all the bit
twiddling explained in *note Data Representation::, but you will
encounter it occasionally in low-level user code as well.
5.4.2 Garbage Collection
------------------------
As explained above, the ‘SCM’ type can represent all Scheme values.
Some values fit entirely into a ‘SCM’ value (such as small integers),
but other values require additional storage in the heap (such as strings
and vectors). This additional storage is managed automatically by
Guile. You don’t need to explicitly deallocate it when a ‘SCM’ value is
no longer used.
Two things must be guaranteed so that Guile is able to manage the
storage automatically: it must know about all blocks of memory that have
ever been allocated for Scheme values, and it must know about all Scheme
values that are still being used. Given this knowledge, Guile can
periodically free all blocks that have been allocated but are not used
by any active Scheme values. This activity is called “garbage
collection”.
Guile’s garbage collector will automatically discover references to
‘SCM’ objects that originate in global variables, static data sections,
function arguments or local variables on the C and Scheme stacks, and
values in machine registers. Other references to ‘SCM’ objects, such as
those in other random data structures in the C heap that contain fields
of type ‘SCM’, can be made visible to the garbage collector by calling
the functions ‘scm_gc_protect_object’ or ‘scm_permanent_object’.
Collectively, these values form the “root set” of garbage collection;
any value on the heap that is referenced directly or indirectly by a
member of the root set is preserved, and all other objects are eligible
for reclamation.
In Guile, garbage collection has two logical phases: the “mark
phase”, in which the collector discovers the set of all live objects,
and the “sweep phase”, in which the collector reclaims the resources
associated with dead objects. The mark phase pauses the program and
traces all ‘SCM’ object references, starting with the root set. The
sweep phase actually runs concurrently with the main program,
incrementally reclaiming memory as needed by allocation.
In the mark phase, the garbage collector traces the Scheme stack and
heap “precisely”. Because the Scheme stack and heap are managed by
Guile, Guile can know precisely where in those data structures it might
find references to other heap objects. This is not the case,
unfortunately, for pointers on the C stack and static data segment.
Instead of requiring the user to inform Guile about all variables in C
that might point to heap objects, Guile traces the C stack and static
data segment “conservatively”. That is to say, Guile just treats every
word on the C stack and every C global variable as a potential reference
in to the Scheme heap(1). Any value that looks like a pointer to a
GC-managed object is treated as such, whether it actually is a reference
or not. Thus, scanning the C stack and static data segment is
guaranteed to find all actual references, but it might also find words
that only accidentally look like references. These “false positives”
might keep ‘SCM’ objects alive that would otherwise be considered dead.
While this might waste memory, keeping an object around longer than it
strictly needs to is harmless. This is why this technique is called
“conservative garbage collection”. In practice, the wasted memory seems
to be no problem, as the static C root set is almost always finite and
small, given that the Scheme stack is separate from the C stack.
The stack of every thread is scanned in this way and the registers of
the CPU and all other memory locations where local variables or function
parameters might show up are included in this scan as well.
The consequence of the conservative scanning is that you can just
declare local variables and function parameters of type ‘SCM’ and be
sure that the garbage collector will not free the corresponding objects.
However, a local variable or function parameter is only protected as
long as it is really on the stack (or in some register). As an
optimization, the C compiler might reuse its location for some other
value and the ‘SCM’ object would no longer be protected. Normally, this
leads to exactly the right behavior: the compiler will only overwrite a
reference when it is no longer needed and thus the object becomes
unprotected precisely when the reference disappears, just as wanted.
There are situations, however, where a ‘SCM’ object needs to be
around longer than its reference from a local variable or function
parameter. This happens, for example, when you retrieve some pointer
from a foreign object and work with that pointer directly. The
reference to the ‘SCM’ foreign object might be dead after the pointer
has been retrieved, but the pointer itself (and the memory pointed to)
is still in use and thus the foreign object must be protected. The
compiler does not know about this connection and might overwrite the
‘SCM’ reference too early.
To get around this problem, you can use ‘scm_remember_upto_here_1’
and its cousins. It will keep the compiler from overwriting the
reference. *Note Foreign Object Memory Management::.
---------- Footnotes ----------
(1) Note that Guile does not scan the C heap for references, so a
reference to a ‘SCM’ object from a memory segment allocated with
‘malloc’ will have to use some other means to keep the ‘SCM’ object
alive. *Note Garbage Collection Functions::.
5.4.3 Control Flow
------------------
Scheme has a more general view of program flow than C, both locally and
non-locally.
Controlling the local flow of control involves things like gotos,
loops, calling functions and returning from them. Non-local control
flow refers to situations where the program jumps across one or more
levels of function activations without using the normal call or return
operations.
The primitive means of C for local control flow is the ‘goto’
statement, together with ‘if’. Loops done with ‘for’, ‘while’ or ‘do’
could in principle be rewritten with just ‘goto’ and ‘if’. In Scheme,
the primitive means for local control flow is the _function call_
(together with ‘if’). Thus, the repetition of some computation in a
loop is ultimately implemented by a function that calls itself, that is,
by recursion.
This approach is theoretically very powerful since it is easier to
reason formally about recursion than about gotos. In C, using recursion
exclusively would not be practical, though, since it would eat up the
stack very quickly. In Scheme, however, it is practical: function calls
that appear in a “tail position” do not use any additional stack space
(*note Tail Calls::).
A function call is in a tail position when it is the last thing the
calling function does. The value returned by the called function is
immediately returned from the calling function. In the following
example, the call to ‘bar-1’ is in a tail position, while the call to
‘bar-2’ is not. (The call to ‘1-’ in ‘foo-2’ is in a tail position,
though.)
(define (foo-1 x)
(bar-1 (1- x)))
(define (foo-2 x)
(1- (bar-2 x)))
Thus, when you take care to recurse only in tail positions, the
recursion will only use constant stack space and will be as good as a
loop constructed from gotos.
Scheme offers a few syntactic abstractions (‘do’ and “named” ‘let’)
that make writing loops slightly easier.
But only Scheme functions can call other functions in a tail
position: C functions can not. This matters when you have, say, two
functions that call each other recursively to form a common loop. The
following (unrealistic) example shows how one might go about determining
whether a non-negative integer N is even or odd.
(define (my-even? n)
(cond ((zero? n) #t)
(else (my-odd? (1- n)))))
(define (my-odd? n)
(cond ((zero? n) #f)
(else (my-even? (1- n)))))
Because the calls to ‘my-even?’ and ‘my-odd?’ are in tail positions,
these two procedures can be applied to arbitrary large integers without
overflowing the stack. (They will still take a lot of time, of course.)
However, when one or both of the two procedures would be rewritten in
C, it could no longer call its companion in a tail position (since C
does not have this concept). You might need to take this consideration
into account when deciding which parts of your program to write in
Scheme and which in C.
In addition to calling functions and returning from them, a Scheme
program can also exit non-locally from a function so that the control
flow returns directly to an outer level. This means that some functions
might not return at all.
Even more, it is not only possible to jump to some outer level of
control, a Scheme program can also jump back into the middle of a
function that has already exited. This might cause some functions to
return more than once.
In general, these non-local jumps are done by invoking
“continuations” that have previously been captured using
‘call-with-current-continuation’. Guile also offers a slightly
restricted set of functions, ‘catch’ and ‘throw’, that can only be used
for non-local exits. This restriction makes them more efficient. Error
reporting (with the function ‘error’) is implemented by invoking
‘throw’, for example. The functions ‘catch’ and ‘throw’ belong to the
topic of “exceptions”.
Since Scheme functions can call C functions and vice versa, C code
can experience the more general control flow of Scheme as well. It is
possible that a C function will not return at all, or will return more
than once. While C does offer ‘setjmp’ and ‘longjmp’ for non-local
exits, it is still an unusual thing for C code. In contrast, non-local
exits are very common in Scheme, mostly to report errors.
You need to be prepared for the non-local jumps in the control flow
whenever you use a function from ‘libguile’: it is best to assume that
any ‘libguile’ function might signal an error or run a pending signal
handler (which in turn can do arbitrary things).
It is often necessary to take cleanup actions when the control leaves
a function non-locally. Also, when the control returns non-locally,
some setup actions might be called for. For example, the Scheme
function ‘with-output-to-port’ needs to modify the global state so that
‘current-output-port’ returns the port passed to ‘with-output-to-port’.
The global output port needs to be reset to its previous value when
‘with-output-to-port’ returns normally or when it is exited non-locally.
Likewise, the port needs to be set again when control enters
non-locally.
Scheme code can use the ‘dynamic-wind’ function to arrange for the
setting and resetting of the global state. C code can use the
corresponding ‘scm_internal_dynamic_wind’ function, or a
‘scm_dynwind_begin’/‘scm_dynwind_end’ pair together with suitable
’dynwind actions’ (*note Dynamic Wind::).
Instead of coping with non-local control flow, you can also prevent
it by erecting a _continuation barrier_, *Note Continuation Barriers::.
The function ‘scm_c_with_continuation_barrier’, for example, is
guaranteed to return exactly once.
5.4.4 Asynchronous Signals
--------------------------
You can not call libguile functions from handlers for POSIX signals, but
you can register Scheme handlers for POSIX signals such as ‘SIGINT’.
These handlers do not run during the actual signal delivery. Instead,
they are run when the program (more precisely, the thread that the
handler has been registered for) reaches the next _safe point_.
The libguile functions themselves have many such safe points.
Consequently, you must be prepared for arbitrary actions anytime you
call a libguile function. For example, even ‘scm_cons’ can contain a
safe point and when a signal handler is pending for your thread, calling
‘scm_cons’ will run this handler and anything might happen, including a
non-local exit although ‘scm_cons’ would not ordinarily do such a thing
on its own.
If you do not want to allow the running of asynchronous signal
handlers, you can block them temporarily with
‘scm_dynwind_block_asyncs’, for example. *Note Asyncs::.
Since signal handling in Guile relies on safe points, you need to
make sure that your functions do offer enough of them. Normally,
calling libguile functions in the normal course of action is all that is
needed. But when a thread might spent a long time in a code section
that calls no libguile function, it is good to include explicit safe
points. This can allow the user to interrupt your code with , for
example.
You can do this with the macro ‘SCM_TICK’. This macro is
syntactically a statement. That is, you could use it like this:
while (1)
{
SCM_TICK;
do_some_work ();
}
Frequent execution of a safe point is even more important in multi
threaded programs, *Note Multi-Threading::.
5.4.5 Multi-Threading
---------------------
Guile can be used in multi-threaded programs just as well as in
single-threaded ones.
Each thread that wants to use functions from libguile must put itself
into _guile mode_ and must then follow a few rules. If it doesn’t want
to honor these rules in certain situations, a thread can temporarily
leave guile mode (but can no longer use libguile functions during that
time, of course).
Threads enter guile mode by calling ‘scm_with_guile’,
‘scm_boot_guile’, or ‘scm_init_guile’. As explained in the reference
documentation for these functions, Guile will then learn about the stack
bounds of the thread and can protect the ‘SCM’ values that are stored in
local variables. When a thread puts itself into guile mode for the
first time, it gets a Scheme representation and is listed by
‘all-threads’, for example.
Threads in guile mode can block (e.g., do blocking I/O) without
causing any problems(1); temporarily leaving guile mode with
‘scm_without_guile’ before blocking slightly improves GC performance,
though. For some common blocking operations, Guile provides convenience
functions. For example, if you want to lock a pthread mutex while in
guile mode, you might want to use ‘scm_pthread_mutex_lock’ which is just
like ‘pthread_mutex_lock’ except that it leaves guile mode while
blocking.
All libguile functions are (intended to be) robust in the face of
multiple threads using them concurrently. This means that there is no
risk of the internal data structures of libguile becoming corrupted in
such a way that the process crashes.
A program might still produce nonsensical results, though. Taking
hashtables as an example, Guile guarantees that you can use them from
multiple threads concurrently and a hashtable will always remain a valid
hashtable and Guile will not crash when you access it. It does not
guarantee, however, that inserting into it concurrently from two threads
will give useful results: only one insertion might actually happen, none
might happen, or the table might in general be modified in a totally
arbitrary manner. (It will still be a valid hashtable, but not the one
that you might have expected.) Guile might also signal an error when it
detects a harmful race condition.
Thus, you need to put in additional synchronizations when multiple
threads want to use a single hashtable, or any other mutable Scheme
object.
When writing C code for use with libguile, you should try to make it
robust as well. An example that converts a list into a vector will help
to illustrate. Here is a correct version:
SCM
my_list_to_vector (SCM list)
{
SCM vector = scm_make_vector (scm_length (list), SCM_UNDEFINED);
size_t len, i;
len = scm_c_vector_length (vector);
i = 0;
while (i < len && scm_is_pair (list))
{
scm_c_vector_set_x (vector, i, scm_car (list));
list = scm_cdr (list);
i++;
}
return vector;
}
The first thing to note is that storing into a ‘SCM’ location
concurrently from multiple threads is guaranteed to be robust: you don’t
know which value wins but it will in any case be a valid ‘SCM’ value.
But there is no guarantee that the list referenced by LIST is not
modified in another thread while the loop iterates over it. Thus, while
copying its elements into the vector, the list might get longer or
shorter. For this reason, the loop must check both that it doesn’t
overrun the vector and that it doesn’t overrun the list. Otherwise,
‘scm_c_vector_set_x’ would raise an error if the index is out of range,
and ‘scm_car’ and ‘scm_cdr’ would raise an error if the value is not a
pair.
It is safe to use ‘scm_car’ and ‘scm_cdr’ on the local variable LIST
once it is known that the variable contains a pair. The contents of the
pair might change spontaneously, but it will always stay a valid pair
(and a local variable will of course not spontaneously point to a
different Scheme object).
Likewise, a vector such as the one returned by ‘scm_make_vector’ is
guaranteed to always stay the same length so that it is safe to only use
scm_c_vector_length once and store the result. (In the example, VECTOR
is safe anyway since it is a fresh object that no other thread can
possibly know about until it is returned from ‘my_list_to_vector’.)
Of course the behavior of ‘my_list_to_vector’ is suboptimal when LIST
does indeed get asynchronously lengthened or shortened in another
thread. But it is robust: it will always return a valid vector. That
vector might be shorter than expected, or its last elements might be
unspecified, but it is a valid vector and if a program wants to rule out
these cases, it must avoid modifying the list asynchronously.
Here is another version that is also correct:
SCM
my_pedantic_list_to_vector (SCM list)
{
SCM vector = scm_make_vector (scm_length (list), SCM_UNDEFINED);
size_t len, i;
len = scm_c_vector_length (vector);
i = 0;
while (i < len)
{
scm_c_vector_set_x (vector, i, scm_car (list));
list = scm_cdr (list);
i++;
}
return vector;
}
This version relies on the error-checking behavior of ‘scm_car’ and
‘scm_cdr’. When the list is shortened (that is, when LIST holds a
non-pair), ‘scm_car’ will throw an error. This might be preferable to
just returning a half-initialized vector.
The API for accessing vectors and arrays of various kinds from C
takes a slightly different approach to thread-robustness. In order to
get at the raw memory that stores the elements of an array, you need to
_reserve_ that array as long as you need the raw memory. During the
time an array is reserved, its elements can still spontaneously change
their values, but the memory itself and other things like the size of
the array are guaranteed to stay fixed. Any operation that would change
these parameters of an array that is currently reserved will signal an
error. In order to avoid these errors, a program should of course put
suitable synchronization mechanisms in place. As you can see, Guile
itself is again only concerned about robustness, not about correctness:
without proper synchronization, your program will likely not be correct,
but the worst consequence is an error message.
Real thread-safety often requires that a critical section of code is
executed in a certain restricted manner. A common requirement is that
the code section is not entered a second time when it is already being
executed. Locking a mutex while in that section ensures that no other
thread will start executing it, blocking asyncs ensures that no
asynchronous code enters the section again from the current thread, and
the error checking of Guile mutexes guarantees that an error is
signalled when the current thread accidentally reenters the critical
section via recursive function calls.
Guile provides two mechanisms to support critical sections as
outlined above. You can either use the macros
‘SCM_CRITICAL_SECTION_START’ and ‘SCM_CRITICAL_SECTION_END’ for very
simple sections; or use a dynwind context together with a call to
‘scm_dynwind_critical_section’.
The macros only work reliably for critical sections that are
guaranteed to not cause a non-local exit. They also do not detect an
accidental reentry by the current thread. Thus, you should probably
only use them to delimit critical sections that do not contain calls to
libguile functions or to other external functions that might do
complicated things.
The function ‘scm_dynwind_critical_section’, on the other hand, will
correctly deal with non-local exits because it requires a dynwind
context. Also, by using a separate mutex for each critical section, it
can detect accidental reentries.
---------- Footnotes ----------
(1) In Guile 1.8, a thread blocking in guile mode would prevent
garbage collection to occur. Thus, threads had to leave guile mode
whenever they could block. This is no longer needed with Guile 2.X.
5.5 Defining New Foreign Object Types
=====================================
The “foreign object type” facility is Guile’s mechanism for importing
object and types from C or other languages into Guile’s system. If you
have a C ‘struct foo’ type, for example, you can define a corresponding
Guile foreign object type that allows Scheme code to handle ‘struct foo
*’ objects.
To define a new foreign object type, the programmer provides Guile
with some essential information about the type — what its name is, how
many fields it has, and its finalizer (if any) — and Guile allocates a
fresh type for it. Foreign objects can be accessed from Scheme or from
C.
5.5.1 Defining Foreign Object Types
-----------------------------------
To create a new foreign object type from C, call
‘scm_make_foreign_object_type’. It returns a value of type ‘SCM’ which
identifies the new type.
Here is how one might declare a new type representing eight-bit
gray-scale images:
#include
struct image {
int width, height;
char *pixels;
/* The name of this image */
SCM name;
/* A function to call when this image is
modified, e.g., to update the screen,
or SCM_BOOL_F if no action necessary */
SCM update_func;
};
static SCM image_type;
void
init_image_type (void)
{
SCM name, slots;
scm_t_struct_finalize finalizer;
name = scm_from_utf8_symbol ("image");
slots = scm_list_1 (scm_from_utf8_symbol ("data"));
finalizer = NULL;
image_type =
scm_make_foreign_object_type (name, slots, finalizer);
}
The result is an initialized ‘image_type’ value that identifies the
new foreign object type. The next section describes how to create
foreign objects and how to access their slots.
5.5.2 Creating Foreign Objects
------------------------------
Foreign objects contain zero or more “slots” of data. A slot can hold a
pointer, an integer that fits into a ‘size_t’ or ‘ssize_t’, or a ‘SCM’
value.
All objects of a given foreign type have the same number of slots.
In the example from the previous section, the ‘image’ type has one slot,
because the slots list passed to ‘scm_make_foreign_object_type’ is of
length one. (The actual names given to slots are unimportant for most
users of the C interface, but can be used on the Scheme side to
introspect on the foreign object.)
To construct a foreign object and initialize its first slot, call
‘scm_make_foreign_object_1 (TYPE, FIRST_SLOT_VALUE)’. There are
similarly named constructors for initializing 0, 1, 2, or 3 slots, or
initializing N slots via an array. *Note Foreign Objects::, for full
details. Any fields that are not explicitly initialized are set to 0.
To get or set the value of a slot by index, you can use the
‘scm_foreign_object_ref’ and ‘scm_foreign_object_set_x’ functions.
These functions take and return values as ‘void *’ pointers; there are
corresponding convenience procedures like ‘_signed_ref’,
‘_unsigned_set_x’ and so on for dealing with slots as signed or unsigned
integers.
Foreign objects fields that are pointers can be tricky to manage. If
possible, it is best that all memory that is referenced by a foreign
object be managed by the garbage collector. That way, the GC can
automatically ensure that memory is accessible when it is needed, and
freed when it becomes inaccessible. If this is not the case for your
program – for example, if you are exposing an object to Scheme that was
allocated by some other, Guile-unaware part of your program – then you
will probably need to implement a finalizer. *Note Foreign Object
Memory Management::, for more.
Continuing the example from the previous section, if the global
variable ‘image_type’ contains the type returned by
‘scm_make_foreign_object_type’, here is how we could construct a foreign
object whose “data” field contains a pointer to a freshly allocated
‘struct image’:
SCM
make_image (SCM name, SCM s_width, SCM s_height)
{
struct image *image;
int width = scm_to_int (s_width);
int height = scm_to_int (s_height);
/* Allocate the `struct image'. Because we
use scm_gc_malloc, this memory block will
be automatically reclaimed when it becomes
inaccessible, and its members will be traced
by the garbage collector. */
image = (struct image *)
scm_gc_malloc (sizeof (struct image), "image");
image->width = width;
image->height = height;
/* Allocating the pixels with
scm_gc_malloc_pointerless means that the
pixels data is collectable by GC, but
that GC shouldn't spend time tracing its
contents for nested pointers because there
aren't any. */
image->pixels =
scm_gc_malloc_pointerless (width * height, "image pixels");
image->name = name;
image->update_func = SCM_BOOL_F;
/* Now wrap the struct image* in a new foreign
object, and return that object. */
return scm_make_foreign_object_1 (image_type, image);
}
We use ‘scm_gc_malloc_pointerless’ for the pixel buffer to tell the
garbage collector not to scan it for pointers. Calls to
‘scm_gc_malloc’, ‘scm_make_foreign_object_1’, and
‘scm_gc_malloc_pointerless’ raise an exception in out-of-memory
conditions; the garbage collector is able to reclaim previously
allocated memory if that happens.
5.5.3 Type Checking of Foreign Objects
--------------------------------------
Functions that operate on foreign objects should check that the passed
‘SCM’ value indeed is of the correct type before accessing its data.
They can do this with ‘scm_assert_foreign_object_type’.
For example, here is a simple function that operates on an image
object, and checks the type of its argument.
SCM
clear_image (SCM image_obj)
{
int area;
struct image *image;
scm_assert_foreign_object_type (image_type, image_obj);
image = scm_foreign_object_ref (image_obj, 0);
area = image->width * image->height;
memset (image->pixels, 0, area);
/* Invoke the image's update function. */
if (scm_is_true (image->update_func))
scm_call_0 (image->update_func);
return SCM_UNSPECIFIED;
}
5.5.4 Foreign Object Memory Management
--------------------------------------
Once a foreign object has been released to the tender mercies of the
Scheme system, it must be prepared to survive garbage collection. In
the example above, all the memory associated with the foreign object is
managed by the garbage collector because we used the ‘scm_gc_’
allocation functions. Thus, no special care must be taken: the garbage
collector automatically scans them and reclaims any unused memory.
However, when data associated with a foreign object is managed in
some other way—e.g., ‘malloc’’d memory or file descriptors—it is
possible to specify a “finalizer” function to release those resources
when the foreign object is reclaimed.
As discussed in *note Garbage Collection::, Guile’s garbage collector
will reclaim inaccessible memory as needed. This reclamation process
runs concurrently with the main program. When Guile analyzes the heap
and determines that an object’s memory can be reclaimed, that memory is
put on a “free list” of objects that can be reclaimed. Usually that’s
the end of it—the object is available for immediate re-use. However
some objects can have “finalizers” associated with them—functions that
are called on reclaimable objects to effect any external cleanup
actions.
Finalizers are tricky business and it is best to avoid them. They
can be invoked at unexpected times, or not at all—for example, they are
not invoked on process exit. They don’t help the garbage collector do
its job; in fact, they are a hindrance. Furthermore, they perturb the
garbage collector’s internal accounting. The GC decides to scan the
heap when it thinks that it is necessary, after some amount of
allocation. Finalizable objects almost always represent an amount of
allocation that is invisible to the garbage collector. The effect can
be that the actual resource usage of a system with finalizable objects
is higher than what the GC thinks it should be.
All those caveats aside, some foreign object types will need
finalizers. For example, if we had a foreign object type that wrapped
file descriptors—and we aren’t suggesting this, as Guile already has
ports —then you might define the type like this:
static SCM file_type;
static void
finalize_file (SCM file)
{
int fd = scm_foreign_object_signed_ref (file, 0);
if (fd >= 0)
{
scm_foreign_object_signed_set_x (file, 0, -1);
close (fd);
}
}
static void
init_file_type (void)
{
SCM name, slots;
scm_t_struct_finalize finalizer;
name = scm_from_utf8_symbol ("file");
slots = scm_list_1 (scm_from_utf8_symbol ("fd"));
finalizer = finalize_file;
image_type =
scm_make_foreign_object_type (name, slots, finalizer);
}
static SCM
make_file (int fd)
{
return scm_make_foreign_object_1 (file_type, (void *) fd);
}
Note that the finalizer may be invoked in ways and at times you might
not expect. In a Guile built without threading support, finalizers are
invoked via “asyncs”, which interleaves them with running Scheme code;
*note Asyncs::. If the user’s Guile is built with support for threads,
the finalizer will probably be called by a dedicated finalization
thread, unless the user invokes ‘scm_run_finalizers ()’ explicitly.
In either case, finalizers run concurrently with the main program,
and so they need to be async-safe and thread-safe. If for some reason
this is impossible, perhaps because you are embedding Guile in some
application that is not itself thread-safe, you have a few options. One
is to use guardians instead of finalizers, and arrange to pump the
guardians for finalizable objects. *Note Guardians::, for more
information. The other option is to disable automatic finalization
entirely, and arrange to call ‘scm_run_finalizers ()’ at appropriate
points. *Note Foreign Objects::, for more on these interfaces.
Finalizers are allowed to allocate memory, access GC-managed memory,
and in general can do anything any Guile user code can do. This was not
the case in Guile 1.8, where finalizers were much more restricted. In
particular, in Guile 2.0, finalizers can resuscitate objects. We do not
recommend that users avail themselves of this possibility, however, as a
resuscitated object can re-expose other finalizable objects that have
been already finalized back to Scheme. These objects will not be
finalized again, but they could cause use-after-free problems to code
that handles objects of that particular foreign object type. To guard
against this possibility, robust finalization routines should clear
state from the foreign object, as in the above ‘free_file’ example.
One final caveat. Foreign object finalizers are associated with the
lifetime of a foreign object, not of its fields. If you access a field
of a finalizable foreign object, and do not arrange to keep a reference
on the foreign object itself, it could be that the outer foreign object
gets finalized while you are working with its field.
For example, consider a procedure to read some data from a file, from
our example above.
SCM
read_bytes (SCM file, SCM n)
{
int fd;
SCM buf;
size_t len, pos;
scm_assert_foreign_object_type (file_type, file);
fd = scm_foreign_object_signed_ref (file, 0);
if (fd < 0)
scm_wrong_type_arg_msg ("read-bytes", SCM_ARG1,
file, "open file");
len = scm_to_size_t (n);
SCM buf = scm_c_make_bytevector (scm_to_size_t (n));
pos = 0;
while (pos < len)
{
char *bytes = SCM_BYTEVECTOR_CONTENTS (buf);
ssize_t count = read (fd, bytes + pos, len - pos);
if (count < 0)
scm_syserror ("read-bytes");
if (count == 0)
break;
pos += count;
}
scm_remember_upto_here_1 (file);
return scm_values (scm_list_2 (buf, scm_from_size_t (pos)));
}
After the prelude, only the ‘fd’ value is used and the C compiler has
no reason to keep the ‘file’ object around. If ‘scm_c_make_bytevector’
results in a garbage collection, ‘file’ might not be on the stack or
anywhere else and could be finalized, leaving ‘read’ to read a closed
(or, in a multi-threaded program, possibly re-used) file descriptor.
The use of ‘scm_remember_upto_here_1’ prevents this, by creating a
reference to ‘file’ after all data accesses. *Note Garbage Collection
Functions::.
‘scm_remember_upto_here_1’ is only needed on finalizable objects,
because garbage collection of other values is invisible to the program –
it happens when needed, and is not observable. But if you can, save
yourself the headache and build your program in such a way that it
doesn’t need finalization.
5.5.5 Foreign Objects and Scheme
--------------------------------
It is also possible to create foreign objects and object types from
Scheme, and to access fields of foreign objects from Scheme. For
example, the file example from the last section could be equivalently
expressed as:
(define-module (my-file)
#:use-module (system foreign-object)
#:use-module ((oop goops) #:select (make))
#:export (make-file))
(define (finalize-file file)
(let ((fd (struct-ref file 0)))
(unless (< fd 0)
(struct-set! file 0 -1)
(close-fdes fd))))
(define
(make-foreign-object-type ' '(fd)
#:finalizer finalize-file))
(define (make-file fd)
(make #:fd fd))
Here we see that the result of ‘make-foreign-object-type’, which is
the equivalent of ‘scm_make_foreign_object_type’, is a struct vtable.
*Note Vtables::, for more information. To instantiate the foreign
object, which is really a Guile struct, we use ‘make’. (We could have
used ‘make-struct/no-tail’, but as an implementation detail, finalizers
are attached in the ‘initialize’ method called by ‘make’). To access
the fields, we use ‘struct-ref’ and ‘struct-set!’. *Note Structure
Basics::.
There is a convenience syntax, ‘define-foreign-object-type’, that
defines a type along with a constructor, and getters for the fields. An
appropriate invocation of ‘define-foreign-object-type’ for the file
object type could look like this:
(use-modules (system foreign-object))
(define-foreign-object-type
make-file
(fd)
#:finalizer finalize-file)
This defines the ‘’ type with one field, a ‘make-file’
constructor, and a getter for the ‘fd’ field, bound to ‘fd’.
Foreign object types are not only vtables but are actually GOOPS
classes, as hinted at above. *Note GOOPS::, for more on Guile’s
object-oriented programming system. Thus one can define print and
equality methods using GOOPS:
(use-modules (oop goops))
(define-method (write (file ) port)
;; Assuming existence of the `fd' getter
(format port "#< ~a>" (fd file)))
(define-method (equal? (a ) (b ))
(eqv? (fd a) (fd b)))
One can even sub-class foreign types.
(define-class ()
(name #:init-keyword #:name #:init-value #f #:accessor name))
The question arises of how to construct these values, given that
‘make-file’ returns a plain old ‘’ object. It turns out that you
can use the GOOPS construction interface, where every field of the
foreign object has an associated initialization keyword argument.
(define* (my-open-file name #:optional (flags O_RDONLY))
(make #:fd (open-fdes name flags) #:name name))
(define-method (write (file ) port)
(format port "#< ~s ~a>" (name file) (fd file)))
*Note Foreign Objects::, for full documentation on the Scheme
interface to foreign objects. *Note GOOPS::, for more on GOOPS.
As a final note, you might wonder how this system supports
encapsulation of sensitive values. First, we have to recognize that
some facilities are essentially unsafe and have global scope. For
example, in C, the integrity and confidentiality of a part of a program
is at the mercy of every other part of that program – because any part
of the program can read and write anything in its address space. At the
same time, principled access to structured data is organized in C on
lexical boundaries; if you don’t expose accessors for your object, you
trust other parts of the program not to work around that barrier.
The situation is not dissimilar in Scheme. Although Scheme’s unsafe
constructs are fewer in number than in C, they do exist. The ‘(system
foreign)’ module can be used to violate confidentiality and integrity,
and shouldn’t be exposed to untrusted code. Although ‘struct-ref’ and
‘struct-set!’ are less unsafe, they still have a cross-cutting
capability of drilling through abstractions. Performing a ‘struct-set!’
on a foreign object slot could cause unsafe foreign code to crash.
Ultimately, structures in Scheme are capabilities for abstraction, and
not abstractions themselves.
That leaves us with the lexical capabilities, like constructors and
accessors. Here is where encapsulation lies: the practical degree to
which the innards of your foreign objects are exposed is the degree to
which their accessors are lexically available in user code. If you want
to allow users to reference fields of your foreign object, provide them
with a getter. Otherwise you should assume that the only access to your
object may come from your code, which has the relevant authority, or via
code with access to cross-cutting ‘struct-ref’ and such, which also has
the cross-cutting authority.
5.6 Function Snarfing
=====================
When writing C code for use with Guile, you typically define a set of C
functions, and then make some of them visible to the Scheme world by
calling ‘scm_c_define_gsubr’ or related functions. If you have many
functions to publish, it can sometimes be annoying to keep the list of
calls to ‘scm_c_define_gsubr’ in sync with the list of function
definitions.
Guile provides the ‘guile-snarf’ program to manage this problem.
Using this tool, you can keep all the information needed to define the
function alongside the function definition itself; ‘guile-snarf’ will
extract this information from your source code, and automatically
generate a file of calls to ‘scm_c_define_gsubr’ which you can
‘#include’ into an initialization function.
The snarfing mechanism works for many kind of initialization actions,
not just for collecting calls to ‘scm_c_define_gsubr’. For a full list
of what can be done, *Note Snarfing Macros::.
The ‘guile-snarf’ program is invoked like this:
guile-snarf [-o OUTFILE] [CPP-ARGS ...]
This command will extract initialization actions to OUTFILE. When no
OUTFILE has been specified or when OUTFILE is ‘-’, standard output will
be used. The C preprocessor is called with CPP-ARGS (which usually
include an input file) and the output is filtered to extract the
initialization actions.
If there are errors during processing, OUTFILE is deleted and the
program exits with non-zero status.
During snarfing, the pre-processor macro ‘SCM_MAGIC_SNARFER’ is
defined. You could use this to avoid including snarfer output files
that don’t yet exist by writing code like this:
#ifndef SCM_MAGIC_SNARFER
#include "foo.x"
#endif
Here is how you might define the Scheme function ‘clear-image’,
implemented by the C function ‘clear_image’:
#include
SCM_DEFINE (clear_image, "clear-image", 1, 0, 0,
(SCM image),
"Clear the image.")
{
/* C code to clear the image in image... */
}
void
init_image_type ()
{
#include "image-type.x"
}
The ‘SCM_DEFINE’ declaration says that the C function ‘clear_image’
implements a Scheme function called ‘clear-image’, which takes one
required argument (of type ‘SCM’ and named ‘image’), no optional
arguments, and no rest argument. The string ‘"Clear the image."’
provides a short help text for the function, it is called a “docstring”.
‘SCM_DEFINE’ macro also defines a static array of characters
initialized to the Scheme name of the function. In this case,
‘s_clear_image’ is set to the C string, "clear-image". You might want
to use this symbol when generating error messages.
Assuming the text above lives in a file named ‘image-type.c’, you
will need to execute the following command to prepare this file for
compilation:
guile-snarf -o image-type.x image-type.c
This scans ‘image-type.c’ for ‘SCM_DEFINE’ declarations, and writes
to ‘image-type.x’ the output:
scm_c_define_gsubr ("clear-image", 1, 0, 0, (SCM (*)() ) clear_image);
When compiled normally, ‘SCM_DEFINE’ is a macro which expands to the
function header for ‘clear_image’.
Note that the output file name matches the ‘#include’ from the input
file. Also, you still need to provide all the same information you
would if you were using ‘scm_c_define_gsubr’ yourself, but you can place
the information near the function definition itself, so it is less
likely to become incorrect or out-of-date.
If you have many files that ‘guile-snarf’ must process, you should
consider using a fragment like the following in your Makefile:
snarfcppopts = $(DEFS) $(INCLUDES) $(CPPFLAGS) $(CFLAGS)
.SUFFIXES: .x
.c.x:
guile-snarf -o $@ $< $(snarfcppopts)
This tells make to run ‘guile-snarf’ to produce each needed ‘.x’ file
from the corresponding ‘.c’ file.
The program ‘guile-snarf’ passes its command-line arguments directly
to the C preprocessor, which it uses to extract the information it needs
from the source code. this means you can pass normal compilation flags
to ‘guile-snarf’ to define preprocessor symbols, add header file
directories, and so on.
5.7 An Overview of Guile Programming
====================================
Guile is designed as an extension language interpreter that is
straightforward to integrate with applications written in C (and C++).
The big win here for the application developer is that Guile
integration, as the Guile web page says, “lowers your project’s
hacktivation energy.” Lowering the hacktivation energy means that you,
as the application developer, _and your users_, reap the benefits that
flow from being able to extend the application in a high level extension
language rather than in plain old C.
In abstract terms, it’s difficult to explain what this really means
and what the integration process involves, so instead let’s begin by
jumping straight into an example of how you might integrate Guile into
an existing program, and what you could expect to gain by so doing.
With that example under our belts, we’ll then return to a more general
analysis of the arguments involved and the range of programming options
available.
5.7.1 How One Might Extend Dia Using Guile
------------------------------------------
Dia is a free software program for drawing schematic diagrams like flow
charts and floor plans ( :’, you get a prompt where you can type in
any Elisp code, which Emacs will then evaluate. Or, again like Emacs,
you could provide a mechanism (such as an init file) to allow Scheme
code to be associated with a particular key sequence, and evaluate the
code when that key sequence is entered.
In either case, once you have the Scheme code that you want to
evaluate, as a null terminated string, you can tell Guile to evaluate it
by calling the ‘scm_c_eval_string’ function.
5.7.1.6 Top-level Structure of Guile-enabled Dia
................................................
Let’s assume that the pre-Guile Dia code looks structurally like this:
• ‘main ()’
• do lots of initialization and setup stuff
• enter Gtk main loop
When you add Guile to a program, one (rather technical) requirement
is that Guile’s garbage collector needs to know where the bottom of the
C stack is. The easiest way to ensure this is to use ‘scm_boot_guile’
like this:
• ‘main ()’
• do lots of initialization and setup stuff
• ‘scm_boot_guile (argc, argv, inner_main, NULL)’
• ‘inner_main ()’
• define all foreign object types
• export primitives to Scheme using ‘scm_c_define_gsubr’
• enter Gtk main loop
In other words, you move the guts of what was previously in your
‘main’ function into a new function called ‘inner_main’, and then add a
‘scm_boot_guile’ call, with ‘inner_main’ as a parameter, to the end of
‘main’.
Assuming that you are using foreign objects and have written
primitive code as described in the preceding subsections, you also need
to insert calls to declare your new foreign objects and export the
primitives to Scheme. These declarations must happen _inside_ the
dynamic scope of the ‘scm_boot_guile’ call, but also _before_ any code
is run that could possibly use them — the beginning of ‘inner_main’ is
an ideal place for this.
5.7.1.7 Going Further with Dia and Guile
........................................
The steps described so far implement an initial Guile integration that
already gives a lot of additional power to Dia application users. But
there are further steps that you could take, and it’s interesting to
consider a few of these.
In general, you could progressively move more of Dia’s source code
from C into Scheme. This might make the code more maintainable and
extensible, and it could open the door to new programming paradigms that
are tricky to effect in C but straightforward in Scheme.
A specific example of this is that you could use the guile-gtk
package, which provides Scheme-level procedures for most of the Gtk+
library, to move the code that lays out and displays Dia objects from C
to Scheme.
As you follow this path, it naturally becomes less useful to maintain
a distinction between Dia’s original non-Guile-related source code, and
its later code implementing foreign objects and primitives for the
Scheme world.
For example, suppose that the original source code had a
‘dia_change_fill_pattern’ function:
void dia_change_fill_pattern (struct dia_shape * shape,
struct dia_pattern * pattern)
{
/* real pattern change work */
}
During initial Guile integration, you add a ‘change_fill_pattern’
primitive for Scheme purposes, which accesses the underlying structures
from its foreign object values and uses ‘dia_change_fill_pattern’ to do
the real work:
SCM change_fill_pattern (SCM shape, SCM pattern)
{
struct dia_shape * d_shape;
struct dia_pattern * d_pattern;
...
dia_change_fill_pattern (d_shape, d_pattern);
return SCM_UNSPECIFIED;
}
At this point, it makes sense to keep ‘dia_change_fill_pattern’ and
‘change_fill_pattern’ separate, because ‘dia_change_fill_pattern’ can
also be called without going through Scheme at all, say because the user
clicks a button which causes a C-registered Gtk+ callback to be called.
But, if the code for creating buttons and registering their callbacks
is moved into Scheme (using guile-gtk), it may become true that
‘dia_change_fill_pattern’ can no longer be called other than through
Scheme. In which case, it makes sense to abolish it and move its
contents directly into ‘change_fill_pattern’, like this:
SCM change_fill_pattern (SCM shape, SCM pattern)
{
struct dia_shape * d_shape;
struct dia_pattern * d_pattern;
...
/* real pattern change work */
return SCM_UNSPECIFIED;
}
So further Guile integration progressively _reduces_ the amount of
functional C code that you have to maintain over the long term.
A similar argument applies to data representation. In the discussion
of foreign objects earlier, issues arose because of the different memory
management and lifetime models that normally apply to data structures in
C and in Scheme. However, with further Guile integration, you can
resolve this issue in a more radical way by allowing all your data
structures to be under the control of the garbage collector, and kept
alive by references from the Scheme world. Instead of maintaining an
array or linked list of shapes in C, you would instead maintain a list
in Scheme.
Rather like the coalescing of ‘dia_change_fill_pattern’ and
‘change_fill_pattern’, the practical upshot of such a change is that you
would no longer have to keep the ‘dia_shape’ and ‘dia_guile_shape’
structures separate, and so wouldn’t need to worry about the pointers
between them. Instead, you could change the foreign object definition
to wrap the ‘dia_shape’ structure directly, and send ‘dia_guile_shape’
off to the scrap yard. Cut out the middle man!
Finally, we come to the holy grail of Guile’s free software /
extension language approach. Once you have a Scheme representation for
interesting Dia data types like shapes, and a handy bunch of primitives
for manipulating them, it suddenly becomes clear that you have a bundle
of functionality that could have far-ranging use beyond Dia itself. In
other words, the data types and primitives could now become a library,
and Dia becomes just one of the many possible applications using that
library — albeit, at this early stage, a rather important one!
In this model, Guile becomes just the glue that binds everything
together. Imagine an application that usefully combined functionality
from Dia, Gnumeric and GnuCash — it’s tricky right now, because no such
application yet exists; but it’ll happen some day ...
5.7.2 Why Scheme is More Hackable Than C
----------------------------------------
Underlying Guile’s value proposition is the assumption that programming
in a high level language, specifically Guile’s implementation of Scheme,
is necessarily better in some way than programming in C. What do we mean
by this claim, and how can we be so sure?
One class of advantages applies not only to Scheme, but more
generally to any interpretable, high level, scripting language, such as
Emacs Lisp, Python, Ruby, or TeX’s macro language. Common features of
all such languages, when compared to C, are that:
• They lend themselves to rapid and experimental development cycles,
owing usually to a combination of their interpretability and the
integrated development environment in which they are used.
• They free developers from some of the low level bookkeeping tasks
associated with C programming, notably memory management.
• They provide high level features such as container objects and
exception handling that make common programming tasks easier.
In the case of Scheme, particular features that make programming
easier — and more fun! — are its powerful mechanisms for abstracting
parts of programs (closures — *note About Closure::) and for iteration
(*note while do::).
The evidence in support of this argument is empirical: the huge
amount of code that has been written in extension languages for
applications that support this mechanism. Most notable are extensions
written in Emacs Lisp for GNU Emacs, in TeX’s macro language for TeX,
and in Script-Fu for the Gimp, but there is increasingly now a
significant code eco-system for Guile-based applications as well, such
as Lilypond and GnuCash. It is close to inconceivable that similar
amounts of functionality could have been added to these applications
just by writing new code in their base implementation languages.
5.7.3 Example: Using Guile for an Application Testbed
-----------------------------------------------------
As an example of what this means in practice, imagine writing a testbed
for an application that is tested by submitting various requests (via a
C interface) and validating the output received. Suppose further that
the application keeps an idea of its current state, and that the
“correct” output for a given request may depend on the current
application state. A complete “white box”(1) test plan for this
application would aim to submit all possible requests in each
distinguishable state, and validate the output for all request/state
combinations.
To write all this test code in C would be very tedious. Suppose
instead that the testbed code adds a single new C function, to submit an
arbitrary request and return the response, and then uses Guile to export
this function as a Scheme procedure. The rest of the testbed can then
be written in Scheme, and so benefits from all the advantages of
programming in Scheme that were described in the previous section.
(In this particular example, there is an additional benefit of
writing most of the testbed in Scheme. A common problem for white box
testing is that mistakes and mistaken assumptions in the application
under test can easily be reproduced in the testbed code. It is more
difficult to copy mistakes like this when the testbed is written in a
different language from the application.)
---------- Footnotes ----------
(1) A “white box” test plan is one that incorporates knowledge of the
internal design of the application under test.
5.7.4 A Choice of Programming Options
-------------------------------------
The preceding arguments and example point to a model of Guile
programming that is applicable in many cases. According to this model,
Guile programming involves a balance between C and Scheme programming,
with the aim being to extract the greatest possible Scheme level benefit
from the least amount of C level work.
The C level work required in this model usually consists of packaging
and exporting functions and application objects such that they can be
seen and manipulated on the Scheme level. To help with this, Guile’s C
language interface includes utility features that aim to make this kind
of integration very easy for the application developer.
This model, though, is really just one of a range of possible
programming options. If all of the functionality that you need is
available from Scheme, you could choose instead to write your whole
application in Scheme (or one of the other high level languages that
Guile supports through translation), and simply use Guile as an
interpreter for Scheme. (In the future, we hope that Guile will also be
able to compile Scheme code, so lessening the performance gap between C
and Scheme code.) Or, at the other end of the C–Scheme scale, you could
write the majority of your application in C, and only call out to Guile
occasionally for specific actions such as reading a configuration file
or executing a user-specified extension. The choices boil down to two
basic questions:
• Which parts of the application do you write in C, and which in
Scheme (or another high level translated language)?
• How do you design the interface between the C and Scheme parts of
your application?
These are of course design questions, and the right design for any
given application will always depend upon the particular requirements
that you are trying to meet. In the context of Guile, however, there
are some generally applicable considerations that can help you when
designing your answers.
5.7.4.1 What Functionality is Already Available?
................................................
Suppose, for the sake of argument, that you would prefer to write your
whole application in Scheme. Then the API available to you consists of:
• standard Scheme
• plus the extensions to standard Scheme provided by Guile in its
core distribution
• plus any additional functionality that you or others have packaged
so that it can be loaded as a Guile Scheme module.
A module in the last category can either be a pure Scheme module — in
other words a collection of utility procedures coded in Scheme — or a
module that provides a Scheme interface to an extension library coded in
C — in other words a nice package where someone else has done the work
of wrapping up some useful C code for you. The set of available modules
is growing quickly and already includes such useful examples as ‘(gtk
gtk)’, which makes Gtk+ drawing functions available in Scheme, and
‘(database postgres)’, which provides SQL access to a Postgres database.
Given the growing collection of pre-existing modules, it is quite
feasible that your application could be implemented by combining a
selection of these modules together with new application code written in
Scheme.
If this approach is not enough, because the functionality that your
application needs is not already available in this form, and it is
impossible to write the new functionality in Scheme, you will need to
write some C code. If the required function is already available in C
(e.g. in a library), all you need is a little glue to connect it to the
world of Guile. If not, you need both to write the basic code and to
plumb it into Guile.
In either case, two general considerations are important. Firstly,
what is the interface by which the functionality is presented to the
Scheme world? Does the interface consist only of function calls (for
example, a simple drawing interface), or does it need to include
“objects” of some kind that can be passed between C and Scheme and
manipulated by both worlds. Secondly, how does the lifetime and memory
management of objects in the C code relate to the garbage collection
governed approach of Scheme objects? In the case where the basic C code
is not already written, most of the difficulties of memory management
can be avoided by using Guile’s C interface features from the start.
For the full documentation on writing C code for Guile and connecting
existing C code to the Guile world, see *note Defining New Foreign
Object Types::, *note Primitive Procedures::, and *note Foreign Function
Interface::.
5.7.4.2 Functional and Performance Constraints
..............................................
5.7.4.3 Your Preferred Programming Style
........................................
5.7.4.4 What Controls Program Execution?
........................................
5.7.5 How About Application Users?
----------------------------------
So far we have considered what Guile programming means for an
application developer. But what if you are instead _using_ an existing
Guile-based application, and want to know what your options are for
programming and extending this application?
The answer to this question varies from one application to another,
because the options available depend inevitably on whether the
application developer has provided any hooks for you to hang your own
code on and, if there are such hooks, what they allow you to do.(1) For
example...
• If the application permits you to load and execute any Guile code,
the world is your oyster. You can extend the application in any
way that you choose.
• A more cautious application might allow you to load and execute
Guile code, but only in a “safe” environment, where the interface
available is restricted by the application from the standard Guile
API.
• Or a really fearful application might not provide a hook to really
execute user code at all, but just use Scheme syntax as a
convenient way for users to specify application data or
configuration options.
In the last two cases, what you can do is, by definition, restricted
by the application, and you should refer to the application’s own manual
to find out your options.
The most well known example of the first case is Emacs, with its
extension language Emacs Lisp: as well as being a text editor, Emacs
supports the loading and execution of arbitrary Emacs Lisp code. The
result of such openness has been dramatic: Emacs now benefits from
user-contributed Emacs Lisp libraries that extend the basic editing
function to do everything from reading news to psychoanalysis and
playing adventure games. The only limitation is that extensions are
restricted to the functionality provided by Emacs’s built-in set of
primitive operations. For example, you can interact and display data by
manipulating the contents of an Emacs buffer, but you can’t pop-up and
draw a window with a layout that is totally different to the Emacs
standard.
This situation with a Guile application that supports the loading of
arbitrary user code is similar, except perhaps even more so, because
Guile also supports the loading of extension libraries written in C.
This last point enables user code to add new primitive operations to
Guile, and so to bypass the limitation present in Emacs Lisp.
At this point, the distinction between an application developer and
an application user becomes rather blurred. Instead of seeing yourself
as a user extending an application, you could equally well say that you
are developing a new application of your own using some of the primitive
functionality provided by the original application. As such, all the
discussions of the preceding sections of this chapter are relevant to
how you can proceed with developing your extension.
---------- Footnotes ----------
(1) Of course, in the world of free software, you always have the
freedom to modify the application’s source code to your own
requirements. Here we are concerned with the extension options that the
application has provided for without your needing to modify its source
code.
5.8 Autoconf Support
====================
Autoconf, a part of the GNU build system, makes it easy for users to
build your package. This section documents Guile’s Autoconf support.
5.8.1 Autoconf Background
-------------------------
As explained in the ‘GNU Autoconf Manual’, any package needs
configuration at build-time (*note Introduction: (autoconf)Top.). If
your package uses Guile (or uses a package that in turn uses Guile), you
probably need to know what specific Guile features are available and
details about them.
The way to do this is to write feature tests and arrange for their
execution by the ‘configure’ script, typically by adding the tests to
‘configure.ac’, and running ‘autoconf’ to create ‘configure’. Users of
your package then run ‘configure’ in the normal way.
Macros are a way to make common feature tests easy to express.
Autoconf provides a wide range of macros (*note (autoconf)Existing
Tests::), and Guile installation provides Guile-specific tests in the
areas of: program detection, compilation flags reporting, and Scheme
module checks.
5.8.2 Autoconf Macros
---------------------
As mentioned earlier in this chapter, Guile supports parallel
installation, and uses ‘pkg-config’ to let the user choose which version
of Guile they are interested in. ‘pkg-config’ has its own set of
Autoconf macros that are probably installed on most every development
system. The most useful of these macros is ‘PKG_CHECK_MODULES’.
PKG_CHECK_MODULES([GUILE], [guile-3.0])
This example looks for Guile and sets the ‘GUILE_CFLAGS’ and
‘GUILE_LIBS’ variables accordingly, or prints an error and exits if
Guile was not found.
Guile comes with additional Autoconf macros providing more
information, installed as ‘PREFIX/share/aclocal/guile.m4’. Their names
all begin with ‘GUILE_’.
-- Autoconf Macro: GUILE_PKG [VERSIONS]
This macro runs the ‘pkg-config’ tool to find development files for
an available version of Guile.
By default, this macro will search for the latest stable version of
Guile (e.g. 3.0), falling back to the previous stable version
(e.g. 2.2) if it is available. If no guile-VERSION.pc file is
found, an error is signalled. The found version is stored in
GUILE_EFFECTIVE_VERSION.
If ‘GUILE_PROGS’ was already invoked, this macro ensures that the
development files have the same effective version as the Guile
program.
GUILE_EFFECTIVE_VERSION is marked for substitution, as by
‘AC_SUBST’.
-- Autoconf Macro: GUILE_FLAGS
This macro runs the ‘pkg-config’ tool to find out how to compile
and link programs against Guile. It sets four variables:
GUILE_CFLAGS, GUILE_LDFLAGS, GUILE_LIBS, and GUILE_LTLIBS.
GUILE_CFLAGS: flags to pass to a C or C++ compiler to build code
that uses Guile header files. This is almost always just one or
more ‘-I’ flags.
GUILE_LDFLAGS: flags to pass to the compiler to link a program
against Guile. This includes ‘-lguile-VERSION’ for the Guile
library itself, and may also include one or more ‘-L’ flag to tell
the compiler where to find the libraries. But it does not include
flags that influence the program’s runtime search path for
libraries, and will therefore lead to a program that fails to
start, unless all necessary libraries are installed in a standard
location such as ‘/usr/lib’.
GUILE_LIBS and GUILE_LTLIBS: flags to pass to the compiler or to
libtool, respectively, to link a program against Guile. It
includes flags that augment the program’s runtime search path for
libraries, so that shared libraries will be found at the location
where they were during linking, even in non-standard locations.
GUILE_LIBS is to be used when linking the program directly with the
compiler, whereas GUILE_LTLIBS is to be used when linking the
program is done through libtool.
The variables are marked for substitution, as by ‘AC_SUBST’.
-- Autoconf Macro: GUILE_SITE_DIR
This looks for Guile’s "site" directories. The variable GUILE_SITE
will be set to Guile’s "site" directory for Scheme source files
(usually something like PREFIX/share/guile/site).
GUILE_SITE_CCACHE will be set to the directory for compiled Scheme
files also known as ‘.go’ files (usually something like
PREFIX/lib/guile/GUILE_EFFECTIVE_VERSION/site-ccache).
GUILE_EXTENSION will be set to the directory for compiled C
extensions (usually something like
PREFIX/lib/guile/GUILE_EFFECTIVE_VERSION/extensions). The latter
two are set to blank if the particular version of Guile does not
support them. Note that this macro will run the macros ‘GUILE_PKG’
and ‘GUILE_PROGS’ if they have not already been run.
The variables are marked for substitution, as by ‘AC_SUBST’.
-- Autoconf Macro: GUILE_PROGS [VERSION]
This macro looks for programs ‘guile’ and ‘guild’, setting
variables GUILE and GUILD to their paths, respectively. The macro
will attempt to find ‘guile’ with the suffix of ‘-X.Y’, followed by
looking for it with the suffix ‘X.Y’, and then fall back to looking
for ‘guile’ with no suffix. If ‘guile’ is still not found, signal
an error. The suffix, if any, that was required to find ‘guile’
will be used for ‘guild’ as well.
By default, this macro will search for the latest stable version of
Guile (e.g. 3.0). x.y or x.y.z versions can be specified. If an
older version is found, the macro will signal an error.
The effective version of the found ‘guile’ is set to
GUILE_EFFECTIVE_VERSION. This macro ensures that the effective
version is compatible with the result of a previous invocation of
‘GUILE_FLAGS’, if any.
As a legacy interface, it also looks for ‘guile-config’ and
‘guile-tools’, setting GUILE_CONFIG and GUILE_TOOLS.
The variables are marked for substitution, as by ‘AC_SUBST’.
-- Autoconf Macro: GUILE_CHECK_RETVAL var check
VAR is a shell variable name to be set to the return value. CHECK
is a Guile Scheme expression, evaluated with "$GUILE -c", and
returning either 0 or non-#f to indicate the check passed. Non-0
number or #f indicates failure. Avoid using the character "#"
since that confuses autoconf.
-- Autoconf Macro: GUILE_MODULE_CHECK var module featuretest
description
VAR is a shell variable name to be set to "yes" or "no". MODULE is
a list of symbols, like: (ice-9 common-list). FEATURETEST is an
expression acceptable to GUILE_CHECK, q.v. DESCRIPTION is a
present-tense verb phrase (passed to AC_MSG_CHECKING).
-- Autoconf Macro: GUILE_MODULE_AVAILABLE var module
VAR is a shell variable name to be set to "yes" or "no". MODULE is
a list of symbols, like: (ice-9 common-list).
-- Autoconf Macro: GUILE_MODULE_REQUIRED symlist
SYMLIST is a list of symbols, WITHOUT surrounding parens, like:
ice-9 common-list.
-- Autoconf Macro: GUILE_MODULE_EXPORTS var module modvar
VAR is a shell variable to be set to "yes" or "no". MODULE is a
list of symbols, like: (ice-9 common-list). MODVAR is the Guile
Scheme variable to check.
-- Autoconf Macro: GUILE_MODULE_REQUIRED_EXPORT module modvar
MODULE is a list of symbols, like: (ice-9 common-list). MODVAR is
the Guile Scheme variable to check.
5.8.3 Using Autoconf Macros
---------------------------
Using the autoconf macros is straightforward: Add the macro "calls"
(actually instantiations) to ‘configure.ac’, run ‘aclocal’, and finally,
run ‘autoconf’. If your system doesn’t have guile.m4 installed, place
the desired macro definitions (‘AC_DEFUN’ forms) in ‘acinclude.m4’, and
‘aclocal’ will do the right thing.
Some of the macros can be used inside normal shell constructs: ‘if
foo ; then GUILE_BAZ ; fi’, but this is not guaranteed. It’s probably a
good idea to instantiate macros at top-level.
We now include two examples, one simple and one complicated.
The first example is for a package that uses libguile, and thus needs
to know how to compile and link against it. So we use
‘PKG_CHECK_MODULES’ to set the vars ‘GUILE_CFLAGS’ and ‘GUILE_LIBS’,
which are automatically substituted in the Makefile.
In configure.ac:
PKG_CHECK_MODULES([GUILE], [guile-3.0])
In Makefile.in:
GUILE_CFLAGS = @GUILE_CFLAGS@
GUILE_LIBS = @GUILE_LIBS@
myprog.o: myprog.c
$(CC) -o $ $(GUILE_CFLAGS) $<
myprog: myprog.o
$(CC) -o $ $< $(GUILE_LIBS)
The second example is for a package of Guile Scheme modules that uses
an external program and other Guile Scheme modules (some might call this
a "pure scheme" package). So we use the ‘GUILE_SITE_DIR’ macro, a
regular ‘AC_PATH_PROG’ macro, and the ‘GUILE_MODULE_AVAILABLE’ macro.
In configure.ac:
GUILE_SITE_DIR
probably_wont_work=""
# pgtype pgtable
GUILE_MODULE_AVAILABLE(have_guile_pg, (database postgres))
test $have_guile_pg = no &&
probably_wont_work="(my pgtype) (my pgtable) $probably_wont_work"
# gpgutils
AC_PATH_PROG(GNUPG,gpg)
test x"$GNUPG" = x &&
probably_wont_work="(my gpgutils) $probably_wont_work"
if test ! "$probably_wont_work" = "" ; then
p=" ***"
echo
echo "$p"
echo "$p NOTE:"
echo "$p The following modules probably won't work:"
echo "$p $probably_wont_work"
echo "$p They can be installed anyway, and will work if their"
echo "$p dependencies are installed later. Please see README."
echo "$p"
echo
fi
In Makefile.in:
instdir = @GUILE_SITE@/my
install:
$(INSTALL) my/*.scm $(instdir)
6 API Reference
***************
Guile provides an application programming interface (“API”) to
developers in two core languages: Scheme and C. This part of the manual
contains reference documentation for all of the functionality that is
available through both Scheme and C interfaces.
6.1 Overview of the Guile API
=============================
Guile’s application programming interface (“API”) makes functionality
available that an application developer can use in either C or Scheme
programming. The interface consists of “elements” that may be macros,
functions or variables in C, and procedures, variables, syntax or other
types of object in Scheme.
Many elements are available to both Scheme and C, in a form that is
appropriate. For example, the ‘assq’ Scheme procedure is also available
as ‘scm_assq’ to C code. These elements are documented only once,
addressing both the Scheme and C aspects of them.
The Scheme name of an element is related to its C name in a regular
way. Also, a C function takes its parameters in a systematic way.
Normally, the name of a C function can be derived given its Scheme
name, using some simple textual transformations:
• Replace ‘-’ (hyphen) with ‘_’ (underscore).
• Replace ‘?’ (question mark) with ‘_p’.
• Replace ‘!’ (exclamation point) with ‘_x’.
• Replace internal ‘->’ with ‘_to_’.
• Replace ‘<=’ (less than or equal) with ‘_leq’.
• Replace ‘>=’ (greater than or equal) with ‘_geq’.
• Replace ‘<’ (less than) with ‘_less’.
• Replace ‘>’ (greater than) with ‘_gr’.
• Prefix with ‘scm_’.
A C function always takes a fixed number of arguments of type ‘SCM’,
even when the corresponding Scheme function takes a variable number.
For some Scheme functions, some last arguments are optional; the
corresponding C function must always be invoked with all optional
arguments specified. To get the effect as if an argument has not been
specified, pass ‘SCM_UNDEFINED’ as its value. You can not do this for
an argument in the middle; when one argument is ‘SCM_UNDEFINED’ all the
ones following it must be ‘SCM_UNDEFINED’ as well.
Some Scheme functions take an arbitrary number of _rest_ arguments;
the corresponding C function must be invoked with a list of all these
arguments. This list is always the last argument of the C function.
These two variants can also be combined.
The type of the return value of a C function that corresponds to a
Scheme function is always ‘SCM’. In the descriptions below, types are
therefore often omitted but for the return value and for the arguments.
6.2 Deprecation
===============
From time to time functions and other features of Guile become obsolete.
Guile’s “deprecation” is a mechanism that can help you cope with this.
When you use a feature that is deprecated, you will likely get a
warning message at run-time. Also, if you have a new enough toolchain,
using a deprecated function from ‘libguile’ will cause a link-time
warning.
The primary source for information about just what interfaces are
deprecated in a given release is the file ‘NEWS’. That file also
documents what you should use instead of the obsoleted things.
The file ‘README’ contains instructions on how to control the
inclusion or removal of the deprecated features from the public API of
Guile, and how to control the deprecation warning messages.
The idea behind this mechanism is that normally all deprecated
interfaces are available, but you get feedback when compiling and
running code that uses them, so that you can migrate to the newer APIs
at your leisure.
6.3 The SCM Type
================
Guile represents all Scheme values with the single C type ‘SCM’. For an
introduction to this topic, *Note Dynamic Types::.
-- C Type: SCM
‘SCM’ is the user level abstract C type that is used to represent
all of Guile’s Scheme objects, no matter what the Scheme object
type is. No C operation except assignment is guaranteed to work
with variables of type ‘SCM’, so you should only use macros and
functions to work with ‘SCM’ values. Values are converted between
C data types and the ‘SCM’ type with utility functions and macros.
-- C Type: scm_t_bits
‘scm_t_bits’ is an unsigned integral data type that is guaranteed
to be large enough to hold all information that is required to
represent any Scheme object. While this data type is mostly used
to implement Guile’s internals, the use of this type is also
necessary to write certain kinds of extensions to Guile.
-- C Type: scm_t_signed_bits
This is a signed integral type of the same size as ‘scm_t_bits’.
-- C Macro: scm_t_bits SCM_UNPACK (SCM X)
Transforms the ‘SCM’ value X into its representation as an integral
type. Only after applying ‘SCM_UNPACK’ it is possible to access
the bits and contents of the ‘SCM’ value.
-- C Macro: SCM SCM_PACK (scm_t_bits X)
Takes a valid integral representation of a Scheme object and
transforms it into its representation as a ‘SCM’ value.
6.4 Initializing Guile
======================
Each thread that wants to use functions from the Guile API needs to put
itself into guile mode with either ‘scm_with_guile’ or ‘scm_init_guile’.
The global state of Guile is initialized automatically when the first
thread enters guile mode.
When a thread wants to block outside of a Guile API function, it
should leave guile mode temporarily with ‘scm_without_guile’, *Note
Blocking::.
Threads that are created by ‘call-with-new-thread’ or
‘scm_spawn_thread’ start out in guile mode so you don’t need to
initialize them.
-- C Function: void * scm_with_guile (void *(*func)(void *), void
*data)
Call FUNC, passing it DATA and return what FUNC returns. While
FUNC is running, the current thread is in guile mode and can thus
use the Guile API.
When ‘scm_with_guile’ is called from guile mode, the thread remains
in guile mode when ‘scm_with_guile’ returns.
Otherwise, it puts the current thread into guile mode and, if
needed, gives it a Scheme representation that is contained in the
list returned by ‘all-threads’, for example. This Scheme
representation is not removed when ‘scm_with_guile’ returns so that
a given thread is always represented by the same Scheme value
during its lifetime, if at all.
When this is the first thread that enters guile mode, the global
state of Guile is initialized before calling ‘func’.
The function FUNC is called via ‘scm_with_continuation_barrier’;
thus, ‘scm_with_guile’ returns exactly once.
When ‘scm_with_guile’ returns, the thread is no longer in guile
mode (except when ‘scm_with_guile’ was called from guile mode, see
above). Thus, only ‘func’ can store ‘SCM’ variables on the stack
and be sure that they are protected from the garbage collector.
See ‘scm_init_guile’ for another approach at initializing Guile
that does not have this restriction.
It is OK to call ‘scm_with_guile’ while a thread has temporarily
left guile mode via ‘scm_without_guile’. It will then simply
temporarily enter guile mode again.
-- C Function: void scm_init_guile ()
Arrange things so that all of the code in the current thread
executes as if from within a call to ‘scm_with_guile’. That is,
all functions called by the current thread can assume that ‘SCM’
values on their stack frames are protected from the garbage
collector (except when the thread has explicitly left guile mode,
of course).
When ‘scm_init_guile’ is called from a thread that already has been
in guile mode once, nothing happens. This behavior matters when
you call ‘scm_init_guile’ while the thread has only temporarily
left guile mode: in that case the thread will not be in guile mode
after ‘scm_init_guile’ returns. Thus, you should not use
‘scm_init_guile’ in such a scenario.
When a uncaught throw happens in a thread that has been put into
guile mode via ‘scm_init_guile’, a short message is printed to the
current error port and the thread is exited via ‘scm_pthread_exit
(NULL)’. No restrictions are placed on continuations.
The function ‘scm_init_guile’ might not be available on all
platforms since it requires some stack-bounds-finding magic that
might not have been ported to all platforms that Guile runs on.
Thus, if you can, it is better to use ‘scm_with_guile’ or its
variation ‘scm_boot_guile’ instead of this function.
-- C Function: void scm_boot_guile (int ARGC, char **ARGV, void
(*MAIN_FUNC) (void *DATA, int ARGC, char **ARGV), void *DATA)
Enter guile mode as with ‘scm_with_guile’ and call MAIN_FUNC,
passing it DATA, ARGC, and ARGV as indicated. When MAIN_FUNC
returns, ‘scm_boot_guile’ calls ‘exit (0)’; ‘scm_boot_guile’ never
returns. If you want some other exit value, have MAIN_FUNC call
‘exit’ itself. If you don’t want to exit at all, use
‘scm_with_guile’ instead of ‘scm_boot_guile’.
The function ‘scm_boot_guile’ arranges for the Scheme
‘command-line’ function to return the strings given by ARGC and
ARGV. If MAIN_FUNC modifies ARGC or ARGV, it should call
‘scm_set_program_arguments’ with the final list, so Scheme code
will know which arguments have been processed (*note Runtime
Environment::).
-- C Function: void scm_shell (int ARGC, char **ARGV)
Process command-line arguments in the manner of the ‘guile’
executable. This includes loading the normal Guile initialization
files, interacting with the user or running any scripts or
expressions specified by ‘-s’ or ‘-e’ options, and then exiting.
*Note Invoking Guile::, for more details.
Since this function does not return, you must do all
application-specific initialization before calling this function.
6.5 Snarfing Macros
===================
The following macros do two different things: when compiled normally,
they expand in one way; when processed during snarfing, they cause the
‘guile-snarf’ program to pick up some initialization code, *Note
Function Snarfing::.
The descriptions below use the term ‘normally’ to refer to the case
when the code is compiled normally, and ‘while snarfing’ when the code
is processed by ‘guile-snarf’.
-- C Macro: SCM_SNARF_INIT (code)
Normally, ‘SCM_SNARF_INIT’ expands to nothing; while snarfing, it
causes CODE to be included in the initialization action file,
followed by a semicolon.
This is the fundamental macro for snarfing initialization actions.
The more specialized macros below use it internally.
-- C Macro: SCM_DEFINE (c_name, scheme_name, req, opt, var, arglist,
docstring)
Normally, this macro expands into
static const char s_C_NAME[] = SCHEME_NAME;
SCM
C_NAME ARGLIST
While snarfing, it causes
scm_c_define_gsubr (s_C_NAME, REQ, OPT, VAR,
C_NAME);
to be added to the initialization actions. Thus, you can use it to
declare a C function named C_NAME that will be made available to
Scheme with the name SCHEME_NAME.
Note that the ARGLIST argument must have parentheses around it.
-- C Macro: SCM_SYMBOL (c_name, scheme_name)
-- C Macro: SCM_GLOBAL_SYMBOL (c_name, scheme_name)
Normally, these macros expand into
static SCM C_NAME
or
SCM C_NAME
respectively. While snarfing, they both expand into the
initialization code
C_NAME = scm_permanent_object (scm_from_locale_symbol (SCHEME_NAME));
Thus, you can use them declare a static or global variable of type
‘SCM’ that will be initialized to the symbol named SCHEME_NAME.
-- C Macro: SCM_KEYWORD (c_name, scheme_name)
-- C Macro: SCM_GLOBAL_KEYWORD (c_name, scheme_name)
Normally, these macros expand into
static SCM C_NAME
or
SCM C_NAME
respectively. While snarfing, they both expand into the
initialization code
C_NAME = scm_permanent_object (scm_c_make_keyword (SCHEME_NAME));
Thus, you can use them declare a static or global variable of type
‘SCM’ that will be initialized to the keyword named SCHEME_NAME.
-- C Macro: SCM_VARIABLE (c_name, scheme_name)
-- C Macro: SCM_GLOBAL_VARIABLE (c_name, scheme_name)
These macros are equivalent to ‘SCM_VARIABLE_INIT’ and
‘SCM_GLOBAL_VARIABLE_INIT’, respectively, with a VALUE of
‘SCM_BOOL_F’.
-- C Macro: SCM_VARIABLE_INIT (c_name, scheme_name, value)
-- C Macro: SCM_GLOBAL_VARIABLE_INIT (c_name, scheme_name, value)
Normally, these macros expand into
static SCM C_NAME
or
SCM C_NAME
respectively. While snarfing, they both expand into the
initialization code
C_NAME = scm_permanent_object (scm_c_define (SCHEME_NAME, VALUE));
Thus, you can use them declare a static or global C variable of
type ‘SCM’ that will be initialized to the object representing the
Scheme variable named SCHEME_NAME in the current module. The
variable will be defined when it doesn’t already exist. It is
always set to VALUE.
6.6 Data Types
==============
Guile’s data types form a powerful built-in library of representations
and functionality that you can apply to your problem domain. This
chapter surveys the data types built-in to Guile, from the simple to the
complex.
6.6.1 Booleans
--------------
The two boolean values are ‘#t’ for true and ‘#f’ for false. They can
also be written as ‘#true’ and ‘#false’, as per R7RS.
Boolean values are returned by predicate procedures, such as the
general equality predicates ‘eq?’, ‘eqv?’ and ‘equal?’ (*note
Equality::) and numerical and string comparison operators like
‘string=?’ (*note String Comparison::) and ‘<=’ (*note Comparison::).
(<= 3 8)
⇒ #t
(<= 3 -3)
⇒ #f
(equal? "house" "houses")
⇒ #f
(eq? #f #f)
⇒
#t
In test condition contexts like ‘if’ and ‘cond’ (*note
Conditionals::), where a group of subexpressions will be evaluated only
if a CONDITION expression evaluates to “true”, “true” means any value at
all except ‘#f’.
(if #t "yes" "no")
⇒ "yes"
(if 0 "yes" "no")
⇒ "yes"
(if #f "yes" "no")
⇒ "no"
A result of this asymmetry is that typical Scheme source code more
often uses ‘#f’ explicitly than ‘#t’: ‘#f’ is necessary to represent an
‘if’ or ‘cond’ false value, whereas ‘#t’ is not necessary to represent
an ‘if’ or ‘cond’ true value.
It is important to note that ‘#f’ is *not* equivalent to any other
Scheme value. In particular, ‘#f’ is not the same as the number 0 (like
in C and C++), and not the same as the “empty list” (like in some Lisp
dialects).
In C, the two Scheme boolean values are available as the two
constants ‘SCM_BOOL_T’ for ‘#t’ and ‘SCM_BOOL_F’ for ‘#f’. Care must be
taken with the false value ‘SCM_BOOL_F’: it is not false when used in C
conditionals. In order to test for it, use ‘scm_is_false’ or
‘scm_is_true’.
-- Scheme Procedure: not x
-- C Function: scm_not (x)
Return ‘#t’ if X is ‘#f’, else return ‘#f’.
-- Scheme Procedure: boolean? obj
-- C Function: scm_boolean_p (obj)
Return ‘#t’ if OBJ is either ‘#t’ or ‘#f’, else return ‘#f’.
-- C Macro: SCM SCM_BOOL_T
The ‘SCM’ representation of the Scheme object ‘#t’.
-- C Macro: SCM SCM_BOOL_F
The ‘SCM’ representation of the Scheme object ‘#f’.
-- C Function: int scm_is_true (SCM obj)
Return ‘0’ if OBJ is ‘#f’, else return ‘1’.
-- C Function: int scm_is_false (SCM obj)
Return ‘1’ if OBJ is ‘#f’, else return ‘0’.
-- C Function: int scm_is_bool (SCM obj)
Return ‘1’ if OBJ is either ‘#t’ or ‘#f’, else return ‘0’.
-- C Function: SCM scm_from_bool (int val)
Return ‘#f’ if VAL is ‘0’, else return ‘#t’.
-- C Function: int scm_to_bool (SCM val)
Return ‘1’ if VAL is ‘SCM_BOOL_T’, return ‘0’ when VAL is
‘SCM_BOOL_F’, else signal a ‘wrong type’ error.
You should probably use ‘scm_is_true’ instead of this function when
you just want to test a ‘SCM’ value for trueness.
6.6.2 Numerical data types
--------------------------
Guile supports a rich “tower” of numerical types — integer, rational,
real and complex — and provides an extensive set of mathematical and
scientific functions for operating on numerical data. This section of
the manual documents those types and functions.
You may also find it illuminating to read R5RS’s presentation of
numbers in Scheme, which is particularly clear and accessible: see *note
(r5rs)Numbers::.
6.6.2.1 Scheme’s Numerical “Tower”
..................................
Scheme’s numerical “tower” consists of the following categories of
numbers:
“integers”
Whole numbers, positive or negative; e.g. –5, 0, 18.
“rationals”
The set of numbers that can be expressed as P/Q where P and Q are
integers; e.g. 9/16 works, but pi (an irrational number) doesn’t.
These include integers (N/1).
“real numbers”
The set of numbers that describes all possible positions along a
one-dimensional line. This includes rationals as well as
irrational numbers.
“complex numbers”
The set of numbers that describes all possible positions in a two
dimensional space. This includes real as well as imaginary numbers
(A+Bi, where A is the “real part”, B is the “imaginary part”, and i
is the square root of −1.)
It is called a tower because each category “sits on” the one that
follows it, in the sense that every integer is also a rational, every
rational is also real, and every real number is also a complex number
(but with zero imaginary part).
In addition to the classification into integers, rationals, reals and
complex numbers, Scheme also distinguishes between whether a number is
represented exactly or not. For example, the result of 2*sin(pi/4) is
exactly 2^(1/2), but Guile can represent neither pi/4 nor 2^(1/2)
exactly. Instead, it stores an inexact approximation, using the C type
‘double’.
Guile can represent exact rationals of any magnitude, inexact
rationals that fit into a C ‘double’, and inexact complex numbers with
‘double’ real and imaginary parts.
The ‘number?’ predicate may be applied to any Scheme value to
discover whether the value is any of the supported numerical types.
-- Scheme Procedure: number? obj
-- C Function: scm_number_p (obj)
Return ‘#t’ if OBJ is any kind of number, else ‘#f’.
For example:
(number? 3)
⇒ #t
(number? "hello there!")
⇒ #f
(define pi 3.141592654)
(number? pi)
⇒ #t
-- C Function: int scm_is_number (SCM obj)
This is equivalent to ‘scm_is_true (scm_number_p (obj))’.
The next few subsections document each of Guile’s numerical data
types in detail.
6.6.2.2 Integers
................
Integers are whole numbers, that is numbers with no fractional part,
such as 2, 83, and −3789.
Integers in Guile can be arbitrarily big, as shown by the following
example.
(define (factorial n)
(let loop ((n n) (product 1))
(if (= n 0)
product
(loop (- n 1) (* product n)))))
(factorial 3)
⇒ 6
(factorial 20)
⇒ 2432902008176640000
(- (factorial 45))
⇒ -119622220865480194561963161495657715064383733760000000000
Readers whose background is in programming languages where integers
are limited by the need to fit into just 4 or 8 bytes of memory may find
this surprising, or suspect that Guile’s representation of integers is
inefficient. In fact, Guile achieves a near optimal balance of
convenience and efficiency by using the host computer’s native
representation of integers where possible, and a more general
representation where the required number does not fit in the native
form. Conversion between these two representations is automatic and
completely invisible to the Scheme level programmer.
C has a host of different integer types, and Guile offers a host of
functions to convert between them and the ‘SCM’ representation. For
example, a C ‘int’ can be handled with ‘scm_to_int’ and ‘scm_from_int’.
Guile also defines a few C integer types of its own, to help with
differences between systems.
C integer types that are not covered can be handled with the generic
‘scm_to_signed_integer’ and ‘scm_from_signed_integer’ for signed types,
or with ‘scm_to_unsigned_integer’ and ‘scm_from_unsigned_integer’ for
unsigned types.
Scheme integers can be exact and inexact. For example, a number
written as ‘3.0’ with an explicit decimal-point is inexact, but it is
also an integer. The functions ‘integer?’ and ‘scm_is_integer’ report
true for such a number, but the functions ‘exact-integer?’,
‘scm_is_exact_integer’, ‘scm_is_signed_integer’, and
‘scm_is_unsigned_integer’ only allow exact integers and thus report
false. Likewise, the conversion functions like ‘scm_to_signed_integer’
only accept exact integers.
The motivation for this behavior is that the inexactness of a number
should not be lost silently. If you want to allow inexact integers, you
can explicitly insert a call to ‘inexact->exact’ or to its C equivalent
‘scm_inexact_to_exact’. (Only inexact integers will be converted by
this call into exact integers; inexact non-integers will become exact
fractions.)
-- Scheme Procedure: integer? x
-- C Function: scm_integer_p (x)
Return ‘#t’ if X is an exact or inexact integer number, else return
‘#f’.
(integer? 487)
⇒ #t
(integer? 3.0)
⇒ #t
(integer? -3.4)
⇒ #f
(integer? +inf.0)
⇒ #f
-- C Function: int scm_is_integer (SCM x)
This is equivalent to ‘scm_is_true (scm_integer_p (x))’.
-- Scheme Procedure: exact-integer? x
-- C Function: scm_exact_integer_p (x)
Return ‘#t’ if X is an exact integer number, else return ‘#f’.
(exact-integer? 37)
⇒ #t
(exact-integer? 3.0)
⇒ #f
-- C Function: int scm_is_exact_integer (SCM x)
This is equivalent to ‘scm_is_true (scm_exact_integer_p (x))’.
-- C Type: scm_t_int8
-- C Type: scm_t_uint8
-- C Type: scm_t_int16
-- C Type: scm_t_uint16
-- C Type: scm_t_int32
-- C Type: scm_t_uint32
-- C Type: scm_t_int64
-- C Type: scm_t_uint64
-- C Type: scm_t_intmax
-- C Type: scm_t_uintmax
The C types are equivalent to the corresponding ISO C types but are
defined on all platforms, with the exception of ‘scm_t_int64’ and
‘scm_t_uint64’, which are only defined when a 64-bit type is
available. For example, ‘scm_t_int8’ is equivalent to ‘int8_t’.
You can regard these definitions as a stop-gap measure until all
platforms provide these types. If you know that all the platforms
that you are interested in already provide these types, it is
better to use them directly instead of the types provided by Guile.
-- C Function: int scm_is_signed_integer (SCM x, scm_t_intmax min,
scm_t_intmax max)
-- C Function: int scm_is_unsigned_integer (SCM x, scm_t_uintmax min,
scm_t_uintmax max)
Return ‘1’ when X represents an exact integer that is between MIN
and MAX, inclusive.
These functions can be used to check whether a ‘SCM’ value will fit
into a given range, such as the range of a given C integer type.
If you just want to convert a ‘SCM’ value to a given C integer
type, use one of the conversion functions directly.
-- C Function: scm_t_intmax scm_to_signed_integer (SCM x, scm_t_intmax
min, scm_t_intmax max)
-- C Function: scm_t_uintmax scm_to_unsigned_integer (SCM x,
scm_t_uintmax min, scm_t_uintmax max)
When X represents an exact integer that is between MIN and MAX
inclusive, return that integer. Else signal an error, either a
‘wrong-type’ error when X is not an exact integer, or an
‘out-of-range’ error when it doesn’t fit the given range.
-- C Function: SCM scm_from_signed_integer (scm_t_intmax x)
-- C Function: SCM scm_from_unsigned_integer (scm_t_uintmax x)
Return the ‘SCM’ value that represents the integer X. This
function will always succeed and will always return an exact
number.
-- C Function: char scm_to_char (SCM x)
-- C Function: signed char scm_to_schar (SCM x)
-- C Function: unsigned char scm_to_uchar (SCM x)
-- C Function: short scm_to_short (SCM x)
-- C Function: unsigned short scm_to_ushort (SCM x)
-- C Function: int scm_to_int (SCM x)
-- C Function: unsigned int scm_to_uint (SCM x)
-- C Function: long scm_to_long (SCM x)
-- C Function: unsigned long scm_to_ulong (SCM x)
-- C Function: long long scm_to_long_long (SCM x)
-- C Function: unsigned long long scm_to_ulong_long (SCM x)
-- C Function: size_t scm_to_size_t (SCM x)
-- C Function: ssize_t scm_to_ssize_t (SCM x)
-- C Function: scm_t_uintptr scm_to_uintptr_t (SCM x)
-- C Function: scm_t_ptrdiff scm_to_ptrdiff_t (SCM x)
-- C Function: scm_t_int8 scm_to_int8 (SCM x)
-- C Function: scm_t_uint8 scm_to_uint8 (SCM x)
-- C Function: scm_t_int16 scm_to_int16 (SCM x)
-- C Function: scm_t_uint16 scm_to_uint16 (SCM x)
-- C Function: scm_t_int32 scm_to_int32 (SCM x)
-- C Function: scm_t_uint32 scm_to_uint32 (SCM x)
-- C Function: scm_t_int64 scm_to_int64 (SCM x)
-- C Function: scm_t_uint64 scm_to_uint64 (SCM x)
-- C Function: scm_t_intmax scm_to_intmax (SCM x)
-- C Function: scm_t_uintmax scm_to_uintmax (SCM x)
-- C Function: scm_t_intptr scm_to_intptr_t (SCM x)
-- C Function: scm_t_uintptr scm_to_uintptr_t (SCM x)
When X represents an exact integer that fits into the indicated C
type, return that integer. Else signal an error, either a
‘wrong-type’ error when X is not an exact integer, or an
‘out-of-range’ error when it doesn’t fit the given range.
The functions ‘scm_to_long_long’, ‘scm_to_ulong_long’,
‘scm_to_int64’, and ‘scm_to_uint64’ are only available when the
corresponding types are.
-- C Function: SCM scm_from_char (char x)
-- C Function: SCM scm_from_schar (signed char x)
-- C Function: SCM scm_from_uchar (unsigned char x)
-- C Function: SCM scm_from_short (short x)
-- C Function: SCM scm_from_ushort (unsigned short x)
-- C Function: SCM scm_from_int (int x)
-- C Function: SCM scm_from_uint (unsigned int x)
-- C Function: SCM scm_from_long (long x)
-- C Function: SCM scm_from_ulong (unsigned long x)
-- C Function: SCM scm_from_long_long (long long x)
-- C Function: SCM scm_from_ulong_long (unsigned long long x)
-- C Function: SCM scm_from_size_t (size_t x)
-- C Function: SCM scm_from_ssize_t (ssize_t x)
-- C Function: SCM scm_from_uintptr_t (uintptr_t x)
-- C Function: SCM scm_from_ptrdiff_t (scm_t_ptrdiff x)
-- C Function: SCM scm_from_int8 (scm_t_int8 x)
-- C Function: SCM scm_from_uint8 (scm_t_uint8 x)
-- C Function: SCM scm_from_int16 (scm_t_int16 x)
-- C Function: SCM scm_from_uint16 (scm_t_uint16 x)
-- C Function: SCM scm_from_int32 (scm_t_int32 x)
-- C Function: SCM scm_from_uint32 (scm_t_uint32 x)
-- C Function: SCM scm_from_int64 (scm_t_int64 x)
-- C Function: SCM scm_from_uint64 (scm_t_uint64 x)
-- C Function: SCM scm_from_intmax (scm_t_intmax x)
-- C Function: SCM scm_from_uintmax (scm_t_uintmax x)
-- C Function: SCM scm_from_intptr_t (scm_t_intptr x)
-- C Function: SCM scm_from_uintptr_t (scm_t_uintptr x)
Return the ‘SCM’ value that represents the integer X. These
functions will always succeed and will always return an exact
number.
-- C Function: void scm_to_mpz (SCM val, mpz_t rop)
Assign VAL to the multiple precision integer ROP. VAL must be an
exact integer, otherwise an error will be signalled. ROP must have
been initialized with ‘mpz_init’ before this function is called.
When ROP is no longer needed the occupied space must be freed with
‘mpz_clear’. *Note (gmp)Initializing Integers::, for details.
-- C Function: SCM scm_from_mpz (mpz_t val)
Return the ‘SCM’ value that represents VAL.
6.6.2.3 Real and Rational Numbers
.................................
Mathematically, the real numbers are the set of numbers that describe
all possible points along a continuous, infinite, one-dimensional line.
The rational numbers are the set of all numbers that can be written as
fractions P/Q, where P and Q are integers. All rational numbers are
also real, but there are real numbers that are not rational, for example
the square root of 2, and pi.
Guile can represent both exact and inexact rational numbers, but it
cannot represent precise finite irrational numbers. Exact rationals are
represented by storing the numerator and denominator as two exact
integers. Inexact rationals are stored as floating point numbers using
the C type ‘double’.
Exact rationals are written as a fraction of integers. There must be
no whitespace around the slash:
1/2
-22/7
Even though the actual encoding of inexact rationals is in binary, it
may be helpful to think of it as a decimal number with a limited number
of significant figures and a decimal point somewhere, since this
corresponds to the standard notation for non-whole numbers. For
example:
0.34
-0.00000142857931198
-5648394822220000000000.0
4.0
The limited precision of Guile’s encoding means that any finite
“real” number in Guile can be written in a rational form, by multiplying
and then dividing by sufficient powers of 10 (or in fact, 2). For
example, ‘-0.00000142857931198’ is the same as −142857931198 divided by
100000000000000000. In Guile’s current incarnation, therefore, the
‘rational?’ and ‘real?’ predicates are equivalent for finite numbers.
Dividing by an exact zero leads to a error message, as one might
expect. However, dividing by an inexact zero does not produce an error.
Instead, the result of the division is either plus or minus infinity,
depending on the sign of the divided number and the sign of the zero
divisor (some platforms support signed zeroes ‘-0.0’ and ‘+0.0’; ‘0.0’
is the same as ‘+0.0’).
Dividing zero by an inexact zero yields a NaN (‘not a number’) value,
although they are actually considered numbers by Scheme. Attempts to
compare a NaN value with any number (including itself) using ‘=’, ‘<’,
‘>’, ‘<=’ or ‘>=’ always returns ‘#f’. Although a NaN value is not ‘=’
to itself, it is both ‘eqv?’ and ‘equal?’ to itself and other NaN
values. However, the preferred way to test for them is by using ‘nan?’.
The real NaN values and infinities are written ‘+nan.0’, ‘+inf.0’ and
‘-inf.0’. This syntax is also recognized by ‘read’ as an extension to
the usual Scheme syntax. These special values are considered by Scheme
to be inexact real numbers but not rational. Note that non-real complex
numbers may also contain infinities or NaN values in their real or
imaginary parts. To test a real number to see if it is infinite, a NaN
value, or neither, use ‘inf?’, ‘nan?’, or ‘finite?’, respectively.
Every real number in Scheme belongs to precisely one of those three
classes.
On platforms that follow IEEE 754 for their floating point
arithmetic, the ‘+inf.0’, ‘-inf.0’, and ‘+nan.0’ values are implemented
using the corresponding IEEE 754 values. They behave in arithmetic
operations like IEEE 754 describes it, i.e., ‘(= +nan.0 +nan.0)’ ⇒ ‘#f’.
-- Scheme Procedure: real? obj
-- C Function: scm_real_p (obj)
Return ‘#t’ if OBJ is a real number, else ‘#f’. Note that the sets
of integer and rational values form subsets of the set of real
numbers, so the predicate will also be fulfilled if OBJ is an
integer number or a rational number.
-- Scheme Procedure: rational? x
-- C Function: scm_rational_p (x)
Return ‘#t’ if X is a rational number, ‘#f’ otherwise. Note that
the set of integer values forms a subset of the set of rational
numbers, i.e. the predicate will also be fulfilled if X is an
integer number.
-- Scheme Procedure: rationalize x eps
-- C Function: scm_rationalize (x, eps)
Returns the _simplest_ rational number differing from X by no more
than EPS.
As required by R5RS, ‘rationalize’ only returns an exact result
when both its arguments are exact. Thus, you might need to use
‘inexact->exact’ on the arguments.
(rationalize (inexact->exact 1.2) 1/100)
⇒ 6/5
-- Scheme Procedure: inf? x
-- C Function: scm_inf_p (x)
Return ‘#t’ if the real number X is ‘+inf.0’ or ‘-inf.0’.
Otherwise return ‘#f’.
-- Scheme Procedure: nan? x
-- C Function: scm_nan_p (x)
Return ‘#t’ if the real number X is ‘+nan.0’, or ‘#f’ otherwise.
-- Scheme Procedure: finite? x
-- C Function: scm_finite_p (x)
Return ‘#t’ if the real number X is neither infinite nor a NaN,
‘#f’ otherwise.
-- Scheme Procedure: nan
-- C Function: scm_nan ()
Return ‘+nan.0’, a NaN value.
-- Scheme Procedure: inf
-- C Function: scm_inf ()
Return ‘+inf.0’, positive infinity.
-- Scheme Procedure: numerator x
-- C Function: scm_numerator (x)
Return the numerator of the rational number X.
-- Scheme Procedure: denominator x
-- C Function: scm_denominator (x)
Return the denominator of the rational number X.
-- C Function: int scm_is_real (SCM val)
-- C Function: int scm_is_rational (SCM val)
Equivalent to ‘scm_is_true (scm_real_p (val))’ and ‘scm_is_true
(scm_rational_p (val))’, respectively.
-- C Function: double scm_to_double (SCM val)
Returns the number closest to VAL that is representable as a
‘double’. Returns infinity for a VAL that is too large in
magnitude. The argument VAL must be a real number.
-- C Function: SCM scm_from_double (double val)
Return the ‘SCM’ value that represents VAL. The returned value is
inexact according to the predicate ‘inexact?’, but it will be
exactly equal to VAL.
6.6.2.4 Complex Numbers
.......................
Complex numbers are the set of numbers that describe all possible points
in a two-dimensional space. The two coordinates of a particular point
in this space are known as the “real” and “imaginary” parts of the
complex number that describes that point.
In Guile, complex numbers are written in rectangular form as the sum
of their real and imaginary parts, using the symbol ‘i’ to indicate the
imaginary part.
3+4i
⇒
3.0+4.0i
(* 3-8i 2.3+0.3i)
⇒
9.3-17.5i
Polar form can also be used, with an ‘@’ between magnitude and angle,
1@3.141592 ⇒ -1.0 (approx)
-1@1.57079 ⇒ 0.0-1.0i (approx)
Guile represents a complex number as a pair of inexact reals, so the
real and imaginary parts of a complex number have the same properties of
inexactness and limited precision as single inexact real numbers.
Note that each part of a complex number may contain any inexact real
value, including the special values ‘+nan.0’, ‘+inf.0’ and ‘-inf.0’, as
well as either of the signed zeroes ‘0.0’ or ‘-0.0’.
-- Scheme Procedure: complex? z
-- C Function: scm_complex_p (z)
Return ‘#t’ if Z is a complex number, ‘#f’ otherwise. Note that
the sets of real, rational and integer values form subsets of the
set of complex numbers, i.e. the predicate will also be fulfilled
if Z is a real, rational or integer number.
-- C Function: int scm_is_complex (SCM val)
Equivalent to ‘scm_is_true (scm_complex_p (val))’.
6.6.2.5 Exact and Inexact Numbers
.................................
R5RS requires that, with few exceptions, a calculation involving inexact
numbers always produces an inexact result. To meet this requirement,
Guile distinguishes between an exact integer value such as ‘5’ and the
corresponding inexact integer value which, to the limited precision
available, has no fractional part, and is printed as ‘5.0’. Guile will
only convert the latter value to the former when forced to do so by an
invocation of the ‘inexact->exact’ procedure.
The only exception to the above requirement is when the values of the
inexact numbers do not affect the result. For example ‘(expt n 0)’ is
‘1’ for any value of ‘n’, therefore ‘(expt 5.0 0)’ is permitted to
return an exact ‘1’.
-- Scheme Procedure: exact? z
-- C Function: scm_exact_p (z)
Return ‘#t’ if the number Z is exact, ‘#f’ otherwise.
(exact? 2)
⇒ #t
(exact? 0.5)
⇒ #f
(exact? (/ 2))
⇒ #t
-- C Function: int scm_is_exact (SCM z)
Return a ‘1’ if the number Z is exact, and ‘0’ otherwise. This is
equivalent to ‘scm_is_true (scm_exact_p (z))’.
An alternate approch to testing the exactness of a number is to use
‘scm_is_signed_integer’ or ‘scm_is_unsigned_integer’.
-- Scheme Procedure: inexact? z
-- C Function: scm_inexact_p (z)
Return ‘#t’ if the number Z is inexact, ‘#f’ else.
-- C Function: int scm_is_inexact (SCM z)
Return a ‘1’ if the number Z is inexact, and ‘0’ otherwise. This
is equivalent to ‘scm_is_true (scm_inexact_p (z))’.
-- Scheme Procedure: inexact->exact z
-- C Function: scm_inexact_to_exact (z)
Return an exact number that is numerically closest to Z, when there
is one. For inexact rationals, Guile returns the exact rational
that is numerically equal to the inexact rational. Inexact complex
numbers with a non-zero imaginary part can not be made exact.
(inexact->exact 0.5)
⇒ 1/2
The following happens because 12/10 is not exactly representable as
a ‘double’ (on most platforms). However, when reading a decimal
number that has been marked exact with the “#e” prefix, Guile is
able to represent it correctly.
(inexact->exact 1.2)
⇒ 5404319552844595/4503599627370496
#e1.2
⇒ 6/5
-- Scheme Procedure: exact->inexact z
-- C Function: scm_exact_to_inexact (z)
Convert the number Z to its inexact representation.
6.6.2.6 Read Syntax for Numerical Data
......................................
The read syntax for integers is a string of digits, optionally preceded
by a minus or plus character, a code indicating the base in which the
integer is encoded, and a code indicating whether the number is exact or
inexact. The supported base codes are:
‘#b’
‘#B’
the integer is written in binary (base 2)
‘#o’
‘#O’
the integer is written in octal (base 8)
‘#d’
‘#D’
the integer is written in decimal (base 10)
‘#x’
‘#X’
the integer is written in hexadecimal (base 16)
If the base code is omitted, the integer is assumed to be decimal.
The following examples show how these base codes are used.
-13
⇒ -13
#d-13
⇒ -13
#x-13
⇒ -19
#b+1101
⇒ 13
#o377
⇒ 255
The codes for indicating exactness (which can, incidentally, be
applied to all numerical values) are:
‘#e’
‘#E’
the number is exact
‘#i’
‘#I’
the number is inexact.
If the exactness indicator is omitted, the number is exact unless it
contains a radix point. Since Guile can not represent exact complex
numbers, an error is signalled when asking for them.
(exact? 1.2)
⇒ #f
(exact? #e1.2)
⇒ #t
(exact? #e+1i)
ERROR: Wrong type argument
Guile also understands the syntax ‘+inf.0’ and ‘-inf.0’ for plus and
minus infinity, respectively. The value must be written exactly as
shown, that is, they always must have a sign and exactly one zero digit
after the decimal point. It also understands ‘+nan.0’ and ‘-nan.0’ for
the special ‘not-a-number’ value. The sign is ignored for
‘not-a-number’ and the value is always printed as ‘+nan.0’.
6.6.2.7 Operations on Integer Values
....................................
-- Scheme Procedure: odd? n
-- C Function: scm_odd_p (n)
Return ‘#t’ if N is an odd number, ‘#f’ otherwise.
-- Scheme Procedure: even? n
-- C Function: scm_even_p (n)
Return ‘#t’ if N is an even number, ‘#f’ otherwise.
-- Scheme Procedure: quotient n d
-- Scheme Procedure: remainder n d
-- C Function: scm_quotient (n, d)
-- C Function: scm_remainder (n, d)
Return the quotient or remainder from N divided by D. The quotient
is rounded towards zero, and the remainder will have the same sign
as N. In all cases quotient and remainder satisfy N = Q*D + R.
(remainder 13 4) ⇒ 1
(remainder -13 4) ⇒ -1
See also ‘truncate-quotient’, ‘truncate-remainder’ and related
operations in *note Arithmetic::.
-- Scheme Procedure: modulo n d
-- C Function: scm_modulo (n, d)
Return the remainder from N divided by D, with the same sign as D.
(modulo 13 4) ⇒ 1
(modulo -13 4) ⇒ 3
(modulo 13 -4) ⇒ -3
(modulo -13 -4) ⇒ -1
See also ‘floor-quotient’, ‘floor-remainder’ and related operations
in *note Arithmetic::.
-- Scheme Procedure: gcd x...
-- C Function: scm_gcd (x, y)
Return the greatest common divisor of all arguments. If called
without arguments, 0 is returned.
The C function ‘scm_gcd’ always takes two arguments, while the
Scheme function can take an arbitrary number.
-- Scheme Procedure: lcm x...
-- C Function: scm_lcm (x, y)
Return the least common multiple of the arguments. If called
without arguments, 1 is returned.
The C function ‘scm_lcm’ always takes two arguments, while the
Scheme function can take an arbitrary number.
-- Scheme Procedure: modulo-expt n k m
-- C Function: scm_modulo_expt (n, k, m)
Return N raised to the integer exponent K, modulo M.
(modulo-expt 2 3 5)
⇒ 3
-- Scheme Procedure: exact-integer-sqrt K
-- C Function: void scm_exact_integer_sqrt (SCM K, SCM *S, SCM *R)
Return two exact non-negative integers S and R such that K = S^2 +
R and S^2 <= K < (S + 1)^2. An error is raised if K is not an
exact non-negative integer.
(exact-integer-sqrt 10) ⇒ 3 and 1
6.6.2.8 Comparison Predicates
.............................
The C comparison functions below always takes two arguments, while the
Scheme functions can take an arbitrary number. Also keep in mind that
the C functions return one of the Scheme boolean values ‘SCM_BOOL_T’ or
‘SCM_BOOL_F’ which are both true as far as C is concerned. Thus, always
write ‘scm_is_true (scm_num_eq_p (x, y))’ when testing the two Scheme
numbers ‘x’ and ‘y’ for equality, for example.
-- Scheme Procedure: =
-- C Function: scm_num_eq_p (x, y)
Return ‘#t’ if all parameters are numerically equal.
-- Scheme Procedure: <
-- C Function: scm_less_p (x, y)
Return ‘#t’ if the list of parameters is monotonically increasing.
-- Scheme Procedure: >
-- C Function: scm_gr_p (x, y)
Return ‘#t’ if the list of parameters is monotonically decreasing.
-- Scheme Procedure: <=
-- C Function: scm_leq_p (x, y)
Return ‘#t’ if the list of parameters is monotonically
non-decreasing.
-- Scheme Procedure: >=
-- C Function: scm_geq_p (x, y)
Return ‘#t’ if the list of parameters is monotonically
non-increasing.
-- Scheme Procedure: zero? z
-- C Function: scm_zero_p (z)
Return ‘#t’ if Z is an exact or inexact number equal to zero.
-- Scheme Procedure: positive? x
-- C Function: scm_positive_p (x)
Return ‘#t’ if X is an exact or inexact number greater than zero.
-- Scheme Procedure: negative? x
-- C Function: scm_negative_p (x)
Return ‘#t’ if X is an exact or inexact number less than zero.
6.6.2.9 Converting Numbers To and From Strings
..............................................
The following procedures read and write numbers according to their
external representation as defined by R5RS (*note R5RS Lexical
Structure: (r5rs)Lexical structure.). *Note the ‘(ice-9 i18n)’ module:
Number Input and Output, for locale-dependent number parsing.
-- Scheme Procedure: number->string n [radix]
-- C Function: scm_number_to_string (n, radix)
Return a string holding the external representation of the number N
in the given RADIX. If N is inexact, a radix of 10 will be used.
-- Scheme Procedure: string->number string [radix]
-- C Function: scm_string_to_number (string, radix)
Return a number of the maximally precise representation expressed
by the given STRING. RADIX must be an exact integer, either 2, 8,
10, or 16. If supplied, RADIX is a default radix that may be
overridden by an explicit radix prefix in STRING (e.g. "#o177").
If RADIX is not supplied, then the default radix is 10. If string
is not a syntactically valid notation for a number, then
‘string->number’ returns ‘#f’.
-- C Function: SCM scm_c_locale_stringn_to_number (const char *string,
size_t len, unsigned radix)
As per ‘string->number’ above, but taking a C string, as pointer
and length. The string characters should be in the current locale
encoding (‘locale’ in the name refers only to that, there’s no
locale-dependent parsing).
6.6.2.10 Complex Number Operations
..................................
-- Scheme Procedure: make-rectangular real_part imaginary_part
-- C Function: scm_make_rectangular (real_part, imaginary_part)
Return a complex number constructed of the given REAL-PART and
IMAGINARY-PART parts.
-- Scheme Procedure: make-polar mag ang
-- C Function: scm_make_polar (mag, ang)
Return the complex number MAG * e^(i * ANG).
-- Scheme Procedure: real-part z
-- C Function: scm_real_part (z)
Return the real part of the number Z.
-- Scheme Procedure: imag-part z
-- C Function: scm_imag_part (z)
Return the imaginary part of the number Z.
-- Scheme Procedure: magnitude z
-- C Function: scm_magnitude (z)
Return the magnitude of the number Z. This is the same as ‘abs’
for real arguments, but also allows complex numbers.
-- Scheme Procedure: angle z
-- C Function: scm_angle (z)
Return the angle of the complex number Z.
-- C Function: SCM scm_c_make_rectangular (double re, double im)
-- C Function: SCM scm_c_make_polar (double x, double y)
Like ‘scm_make_rectangular’ or ‘scm_make_polar’, respectively, but
these functions take ‘double’s as their arguments.
-- C Function: double scm_c_real_part (z)
-- C Function: double scm_c_imag_part (z)
Returns the real or imaginary part of Z as a ‘double’.
-- C Function: double scm_c_magnitude (z)
-- C Function: double scm_c_angle (z)
Returns the magnitude or angle of Z as a ‘double’.
6.6.2.11 Arithmetic Functions
.............................
The C arithmetic functions below always takes two arguments, while the
Scheme functions can take an arbitrary number. When you need to invoke
them with just one argument, for example to compute the equivalent of
‘(- x)’, pass ‘SCM_UNDEFINED’ as the second one: ‘scm_difference (x,
SCM_UNDEFINED)’.
-- Scheme Procedure: + z1 ...
-- C Function: scm_sum (z1, z2)
Return the sum of all parameter values. Return 0 if called without
any parameters.
-- Scheme Procedure: - z1 z2 ...
-- C Function: scm_difference (z1, z2)
If called with one argument Z1, -Z1 is returned. Otherwise the sum
of all but the first argument are subtracted from the first
argument.
-- Scheme Procedure: * z1 ...
-- C Function: scm_product (z1, z2)
Return the product of all arguments. If called without arguments,
1 is returned.
-- Scheme Procedure: / z1 z2 ...
-- C Function: scm_divide (z1, z2)
Divide the first argument by the product of the remaining
arguments. If called with one argument Z1, 1/Z1 is returned.
-- Scheme Procedure: 1+ z
-- C Function: scm_oneplus (z)
Return Z + 1.
-- Scheme Procedure: 1- z
-- C function: scm_oneminus (z)
Return Z - 1.
-- Scheme Procedure: abs x
-- C Function: scm_abs (x)
Return the absolute value of X.
X must be a number with zero imaginary part. To calculate the
magnitude of a complex number, use ‘magnitude’ instead.
-- Scheme Procedure: max x1 x2 ...
-- C Function: scm_max (x1, x2)
Return the maximum of all parameter values.
-- Scheme Procedure: min x1 x2 ...
-- C Function: scm_min (x1, x2)
Return the minimum of all parameter values.
-- Scheme Procedure: truncate x
-- C Function: scm_truncate_number (x)
Round the inexact number X towards zero.
-- Scheme Procedure: round x
-- C Function: scm_round_number (x)
Round the inexact number X to the nearest integer. When exactly
halfway between two integers, round to the even one.
-- Scheme Procedure: floor x
-- C Function: scm_floor (x)
Round the number X towards minus infinity.
-- Scheme Procedure: ceiling x
-- C Function: scm_ceiling (x)
Round the number X towards infinity.
-- C Function: double scm_c_truncate (double x)
-- C Function: double scm_c_round (double x)
Like ‘scm_truncate_number’ or ‘scm_round_number’, respectively, but
these functions take and return ‘double’ values.
-- Scheme Procedure: euclidean/ X Y
-- Scheme Procedure: euclidean-quotient X Y
-- Scheme Procedure: euclidean-remainder X Y
-- C Function: void scm_euclidean_divide (SCM X, SCM Y, SCM *Q, SCM *R)
-- C Function: SCM scm_euclidean_quotient (SCM X, SCM Y)
-- C Function: SCM scm_euclidean_remainder (SCM X, SCM Y)
These procedures accept two real numbers X and Y, where the divisor
Y must be non-zero. ‘euclidean-quotient’ returns the integer Q and
‘euclidean-remainder’ returns the real number R such that X = Q*Y +
R and 0 <= R < |Y|. ‘euclidean/’ returns both Q and R, and is more
efficient than computing each separately. Note that when Y > 0,
‘euclidean-quotient’ returns floor(X/Y), otherwise it returns
ceiling(X/Y).
Note that these operators are equivalent to the R6RS operators
‘div’, ‘mod’, and ‘div-and-mod’.
(euclidean-quotient 123 10) ⇒ 12
(euclidean-remainder 123 10) ⇒ 3
(euclidean/ 123 10) ⇒ 12 and 3
(euclidean/ 123 -10) ⇒ -12 and 3
(euclidean/ -123 10) ⇒ -13 and 7
(euclidean/ -123 -10) ⇒ 13 and 7
(euclidean/ -123.2 -63.5) ⇒ 2.0 and 3.8
(euclidean/ 16/3 -10/7) ⇒ -3 and 22/21
-- Scheme Procedure: floor/ X Y
-- Scheme Procedure: floor-quotient X Y
-- Scheme Procedure: floor-remainder X Y
-- C Function: void scm_floor_divide (SCM X, SCM Y, SCM *Q, SCM *R)
-- C Function: SCM scm_floor_quotient (X, Y)
-- C Function: SCM scm_floor_remainder (X, Y)
These procedures accept two real numbers X and Y, where the divisor
Y must be non-zero. ‘floor-quotient’ returns the integer Q and
‘floor-remainder’ returns the real number R such that Q =
floor(X/Y) and X = Q*Y + R. ‘floor/’ returns both Q and R, and is
more efficient than computing each separately. Note that R, if
non-zero, will have the same sign as Y.
When X and Y are integers, ‘floor-remainder’ is equivalent to the
R5RS integer-only operator ‘modulo’.
(floor-quotient 123 10) ⇒ 12
(floor-remainder 123 10) ⇒ 3
(floor/ 123 10) ⇒ 12 and 3
(floor/ 123 -10) ⇒ -13 and -7
(floor/ -123 10) ⇒ -13 and 7
(floor/ -123 -10) ⇒ 12 and -3
(floor/ -123.2 -63.5) ⇒ 1.0 and -59.7
(floor/ 16/3 -10/7) ⇒ -4 and -8/21
-- Scheme Procedure: ceiling/ X Y
-- Scheme Procedure: ceiling-quotient X Y
-- Scheme Procedure: ceiling-remainder X Y
-- C Function: void scm_ceiling_divide (SCM X, SCM Y, SCM *Q, SCM *R)
-- C Function: SCM scm_ceiling_quotient (X, Y)
-- C Function: SCM scm_ceiling_remainder (X, Y)
These procedures accept two real numbers X and Y, where the divisor
Y must be non-zero. ‘ceiling-quotient’ returns the integer Q and
‘ceiling-remainder’ returns the real number R such that Q =
ceiling(X/Y) and X = Q*Y + R. ‘ceiling/’ returns both Q and R, and
is more efficient than computing each separately. Note that R, if
non-zero, will have the opposite sign of Y.
(ceiling-quotient 123 10) ⇒ 13
(ceiling-remainder 123 10) ⇒ -7
(ceiling/ 123 10) ⇒ 13 and -7
(ceiling/ 123 -10) ⇒ -12 and 3
(ceiling/ -123 10) ⇒ -12 and -3
(ceiling/ -123 -10) ⇒ 13 and 7
(ceiling/ -123.2 -63.5) ⇒ 2.0 and 3.8
(ceiling/ 16/3 -10/7) ⇒ -3 and 22/21
-- Scheme Procedure: truncate/ X Y
-- Scheme Procedure: truncate-quotient X Y
-- Scheme Procedure: truncate-remainder X Y
-- C Function: void scm_truncate_divide (SCM X, SCM Y, SCM *Q, SCM *R)
-- C Function: SCM scm_truncate_quotient (X, Y)
-- C Function: SCM scm_truncate_remainder (X, Y)
These procedures accept two real numbers X and Y, where the divisor
Y must be non-zero. ‘truncate-quotient’ returns the integer Q and
‘truncate-remainder’ returns the real number R such that Q is X/Y
rounded toward zero, and X = Q*Y + R. ‘truncate/’ returns both Q
and R, and is more efficient than computing each separately. Note
that R, if non-zero, will have the same sign as X.
When X and Y are integers, these operators are equivalent to the
R5RS integer-only operators ‘quotient’ and ‘remainder’.
(truncate-quotient 123 10) ⇒ 12
(truncate-remainder 123 10) ⇒ 3
(truncate/ 123 10) ⇒ 12 and 3
(truncate/ 123 -10) ⇒ -12 and 3
(truncate/ -123 10) ⇒ -12 and -3
(truncate/ -123 -10) ⇒ 12 and -3
(truncate/ -123.2 -63.5) ⇒ 1.0 and -59.7
(truncate/ 16/3 -10/7) ⇒ -3 and 22/21
-- Scheme Procedure: centered/ X Y
-- Scheme Procedure: centered-quotient X Y
-- Scheme Procedure: centered-remainder X Y
-- C Function: void scm_centered_divide (SCM X, SCM Y, SCM *Q, SCM *R)
-- C Function: SCM scm_centered_quotient (SCM X, SCM Y)
-- C Function: SCM scm_centered_remainder (SCM X, SCM Y)
These procedures accept two real numbers X and Y, where the divisor
Y must be non-zero. ‘centered-quotient’ returns the integer Q and
‘centered-remainder’ returns the real number R such that X = Q*Y +
R and -|Y/2| <= R < |Y/2|. ‘centered/’ returns both Q and R, and
is more efficient than computing each separately.
Note that ‘centered-quotient’ returns X/Y rounded to the nearest
integer. When X/Y lies exactly half-way between two integers, the
tie is broken according to the sign of Y. If Y > 0, ties are
rounded toward positive infinity, otherwise they are rounded toward
negative infinity. This is a consequence of the requirement that
-|Y/2| <= R < |Y/2|.
Note that these operators are equivalent to the R6RS operators
‘div0’, ‘mod0’, and ‘div0-and-mod0’.
(centered-quotient 123 10) ⇒ 12
(centered-remainder 123 10) ⇒ 3
(centered/ 123 10) ⇒ 12 and 3
(centered/ 123 -10) ⇒ -12 and 3
(centered/ -123 10) ⇒ -12 and -3
(centered/ -123 -10) ⇒ 12 and -3
(centered/ 125 10) ⇒ 13 and -5
(centered/ 127 10) ⇒ 13 and -3
(centered/ 135 10) ⇒ 14 and -5
(centered/ -123.2 -63.5) ⇒ 2.0 and 3.8
(centered/ 16/3 -10/7) ⇒ -4 and -8/21
-- Scheme Procedure: round/ X Y
-- Scheme Procedure: round-quotient X Y
-- Scheme Procedure: round-remainder X Y
-- C Function: void scm_round_divide (SCM X, SCM Y, SCM *Q, SCM *R)
-- C Function: SCM scm_round_quotient (X, Y)
-- C Function: SCM scm_round_remainder (X, Y)
These procedures accept two real numbers X and Y, where the divisor
Y must be non-zero. ‘round-quotient’ returns the integer Q and
‘round-remainder’ returns the real number R such that X = Q*Y + R
and Q is X/Y rounded to the nearest integer, with ties going to the
nearest even integer. ‘round/’ returns both Q and R, and is more
efficient than computing each separately.
Note that ‘round/’ and ‘centered/’ are almost equivalent, but their
behavior differs when X/Y lies exactly half-way between two
integers. In this case, ‘round/’ chooses the nearest even integer,
whereas ‘centered/’ chooses in such a way to satisfy the constraint
-|Y/2| <= R < |Y/2|, which is stronger than the corresponding
constraint for ‘round/’, -|Y/2| <= R <= |Y/2|. In particular, when
X and Y are integers, the number of possible remainders returned by
‘centered/’ is |Y|, whereas the number of possible remainders
returned by ‘round/’ is |Y|+1 when Y is even.
(round-quotient 123 10) ⇒ 12
(round-remainder 123 10) ⇒ 3
(round/ 123 10) ⇒ 12 and 3
(round/ 123 -10) ⇒ -12 and 3
(round/ -123 10) ⇒ -12 and -3
(round/ -123 -10) ⇒ 12 and -3
(round/ 125 10) ⇒ 12 and 5
(round/ 127 10) ⇒ 13 and -3
(round/ 135 10) ⇒ 14 and -5
(round/ -123.2 -63.5) ⇒ 2.0 and 3.8
(round/ 16/3 -10/7) ⇒ -4 and -8/21
6.6.2.12 Scientific Functions
.............................
The following procedures accept any kind of number as arguments,
including complex numbers.
-- Scheme Procedure: sqrt z
Return the square root of Z. Of the two possible roots (positive
and negative), the one with a positive real part is returned, or if
that’s zero then a positive imaginary part. Thus,
(sqrt 9.0) ⇒ 3.0
(sqrt -9.0) ⇒ 0.0+3.0i
(sqrt 1.0+1.0i) ⇒ 1.09868411346781+0.455089860562227i
(sqrt -1.0-1.0i) ⇒ 0.455089860562227-1.09868411346781i
-- Scheme Procedure: expt z1 z2
Return Z1 raised to the power of Z2.
-- Scheme Procedure: sin z
Return the sine of Z.
-- Scheme Procedure: cos z
Return the cosine of Z.
-- Scheme Procedure: tan z
Return the tangent of Z.
-- Scheme Procedure: asin z
Return the arcsine of Z.
-- Scheme Procedure: acos z
Return the arccosine of Z.
-- Scheme Procedure: atan z
-- Scheme Procedure: atan y x
Return the arctangent of Z, or of Y/X.
-- Scheme Procedure: exp z
Return e to the power of Z, where e is the base of natural
logarithms (2.71828...).
-- Scheme Procedure: log z
Return the natural logarithm of Z.
-- Scheme Procedure: log10 z
Return the base 10 logarithm of Z.
-- Scheme Procedure: sinh z
Return the hyperbolic sine of Z.
-- Scheme Procedure: cosh z
Return the hyperbolic cosine of Z.
-- Scheme Procedure: tanh z
Return the hyperbolic tangent of Z.
-- Scheme Procedure: asinh z
Return the hyperbolic arcsine of Z.
-- Scheme Procedure: acosh z
Return the hyperbolic arccosine of Z.
-- Scheme Procedure: atanh z
Return the hyperbolic arctangent of Z.
6.6.2.13 Bitwise Operations
...........................
For the following bitwise functions, negative numbers are treated as
infinite precision twos-complements. For instance -6 is bits ...111010,
with infinitely many ones on the left. It can be seen that adding 6
(binary 110) to such a bit pattern gives all zeros.
-- Scheme Procedure: logand n1 n2 ...
-- C Function: scm_logand (n1, n2)
Return the bitwise AND of the integer arguments.
(logand) ⇒ -1
(logand 7) ⇒ 7
(logand #b111 #b011 #b001) ⇒ 1
-- Scheme Procedure: logior n1 n2 ...
-- C Function: scm_logior (n1, n2)
Return the bitwise OR of the integer arguments.
(logior) ⇒ 0
(logior 7) ⇒ 7
(logior #b000 #b001 #b011) ⇒ 3
-- Scheme Procedure: logxor n1 n2 ...
-- C Function: scm_loxor (n1, n2)
Return the bitwise XOR of the integer arguments. A bit is set in
the result if it is set in an odd number of arguments.
(logxor) ⇒ 0
(logxor 7) ⇒ 7
(logxor #b000 #b001 #b011) ⇒ 2
(logxor #b000 #b001 #b011 #b011) ⇒ 1
-- Scheme Procedure: lognot n
-- C Function: scm_lognot (n)
Return the integer which is the ones-complement of the integer
argument, ie. each 0 bit is changed to 1 and each 1 bit to 0.
(number->string (lognot #b10000000) 2)
⇒ "-10000001"
(number->string (lognot #b0) 2)
⇒ "-1"
-- Scheme Procedure: logtest j k
-- C Function: scm_logtest (j, k)
Test whether J and K have any 1 bits in common. This is equivalent
to ‘(not (zero? (logand j k)))’, but without actually calculating
the ‘logand’, just testing for non-zero.
(logtest #b0100 #b1011) ⇒ #f
(logtest #b0100 #b0111) ⇒ #t
-- Scheme Procedure: logbit? index j
-- C Function: scm_logbit_p (index, j)
Test whether bit number INDEX in J is set. INDEX starts from 0 for
the least significant bit.
(logbit? 0 #b1101) ⇒ #t
(logbit? 1 #b1101) ⇒ #f
(logbit? 2 #b1101) ⇒ #t
(logbit? 3 #b1101) ⇒ #t
(logbit? 4 #b1101) ⇒ #f
-- Scheme Procedure: ash n count
-- C Function: scm_ash (n, count)
Return floor(n * 2^{count}). N and COUNT must be exact integers.
With N viewed as an infinite-precision twos-complement integer,
‘ash’ means a left shift introducing zero bits when COUNT is
positive, or a right shift dropping bits when COUNT is negative.
This is an “arithmetic” shift.
(number->string (ash #b1 3) 2) ⇒ "1000"
(number->string (ash #b1010 -1) 2) ⇒ "101"
;; -23 is bits ...11101001, -6 is bits ...111010
(ash -23 -2) ⇒ -6
-- Scheme Procedure: round-ash n count
-- C Function: scm_round_ash (n, count)
Return round(n * 2^count). N and COUNT must be exact integers.
With N viewed as an infinite-precision twos-complement integer,
‘round-ash’ means a left shift introducing zero bits when COUNT is
positive, or a right shift rounding to the nearest integer (with
ties going to the nearest even integer) when COUNT is negative.
This is a rounded “arithmetic” shift.
(number->string (round-ash #b1 3) 2) ⇒ \"1000\"
(number->string (round-ash #b1010 -1) 2) ⇒ \"101\"
(number->string (round-ash #b1010 -2) 2) ⇒ \"10\"
(number->string (round-ash #b1011 -2) 2) ⇒ \"11\"
(number->string (round-ash #b1101 -2) 2) ⇒ \"11\"
(number->string (round-ash #b1110 -2) 2) ⇒ \"100\"
-- Scheme Procedure: logcount n
-- C Function: scm_logcount (n)
Return the number of bits in integer N. If N is positive, the
1-bits in its binary representation are counted. If negative, the
0-bits in its two’s-complement binary representation are counted.
If zero, 0 is returned.
(logcount #b10101010)
⇒ 4
(logcount 0)
⇒ 0
(logcount -2)
⇒ 1
-- Scheme Procedure: integer-length n
-- C Function: scm_integer_length (n)
Return the number of bits necessary to represent N.
For positive N this is how many bits to the most significant one
bit. For negative N it’s how many bits to the most significant
zero bit in twos complement form.
(integer-length #b10101010) ⇒ 8
(integer-length #b1111) ⇒ 4
(integer-length 0) ⇒ 0
(integer-length -1) ⇒ 0
(integer-length -256) ⇒ 8
(integer-length -257) ⇒ 9
-- Scheme Procedure: integer-expt n k
-- C Function: scm_integer_expt (n, k)
Return N raised to the power K. K must be an exact integer, N can
be any number.
Negative K is supported, and results in 1/n^abs(k) in the usual
way. N^0 is 1, as usual, and that includes 0^0 is 1.
(integer-expt 2 5) ⇒ 32
(integer-expt -3 3) ⇒ -27
(integer-expt 5 -3) ⇒ 1/125
(integer-expt 0 0) ⇒ 1
-- Scheme Procedure: bit-extract n start end
-- C Function: scm_bit_extract (n, start, end)
Return the integer composed of the START (inclusive) through END
(exclusive) bits of N. The STARTth bit becomes the 0-th bit in the
result.
(number->string (bit-extract #b1101101010 0 4) 2)
⇒ "1010"
(number->string (bit-extract #b1101101010 4 9) 2)
⇒ "10110"
6.6.2.14 Random Number Generation
.................................
Pseudo-random numbers are generated from a random state object, which
can be created with ‘seed->random-state’ or ‘datum->random-state’. An
external representation (i.e. one which can written with ‘write’ and
read with ‘read’) of a random state object can be obtained via
‘random-state->datum’. The STATE parameter to the various functions
below is optional, it defaults to the state object in the
‘*random-state*’ variable.
-- Scheme Procedure: copy-random-state [state]
-- C Function: scm_copy_random_state (state)
Return a copy of the random state STATE.
-- Scheme Procedure: random n [state]
-- C Function: scm_random (n, state)
Return a number in [0, N).
Accepts a positive integer or real n and returns a number of the
same type between zero (inclusive) and N (exclusive). The values
returned have a uniform distribution.
-- Scheme Procedure: random:exp [state]
-- C Function: scm_random_exp (state)
Return an inexact real in an exponential distribution with mean 1.
For an exponential distribution with mean U use ‘(* U
(random:exp))’.
-- Scheme Procedure: random:hollow-sphere! vect [state]
-- C Function: scm_random_hollow_sphere_x (vect, state)
Fills VECT with inexact real random numbers the sum of whose
squares is equal to 1.0. Thinking of VECT as coordinates in space
of dimension N = ‘(vector-length VECT)’, the coordinates are
uniformly distributed over the surface of the unit n-sphere.
-- Scheme Procedure: random:normal [state]
-- C Function: scm_random_normal (state)
Return an inexact real in a normal distribution. The distribution
used has mean 0 and standard deviation 1. For a normal
distribution with mean M and standard deviation D use ‘(+ M (* D
(random:normal)))’.
-- Scheme Procedure: random:normal-vector! vect [state]
-- C Function: scm_random_normal_vector_x (vect, state)
Fills VECT with inexact real random numbers that are independent
and standard normally distributed (i.e., with mean 0 and variance
1).
-- Scheme Procedure: random:solid-sphere! vect [state]
-- C Function: scm_random_solid_sphere_x (vect, state)
Fills VECT with inexact real random numbers the sum of whose
squares is less than 1.0. Thinking of VECT as coordinates in space
of dimension N = ‘(vector-length VECT)’, the coordinates are
uniformly distributed within the unit N-sphere.
-- Scheme Procedure: random:uniform [state]
-- C Function: scm_random_uniform (state)
Return a uniformly distributed inexact real random number in [0,1).
-- Scheme Procedure: seed->random-state seed
-- C Function: scm_seed_to_random_state (seed)
Return a new random state using SEED.
-- Scheme Procedure: datum->random-state datum
-- C Function: scm_datum_to_random_state (datum)
Return a new random state from DATUM, which should have been
obtained by ‘random-state->datum’.
-- Scheme Procedure: random-state->datum state
-- C Function: scm_random_state_to_datum (state)
Return a datum representation of STATE that may be written out and
read back with the Scheme reader.
-- Scheme Procedure: random-state-from-platform
-- C Function: scm_random_state_from_platform ()
Construct a new random state seeded from a platform-specific source
of entropy, appropriate for use in non-security-critical
applications. Currently ‘/dev/urandom’ is tried first, or else the
seed is based on the time, date, process ID, an address from a
freshly allocated heap cell, an address from the local stack frame,
and a high-resolution timer if available.
-- Variable: *random-state*
The global random state used by the above functions when the STATE
parameter is not given.
Note that the initial value of ‘*random-state*’ is the same every
time Guile starts up. Therefore, if you don’t pass a STATE parameter to
the above procedures, and you don’t set ‘*random-state*’ to
‘(seed->random-state your-seed)’, where ‘your-seed’ is something that
_isn’t_ the same every time, you’ll get the same sequence of “random”
numbers on every run.
For example, unless the relevant source code has changed, ‘(map
random (cdr (iota 30)))’, if the first use of random numbers since Guile
started up, will always give:
(map random (cdr (iota 19)))
⇒
(0 1 1 2 2 2 1 2 6 7 10 0 5 3 12 5 5 12)
To seed the random state in a sensible way for non-security-critical
applications, do this during initialization of your program:
(set! *random-state* (random-state-from-platform))
6.6.3 Characters
----------------
In Scheme, there is a data type to describe a single character.
Defining what exactly a character _is_ can be more complicated than
it seems. Guile follows the advice of R6RS and uses The Unicode
Standard to help define what a character is. So, for Guile, a character
is anything in the Unicode Character Database.
The Unicode Character Database is basically a table of characters
indexed using integers called ’code points’. Valid code points are in
the ranges 0 to ‘#xD7FF’ inclusive or ‘#xE000’ to ‘#x10FFFF’ inclusive,
which is about 1.1 million code points.
Any code point that has been assigned to a character or that has
otherwise been given a meaning by Unicode is called a ’designated code
point’. Most of the designated code points, about 200,000 of them,
indicate characters, accents or other combining marks that modify other
characters, symbols, whitespace, and control characters. Some are not
characters but indicators that suggest how to format or display
neighboring characters.
If a code point is not a designated code point – if it has not been
assigned to a character by The Unicode Standard – it is a ’reserved code
point’, meaning that they are reserved for future use. Most of the code
points, about 800,000, are ’reserved code points’.
By convention, a Unicode code point is written as “U+XXXX” where
“XXXX” is a hexadecimal number. Please note that this convenient
notation is not valid code. Guile does not interpret “U+XXXX” as a
character.
In Scheme, a character literal is written as ‘#\NAME’ where NAME is
the name of the character that you want. Printable characters have
their usual single character name; for example, ‘#\a’ is a lower case
‘a’.
Some of the code points are ’combining characters’ that are not meant
to be printed by themselves but are instead meant to modify the
appearance of the previous character. For combining characters, an
alternate form of the character literal is ‘#\’ followed by U+25CC (a
small, dotted circle), followed by the combining character. This allows
the combining character to be drawn on the circle, not on the backslash
of ‘#\’.
Many of the non-printing characters, such as whitespace characters
and control characters, also have names.
The most commonly used non-printing characters have long character
names, described in the table below.
Character Codepoint
Name
‘#\nul’ U+0000
‘#\alarm’ U+0007
‘#\backspace’ U+0008
‘#\tab’ U+0009
‘#\linefeed’ U+000A
‘#\newline’ U+000A
‘#\vtab’ U+000B
‘#\page’ U+000C
‘#\return’ U+000D
‘#\esc’ U+001B
‘#\space’ U+0020
‘#\delete’ U+007F
There are also short names for all of the “C0 control characters”
(those with code points below 32). The following table lists the short
name for each character.
0 = ‘#\nul’ 1 = ‘#\soh’ 2 = ‘#\stx’ 3 = ‘#\etx’
4 = ‘#\eot’ 5 = ‘#\enq’ 6 = ‘#\ack’ 7 = ‘#\bel’
8 = ‘#\bs’ 9 = ‘#\ht’ 10 = ‘#\lf’ 11 = ‘#\vt’
12 = ‘#\ff’ 13 = ‘#\cr’ 14 = ‘#\so’ 15 = ‘#\si’
16 = ‘#\dle’ 17 = ‘#\dc1’ 18 = ‘#\dc2’ 19 = ‘#\dc3’
20 = ‘#\dc4’ 21 = ‘#\nak’ 22 = ‘#\syn’ 23 = ‘#\etb’
24 = ‘#\can’ 25 = ‘#\em’ 26 = ‘#\sub’ 27 = ‘#\esc’
28 = ‘#\fs’ 29 = ‘#\gs’ 30 = ‘#\rs’ 31 = ‘#\us’
32 = ‘#\sp’
The short name for the “delete” character (code point U+007F) is
‘#\del’.
The R7RS name for the “escape” character (code point U+001B) is
‘#\escape’.
There are also a few alternative names left over for compatibility
with previous versions of Guile.
Alternate Standard
‘#\nl’ ‘#\newline’
‘#\np’ ‘#\page’
‘#\null’ ‘#\nul’
Characters may also be written using their code point values. They
can be written with as an octal number, such as ‘#\10’ for ‘#\bs’ or
‘#\177’ for ‘#\del’.
If one prefers hex to octal, there is an additional syntax for
character escapes: ‘#\xHHHH’ – the letter ’x’ followed by a hexadecimal
number of one to eight digits.
-- Scheme Procedure: char? x
-- C Function: scm_char_p (x)
Return ‘#t’ if X is a character, else ‘#f’.
Fundamentally, the character comparison operations below are numeric
comparisons of the character’s code points.
-- Scheme Procedure: char=? x y
Return ‘#t’ if code point of X is equal to the code point of Y,
else ‘#f’.
-- Scheme Procedure: char? x y
Return ‘#t’ if the code point of X is greater than the code point
of Y, else ‘#f’.
-- Scheme Procedure: char>=? x y
Return ‘#t’ if the code point of X is greater than or equal to the
code point of Y, else ‘#f’.
Case-insensitive character comparisons use _Unicode case folding_.
In case folding comparisons, if a character is lowercase and has an
uppercase form that can be expressed as a single character, it is
converted to uppercase before comparison. All other characters undergo
no conversion before the comparison occurs. This includes the German
sharp S (Eszett) which is not uppercased before conversion because its
uppercase form has two characters. Unicode case folding is language
independent: it uses rules that are generally true, but, it cannot cover
all cases for all languages.
-- Scheme Procedure: char-ci=? x y
Return ‘#t’ if the case-folded code point of X is the same as the
case-folded code point of Y, else ‘#f’.
-- Scheme Procedure: char-ci? x y
Return ‘#t’ if the case-folded code point of X is greater than the
case-folded code point of Y, else ‘#f’.
-- Scheme Procedure: char-ci>=? x y
Return ‘#t’ if the case-folded code point of X is greater than or
equal to the case-folded code point of Y, else ‘#f’.
-- Scheme Procedure: char-alphabetic? chr
-- C Function: scm_char_alphabetic_p (chr)
Return ‘#t’ if CHR is alphabetic, else ‘#f’.
-- Scheme Procedure: char-numeric? chr
-- C Function: scm_char_numeric_p (chr)
Return ‘#t’ if CHR is numeric, else ‘#f’.
-- Scheme Procedure: char-whitespace? chr
-- C Function: scm_char_whitespace_p (chr)
Return ‘#t’ if CHR is whitespace, else ‘#f’.
-- Scheme Procedure: char-upper-case? chr
-- C Function: scm_char_upper_case_p (chr)
Return ‘#t’ if CHR is uppercase, else ‘#f’.
-- Scheme Procedure: char-lower-case? chr
-- C Function: scm_char_lower_case_p (chr)
Return ‘#t’ if CHR is lowercase, else ‘#f’.
-- Scheme Procedure: char-is-both? chr
-- C Function: scm_char_is_both_p (chr)
Return ‘#t’ if CHR is either uppercase or lowercase, else ‘#f’.
-- Scheme Procedure: char-general-category chr
-- C Function: scm_char_general_category (chr)
Return a symbol giving the two-letter name of the Unicode general
category assigned to CHR or ‘#f’ if no named category is assigned.
The following table provides a list of category names along with
their meanings.
Lu Uppercase letter Pf Final quote punctuation
Ll Lowercase letter Po Other punctuation
Lt Titlecase letter Sm Math symbol
Lm Modifier letter Sc Currency symbol
Lo Other letter Sk Modifier symbol
Mn Non-spacing mark So Other symbol
Mc Combining spacing mark Zs Space separator
Me Enclosing mark Zl Line separator
Nd Decimal digit number Zp Paragraph separator
Nl Letter number Cc Control
No Other number Cf Format
Pc Connector punctuation Cs Surrogate
Pd Dash punctuation Co Private use
Ps Open punctuation Cn Unassigned
Pe Close punctuation
Pi Initial quote punctuation
-- Scheme Procedure: char->integer chr
-- C Function: scm_char_to_integer (chr)
Return the code point of CHR.
-- Scheme Procedure: integer->char n
-- C Function: scm_integer_to_char (n)
Return the character that has code point N. The integer N must be
a valid code point. Valid code points are in the ranges 0 to
‘#xD7FF’ inclusive or ‘#xE000’ to ‘#x10FFFF’ inclusive.
-- Scheme Procedure: char-upcase chr
-- C Function: scm_char_upcase (chr)
Return the uppercase character version of CHR.
-- Scheme Procedure: char-downcase chr
-- C Function: scm_char_downcase (chr)
Return the lowercase character version of CHR.
-- Scheme Procedure: char-titlecase chr
-- C Function: scm_char_titlecase (chr)
Return the titlecase character version of CHR if one exists;
otherwise return the uppercase version.
For most characters these will be the same, but the Unicode
Standard includes certain digraph compatibility characters, such as
‘U+01F3’ “dz”, for which the uppercase and titlecase characters are
different (‘U+01F1’ “DZ” and ‘U+01F2’ “Dz” in this case,
respectively).
-- C Function: scm_t_wchar scm_c_upcase (scm_t_wchar C)
-- C Function: scm_t_wchar scm_c_downcase (scm_t_wchar C)
-- C Function: scm_t_wchar scm_c_titlecase (scm_t_wchar C)
These C functions take an integer representation of a Unicode
codepoint and return the codepoint corresponding to its uppercase,
lowercase, and titlecase forms respectively. The type
‘scm_t_wchar’ is a signed, 32-bit integer.
Characters also have “formal names”, which are defined by Unicode.
These names can be accessed in Guile from the ‘(ice-9 unicode)’ module:
(use-modules (ice-9 unicode))
-- Scheme Procedure: char->formal-name chr
Return the formal all-upper-case Unicode name of CH, as a string,
or ‘#f’ if the character has no name.
-- Scheme Procedure: formal-name->char name
Return the character whose formal all-upper-case Unicode name is
NAME, or ‘#f’ if no such character is known.
6.6.4 Character Sets
--------------------
The features described in this section correspond directly to SRFI-14.
The data type “charset” implements sets of characters (*note
Characters::). Because the internal representation of character sets is
not visible to the user, a lot of procedures for handling them are
provided.
Character sets can be created, extended, tested for the membership of
a characters and be compared to other character sets.
6.6.4.1 Character Set Predicates/Comparison
...........................................
Use these procedures for testing whether an object is a character set,
or whether several character sets are equal or subsets of each other.
‘char-set-hash’ can be used for calculating a hash value, maybe for
usage in fast lookup procedures.
-- Scheme Procedure: char-set? obj
-- C Function: scm_char_set_p (obj)
Return ‘#t’ if OBJ is a character set, ‘#f’ otherwise.
-- Scheme Procedure: char-set= char_set ...
-- C Function: scm_char_set_eq (char_sets)
Return ‘#t’ if all given character sets are equal.
-- Scheme Procedure: char-set<= char_set ...
-- C Function: scm_char_set_leq (char_sets)
Return ‘#t’ if every character set CHAR_SETi is a subset of
character set CHAR_SETi+1.
-- Scheme Procedure: char-set-hash cs [bound]
-- C Function: scm_char_set_hash (cs, bound)
Compute a hash value for the character set CS. If BOUND is given
and non-zero, it restricts the returned value to the range 0 ...
BOUND - 1.
6.6.4.2 Iterating Over Character Sets
.....................................
Character set cursors are a means for iterating over the members of a
character sets. After creating a character set cursor with
‘char-set-cursor’, a cursor can be dereferenced with ‘char-set-ref’,
advanced to the next member with ‘char-set-cursor-next’. Whether a
cursor has passed past the last element of the set can be checked with
‘end-of-char-set?’.
Additionally, mapping and (un-)folding procedures for character sets
are provided.
-- Scheme Procedure: char-set-cursor cs
-- C Function: scm_char_set_cursor (cs)
Return a cursor into the character set CS.
-- Scheme Procedure: char-set-ref cs cursor
-- C Function: scm_char_set_ref (cs, cursor)
Return the character at the current cursor position CURSOR in the
character set CS. It is an error to pass a cursor for which
‘end-of-char-set?’ returns true.
-- Scheme Procedure: char-set-cursor-next cs cursor
-- C Function: scm_char_set_cursor_next (cs, cursor)
Advance the character set cursor CURSOR to the next character in
the character set CS. It is an error if the cursor given satisfies
‘end-of-char-set?’.
-- Scheme Procedure: end-of-char-set? cursor
-- C Function: scm_end_of_char_set_p (cursor)
Return ‘#t’ if CURSOR has reached the end of a character set, ‘#f’
otherwise.
-- Scheme Procedure: char-set-fold kons knil cs
-- C Function: scm_char_set_fold (kons, knil, cs)
Fold the procedure KONS over the character set CS, initializing it
with KNIL.
-- Scheme Procedure: char-set-unfold p f g seed [base_cs]
-- C Function: scm_char_set_unfold (p, f, g, seed, base_cs)
This is a fundamental constructor for character sets.
• G is used to generate a series of “seed” values from the
initial seed: SEED, (G SEED), (G^2 SEED), (G^3 SEED), ...
• P tells us when to stop – when it returns true when applied to
one of the seed values.
• F maps each seed value to a character. These characters are
added to the base character set BASE_CS to form the result;
BASE_CS defaults to the empty set.
-- Scheme Procedure: char-set-unfold! p f g seed base_cs
-- C Function: scm_char_set_unfold_x (p, f, g, seed, base_cs)
This is a fundamental constructor for character sets.
• G is used to generate a series of “seed” values from the
initial seed: SEED, (G SEED), (G^2 SEED), (G^3 SEED), ...
• P tells us when to stop – when it returns true when applied to
one of the seed values.
• F maps each seed value to a character. These characters are
added to the base character set BASE_CS to form the result;
BASE_CS defaults to the empty set.
-- Scheme Procedure: char-set-for-each proc cs
-- C Function: scm_char_set_for_each (proc, cs)
Apply PROC to every character in the character set CS. The return
value is not specified.
-- Scheme Procedure: char-set-map proc cs
-- C Function: scm_char_set_map (proc, cs)
Map the procedure PROC over every character in CS. PROC must be a
character -> character procedure.
6.6.4.3 Creating Character Sets
...............................
New character sets are produced with these procedures.
-- Scheme Procedure: char-set-copy cs
-- C Function: scm_char_set_copy (cs)
Return a newly allocated character set containing all characters in
CS.
-- Scheme Procedure: char-set chr ...
-- C Function: scm_char_set (chrs)
Return a character set containing all given characters.
-- Scheme Procedure: list->char-set list [base_cs]
-- C Function: scm_list_to_char_set (list, base_cs)
Convert the character list LIST to a character set. If the
character set BASE_CS is given, the character in this set are also
included in the result.
-- Scheme Procedure: list->char-set! list base_cs
-- C Function: scm_list_to_char_set_x (list, base_cs)
Convert the character list LIST to a character set. The characters
are added to BASE_CS and BASE_CS is returned.
-- Scheme Procedure: string->char-set str [base_cs]
-- C Function: scm_string_to_char_set (str, base_cs)
Convert the string STR to a character set. If the character set
BASE_CS is given, the characters in this set are also included in
the result.
-- Scheme Procedure: string->char-set! str base_cs
-- C Function: scm_string_to_char_set_x (str, base_cs)
Convert the string STR to a character set. The characters from the
string are added to BASE_CS, and BASE_CS is returned.
-- Scheme Procedure: char-set-filter pred cs [base_cs]
-- C Function: scm_char_set_filter (pred, cs, base_cs)
Return a character set containing every character from CS so that
it satisfies PRED. If provided, the characters from BASE_CS are
added to the result.
-- Scheme Procedure: char-set-filter! pred cs base_cs
-- C Function: scm_char_set_filter_x (pred, cs, base_cs)
Return a character set containing every character from CS so that
it satisfies PRED. The characters are added to BASE_CS and BASE_CS
is returned.
-- Scheme Procedure: ucs-range->char-set lower upper [error [base_cs]]
-- C Function: scm_ucs_range_to_char_set (lower, upper, error, base_cs)
Return a character set containing all characters whose character
codes lie in the half-open range [LOWER,UPPER).
If ERROR is a true value, an error is signalled if the specified
range contains characters which are not contained in the
implemented character range. If ERROR is ‘#f’, these characters
are silently left out of the resulting character set.
The characters in BASE_CS are added to the result, if given.
-- Scheme Procedure: ucs-range->char-set! lower upper error base_cs
-- C Function: scm_ucs_range_to_char_set_x (lower, upper, error,
base_cs)
Return a character set containing all characters whose character
codes lie in the half-open range [LOWER,UPPER).
If ERROR is a true value, an error is signalled if the specified
range contains characters which are not contained in the
implemented character range. If ERROR is ‘#f’, these characters
are silently left out of the resulting character set.
The characters are added to BASE_CS and BASE_CS is returned.
-- Scheme Procedure: ->char-set x
-- C Function: scm_to_char_set (x)
Coerces x into a char-set. X may be a string, character or
char-set. A string is converted to the set of its constituent
characters; a character is converted to a singleton set; a char-set
is returned as-is.
6.6.4.4 Querying Character Sets
...............................
Access the elements and other information of a character set with these
procedures.
-- Scheme Procedure: %char-set-dump cs
Returns an association list containing debugging information for
CS. The association list has the following entries.
‘char-set’
The char-set itself
‘len’
The number of groups of contiguous code points the char-set
contains
‘ranges’
A list of lists where each sublist is a range of code points
and their associated characters
The return value of this function cannot be relied upon to be
consistent between versions of Guile and should not be used in
code.
-- Scheme Procedure: char-set-size cs
-- C Function: scm_char_set_size (cs)
Return the number of elements in character set CS.
-- Scheme Procedure: char-set-count pred cs
-- C Function: scm_char_set_count (pred, cs)
Return the number of the elements int the character set CS which
satisfy the predicate PRED.
-- Scheme Procedure: char-set->list cs
-- C Function: scm_char_set_to_list (cs)
Return a list containing the elements of the character set CS.
-- Scheme Procedure: char-set->string cs
-- C Function: scm_char_set_to_string (cs)
Return a string containing the elements of the character set CS.
The order in which the characters are placed in the string is not
defined.
-- Scheme Procedure: char-set-contains? cs ch
-- C Function: scm_char_set_contains_p (cs, ch)
Return ‘#t’ if the character CH is contained in the character set
CS, or ‘#f’ otherwise.
-- Scheme Procedure: char-set-every pred cs
-- C Function: scm_char_set_every (pred, cs)
Return a true value if every character in the character set CS
satisfies the predicate PRED.
-- Scheme Procedure: char-set-any pred cs
-- C Function: scm_char_set_any (pred, cs)
Return a true value if any character in the character set CS
satisfies the predicate PRED.
6.6.4.5 Character-Set Algebra
.............................
Character sets can be manipulated with the common set algebra operation,
such as union, complement, intersection etc. All of these procedures
provide side-effecting variants, which modify their character set
argument(s).
-- Scheme Procedure: char-set-adjoin cs chr ...
-- C Function: scm_char_set_adjoin (cs, chrs)
Add all character arguments to the first argument, which must be a
character set.
-- Scheme Procedure: char-set-delete cs chr ...
-- C Function: scm_char_set_delete (cs, chrs)
Delete all character arguments from the first argument, which must
be a character set.
-- Scheme Procedure: char-set-adjoin! cs chr ...
-- C Function: scm_char_set_adjoin_x (cs, chrs)
Add all character arguments to the first argument, which must be a
character set.
-- Scheme Procedure: char-set-delete! cs chr ...
-- C Function: scm_char_set_delete_x (cs, chrs)
Delete all character arguments from the first argument, which must
be a character set.
-- Scheme Procedure: char-set-complement cs
-- C Function: scm_char_set_complement (cs)
Return the complement of the character set CS.
Note that the complement of a character set is likely to contain many
reserved code points (code points that are not associated with
characters). It may be helpful to modify the output of
‘char-set-complement’ by computing its intersection with the set of
designated code points, ‘char-set:designated’.
-- Scheme Procedure: char-set-union cs ...
-- C Function: scm_char_set_union (char_sets)
Return the union of all argument character sets.
-- Scheme Procedure: char-set-intersection cs ...
-- C Function: scm_char_set_intersection (char_sets)
Return the intersection of all argument character sets.
-- Scheme Procedure: char-set-difference cs1 cs ...
-- C Function: scm_char_set_difference (cs1, char_sets)
Return the difference of all argument character sets.
-- Scheme Procedure: char-set-xor cs ...
-- C Function: scm_char_set_xor (char_sets)
Return the exclusive-or of all argument character sets.
-- Scheme Procedure: char-set-diff+intersection cs1 cs ...
-- C Function: scm_char_set_diff_plus_intersection (cs1, char_sets)
Return the difference and the intersection of all argument
character sets.
-- Scheme Procedure: char-set-complement! cs
-- C Function: scm_char_set_complement_x (cs)
Return the complement of the character set CS.
-- Scheme Procedure: char-set-union! cs1 cs ...
-- C Function: scm_char_set_union_x (cs1, char_sets)
Return the union of all argument character sets.
-- Scheme Procedure: char-set-intersection! cs1 cs ...
-- C Function: scm_char_set_intersection_x (cs1, char_sets)
Return the intersection of all argument character sets.
-- Scheme Procedure: char-set-difference! cs1 cs ...
-- C Function: scm_char_set_difference_x (cs1, char_sets)
Return the difference of all argument character sets.
-- Scheme Procedure: char-set-xor! cs1 cs ...
-- C Function: scm_char_set_xor_x (cs1, char_sets)
Return the exclusive-or of all argument character sets.
-- Scheme Procedure: char-set-diff+intersection! cs1 cs2 cs ...
-- C Function: scm_char_set_diff_plus_intersection_x (cs1, cs2,
char_sets)
Return the difference and the intersection of all argument
character sets.
6.6.4.6 Standard Character Sets
...............................
In order to make the use of the character set data type and procedures
useful, several predefined character set variables exist.
These character sets are locale independent and are not recomputed
upon a ‘setlocale’ call. They contain characters from the whole range
of Unicode code points. For instance, ‘char-set:letter’ contains about
100,000 characters.
-- Scheme Variable: char-set:lower-case
-- C Variable: scm_char_set_lower_case
All lower-case characters.
-- Scheme Variable: char-set:upper-case
-- C Variable: scm_char_set_upper_case
All upper-case characters.
-- Scheme Variable: char-set:title-case
-- C Variable: scm_char_set_title_case
All single characters that function as if they were an upper-case
letter followed by a lower-case letter.
-- Scheme Variable: char-set:letter
-- C Variable: scm_char_set_letter
All letters. This includes ‘char-set:lower-case’,
‘char-set:upper-case’, ‘char-set:title-case’, and many letters that
have no case at all. For example, Chinese and Japanese characters
typically have no concept of case.
-- Scheme Variable: char-set:digit
-- C Variable: scm_char_set_digit
All digits.
-- Scheme Variable: char-set:letter+digit
-- C Variable: scm_char_set_letter_and_digit
The union of ‘char-set:letter’ and ‘char-set:digit’.
-- Scheme Variable: char-set:graphic
-- C Variable: scm_char_set_graphic
All characters which would put ink on the paper.
-- Scheme Variable: char-set:printing
-- C Variable: scm_char_set_printing
The union of ‘char-set:graphic’ and ‘char-set:whitespace’.
-- Scheme Variable: char-set:whitespace
-- C Variable: scm_char_set_whitespace
All whitespace characters.
-- Scheme Variable: char-set:blank
-- C Variable: scm_char_set_blank
All horizontal whitespace characters, which notably includes
‘#\space’ and ‘#\tab’.
-- Scheme Variable: char-set:iso-control
-- C Variable: scm_char_set_iso_control
The ISO control characters are the C0 control characters (U+0000 to
U+001F), delete (U+007F), and the C1 control characters (U+0080 to
U+009F).
-- Scheme Variable: char-set:punctuation
-- C Variable: scm_char_set_punctuation
All punctuation characters, such as the characters
‘!"#%&'()*,-./:;?@[\\]_{}’
-- Scheme Variable: char-set:symbol
-- C Variable: scm_char_set_symbol
All symbol characters, such as the characters ‘$+<=>^`|~’.
-- Scheme Variable: char-set:hex-digit
-- C Variable: scm_char_set_hex_digit
The hexadecimal digits ‘0123456789abcdefABCDEF’.
-- Scheme Variable: char-set:ascii
-- C Variable: scm_char_set_ascii
All ASCII characters.
-- Scheme Variable: char-set:empty
-- C Variable: scm_char_set_empty
The empty character set.
-- Scheme Variable: char-set:designated
-- C Variable: scm_char_set_designated
This character set contains all designated code points. This
includes all the code points to which Unicode has assigned a
character or other meaning.
-- Scheme Variable: char-set:full
-- C Variable: scm_char_set_full
This character set contains all possible code points. This
includes both designated and reserved code points.
6.6.5 Strings
-------------
Strings are fixed-length sequences of characters. They can be created
by calling constructor procedures, but they can also literally get
entered at the REPL or in Scheme source files.
Strings always carry the information about how many characters they
are composed of with them, so there is no special end-of-string
character, like in C. That means that Scheme strings can contain any
character, even the ‘#\nul’ character ‘\0’.
To use strings efficiently, you need to know a bit about how Guile
implements them. In Guile, a string consists of two parts, a head and
the actual memory where the characters are stored. When a string (or a
substring of it) is copied, only a new head gets created, the memory is
usually not copied. The two heads start out pointing to the same
memory.
When one of these two strings is modified, as with ‘string-set!’,
their common memory does get copied so that each string has its own
memory and modifying one does not accidentally modify the other as well.
Thus, Guile’s strings are ‘copy on write’; the actual copying of their
memory is delayed until one string is written to.
This implementation makes functions like ‘substring’ very efficient
in the common case that no modifications are done to the involved
strings.
If you do know that your strings are getting modified right away, you
can use ‘substring/copy’ instead of ‘substring’. This function performs
the copy immediately at the time of creation. This is more efficient,
especially in a multi-threaded program. Also, ‘substring/copy’ can
avoid the problem that a short substring holds on to the memory of a
very large original string that could otherwise be recycled.
If you want to avoid the copy altogether, so that modifications of
one string show up in the other, you can use ‘substring/shared’. The
strings created by this procedure are called “mutation sharing
substrings” since the substring and the original string share
modifications to each other.
If you want to prevent modifications, use ‘substring/read-only’.
Guile provides all procedures of SRFI-13 and a few more.
6.6.5.1 String Read Syntax
..........................
The read syntax for strings is an arbitrarily long sequence of
characters enclosed in double quotes (").
Backslash is an escape character and can be used to insert the
following special characters. \" and \\ are R5RS standard, \| is R7RS
standard, the next seven are R6RS standard — notice they follow C syntax
— and the remaining four are Guile extensions.
\\
Backslash character.
\"
Double quote character (an unescaped " is otherwise the end of the
string).
\|
Vertical bar character.
\a
Bell character (ASCII 7).
\f
Formfeed character (ASCII 12).
\n
Newline character (ASCII 10).
\r
Carriage return character (ASCII 13).
\t
Tab character (ASCII 9).
\v
Vertical tab character (ASCII 11).
\b
Backspace character (ASCII 8).
\0
NUL character (ASCII 0).
\(
Open parenthesis. This is intended for use at the beginning of
lines in multiline strings to avoid confusing Emacs lisp modes.
\ followed by newline (ASCII 10)
Nothing. This way if \ is the last character in a line, the string
will continue with the first character from the next line, without
a line break.
If the ‘hungry-eol-escapes’ reader option is enabled, which is not
the case by default, leading whitespace on the next line is
discarded.
"foo\
bar"
⇒ "foo bar"
(read-enable 'hungry-eol-escapes)
"foo\
bar"
⇒ "foobar"
\xHH
Character code given by two hexadecimal digits. For example \x7f
for an ASCII DEL (127).
\uHHHH
Character code given by four hexadecimal digits. For example
\u0100 for a capital A with macron (U+0100).
\UHHHHHH
Character code given by six hexadecimal digits. For example
\U010402.
The following are examples of string literals:
"foo"
"bar plonk"
"Hello World"
"\"Hi\", he said."
The three escape sequences ‘\xHH’, ‘\uHHHH’ and ‘\UHHHHHH’ were
chosen to not break compatibility with code written for previous
versions of Guile. The R6RS specification suggests a different,
incompatible syntax for hex escapes: ‘\xHHHH;’ – a character code
followed by one to eight hexadecimal digits terminated with a semicolon.
If this escape format is desired instead, it can be enabled with the
reader option ‘r6rs-hex-escapes’.
(read-enable 'r6rs-hex-escapes)
For more on reader options, *Note Scheme Read::.
6.6.5.2 String Predicates
.........................
The following procedures can be used to check whether a given string
fulfills some specified property.
-- Scheme Procedure: string? obj
-- C Function: scm_string_p (obj)
Return ‘#t’ if OBJ is a string, else ‘#f’.
-- C Function: int scm_is_string (SCM obj)
Returns ‘1’ if OBJ is a string, ‘0’ otherwise.
-- Scheme Procedure: string-null? str
-- C Function: scm_string_null_p (str)
Return ‘#t’ if STR’s length is zero, and ‘#f’ otherwise.
(string-null? "") ⇒ #t
y ⇒ "foo"
(string-null? y) ⇒ #f
-- Scheme Procedure: string-any char_pred s [start [end]]
-- C Function: scm_string_any (char_pred, s, start, end)
Check if CHAR_PRED is true for any character in string S.
CHAR_PRED can be a character to check for any equal to that, or a
character set (*note Character Sets::) to check for any in that
set, or a predicate procedure to call.
For a procedure, calls ‘(CHAR_PRED c)’ are made successively on the
characters from START to END. If CHAR_PRED returns true (ie.
non-‘#f’), ‘string-any’ stops and that return value is the return
from ‘string-any’. The call on the last character (ie. at END-1),
if that point is reached, is a tail call.
If there are no characters in S (ie. START equals END) then the
return is ‘#f’.
-- Scheme Procedure: string-every char_pred s [start [end]]
-- C Function: scm_string_every (char_pred, s, start, end)
Check if CHAR_PRED is true for every character in string S.
CHAR_PRED can be a character to check for every character equal to
that, or a character set (*note Character Sets::) to check for
every character being in that set, or a predicate procedure to
call.
For a procedure, calls ‘(CHAR_PRED c)’ are made successively on the
characters from START to END. If CHAR_PRED returns ‘#f’,
‘string-every’ stops and returns ‘#f’. The call on the last
character (ie. at END-1), if that point is reached, is a tail call
and the return from that call is the return from ‘string-every’.
If there are no characters in S (ie. START equals END) then the
return is ‘#t’.
6.6.5.3 String Constructors
...........................
The string constructor procedures create new string objects, possibly
initializing them with some specified character data. See also *Note
String Selection::, for ways to create strings from existing strings.
-- Scheme Procedure: string char...
Return a newly allocated string made from the given character
arguments.
(string #\x #\y #\z) ⇒ "xyz"
(string) ⇒ ""
-- Scheme Procedure: list->string lst
-- C Function: scm_string (lst)
Return a newly allocated string made from a list of characters.
(list->string '(#\a #\b #\c)) ⇒ "abc"
-- Scheme Procedure: reverse-list->string lst
-- C Function: scm_reverse_list_to_string (lst)
Return a newly allocated string made from a list of characters, in
reverse order.
(reverse-list->string '(#\a #\B #\c)) ⇒ "cBa"
-- Scheme Procedure: make-string k [chr]
-- C Function: scm_make_string (k, chr)
Return a newly allocated string of length K. If CHR is given, then
all elements of the string are initialized to CHR, otherwise the
contents of the string are unspecified.
-- C Function: SCM scm_c_make_string (size_t len, SCM chr)
Like ‘scm_make_string’, but expects the length as a ‘size_t’.
-- Scheme Procedure: string-tabulate proc len
-- C Function: scm_string_tabulate (proc, len)
PROC is an integer->char procedure. Construct a string of size LEN
by applying PROC to each index to produce the corresponding string
element. The order in which PROC is applied to the indices is not
specified.
-- Scheme Procedure: string-join ls [delimiter [grammar]]
-- C Function: scm_string_join (ls, delimiter, grammar)
Append the string in the string list LS, using the string DELIMITER
as a delimiter between the elements of LS. DELIMITER defaults to
‘ ’, that is, strings in LS are appended with the space character
in between them. GRAMMAR is a symbol which specifies how the
delimiter is placed between the strings, and defaults to the symbol
‘infix’.
‘infix’
Insert the separator between list elements. An empty string
will produce an empty list.
‘strict-infix’
Like ‘infix’, but will raise an error if given the empty list.
‘suffix’
Insert the separator after every list element.
‘prefix’
Insert the separator before each list element.
6.6.5.4 List/String conversion
..............................
When processing strings, it is often convenient to first convert them
into a list representation by using the procedure ‘string->list’, work
with the resulting list, and then convert it back into a string. These
procedures are useful for similar tasks.
-- Scheme Procedure: string->list str [start [end]]
-- C Function: scm_substring_to_list (str, start, end)
-- C Function: scm_string_to_list (str)
Convert the string STR into a list of characters.
-- Scheme Procedure: string-split str char_pred
-- C Function: scm_string_split (str, char_pred)
Split the string STR into a list of substrings delimited by
appearances of characters that
• equal CHAR_PRED, if it is a character,
• satisfy the predicate CHAR_PRED, if it is a procedure,
• are in the set CHAR_PRED, if it is a character set.
Note that an empty substring between separator characters will
result in an empty string in the result list.
(string-split "root:x:0:0:root:/root:/bin/bash" #\:)
⇒
("root" "x" "0" "0" "root" "/root" "/bin/bash")
(string-split "::" #\:)
⇒
("" "" "")
(string-split "" #\:)
⇒
("")
6.6.5.5 String Selection
........................
Portions of strings can be extracted by these procedures. ‘string-ref’
delivers individual characters whereas ‘substring’ can be used to
extract substrings from longer strings.
-- Scheme Procedure: string-length string
-- C Function: scm_string_length (string)
Return the number of characters in STRING.
-- C Function: size_t scm_c_string_length (SCM str)
Return the number of characters in STR as a ‘size_t’.
-- Scheme Procedure: string-ref str k
-- C Function: scm_string_ref (str, k)
Return character K of STR using zero-origin indexing. K must be a
valid index of STR.
-- C Function: SCM scm_c_string_ref (SCM str, size_t k)
Return character K of STR using zero-origin indexing. K must be a
valid index of STR.
-- Scheme Procedure: string-copy str [start [end]]
-- C Function: scm_substring_copy (str, start, end)
-- C Function: scm_string_copy (str)
Return a copy of the given string STR.
The returned string shares storage with STR initially, but it is
copied as soon as one of the two strings is modified.
-- Scheme Procedure: substring str start [end]
-- C Function: scm_substring (str, start, end)
Return a new string formed from the characters of STR beginning
with index START (inclusive) and ending with index END (exclusive).
STR must be a string, START and END must be exact integers
satisfying:
0 <= START <= END <= ‘(string-length STR)’.
The returned string shares storage with STR initially, but it is
copied as soon as one of the two strings is modified.
-- Scheme Procedure: substring/shared str start [end]
-- C Function: scm_substring_shared (str, start, end)
Like ‘substring’, but the strings continue to share their storage
even if they are modified. Thus, modifications to STR show up in
the new string, and vice versa.
-- Scheme Procedure: substring/copy str start [end]
-- C Function: scm_substring_copy (str, start, end)
Like ‘substring’, but the storage for the new string is copied
immediately.
-- Scheme Procedure: substring/read-only str start [end]
-- C Function: scm_substring_read_only (str, start, end)
Like ‘substring’, but the resulting string can not be modified.
-- C Function: SCM scm_c_substring (SCM str, size_t start, size_t end)
-- C Function: SCM scm_c_substring_shared (SCM str, size_t start,
size_t end)
-- C Function: SCM scm_c_substring_copy (SCM str, size_t start, size_t
end)
-- C Function: SCM scm_c_substring_read_only (SCM str, size_t start,
size_t end)
Like ‘scm_substring’, etc. but the bounds are given as a ‘size_t’.
-- Scheme Procedure: string-take s n
-- C Function: scm_string_take (s, n)
Return the N first characters of S.
-- Scheme Procedure: string-drop s n
-- C Function: scm_string_drop (s, n)
Return all but the first N characters of S.
-- Scheme Procedure: string-take-right s n
-- C Function: scm_string_take_right (s, n)
Return the N last characters of S.
-- Scheme Procedure: string-drop-right s n
-- C Function: scm_string_drop_right (s, n)
Return all but the last N characters of S.
-- Scheme Procedure: string-pad s len [chr [start [end]]]
-- Scheme Procedure: string-pad-right s len [chr [start [end]]]
-- C Function: scm_string_pad (s, len, chr, start, end)
-- C Function: scm_string_pad_right (s, len, chr, start, end)
Take characters START to END from the string S and either pad with
CHR or truncate them to give LEN characters.
‘string-pad’ pads or truncates on the left, so for example
(string-pad "x" 3) ⇒ " x"
(string-pad "abcde" 3) ⇒ "cde"
‘string-pad-right’ pads or truncates on the right, so for example
(string-pad-right "x" 3) ⇒ "x "
(string-pad-right "abcde" 3) ⇒ "abc"
-- Scheme Procedure: string-trim s [char_pred [start [end]]]
-- Scheme Procedure: string-trim-right s [char_pred [start [end]]]
-- Scheme Procedure: string-trim-both s [char_pred [start [end]]]
-- C Function: scm_string_trim (s, char_pred, start, end)
-- C Function: scm_string_trim_right (s, char_pred, start, end)
-- C Function: scm_string_trim_both (s, char_pred, start, end)
Trim occurrences of CHAR_PRED from the ends of S.
‘string-trim’ trims CHAR_PRED characters from the left (start) of
the string, ‘string-trim-right’ trims them from the right (end) of
the string, ‘string-trim-both’ trims from both ends.
CHAR_PRED can be a character, a character set, or a predicate
procedure to call on each character. If CHAR_PRED is not given the
default is whitespace as per ‘char-set:whitespace’ (*note Standard
Character Sets::).
(string-trim " x ") ⇒ "x "
(string-trim-right "banana" #\a) ⇒ "banan"
(string-trim-both ".,xy:;" char-set:punctuation)
⇒ "xy"
(string-trim-both "xyzzy" (lambda (c)
(or (eqv? c #\x)
(eqv? c #\y))))
⇒ "zz"
6.6.5.6 String Modification
...........................
These procedures are for modifying strings in-place. This means that
the result of the operation is not a new string; instead, the original
string’s memory representation is modified.
-- Scheme Procedure: string-set! str k chr
-- C Function: scm_string_set_x (str, k, chr)
Store CHR in element K of STR and return an unspecified value. K
must be a valid index of STR.
-- C Function: void scm_c_string_set_x (SCM str, size_t k, SCM chr)
Like ‘scm_string_set_x’, but the index is given as a ‘size_t’.
-- Scheme Procedure: string-fill! str chr [start [end]]
-- C Function: scm_substring_fill_x (str, chr, start, end)
-- C Function: scm_string_fill_x (str, chr)
Stores CHR in every element of the given STR and returns an
unspecified value.
-- Scheme Procedure: substring-fill! str start end fill
-- C Function: scm_substring_fill_x (str, start, end, fill)
Change every character in STR between START and END to FILL.
(define y (string-copy "abcdefg"))
(substring-fill! y 1 3 #\r)
y
⇒ "arrdefg"
-- Scheme Procedure: substring-move! str1 start1 end1 str2 start2
-- C Function: scm_substring_move_x (str1, start1, end1, str2, start2)
Copy the substring of STR1 bounded by START1 and END1 into STR2
beginning at position START2. STR1 and STR2 can be the same
string.
-- Scheme Procedure: string-copy! target tstart s [start [end]]
-- C Function: scm_string_copy_x (target, tstart, s, start, end)
Copy the sequence of characters from index range [START, END) in
string S to string TARGET, beginning at index TSTART. The
characters are copied left-to-right or right-to-left as needed –
the copy is guaranteed to work, even if TARGET and S are the same
string. It is an error if the copy operation runs off the end of
the target string.
6.6.5.7 String Comparison
.........................
The procedures in this section are similar to the character ordering
predicates (*note Characters::), but are defined on character sequences.
The first set is specified in R5RS and has names that end in ‘?’.
The second set is specified in SRFI-13 and the names have not ending
‘?’.
The predicates ending in ‘-ci’ ignore the character case when
comparing strings. For now, case-insensitive comparison is done using
the R5RS rules, where every lower-case character that has a single
character upper-case form is converted to uppercase before comparison.
See *Note the ‘(ice-9 i18n)’ module: Text Collation, for
locale-dependent string comparison.
-- Scheme Procedure: string=? s1 s2 s3 ...
Lexicographic equality predicate; return ‘#t’ if all strings are
the same length and contain the same characters in the same
positions, otherwise return ‘#f’.
The procedure ‘string-ci=?’ treats upper and lower case letters as
though they were the same character, but ‘string=?’ treats upper
and lower case as distinct characters.
-- Scheme Procedure: string? s1 s2 s3 ...
Lexicographic ordering predicate; return ‘#t’ if, for every pair of
consecutive string arguments STR_I and STR_I+1, STR_I is
lexicographically greater than STR_I+1.
-- Scheme Procedure: string>=? s1 s2 s3 ...
Lexicographic ordering predicate; return ‘#t’ if, for every pair of
consecutive string arguments STR_I and STR_I+1, STR_I is
lexicographically greater than or equal to STR_I+1.
-- Scheme Procedure: string-ci=? s1 s2 s3 ...
Case-insensitive string equality predicate; return ‘#t’ if all
strings are the same length and their component characters match
(ignoring case) at each position; otherwise return ‘#f’.
-- Scheme Procedure: string-ci? s1 s2 s3 ...
Case insensitive lexicographic ordering predicate; return ‘#t’ if,
for every pair of consecutive string arguments STR_I and STR_I+1,
STR_I is lexicographically greater than STR_I+1 regardless of case.
-- Scheme Procedure: string-ci>=? s1 s2 s3 ...
Case insensitive lexicographic ordering predicate; return ‘#t’ if,
for every pair of consecutive string arguments STR_I and STR_I+1,
STR_I is lexicographically greater than or equal to STR_I+1
regardless of case.
-- Scheme Procedure: string-compare s1 s2 proc_lt proc_eq proc_gt
[start1 [end1 [start2 [end2]]]]
-- C Function: scm_string_compare (s1, s2, proc_lt, proc_eq, proc_gt,
start1, end1, start2, end2)
Apply PROC_LT, PROC_EQ, PROC_GT to the mismatch index, depending
upon whether S1 is less than, equal to, or greater than S2. The
mismatch index is the largest index I such that for every 0 <= J <
I, S1[J] = S2[J] – that is, I is the first position that does not
match.
-- Scheme Procedure: string-compare-ci s1 s2 proc_lt proc_eq proc_gt
[start1 [end1 [start2 [end2]]]]
-- C Function: scm_string_compare_ci (s1, s2, proc_lt, proc_eq,
proc_gt, start1, end1, start2, end2)
Apply PROC_LT, PROC_EQ, PROC_GT to the mismatch index, depending
upon whether S1 is less than, equal to, or greater than S2. The
mismatch index is the largest index I such that for every 0 <= J <
I, S1[J] = S2[J] – that is, I is the first position where the
lowercased letters do not match.
-- Scheme Procedure: string= s1 s2 [start1 [end1 [start2 [end2]]]]
-- C Function: scm_string_eq (s1, s2, start1, end1, start2, end2)
Return ‘#f’ if S1 and S2 are not equal, a true value otherwise.
-- Scheme Procedure: string<> s1 s2 [start1 [end1 [start2 [end2]]]]
-- C Function: scm_string_neq (s1, s2, start1, end1, start2, end2)
Return ‘#f’ if S1 and S2 are equal, a true value otherwise.
-- Scheme Procedure: string< s1 s2 [start1 [end1 [start2 [end2]]]]
-- C Function: scm_string_lt (s1, s2, start1, end1, start2, end2)
Return ‘#f’ if S1 is greater or equal to S2, a true value
otherwise.
-- Scheme Procedure: string> s1 s2 [start1 [end1 [start2 [end2]]]]
-- C Function: scm_string_gt (s1, s2, start1, end1, start2, end2)
Return ‘#f’ if S1 is less or equal to S2, a true value otherwise.
-- Scheme Procedure: string<= s1 s2 [start1 [end1 [start2 [end2]]]]
-- C Function: scm_string_le (s1, s2, start1, end1, start2, end2)
Return ‘#f’ if S1 is greater to S2, a true value otherwise.
-- Scheme Procedure: string>= s1 s2 [start1 [end1 [start2 [end2]]]]
-- C Function: scm_string_ge (s1, s2, start1, end1, start2, end2)
Return ‘#f’ if S1 is less to S2, a true value otherwise.
-- Scheme Procedure: string-ci= s1 s2 [start1 [end1 [start2 [end2]]]]
-- C Function: scm_string_ci_eq (s1, s2, start1, end1, start2, end2)
Return ‘#f’ if S1 and S2 are not equal, a true value otherwise.
The character comparison is done case-insensitively.
-- Scheme Procedure: string-ci<> s1 s2 [start1 [end1 [start2 [end2]]]]
-- C Function: scm_string_ci_neq (s1, s2, start1, end1, start2, end2)
Return ‘#f’ if S1 and S2 are equal, a true value otherwise. The
character comparison is done case-insensitively.
-- Scheme Procedure: string-ci< s1 s2 [start1 [end1 [start2 [end2]]]]
-- C Function: scm_string_ci_lt (s1, s2, start1, end1, start2, end2)
Return ‘#f’ if S1 is greater or equal to S2, a true value
otherwise. The character comparison is done case-insensitively.
-- Scheme Procedure: string-ci> s1 s2 [start1 [end1 [start2 [end2]]]]
-- C Function: scm_string_ci_gt (s1, s2, start1, end1, start2, end2)
Return ‘#f’ if S1 is less or equal to S2, a true value otherwise.
The character comparison is done case-insensitively.
-- Scheme Procedure: string-ci<= s1 s2 [start1 [end1 [start2 [end2]]]]
-- C Function: scm_string_ci_le (s1, s2, start1, end1, start2, end2)
Return ‘#f’ if S1 is greater to S2, a true value otherwise. The
character comparison is done case-insensitively.
-- Scheme Procedure: string-ci>= s1 s2 [start1 [end1 [start2 [end2]]]]
-- C Function: scm_string_ci_ge (s1, s2, start1, end1, start2, end2)
Return ‘#f’ if S1 is less to S2, a true value otherwise. The
character comparison is done case-insensitively.
-- Scheme Procedure: string-hash s [bound [start [end]]]
-- C Function: scm_substring_hash (s, bound, start, end)
Compute a hash value for S. The optional argument BOUND is a
non-negative exact integer specifying the range of the hash
function. A positive value restricts the return value to the range
[0,bound).
-- Scheme Procedure: string-hash-ci s [bound [start [end]]]
-- C Function: scm_substring_hash_ci (s, bound, start, end)
Compute a hash value for S. The optional argument BOUND is a
non-negative exact integer specifying the range of the hash
function. A positive value restricts the return value to the range
[0,bound).
Because the same visual appearance of an abstract Unicode character
can be obtained via multiple sequences of Unicode characters, even the
case-insensitive string comparison functions described above may return
‘#f’ when presented with strings containing different representations of
the same character. For example, the Unicode character “LATIN SMALL
LETTER S WITH DOT BELOW AND DOT ABOVE” can be represented with a single
character (U+1E69) or by the character “LATIN SMALL LETTER S” (U+0073)
followed by the combining marks “COMBINING DOT BELOW” (U+0323) and
“COMBINING DOT ABOVE” (U+0307).
For this reason, it is often desirable to ensure that the strings to
be compared are using a mutually consistent representation for every
character. The Unicode standard defines two methods of normalizing the
contents of strings: Decomposition, which breaks composite characters
into a set of constituent characters with an ordering defined by the
Unicode Standard; and composition, which performs the converse.
There are two decomposition operations. “Canonical decomposition”
produces character sequences that share the same visual appearance as
the original characters, while “compatibility decomposition” produces
ones whose visual appearances may differ from the originals but which
represent the same abstract character.
These operations are encapsulated in the following set of
normalization forms:
“NFD”
Characters are decomposed to their canonical forms.
“NFKD”
Characters are decomposed to their compatibility forms.
“NFC”
Characters are decomposed to their canonical forms, then composed.
“NFKC”
Characters are decomposed to their compatibility forms, then
composed.
The functions below put their arguments into one of the forms
described above.
-- Scheme Procedure: string-normalize-nfd s
-- C Function: scm_string_normalize_nfd (s)
Return the ‘NFD’ normalized form of S.
-- Scheme Procedure: string-normalize-nfkd s
-- C Function: scm_string_normalize_nfkd (s)
Return the ‘NFKD’ normalized form of S.
-- Scheme Procedure: string-normalize-nfc s
-- C Function: scm_string_normalize_nfc (s)
Return the ‘NFC’ normalized form of S.
-- Scheme Procedure: string-normalize-nfkc s
-- C Function: scm_string_normalize_nfkc (s)
Return the ‘NFKC’ normalized form of S.
6.6.5.8 String Searching
........................
-- Scheme Procedure: string-index s char_pred [start [end]]
-- C Function: scm_string_index (s, char_pred, start, end)
Search through the string S from left to right, returning the index
of the first occurrence of a character which
• equals CHAR_PRED, if it is character,
• satisfies the predicate CHAR_PRED, if it is a procedure,
• is in the set CHAR_PRED, if it is a character set.
Return ‘#f’ if no match is found.
-- Scheme Procedure: string-rindex s char_pred [start [end]]
-- C Function: scm_string_rindex (s, char_pred, start, end)
Search through the string S from right to left, returning the index
of the last occurrence of a character which
• equals CHAR_PRED, if it is character,
• satisfies the predicate CHAR_PRED, if it is a procedure,
• is in the set if CHAR_PRED is a character set.
Return ‘#f’ if no match is found.
-- Scheme Procedure: string-prefix-length s1 s2 [start1 [end1 [start2
[end2]]]]
-- C Function: scm_string_prefix_length (s1, s2, start1, end1, start2,
end2)
Return the length of the longest common prefix of the two strings.
-- Scheme Procedure: string-prefix-length-ci s1 s2 [start1 [end1
[start2 [end2]]]]
-- C Function: scm_string_prefix_length_ci (s1, s2, start1, end1,
start2, end2)
Return the length of the longest common prefix of the two strings,
ignoring character case.
-- Scheme Procedure: string-suffix-length s1 s2 [start1 [end1 [start2
[end2]]]]
-- C Function: scm_string_suffix_length (s1, s2, start1, end1, start2,
end2)
Return the length of the longest common suffix of the two strings.
-- Scheme Procedure: string-suffix-length-ci s1 s2 [start1 [end1
[start2 [end2]]]]
-- C Function: scm_string_suffix_length_ci (s1, s2, start1, end1,
start2, end2)
Return the length of the longest common suffix of the two strings,
ignoring character case.
-- Scheme Procedure: string-prefix? s1 s2 [start1 [end1 [start2
[end2]]]]
-- C Function: scm_string_prefix_p (s1, s2, start1, end1, start2, end2)
Is S1 a prefix of S2?
-- Scheme Procedure: string-prefix-ci? s1 s2 [start1 [end1 [start2
[end2]]]]
-- C Function: scm_string_prefix_ci_p (s1, s2, start1, end1, start2,
end2)
Is S1 a prefix of S2, ignoring character case?
-- Scheme Procedure: string-suffix? s1 s2 [start1 [end1 [start2
[end2]]]]
-- C Function: scm_string_suffix_p (s1, s2, start1, end1, start2, end2)
Is S1 a suffix of S2?
-- Scheme Procedure: string-suffix-ci? s1 s2 [start1 [end1 [start2
[end2]]]]
-- C Function: scm_string_suffix_ci_p (s1, s2, start1, end1, start2,
end2)
Is S1 a suffix of S2, ignoring character case?
-- Scheme Procedure: string-index-right s char_pred [start [end]]
-- C Function: scm_string_index_right (s, char_pred, start, end)
Search through the string S from right to left, returning the index
of the last occurrence of a character which
• equals CHAR_PRED, if it is character,
• satisfies the predicate CHAR_PRED, if it is a procedure,
• is in the set if CHAR_PRED is a character set.
Return ‘#f’ if no match is found.
-- Scheme Procedure: string-skip s char_pred [start [end]]
-- C Function: scm_string_skip (s, char_pred, start, end)
Search through the string S from left to right, returning the index
of the first occurrence of a character which
• does not equal CHAR_PRED, if it is character,
• does not satisfy the predicate CHAR_PRED, if it is a
procedure,
• is not in the set if CHAR_PRED is a character set.
-- Scheme Procedure: string-skip-right s char_pred [start [end]]
-- C Function: scm_string_skip_right (s, char_pred, start, end)
Search through the string S from right to left, returning the index
of the last occurrence of a character which
• does not equal CHAR_PRED, if it is character,
• does not satisfy the predicate CHAR_PRED, if it is a
procedure,
• is not in the set if CHAR_PRED is a character set.
-- Scheme Procedure: string-count s char_pred [start [end]]
-- C Function: scm_string_count (s, char_pred, start, end)
Return the count of the number of characters in the string S which
• equals CHAR_PRED, if it is character,
• satisfies the predicate CHAR_PRED, if it is a procedure.
• is in the set CHAR_PRED, if it is a character set.
-- Scheme Procedure: string-contains s1 s2 [start1 [end1 [start2
[end2]]]]
-- C Function: scm_string_contains (s1, s2, start1, end1, start2, end2)
Does string S1 contain string S2? Return the index in S1 where S2
occurs as a substring, or false. The optional start/end indices
restrict the operation to the indicated substrings.
-- Scheme Procedure: string-contains-ci s1 s2 [start1 [end1 [start2
[end2]]]]
-- C Function: scm_string_contains_ci (s1, s2, start1, end1, start2,
end2)
Does string S1 contain string S2? Return the index in S1 where S2
occurs as a substring, or false. The optional start/end indices
restrict the operation to the indicated substrings. Character
comparison is done case-insensitively.
6.6.5.9 Alphabetic Case Mapping
...............................
These are procedures for mapping strings to their upper- or lower-case
equivalents, respectively, or for capitalizing strings.
They use the basic case mapping rules for Unicode characters. No
special language or context rules are considered. The resulting strings
are guaranteed to be the same length as the input strings.
*Note the ‘(ice-9 i18n)’ module: Character Case Mapping, for
locale-dependent case conversions.
-- Scheme Procedure: string-upcase str [start [end]]
-- C Function: scm_substring_upcase (str, start, end)
-- C Function: scm_string_upcase (str)
Upcase every character in ‘str’.
-- Scheme Procedure: string-upcase! str [start [end]]
-- C Function: scm_substring_upcase_x (str, start, end)
-- C Function: scm_string_upcase_x (str)
Destructively upcase every character in ‘str’.
(string-upcase! y)
⇒ "ARRDEFG"
y
⇒ "ARRDEFG"
-- Scheme Procedure: string-downcase str [start [end]]
-- C Function: scm_substring_downcase (str, start, end)
-- C Function: scm_string_downcase (str)
Downcase every character in STR.
-- Scheme Procedure: string-downcase! str [start [end]]
-- C Function: scm_substring_downcase_x (str, start, end)
-- C Function: scm_string_downcase_x (str)
Destructively downcase every character in STR.
y
⇒ "ARRDEFG"
(string-downcase! y)
⇒ "arrdefg"
y
⇒ "arrdefg"
-- Scheme Procedure: string-capitalize str
-- C Function: scm_string_capitalize (str)
Return a freshly allocated string with the characters in STR, where
the first character of every word is capitalized.
-- Scheme Procedure: string-capitalize! str
-- C Function: scm_string_capitalize_x (str)
Upcase the first character of every word in STR destructively and
return STR.
y ⇒ "hello world"
(string-capitalize! y) ⇒ "Hello World"
y ⇒ "Hello World"
-- Scheme Procedure: string-titlecase str [start [end]]
-- C Function: scm_string_titlecase (str, start, end)
Titlecase every first character in a word in STR.
-- Scheme Procedure: string-titlecase! str [start [end]]
-- C Function: scm_string_titlecase_x (str, start, end)
Destructively titlecase every first character in a word in STR.
6.6.5.10 Reversing and Appending Strings
........................................
-- Scheme Procedure: string-reverse str [start [end]]
-- C Function: scm_string_reverse (str, start, end)
Reverse the string STR. The optional arguments START and END
delimit the region of STR to operate on.
-- Scheme Procedure: string-reverse! str [start [end]]
-- C Function: scm_string_reverse_x (str, start, end)
Reverse the string STR in-place. The optional arguments START and
END delimit the region of STR to operate on. The return value is
unspecified.
-- Scheme Procedure: string-append arg ...
-- C Function: scm_string_append (args)
Return a newly allocated string whose characters form the
concatenation of the given strings, ARG ....
(let ((h "hello "))
(string-append h "world"))
⇒ "hello world"
-- Scheme Procedure: string-append/shared arg ...
-- C Function: scm_string_append_shared (args)
Like ‘string-append’, but the result may share memory with the
argument strings.
-- Scheme Procedure: string-concatenate ls
-- C Function: scm_string_concatenate (ls)
Append the elements (which must be strings) of LS together into a
single string. Guaranteed to return a freshly allocated string.
-- Scheme Procedure: string-concatenate-reverse ls [final_string [end]]
-- C Function: scm_string_concatenate_reverse (ls, final_string, end)
Without optional arguments, this procedure is equivalent to
(string-concatenate (reverse ls))
If the optional argument FINAL_STRING is specified, it is consed
onto the beginning to LS before performing the list-reverse and
string-concatenate operations. If END is given, only the
characters of FINAL_STRING up to index END are used.
Guaranteed to return a freshly allocated string.
-- Scheme Procedure: string-concatenate/shared ls
-- C Function: scm_string_concatenate_shared (ls)
Like ‘string-concatenate’, but the result may share memory with the
strings in the list LS.
-- Scheme Procedure: string-concatenate-reverse/shared ls [final_string
[end]]
-- C Function: scm_string_concatenate_reverse_shared (ls, final_string,
end)
Like ‘string-concatenate-reverse’, but the result may share memory
with the strings in the LS arguments.
6.6.5.11 Mapping, Folding, and Unfolding
........................................
-- Scheme Procedure: string-map proc s [start [end]]
-- C Function: scm_string_map (proc, s, start, end)
PROC is a char->char procedure, it is mapped over S. The order in
which the procedure is applied to the string elements is not
specified.
-- Scheme Procedure: string-map! proc s [start [end]]
-- C Function: scm_string_map_x (proc, s, start, end)
PROC is a char->char procedure, it is mapped over S. The order in
which the procedure is applied to the string elements is not
specified. The string S is modified in-place, the return value is
not specified.
-- Scheme Procedure: string-for-each proc s [start [end]]
-- C Function: scm_string_for_each (proc, s, start, end)
PROC is mapped over S in left-to-right order. The return value is
not specified.
-- Scheme Procedure: string-for-each-index proc s [start [end]]
-- C Function: scm_string_for_each_index (proc, s, start, end)
Call ‘(PROC i)’ for each index i in S, from left to right.
For example, to change characters to alternately upper and lower
case,
(define str (string-copy "studly"))
(string-for-each-index
(lambda (i)
(string-set! str i
((if (even? i) char-upcase char-downcase)
(string-ref str i))))
str)
str ⇒ "StUdLy"
-- Scheme Procedure: string-fold kons knil s [start [end]]
-- C Function: scm_string_fold (kons, knil, s, start, end)
Fold KONS over the characters of S, with KNIL as the terminating
element, from left to right. KONS must expect two arguments: The
actual character and the last result of KONS’ application.
-- Scheme Procedure: string-fold-right kons knil s [start [end]]
-- C Function: scm_string_fold_right (kons, knil, s, start, end)
Fold KONS over the characters of S, with KNIL as the terminating
element, from right to left. KONS must expect two arguments: The
actual character and the last result of KONS’ application.
-- Scheme Procedure: string-unfold p f g seed [base [make_final]]
-- C Function: scm_string_unfold (p, f, g, seed, base, make_final)
• G is used to generate a series of _seed_ values from the
initial SEED: SEED, (G SEED), (G^2 SEED), (G^3 SEED), ...
• P tells us when to stop – when it returns true when applied to
one of these seed values.
• F maps each seed value to the corresponding character in the
result string. These chars are assembled into the string in a
left-to-right order.
• BASE is the optional initial/leftmost portion of the
constructed string; it default to the empty string.
• MAKE_FINAL is applied to the terminal seed value (on which P
returns true) to produce the final/rightmost portion of the
constructed string. The default is nothing extra.
-- Scheme Procedure: string-unfold-right p f g seed [base [make_final]]
-- C Function: scm_string_unfold_right (p, f, g, seed, base,
make_final)
• G is used to generate a series of _seed_ values from the
initial SEED: SEED, (G SEED), (G^2 SEED), (G^3 SEED), ...
• P tells us when to stop – when it returns true when applied to
one of these seed values.
• F maps each seed value to the corresponding character in the
result string. These chars are assembled into the string in a
right-to-left order.
• BASE is the optional initial/rightmost portion of the
constructed string; it default to the empty string.
• MAKE_FINAL is applied to the terminal seed value (on which P
returns true) to produce the final/leftmost portion of the
constructed string. It defaults to ‘(lambda (x) )’.
6.6.5.12 Miscellaneous String Operations
........................................
-- Scheme Procedure: xsubstring s from [to [start [end]]]
-- C Function: scm_xsubstring (s, from, to, start, end)
This is the _extended substring_ procedure that implements
replicated copying of a substring of some string.
S is a string, START and END are optional arguments that demarcate
a substring of S, defaulting to 0 and the length of S. Replicate
this substring up and down index space, in both the positive and
negative directions. ‘xsubstring’ returns the substring of this
string beginning at index FROM, and ending at TO, which defaults to
FROM + (END - START).
-- Scheme Procedure: string-xcopy! target tstart s sfrom [sto [start
[end]]]
-- C Function: scm_string_xcopy_x (target, tstart, s, sfrom, sto,
start, end)
Exactly the same as ‘xsubstring’, but the extracted text is written
into the string TARGET starting at index TSTART. The operation is
not defined if ‘(eq? TARGET S)’ or these arguments share storage –
you cannot copy a string on top of itself.
-- Scheme Procedure: string-replace s1 s2 [start1 [end1 [start2
[end2]]]]
-- C Function: scm_string_replace (s1, s2, start1, end1, start2, end2)
Return the string S1, but with the characters START1 ... END1
replaced by the characters START2 ... END2 from S2.
-- Scheme Procedure: string-tokenize s [token_set [start [end]]]
-- C Function: scm_string_tokenize (s, token_set, start, end)
Split the string S into a list of substrings, where each substring
is a maximal non-empty contiguous sequence of characters from the
character set TOKEN_SET, which defaults to ‘char-set:graphic’. If
START or END indices are provided, they restrict ‘string-tokenize’
to operating on the indicated substring of S.
-- Scheme Procedure: string-filter char_pred s [start [end]]
-- C Function: scm_string_filter (char_pred, s, start, end)
Filter the string S, retaining only those characters which satisfy
CHAR_PRED.
If CHAR_PRED is a procedure, it is applied to each character as a
predicate, if it is a character, it is tested for equality and if
it is a character set, it is tested for membership.
-- Scheme Procedure: string-delete char_pred s [start [end]]
-- C Function: scm_string_delete (char_pred, s, start, end)
Delete characters satisfying CHAR_PRED from S.
If CHAR_PRED is a procedure, it is applied to each character as a
predicate, if it is a character, it is tested for equality and if
it is a character set, it is tested for membership.
The following additional functions are available in the module
‘(ice-9 string-fun)’. They can be used with:
(use-modules (ice-9 string-fun))
-- Scheme Procedure: string-replace-substring str substring replacement
Return a new string where every instance of SUBSTRING in string STR
has been replaced by REPLACEMENT. For example:
(string-replace-substring "a ring of strings" "ring" "rut")
⇒ "a rut of struts"
6.6.5.13 Representing Strings as Bytes
......................................
Out in the cold world outside of Guile, not all strings are treated in
the same way. Out there there are only bytes, and there are many ways
of representing a strings (sequences of characters) as binary data
(sequences of bytes).
As a user, usually you don’t have to think about this very much.
When you type on your keyboard, your system encodes your keystrokes as
bytes according to the locale that you have configured on your computer.
Guile uses the locale to decode those bytes back into characters –
hopefully the same characters that you typed in.
All is not so clear when dealing with a system with multiple users,
such as a web server. Your web server might get a request from one user
for data encoded in the ISO-8859-1 character set, and then another
request from a different user for UTF-8 data.
Guile provides an “iconv” module for converting between strings and
sequences of bytes. *Note Bytevectors::, for more on how Guile
represents raw byte sequences. This module gets its name from the
common UNIX command of the same name.
Note that often it is sufficient to just read and write strings from
ports instead of using these functions. To do this, specify the port
encoding using ‘set-port-encoding!’. *Note Ports::, for more on ports
and character encodings.
Unlike the rest of the procedures in this section, you have to load
the ‘iconv’ module before having access to these procedures:
(use-modules (ice-9 iconv))
-- Scheme Procedure: string->bytevector string encoding
[conversion-strategy]
Encode STRING as a sequence of bytes.
The string will be encoded in the character set specified by the
ENCODING string. If the string has characters that cannot be
represented in the encoding, by default this procedure raises an
‘encoding-error’. Pass a CONVERSION-STRATEGY argument to specify
other behaviors.
The return value is a bytevector. *Note Bytevectors::, for more on
bytevectors. *Note Ports::, for more on character encodings and
conversion strategies.
-- Scheme Procedure: bytevector->string bytevector encoding
[conversion-strategy]
Decode BYTEVECTOR into a string.
The bytes will be decoded from the character set by the ENCODING
string. If the bytes do not form a valid encoding, by default this
procedure raises an ‘decoding-error’. As with
‘string->bytevector’, pass the optional CONVERSION-STRATEGY
argument to modify this behavior. *Note Ports::, for more on
character encodings and conversion strategies.
-- Scheme Procedure: call-with-output-encoded-string encoding proc
[conversion-strategy]
Like ‘call-with-output-string’, but instead of returning a string,
returns a encoding of the string according to ENCODING, as a
bytevector. This procedure can be more efficient than collecting a
string and then converting it via ‘string->bytevector’.
6.6.5.14 Conversion to/from C
.............................
When creating a Scheme string from a C string or when converting a
Scheme string to a C string, the concept of character encoding becomes
important.
In C, a string is just a sequence of bytes, and the character
encoding describes the relation between these bytes and the actual
characters that make up the string. For Scheme strings, character
encoding is not an issue (most of the time), since in Scheme you usually
treat strings as character sequences, not byte sequences.
Converting to C and converting from C each have their own challenges.
When converting from C to Scheme, it is important that the sequence
of bytes in the C string be valid with respect to its encoding. ASCII
strings, for example, can’t have any bytes greater than 127. An ASCII
byte greater than 127 is considered _ill-formed_ and cannot be converted
into a Scheme character.
Problems can occur in the reverse operation as well. Not all
character encodings can hold all possible Scheme characters. Some
encodings, like ASCII for example, can only describe a small subset of
all possible characters. So, when converting to C, one must first
decide what to do with Scheme characters that can’t be represented in
the C string.
Converting a Scheme string to a C string will often allocate fresh
memory to hold the result. You must take care that this memory is
properly freed eventually. In many cases, this can be achieved by using
‘scm_dynwind_free’ inside an appropriate dynwind context, *Note Dynamic
Wind::.
-- C Function: SCM scm_from_locale_string (const char *str)
-- C Function: SCM scm_from_locale_stringn (const char *str, size_t
len)
Creates a new Scheme string that has the same contents as STR when
interpreted in the character encoding of the current locale.
For ‘scm_from_locale_string’, STR must be null-terminated.
For ‘scm_from_locale_stringn’, LEN specifies the length of STR in
bytes, and STR does not need to be null-terminated. If LEN is
‘(size_t)-1’, then STR does need to be null-terminated and the real
length will be found with ‘strlen’.
If the C string is ill-formed, an error will be raised.
Note that these functions should _not_ be used to convert C string
constants, because there is no guarantee that the current locale
will match that of the execution character set, used for string and
character constants. Most modern C compilers use UTF-8 by default,
so to convert C string constants we recommend
‘scm_from_utf8_string’.
-- C Function: SCM scm_take_locale_string (char *str)
-- C Function: SCM scm_take_locale_stringn (char *str, size_t len)
Like ‘scm_from_locale_string’ and ‘scm_from_locale_stringn’,
respectively, but also frees STR with ‘free’ eventually. Thus, you
can use this function when you would free STR anyway immediately
after creating the Scheme string. In certain cases, Guile can then
use STR directly as its internal representation.
-- C Function: char * scm_to_locale_string (SCM str)
-- C Function: char * scm_to_locale_stringn (SCM str, size_t *lenp)
Returns a C string with the same contents as STR in the character
encoding of the current locale. The C string must be freed with
‘free’ eventually, maybe by using ‘scm_dynwind_free’, *Note Dynamic
Wind::.
For ‘scm_to_locale_string’, the returned string is null-terminated
and an error is signalled when STR contains ‘#\nul’ characters.
For ‘scm_to_locale_stringn’ and LENP not ‘NULL’, STR might contain
‘#\nul’ characters and the length of the returned string in bytes
is stored in ‘*LENP’. The returned string will not be
null-terminated in this case. If LENP is ‘NULL’,
‘scm_to_locale_stringn’ behaves like ‘scm_to_locale_string’.
If a character in STR cannot be represented in the character
encoding of the current locale, the default port conversion
strategy is used. *Note Ports::, for more on conversion
strategies.
If the conversion strategy is ‘error’, an error will be raised. If
it is ‘substitute’, a replacement character, such as a question
mark, will be inserted in its place. If it is ‘escape’, a hex
escape will be inserted in its place.
-- C Function: size_t scm_to_locale_stringbuf (SCM str, char *buf,
size_t max_len)
Puts STR as a C string in the current locale encoding into the
memory pointed to by BUF. The buffer at BUF has room for MAX_LEN
bytes and ‘scm_to_local_stringbuf’ will never store more than that.
No terminating ‘'\0'’ will be stored.
The return value of ‘scm_to_locale_stringbuf’ is the number of
bytes that are needed for all of STR, regardless of whether BUF was
large enough to hold them. Thus, when the return value is larger
than MAX_LEN, only MAX_LEN bytes have been stored and you probably
need to try again with a larger buffer.
For most situations, string conversion should occur using the current
locale, such as with the functions above. But there may be cases where
one wants to convert strings from a character encoding other than the
locale’s character encoding. For these cases, the lower-level functions
‘scm_to_stringn’ and ‘scm_from_stringn’ are provided. These functions
should seldom be necessary if one is properly using locales.
-- C Type: scm_t_string_failed_conversion_handler
This is an enumerated type that can take one of three values:
‘SCM_FAILED_CONVERSION_ERROR’,
‘SCM_FAILED_CONVERSION_QUESTION_MARK’, and
‘SCM_FAILED_CONVERSION_ESCAPE_SEQUENCE’. They are used to indicate
a strategy for handling characters that cannot be converted to or
from a given character encoding. ‘SCM_FAILED_CONVERSION_ERROR’
indicates that a conversion should throw an error if some
characters cannot be converted.
‘SCM_FAILED_CONVERSION_QUESTION_MARK’ indicates that a conversion
should replace unconvertable characters with the question mark
character. And, ‘SCM_FAILED_CONVERSION_ESCAPE_SEQUENCE’ requests
that a conversion should replace an unconvertable character with an
escape sequence.
While all three strategies apply when converting Scheme strings to
C, only ‘SCM_FAILED_CONVERSION_ERROR’ and
‘SCM_FAILED_CONVERSION_QUESTION_MARK’ can be used when converting C
strings to Scheme.
-- C Function: char *scm_to_stringn (SCM str, size_t *lenp, const char
*encoding, scm_t_string_failed_conversion_handler handler)
This function returns a newly allocated C string from the Guile
string STR. The length of the returned string in bytes will be
returned in LENP. The character encoding of the C string is passed
as the ASCII, null-terminated C string ENCODING. The HANDLER
parameter gives a strategy for dealing with characters that cannot
be converted into ENCODING.
If LENP is ‘NULL’, this function will return a null-terminated C
string. It will throw an error if the string contains a null
character.
The Scheme interface to this function is ‘string->bytevector’, from
the ‘ice-9 iconv’ module. *Note Representing Strings as Bytes::.
-- C Function: SCM scm_from_stringn (const char *str, size_t len, const
char *encoding, scm_t_string_failed_conversion_handler
handler)
This function returns a scheme string from the C string STR. The
length in bytes of the C string is input as LEN. The encoding of
the C string is passed as the ASCII, null-terminated C string
‘encoding’. The HANDLER parameters suggests a strategy for dealing
with unconvertable characters.
The Scheme interface to this function is ‘bytevector->string’.
*Note Representing Strings as Bytes::.
The following conversion functions are provided as a convenience for
the most commonly used encodings.
-- C Function: SCM scm_from_latin1_string (const char *str)
-- C Function: SCM scm_from_utf8_string (const char *str)
-- C Function: SCM scm_from_utf32_string (const scm_t_wchar *str)
Return a scheme string from the null-terminated C string STR, which
is ISO-8859-1-, UTF-8-, or UTF-32-encoded. These functions should
be used to convert hard-coded C string constants into Scheme
strings.
-- C Function: SCM scm_from_latin1_stringn (const char *str, size_t
len)
-- C Function: SCM scm_from_utf8_stringn (const char *str, size_t len)
-- C Function: SCM scm_from_utf32_stringn (const scm_t_wchar *str,
size_t len)
Return a scheme string from C string STR, which is ISO-8859-1-,
UTF-8-, or UTF-32-encoded, of length LEN. LEN is the number of
bytes pointed to by STR for ‘scm_from_latin1_stringn’ and
‘scm_from_utf8_stringn’; it is the number of elements (code points)
in STR in the case of ‘scm_from_utf32_stringn’.
-- C function: char *scm_to_latin1_stringn (SCM str, size_t *lenp)
-- C function: char *scm_to_utf8_stringn (SCM str, size_t *lenp)
-- C function: scm_t_wchar *scm_to_utf32_stringn (SCM str, size_t
*lenp)
Return a newly allocated, ISO-8859-1-, UTF-8-, or UTF-32-encoded C
string from Scheme string STR. An error is thrown when STR cannot
be converted to the specified encoding. If LENP is ‘NULL’, the
returned C string will be null terminated, and an error will be
thrown if the C string would otherwise contain null characters. If
LENP is not ‘NULL’, the string is not null terminated, and the
length of the returned string is returned in LENP. The length
returned is the number of bytes for ‘scm_to_latin1_stringn’ and
‘scm_to_utf8_stringn’; it is the number of elements (code points)
for ‘scm_to_utf32_stringn’.
It is not often the case, but sometimes when you are dealing with the
implementation details of a port, you need to encode and decode strings
according to the encoding and conversion strategy of the port. There
are some convenience functions for that purpose as well.
-- C Function: SCM scm_from_port_string (const char *str, SCM port)
-- C Function: SCM scm_from_port_stringn (const char *str, size_t len,
SCM port)
-- C Function: char* scm_to_port_string (SCM str, SCM port)
-- C Function: char* scm_to_port_stringn (SCM str, size_t *lenp, SCM
port)
Like ‘scm_from_stringn’ and friends, except they take their
encoding and conversion strategy from a given port object.
6.6.5.15 String Internals
.........................
Guile stores each string in memory as a contiguous array of Unicode code
points along with an associated set of attributes. If all of the code
points of a string have an integer range between 0 and 255 inclusive,
the code point array is stored as one byte per code point: it is stored
as an ISO-8859-1 (aka Latin-1) string. If any of the code points of the
string has an integer value greater that 255, the code point array is
stored as four bytes per code point: it is stored as a UTF-32 string.
Conversion between the one-byte-per-code-point and
four-bytes-per-code-point representations happens automatically as
necessary.
No API is provided to set the internal representation of strings;
however, there are pair of procedures available to query it. These are
debugging procedures. Using them in production code is discouraged,
since the details of Guile’s internal representation of strings may
change from release to release.
-- Scheme Procedure: string-bytes-per-char str
-- C Function: scm_string_bytes_per_char (str)
Return the number of bytes used to encode a Unicode code point in
string STR. The result is one or four.
-- Scheme Procedure: %string-dump str
-- C Function: scm_sys_string_dump (str)
Returns an association list containing debugging information for
STR. The association list has the following entries.
‘string’
The string itself.
‘start’
The start index of the string into its stringbuf
‘length’
The length of the string
‘shared’
If this string is a substring, it returns its parent string.
Otherwise, it returns ‘#f’
‘read-only’
‘#t’ if the string is read-only
‘stringbuf-chars’
A new string containing this string’s stringbuf’s characters
‘stringbuf-length’
The number of characters in this stringbuf
‘stringbuf-shared’
‘#t’ if this stringbuf is shared
‘stringbuf-wide’
‘#t’ if this stringbuf’s characters are stored in a 32-bit
buffer, or ‘#f’ if they are stored in an 8-bit buffer
6.6.6 Symbols
-------------
Symbols in Scheme are widely used in three ways: as items of discrete
data, as lookup keys for alists and hash tables, and to denote variable
references.
A “symbol” is similar to a string in that it is defined by a sequence
of characters. The sequence of characters is known as the symbol’s
“name”. In the usual case — that is, where the symbol’s name doesn’t
include any characters that could be confused with other elements of
Scheme syntax — a symbol is written in a Scheme program by writing the
sequence of characters that make up the name, _without_ any quotation
marks or other special syntax. For example, the symbol whose name is
“multiply-by-2” is written, simply:
multiply-by-2
Notice how this differs from a _string_ with contents
“multiply-by-2”, which is written with double quotation marks, like
this:
"multiply-by-2"
Looking beyond how they are written, symbols are different from
strings in two important respects.
The first important difference is uniqueness. If the same-looking
string is read twice from two different places in a program, the result
is two _different_ string objects whose contents just happen to be the
same. If, on the other hand, the same-looking symbol is read twice from
two different places in a program, the result is the _same_ symbol
object both times.
Given two read symbols, you can use ‘eq?’ to test whether they are
the same (that is, have the same name). ‘eq?’ is the most efficient
comparison operator in Scheme, and comparing two symbols like this is as
fast as comparing, for example, two numbers. Given two strings, on the
other hand, you must use ‘equal?’ or ‘string=?’, which are much slower
comparison operators, to determine whether the strings have the same
contents.
(define sym1 (quote hello))
(define sym2 (quote hello))
(eq? sym1 sym2) ⇒ #t
(define str1 "hello")
(define str2 "hello")
(eq? str1 str2) ⇒ #f
(equal? str1 str2) ⇒ #t
The second important difference is that symbols, unlike strings, are
not self-evaluating. This is why we need the ‘(quote ...)’s in the
example above: ‘(quote hello)’ evaluates to the symbol named "hello"
itself, whereas an unquoted ‘hello’ is _read_ as the symbol named
"hello" and evaluated as a variable reference ... about which more below
(*note Symbol Variables::).
6.6.6.1 Symbols as Discrete Data
................................
Numbers and symbols are similar to the extent that they both lend
themselves to ‘eq?’ comparison. But symbols are more descriptive than
numbers, because a symbol’s name can be used directly to describe the
concept for which that symbol stands.
For example, imagine that you need to represent some colours in a
computer program. Using numbers, you would have to choose arbitrarily
some mapping between numbers and colours, and then take care to use that
mapping consistently:
;; 1=red, 2=green, 3=purple
(if (eq? (colour-of vehicle) 1)
...)
You can make the mapping more explicit and the code more readable by
defining constants:
(define red 1)
(define green 2)
(define purple 3)
(if (eq? (colour-of vehicle) red)
...)
But the simplest and clearest approach is not to use numbers at all, but
symbols whose names specify the colours that they refer to:
(if (eq? (colour-of vehicle) 'red)
...)
The descriptive advantages of symbols over numbers increase as the
set of concepts that you want to describe grows. Suppose that a car
object can have other properties as well, such as whether it has or
uses:
• automatic or manual transmission
• leaded or unleaded fuel
• power steering (or not).
Then a car’s combined property set could be naturally represented and
manipulated as a list of symbols:
(properties-of vehicle1)
⇒
(red manual unleaded power-steering)
(if (memq 'power-steering (properties-of vehicle1))
(display "Unfit people can drive this vehicle.\n")
(display "You'll need strong arms to drive this vehicle!\n"))
⊣
Unfit people can drive this vehicle.
Remember, the fundamental property of symbols that we are relying on
here is that an occurrence of ‘'red’ in one part of a program is an
_indistinguishable_ symbol from an occurrence of ‘'red’ in another part
of a program; this means that symbols can usefully be compared using
‘eq?’. At the same time, symbols have naturally descriptive names.
This combination of efficiency and descriptive power makes them ideal
for use as discrete data.
6.6.6.2 Symbols as Lookup Keys
..............................
Given their efficiency and descriptive power, it is natural to use
symbols as the keys in an association list or hash table.
To illustrate this, consider a more structured representation of the
car properties example from the preceding subsection. Rather than
mixing all the properties up together in a flat list, we could use an
association list like this:
(define car1-properties '((colour . red)
(transmission . manual)
(fuel . unleaded)
(steering . power-assisted)))
Notice how this structure is more explicit and extensible than the
flat list. For example it makes clear that ‘manual’ refers to the
transmission rather than, say, the windows or the locking of the car.
It also allows further properties to use the same symbols among their
possible values without becoming ambiguous:
(define car1-properties '((colour . red)
(transmission . manual)
(fuel . unleaded)
(steering . power-assisted)
(seat-colour . red)
(locking . manual)))
With a representation like this, it is easy to use the efficient
‘assq-XXX’ family of procedures (*note Association Lists::) to extract
or change individual pieces of information:
(assq-ref car1-properties 'fuel) ⇒ unleaded
(assq-ref car1-properties 'transmission) ⇒ manual
(assq-set! car1-properties 'seat-colour 'black)
⇒
((colour . red)
(transmission . manual)
(fuel . unleaded)
(steering . power-assisted)
(seat-colour . black)
(locking . manual)))
Hash tables also have keys, and exactly the same arguments apply to
the use of symbols in hash tables as in association lists. The hash
value that Guile uses to decide where to add a symbol-keyed entry to a
hash table can be obtained by calling the ‘symbol-hash’ procedure:
-- Scheme Procedure: symbol-hash symbol
-- C Function: scm_symbol_hash (symbol)
Return a hash value for SYMBOL.
See *note Hash Tables:: for information about hash tables in general,
and for why you might choose to use a hash table rather than an
association list.
6.6.6.3 Symbols as Denoting Variables
.....................................
When an unquoted symbol in a Scheme program is evaluated, it is
interpreted as a variable reference, and the result of the evaluation is
the appropriate variable’s value.
For example, when the expression ‘(string-length "abcd")’ is read and
evaluated, the sequence of characters ‘string-length’ is read as the
symbol whose name is "string-length". This symbol is associated with a
variable whose value is the procedure that implements string length
calculation. Therefore evaluation of the ‘string-length’ symbol results
in that procedure.
The details of the connection between an unquoted symbol and the
variable to which it refers are explained elsewhere. See *note Binding
Constructs::, for how associations between symbols and variables are
created, and *note Modules::, for how those associations are affected by
Guile’s module system.
6.6.6.4 Operations Related to Symbols
.....................................
Given any Scheme value, you can determine whether it is a symbol using
the ‘symbol?’ primitive:
-- Scheme Procedure: symbol? obj
-- C Function: scm_symbol_p (obj)
Return ‘#t’ if OBJ is a symbol, otherwise return ‘#f’.
-- C Function: int scm_is_symbol (SCM val)
Equivalent to ‘scm_is_true (scm_symbol_p (val))’.
Once you know that you have a symbol, you can obtain its name as a
string by calling ‘symbol->string’. Note that Guile differs by default
from R5RS on the details of ‘symbol->string’ as regards
case-sensitivity:
-- Scheme Procedure: symbol->string s
-- C Function: scm_symbol_to_string (s)
Return the name of symbol S as a string. By default, Guile reads
symbols case-sensitively, so the string returned will have the same
case variation as the sequence of characters that caused S to be
created.
If Guile is set to read symbols case-insensitively (as specified by
R5RS), and S comes into being as part of a literal expression
(*note (r5rs)Literal expressions::) or by a call to the ‘read’ or
‘string-ci->symbol’ procedures, Guile converts any alphabetic
characters in the symbol’s name to lower case before creating the
symbol object, so the string returned here will be in lower case.
If S was created by ‘string->symbol’, the case of characters in the
string returned will be the same as that in the string that was
passed to ‘string->symbol’, regardless of Guile’s case-sensitivity
setting at the time S was created.
It is an error to apply mutation procedures like ‘string-set!’ to
strings returned by this procedure.
Most symbols are created by writing them literally in code. However
it is also possible to create symbols programmatically using the
following procedures:
-- Scheme Procedure: symbol char...
Return a newly allocated symbol made from the given character
arguments.
(symbol #\x #\y #\z) ⇒ xyz
-- Scheme Procedure: list->symbol lst
Return a newly allocated symbol made from a list of characters.
(list->symbol '(#\a #\b #\c)) ⇒ abc
-- Scheme Procedure: symbol-append arg ...
Return a newly allocated symbol whose characters form the
concatenation of the given symbols, ARG ....
(let ((h 'hello))
(symbol-append h 'world))
⇒ helloworld
-- Scheme Procedure: string->symbol string
-- C Function: scm_string_to_symbol (string)
Return the symbol whose name is STRING. This procedure can create
symbols with names containing special characters or letters in the
non-standard case, but it is usually a bad idea to create such
symbols because in some implementations of Scheme they cannot be
read as themselves.
-- Scheme Procedure: string-ci->symbol str
-- C Function: scm_string_ci_to_symbol (str)
Return the symbol whose name is STR. If Guile is currently reading
symbols case-insensitively, STR is converted to lowercase before
the returned symbol is looked up or created.
The following examples illustrate Guile’s detailed behaviour as
regards the case-sensitivity of symbols:
(read-enable 'case-insensitive) ; R5RS compliant behaviour
(symbol->string 'flying-fish) ⇒ "flying-fish"
(symbol->string 'Martin) ⇒ "martin"
(symbol->string
(string->symbol "Malvina")) ⇒ "Malvina"
(eq? 'mISSISSIppi 'mississippi) ⇒ #t
(string->symbol "mISSISSIppi") ⇒ mISSISSIppi
(eq? 'bitBlt (string->symbol "bitBlt")) ⇒ #f
(eq? 'LolliPop
(string->symbol (symbol->string 'LolliPop))) ⇒ #t
(string=? "K. Harper, M.D."
(symbol->string
(string->symbol "K. Harper, M.D."))) ⇒ #t
(read-disable 'case-insensitive) ; Guile default behaviour
(symbol->string 'flying-fish) ⇒ "flying-fish"
(symbol->string 'Martin) ⇒ "Martin"
(symbol->string
(string->symbol "Malvina")) ⇒ "Malvina"
(eq? 'mISSISSIppi 'mississippi) ⇒ #f
(string->symbol "mISSISSIppi") ⇒ mISSISSIppi
(eq? 'bitBlt (string->symbol "bitBlt")) ⇒ #t
(eq? 'LolliPop
(string->symbol (symbol->string 'LolliPop))) ⇒ #t
(string=? "K. Harper, M.D."
(symbol->string
(string->symbol "K. Harper, M.D."))) ⇒ #t
From C, there are lower level functions that construct a Scheme
symbol from a C string in the current locale encoding.
When you want to do more from C, you should convert between symbols
and strings using ‘scm_symbol_to_string’ and ‘scm_string_to_symbol’ and
work with the strings.
-- C Function: SCM scm_from_latin1_symbol (const char *name)
-- C Function: SCM scm_from_utf8_symbol (const char *name)
Construct and return a Scheme symbol whose name is specified by the
null-terminated C string NAME. These are appropriate when the C
string is hard-coded in the source code.
-- C Function: SCM scm_from_locale_symbol (const char *name)
-- C Function: SCM scm_from_locale_symboln (const char *name, size_t
len)
Construct and return a Scheme symbol whose name is specified by
NAME. For ‘scm_from_locale_symbol’, NAME must be null terminated;
for ‘scm_from_locale_symboln’ the length of NAME is specified
explicitly by LEN.
Note that these functions should _not_ be used when NAME is a C
string constant, because there is no guarantee that the current
locale will match that of the execution character set, used for
string and character constants. Most modern C compilers use UTF-8
by default, so in such cases we recommend ‘scm_from_utf8_symbol’.
-- C Function: SCM scm_take_locale_symbol (char *str)
-- C Function: SCM scm_take_locale_symboln (char *str, size_t len)
Like ‘scm_from_locale_symbol’ and ‘scm_from_locale_symboln’,
respectively, but also frees STR with ‘free’ eventually. Thus, you
can use this function when you would free STR anyway immediately
after creating the Scheme string. In certain cases, Guile can then
use STR directly as its internal representation.
The size of a symbol can also be obtained from C:
-- C Function: size_t scm_c_symbol_length (SCM sym)
Return the number of characters in SYM.
Finally, some applications, especially those that generate new Scheme
code dynamically, need to generate symbols for use in the generated
code. The ‘gensym’ primitive meets this need:
-- Scheme Procedure: gensym [prefix]
-- C Function: scm_gensym (prefix)
Create a new symbol with a name constructed from a prefix and a
counter value. The string PREFIX can be specified as an optional
argument. Default prefix is ‘ g’. The counter is increased by 1
at each call. There is no provision for resetting the counter.
The symbols generated by ‘gensym’ are _likely_ to be unique, since
their names begin with a space and it is only otherwise possible to
generate such symbols if a programmer goes out of their way to do so.
Uniqueness can be guaranteed by instead using uninterned symbols (*note
Symbol Uninterned::), though they can’t be usefully written out and read
back in.
6.6.6.5 Extended Read Syntax for Symbols
........................................
The read syntax for a symbol is a sequence of letters, digits, and
“extended alphabetic characters”, beginning with a character that cannot
begin a number. In addition, the special cases of ‘+’, ‘-’, and ‘...’
are read as symbols even though numbers can begin with ‘+’, ‘-’ or ‘.’.
Extended alphabetic characters may be used within identifiers as if
they were letters. The set of extended alphabetic characters is:
! $ % & * + - . / : < = > ? @ ^ _ ~
In addition to the standard read syntax defined above (which is taken
from R5RS (*note (r5rs)Formal syntax::)), Guile provides an extended
symbol read syntax that allows the inclusion of unusual characters such
as space characters, newlines and parentheses. If (for whatever reason)
you need to write a symbol containing characters not mentioned above,
you can do so as follows.
• Begin the symbol with the characters ‘#{’,
• write the characters of the symbol and
• finish the symbol with the characters ‘}#’.
Here are a few examples of this form of read syntax. The first
symbol needs to use extended syntax because it contains a space
character, the second because it contains a line break, and the last
because it looks like a number.
#{foo bar}#
#{what
ever}#
#{4242}#
Although Guile provides this extended read syntax for symbols,
widespread usage of it is discouraged because it is not portable and not
very readable.
Alternatively, if you enable the ‘r7rs-symbols’ read option (see
*note Scheme Read::), you can write arbitrary symbols using the same
notation used for strings, except delimited by vertical bars instead of
double quotes.
|foo bar|
|\x3BB; is a greek lambda|
|\| is a vertical bar|
Note that there’s also an ‘r7rs-symbols’ print option (*note Scheme
Write::). To enable the use of this notation, evaluate one or both of
the following expressions:
(read-enable 'r7rs-symbols)
(print-enable 'r7rs-symbols)
6.6.6.6 Uninterned Symbols
..........................
What makes symbols useful is that they are automatically kept unique.
There are no two symbols that are distinct objects but have the same
name. But of course, there is no rule without exception. In addition
to the normal symbols that have been discussed up to now, you can also
create special “uninterned” symbols that behave slightly differently.
To understand what is different about them and why they might be
useful, we look at how normal symbols are actually kept unique.
Whenever Guile wants to find the symbol with a specific name, for
example during ‘read’ or when executing ‘string->symbol’, it first looks
into a table of all existing symbols to find out whether a symbol with
the given name already exists. When this is the case, Guile just
returns that symbol. When not, a new symbol with the name is created
and entered into the table so that it can be found later.
Sometimes you might want to create a symbol that is guaranteed
‘fresh’, i.e. a symbol that did not exist previously. You might also
want to somehow guarantee that no one else will ever unintentionally
stumble across your symbol in the future. These properties of a symbol
are often needed when generating code during macro expansion. When
introducing new temporary variables, you want to guarantee that they
don’t conflict with variables in other people’s code.
The simplest way to arrange for this is to create a new symbol but
not enter it into the global table of all symbols. That way, no one
will ever get access to your symbol by chance. Symbols that are not in
the table are called “uninterned”. Of course, symbols that _are_ in the
table are called “interned”.
You create new uninterned symbols with the function ‘make-symbol’.
You can test whether a symbol is interned or not with
‘symbol-interned?’.
Uninterned symbols break the rule that the name of a symbol uniquely
identifies the symbol object. Because of this, they can not be written
out and read back in like interned symbols. Currently, Guile has no
support for reading uninterned symbols. Note that the function ‘gensym’
does not return uninterned symbols for this reason.
-- Scheme Procedure: make-symbol name
-- C Function: scm_make_symbol (name)
Return a new uninterned symbol with the name NAME. The returned
symbol is guaranteed to be unique and future calls to
‘string->symbol’ will not return it.
-- Scheme Procedure: symbol-interned? symbol
-- C Function: scm_symbol_interned_p (symbol)
Return ‘#t’ if SYMBOL is interned, otherwise return ‘#f’.
For example:
(define foo-1 (string->symbol "foo"))
(define foo-2 (string->symbol "foo"))
(define foo-3 (make-symbol "foo"))
(define foo-4 (make-symbol "foo"))
(eq? foo-1 foo-2)
⇒ #t
; Two interned symbols with the same name are the same object,
(eq? foo-1 foo-3)
⇒ #f
; but a call to make-symbol with the same name returns a
; distinct object.
(eq? foo-3 foo-4)
⇒ #f
; A call to make-symbol always returns a new object, even for
; the same name.
foo-3
⇒ #
; Uninterned symbols print differently from interned symbols,
(symbol? foo-3)
⇒ #t
; but they are still symbols,
(symbol-interned? foo-3)
⇒ #f
; just not interned.
6.6.7 Keywords
--------------
Keywords are self-evaluating objects with a convenient read syntax that
makes them easy to type.
Guile’s keyword support conforms to R5RS, and adds a (switchable)
read syntax extension to permit keywords to begin with ‘:’ as well as
‘#:’, or to end with ‘:’.
6.6.7.1 Why Use Keywords?
.........................
Keywords are useful in contexts where a program or procedure wants to be
able to accept a large number of optional arguments without making its
interface unmanageable.
To illustrate this, consider a hypothetical ‘make-window’ procedure,
which creates a new window on the screen for drawing into using some
graphical toolkit. There are many parameters that the caller might like
to specify, but which could also be sensibly defaulted, for example:
• color depth – Default: the color depth for the screen
• background color – Default: white
• width – Default: 600
• height – Default: 400
If ‘make-window’ did not use keywords, the caller would have to pass
in a value for each possible argument, remembering the correct argument
order and using a special value to indicate the default value for that
argument:
(make-window 'default ;; Color depth
'default ;; Background color
800 ;; Width
100 ;; Height
...) ;; More make-window arguments
With keywords, on the other hand, defaulted arguments are omitted,
and non-default arguments are clearly tagged by the appropriate keyword.
As a result, the invocation becomes much clearer:
(make-window #:width 800 #:height 100)
On the other hand, for a simpler procedure with few arguments, the
use of keywords would be a hindrance rather than a help. The primitive
procedure ‘cons’, for example, would not be improved if it had to be
invoked as
(cons #:car x #:cdr y)
So the decision whether to use keywords or not is purely pragmatic:
use them if they will clarify the procedure invocation at point of call.
6.6.7.2 Coding With Keywords
............................
If a procedure wants to support keywords, it should take a rest argument
and then use whatever means is convenient to extract keywords and their
corresponding arguments from the contents of that rest argument.
The following example illustrates the principle: the code for
‘make-window’ uses a helper procedure called ‘get-keyword-value’ to
extract individual keyword arguments from the rest argument.
(define (get-keyword-value args keyword default)
(let ((kv (memq keyword args)))
(if (and kv (>= (length kv) 2))
(cadr kv)
default)))
(define (make-window . args)
(let ((depth (get-keyword-value args #:depth screen-depth))
(bg (get-keyword-value args #:bg "white"))
(width (get-keyword-value args #:width 800))
(height (get-keyword-value args #:height 100))
...)
...))
But you don’t need to write ‘get-keyword-value’. The ‘(ice-9
optargs)’ module provides a set of powerful macros that you can use to
implement keyword-supporting procedures like this:
(use-modules (ice-9 optargs))
(define (make-window . args)
(let-keywords args #f ((depth screen-depth)
(bg "white")
(width 800)
(height 100))
...))
Or, even more economically, like this:
(use-modules (ice-9 optargs))
(define* (make-window #:key (depth screen-depth)
(bg "white")
(width 800)
(height 100))
...)
For further details on ‘let-keywords’, ‘define*’ and other facilities
provided by the ‘(ice-9 optargs)’ module, see *note Optional
Arguments::.
To handle keyword arguments from procedures implemented in C, use
‘scm_c_bind_keyword_arguments’ (*note Keyword Procedures::).
6.6.7.3 Keyword Read Syntax
...........................
Guile, by default, only recognizes a keyword syntax that is compatible
with R5RS. A token of the form ‘#:NAME’, where ‘NAME’ has the same
syntax as a Scheme symbol (*note Symbol Read Syntax::), is the external
representation of the keyword named ‘NAME’. Keyword objects print using
this syntax as well, so values containing keyword objects can be read
back into Guile. When used in an expression, keywords are self-quoting
objects.
If the ‘keywords’ read option is set to ‘'prefix’, Guile also
recognizes the alternative read syntax ‘:NAME’. Otherwise, tokens of
the form ‘:NAME’ are read as symbols, as required by R5RS.
If the ‘keywords’ read option is set to ‘'postfix’, Guile recognizes
the SRFI-88 read syntax ‘NAME:’ (*note SRFI-88::). Otherwise, tokens of
this form are read as symbols.
To enable and disable the alternative non-R5RS keyword syntax, you
use the ‘read-set!’ procedure documented *note Scheme Read::. Note that
the ‘prefix’ and ‘postfix’ syntax are mutually exclusive.
(read-set! keywords 'prefix)
#:type
⇒
#:type
:type
⇒
#:type
(read-set! keywords 'postfix)
type:
⇒
#:type
:type
⇒
:type
(read-set! keywords #f)
#:type
⇒
#:type
:type
⊣
ERROR: In expression :type:
ERROR: Unbound variable: :type
ABORT: (unbound-variable)
6.6.7.4 Keyword Procedures
..........................
-- Scheme Procedure: keyword? obj
-- C Function: scm_keyword_p (obj)
Return ‘#t’ if the argument OBJ is a keyword, else ‘#f’.
-- Scheme Procedure: keyword->symbol keyword
-- C Function: scm_keyword_to_symbol (keyword)
Return the symbol with the same name as KEYWORD.
-- Scheme Procedure: symbol->keyword symbol
-- C Function: scm_symbol_to_keyword (symbol)
Return the keyword with the same name as SYMBOL.
-- C Function: int scm_is_keyword (SCM obj)
Equivalent to ‘scm_is_true (scm_keyword_p (OBJ))’.
-- C Function: SCM scm_from_locale_keyword (const char *name)
-- C Function: SCM scm_from_locale_keywordn (const char *name, size_t
len)
Equivalent to ‘scm_symbol_to_keyword (scm_from_locale_symbol
(NAME))’ and ‘scm_symbol_to_keyword (scm_from_locale_symboln (NAME,
LEN))’, respectively.
Note that these functions should _not_ be used when NAME is a C
string constant, because there is no guarantee that the current
locale will match that of the execution character set, used for
string and character constants. Most modern C compilers use UTF-8
by default, so in such cases we recommend ‘scm_from_utf8_keyword’.
-- C Function: SCM scm_from_latin1_keyword (const char *name)
-- C Function: SCM scm_from_utf8_keyword (const char *name)
Equivalent to ‘scm_symbol_to_keyword (scm_from_latin1_symbol
(NAME))’ and ‘scm_symbol_to_keyword (scm_from_utf8_symbol (NAME))’,
respectively.
-- C Function: void scm_c_bind_keyword_arguments (const char *subr, SCM
rest, scm_t_keyword_arguments_flags flags, SCM keyword1, SCM
*argp1, ..., SCM keywordN, SCM *argpN, SCM_UNDEFINED)
Extract the specified keyword arguments from REST, which is not
modified. If the keyword argument KEYWORD1 is present in REST with
an associated value, that value is stored in the variable pointed
to by ARGP1, otherwise the variable is left unchanged. Similarly
for the other keywords and argument pointers up to KEYWORDN and
ARGPN. The argument list to ‘scm_c_bind_keyword_arguments’ must be
terminated by ‘SCM_UNDEFINED’.
Note that since the variables pointed to by ARGP1 through ARGPN are
left unchanged if the associated keyword argument is not present,
they should be initialized to their default values before calling
‘scm_c_bind_keyword_arguments’. Alternatively, you can initialize
them to ‘SCM_UNDEFINED’ before the call, and then use ‘SCM_UNBNDP’
after the call to see which ones were provided.
If an unrecognized keyword argument is present in REST and FLAGS
does not contain ‘SCM_ALLOW_OTHER_KEYS’, or if non-keyword
arguments are present and FLAGS does not contain
‘SCM_ALLOW_NON_KEYWORD_ARGUMENTS’, an exception is raised. SUBR
should be the name of the procedure receiving the keyword
arguments, for purposes of error reporting.
For example:
SCM k_delimiter;
SCM k_grammar;
SCM sym_infix;
SCM my_string_join (SCM strings, SCM rest)
{
SCM delimiter = SCM_UNDEFINED;
SCM grammar = sym_infix;
scm_c_bind_keyword_arguments ("my-string-join", rest, 0,
k_delimiter, &delimiter,
k_grammar, &grammar,
SCM_UNDEFINED);
if (SCM_UNBNDP (delimiter))
delimiter = scm_from_utf8_string (" ");
return scm_string_join (strings, delimiter, grammar);
}
void my_init ()
{
k_delimiter = scm_from_utf8_keyword ("delimiter");
k_grammar = scm_from_utf8_keyword ("grammar");
sym_infix = scm_from_utf8_symbol ("infix");
scm_c_define_gsubr ("my-string-join", 1, 0, 1, my_string_join);
}
6.6.8 Pairs
-----------
Pairs are used to combine two Scheme objects into one compound object.
Hence the name: A pair stores a pair of objects.
The data type “pair” is extremely important in Scheme, just like in
any other Lisp dialect. The reason is that pairs are not only used to
make two values available as one object, but that pairs are used for
constructing lists of values. Because lists are so important in Scheme,
they are described in a section of their own (*note Lists::).
Pairs can literally get entered in source code or at the REPL, in the
so-called “dotted list” syntax. This syntax consists of an opening
parentheses, the first element of the pair, a dot, the second element
and a closing parentheses. The following example shows how a pair
consisting of the two numbers 1 and 2, and a pair containing the symbols
‘foo’ and ‘bar’ can be entered. It is very important to write the
whitespace before and after the dot, because otherwise the Scheme parser
would not be able to figure out where to split the tokens.
(1 . 2)
(foo . bar)
But beware, if you want to try out these examples, you have to
“quote” the expressions. More information about quotation is available
in the section *note Expression Syntax::. The correct way to try these
examples is as follows.
'(1 . 2)
⇒
(1 . 2)
'(foo . bar)
⇒
(foo . bar)
A new pair is made by calling the procedure ‘cons’ with two
arguments. Then the argument values are stored into a newly allocated
pair, and the pair is returned. The name ‘cons’ stands for "construct".
Use the procedure ‘pair?’ to test whether a given Scheme object is a
pair or not.
-- Scheme Procedure: cons x y
-- C Function: scm_cons (x, y)
Return a newly allocated pair whose car is X and whose cdr is Y.
The pair is guaranteed to be different (in the sense of ‘eq?’) from
every previously existing object.
-- Scheme Procedure: pair? x
-- C Function: scm_pair_p (x)
Return ‘#t’ if X is a pair; otherwise return ‘#f’.
-- C Function: int scm_is_pair (SCM x)
Return 1 when X is a pair; otherwise return 0.
The two parts of a pair are traditionally called “car” and “cdr”.
They can be retrieved with procedures of the same name (‘car’ and
‘cdr’), and can be modified with the procedures ‘set-car!’ and
‘set-cdr!’.
Since a very common operation in Scheme programs is to access the car
of a car of a pair, or the car of the cdr of a pair, etc., the
procedures called ‘caar’, ‘cadr’ and so on are also predefined.
However, using these procedures is often detrimental to readability, and
error-prone. Thus, accessing the contents of a list is usually better
achieved using pattern matching techniques (*note Pattern Matching::).
-- Scheme Procedure: car pair
-- Scheme Procedure: cdr pair
-- C Function: scm_car (pair)
-- C Function: scm_cdr (pair)
Return the car or the cdr of PAIR, respectively.
-- C Macro: SCM SCM_CAR (SCM pair)
-- C Macro: SCM SCM_CDR (SCM pair)
These two macros are the fastest way to access the car or cdr of a
pair; they can be thought of as compiling into a single memory
reference.
These macros do no checking at all. The argument PAIR must be a
valid pair.
-- Scheme Procedure: cddr pair
-- Scheme Procedure: cdar pair
-- Scheme Procedure: cadr pair
-- Scheme Procedure: caar pair
-- Scheme Procedure: cdddr pair
-- Scheme Procedure: cddar pair
-- Scheme Procedure: cdadr pair
-- Scheme Procedure: cdaar pair
-- Scheme Procedure: caddr pair
-- Scheme Procedure: cadar pair
-- Scheme Procedure: caadr pair
-- Scheme Procedure: caaar pair
-- Scheme Procedure: cddddr pair
-- Scheme Procedure: cdddar pair
-- Scheme Procedure: cddadr pair
-- Scheme Procedure: cddaar pair
-- Scheme Procedure: cdaddr pair
-- Scheme Procedure: cdadar pair
-- Scheme Procedure: cdaadr pair
-- Scheme Procedure: cdaaar pair
-- Scheme Procedure: cadddr pair
-- Scheme Procedure: caddar pair
-- Scheme Procedure: cadadr pair
-- Scheme Procedure: cadaar pair
-- Scheme Procedure: caaddr pair
-- Scheme Procedure: caadar pair
-- Scheme Procedure: caaadr pair
-- Scheme Procedure: caaaar pair
-- C Function: scm_cddr (pair)
-- C Function: scm_cdar (pair)
-- C Function: scm_cadr (pair)
-- C Function: scm_caar (pair)
-- C Function: scm_cdddr (pair)
-- C Function: scm_cddar (pair)
-- C Function: scm_cdadr (pair)
-- C Function: scm_cdaar (pair)
-- C Function: scm_caddr (pair)
-- C Function: scm_cadar (pair)
-- C Function: scm_caadr (pair)
-- C Function: scm_caaar (pair)
-- C Function: scm_cddddr (pair)
-- C Function: scm_cdddar (pair)
-- C Function: scm_cddadr (pair)
-- C Function: scm_cddaar (pair)
-- C Function: scm_cdaddr (pair)
-- C Function: scm_cdadar (pair)
-- C Function: scm_cdaadr (pair)
-- C Function: scm_cdaaar (pair)
-- C Function: scm_cadddr (pair)
-- C Function: scm_caddar (pair)
-- C Function: scm_cadadr (pair)
-- C Function: scm_cadaar (pair)
-- C Function: scm_caaddr (pair)
-- C Function: scm_caadar (pair)
-- C Function: scm_caaadr (pair)
-- C Function: scm_caaaar (pair)
These procedures are compositions of ‘car’ and ‘cdr’, where for
example ‘caddr’ could be defined by
(define caddr (lambda (x) (car (cdr (cdr x)))))
‘cadr’, ‘caddr’ and ‘cadddr’ pick out the second, third or fourth
elements of a list, respectively. SRFI-1 provides the same under
the names ‘second’, ‘third’ and ‘fourth’ (*note SRFI-1
Selectors::).
-- Scheme Procedure: set-car! pair value
-- C Function: scm_set_car_x (pair, value)
Stores VALUE in the car field of PAIR. The value returned by
‘set-car!’ is unspecified.
-- Scheme Procedure: set-cdr! pair value
-- C Function: scm_set_cdr_x (pair, value)
Stores VALUE in the cdr field of PAIR. The value returned by
‘set-cdr!’ is unspecified.
6.6.9 Lists
-----------
A very important data type in Scheme—as well as in all other Lisp
dialects—is the data type “list”.(1)
This is the short definition of what a list is:
• Either the empty list ‘()’,
• or a pair which has a list in its cdr.
---------- Footnotes ----------
(1) Strictly speaking, Scheme does not have a real datatype “list”.
Lists are made up of “chained pairs”, and only exist by definition—a
list is a chain of pairs which looks like a list.
6.6.9.1 List Read Syntax
........................
The syntax for lists is an opening parentheses, then all the elements of
the list (separated by whitespace) and finally a closing
parentheses.(1).
(1 2 3) ; a list of the numbers 1, 2 and 3
("foo" bar 3.1415) ; a string, a symbol and a real number
() ; the empty list
The last example needs a bit more explanation. A list with no
elements, called the “empty list”, is special in some ways. It is used
for terminating lists by storing it into the cdr of the last pair that
makes up a list. An example will clear that up:
(car '(1))
⇒
1
(cdr '(1))
⇒
()
This example also shows that lists have to be quoted when written
(*note Expression Syntax::), because they would otherwise be mistakingly
taken as procedure applications (*note Simple Invocation::).
---------- Footnotes ----------
(1) Note that there is no separation character between the list
elements, like a comma or a semicolon.
6.6.9.2 List Predicates
.......................
Often it is useful to test whether a given Scheme object is a list or
not. List-processing procedures could use this information to test
whether their input is valid, or they could do different things
depending on the datatype of their arguments.
-- Scheme Procedure: list? x
-- C Function: scm_list_p (x)
Return ‘#t’ if X is a proper list, else ‘#f’.
The predicate ‘null?’ is often used in list-processing code to tell
whether a given list has run out of elements. That is, a loop somehow
deals with the elements of a list until the list satisfies ‘null?’.
Then, the algorithm terminates.
-- Scheme Procedure: null? x
-- C Function: scm_null_p (x)
Return ‘#t’ if X is the empty list, else ‘#f’.
-- C Function: int scm_is_null (SCM x)
Return 1 when X is the empty list; otherwise return 0.
6.6.9.3 List Constructors
.........................
This section describes the procedures for constructing new lists.
‘list’ simply returns a list where the elements are the arguments,
‘cons*’ is similar, but the last argument is stored in the cdr of the
last pair of the list.
-- Scheme Procedure: list elem ...
-- C Function: scm_list_1 (elem1)
-- C Function: scm_list_2 (elem1, elem2)
-- C Function: scm_list_3 (elem1, elem2, elem3)
-- C Function: scm_list_4 (elem1, elem2, elem3, elem4)
-- C Function: scm_list_5 (elem1, elem2, elem3, elem4, elem5)
-- C Function: scm_list_n (elem1, ..., elemN, SCM_UNDEFINED)
Return a new list containing elements ELEM ....
‘scm_list_n’ takes a variable number of arguments, terminated by
the special ‘SCM_UNDEFINED’. That final ‘SCM_UNDEFINED’ is not
included in the list. None of ELEM ... can themselves be
‘SCM_UNDEFINED’, or ‘scm_list_n’ will terminate at that point.
-- Scheme Procedure: cons* arg1 arg2 ...
Like ‘list’, but the last arg provides the tail of the constructed
list, returning ‘(cons ARG1 (cons ARG2 (cons ... ARGN)))’.
Requires at least one argument. If given one argument, that
argument is returned as result. This function is called ‘list*’ in
some other Schemes and in Common LISP.
-- Scheme Procedure: list-copy lst
-- C Function: scm_list_copy (lst)
Return a (newly-created) copy of LST.
-- Scheme Procedure: make-list n [init]
Create a list containing of N elements, where each element is
initialized to INIT. INIT defaults to the empty list ‘()’ if not
given.
Note that ‘list-copy’ only makes a copy of the pairs which make up
the spine of the lists. The list elements are not copied, which means
that modifying the elements of the new list also modifies the elements
of the old list. On the other hand, applying procedures like ‘set-cdr!’
or ‘delv!’ to the new list will not alter the old list. If you also
need to copy the list elements (making a deep copy), use the procedure
‘copy-tree’ from ‘(ice-9 copy-tree)’ (*note Copying::).
6.6.9.4 List Selection
......................
These procedures are used to get some information about a list, or to
retrieve one or more elements of a list.
-- Scheme Procedure: length lst
-- C Function: scm_length (lst)
Return the number of elements in list LST.
-- Scheme Procedure: last-pair lst
-- C Function: scm_last_pair (lst)
Return the last pair in LST, signalling an error if LST is
circular.
-- Scheme Procedure: list-ref list k
-- C Function: scm_list_ref (list, k)
Return the Kth element from LIST.
-- Scheme Procedure: list-tail lst k
-- Scheme Procedure: list-cdr-ref lst k
-- C Function: scm_list_tail (lst, k)
Return the "tail" of LST beginning with its Kth element. The first
element of the list is considered to be element 0.
‘list-tail’ and ‘list-cdr-ref’ are identical. It may help to think
of ‘list-cdr-ref’ as accessing the Kth cdr of the list, or
returning the results of cdring K times down LST.
-- Scheme Procedure: list-head lst k
-- C Function: scm_list_head (lst, k)
Copy the first K elements from LST into a new list, and return it.
6.6.9.5 Append and Reverse
..........................
‘append’ and ‘append!’ are used to concatenate two or more lists in
order to form a new list. ‘reverse’ and ‘reverse!’ return lists with
the same elements as their arguments, but in reverse order. The
procedure variants with an ‘!’ directly modify the pairs which form the
list, whereas the other procedures create new pairs. This is why you
should be careful when using the side-effecting variants.
-- Scheme Procedure: append lst ... obj
-- Scheme Procedure: append
-- Scheme Procedure: append! lst ... obj
-- Scheme Procedure: append!
-- C Function: scm_append (lstlst)
-- C Function: scm_append_x (lstlst)
Return a list comprising all the elements of lists LST ... OBJ. If
called with no arguments, return the empty list.
(append '(x) '(y)) ⇒ (x y)
(append '(a) '(b c d)) ⇒ (a b c d)
(append '(a (b)) '((c))) ⇒ (a (b) (c))
The last argument OBJ may actually be any object; an improper list
results if the last argument is not a proper list.
(append '(a b) '(c . d)) ⇒ (a b c . d)
(append '() 'a) ⇒ a
‘append’ doesn’t modify the given lists, but the return may share
structure with the final OBJ. ‘append!’ is permitted, but not
required, to modify the given lists to form its return.
For ‘scm_append’ and ‘scm_append_x’, LSTLST is a list of the list
operands LST ... OBJ. That LSTLST itself is not modified or used
in the return.
-- Scheme Procedure: reverse lst
-- Scheme Procedure: reverse! lst [newtail]
-- C Function: scm_reverse (lst)
-- C Function: scm_reverse_x (lst, newtail)
Return a list comprising the elements of LST, in reverse order.
‘reverse’ constructs a new list. ‘reverse!’ is permitted, but not
required, to modify LST in constructing its return.
For ‘reverse!’, the optional NEWTAIL is appended to the result.
NEWTAIL isn’t reversed, it simply becomes the list tail. For
‘scm_reverse_x’, the NEWTAIL parameter is mandatory, but can be
‘SCM_EOL’ if no further tail is required.
6.6.9.6 List Modification
.........................
The following procedures modify an existing list, either by changing
elements of the list, or by changing the list structure itself.
-- Scheme Procedure: list-set! list k val
-- C Function: scm_list_set_x (list, k, val)
Set the Kth element of LIST to VAL.
-- Scheme Procedure: list-cdr-set! list k val
-- C Function: scm_list_cdr_set_x (list, k, val)
Set the Kth cdr of LIST to VAL.
-- Scheme Procedure: delq item lst
-- C Function: scm_delq (item, lst)
Return a newly-created copy of LST with elements ‘eq?’ to ITEM
removed. This procedure mirrors ‘memq’: ‘delq’ compares elements
of LST against ITEM with ‘eq?’.
-- Scheme Procedure: delv item lst
-- C Function: scm_delv (item, lst)
Return a newly-created copy of LST with elements ‘eqv?’ to ITEM
removed. This procedure mirrors ‘memv’: ‘delv’ compares elements
of LST against ITEM with ‘eqv?’.
-- Scheme Procedure: delete item lst
-- C Function: scm_delete (item, lst)
Return a newly-created copy of LST with elements ‘equal?’ to ITEM
removed. This procedure mirrors ‘member’: ‘delete’ compares
elements of LST against ITEM with ‘equal?’.
See also SRFI-1 which has an extended ‘delete’ (*note SRFI-1
Deleting::), and also an ‘lset-difference’ which can delete
multiple ITEMs in one call (*note SRFI-1 Set Operations::).
-- Scheme Procedure: delq! item lst
-- Scheme Procedure: delv! item lst
-- Scheme Procedure: delete! item lst
-- C Function: scm_delq_x (item, lst)
-- C Function: scm_delv_x (item, lst)
-- C Function: scm_delete_x (item, lst)
These procedures are destructive versions of ‘delq’, ‘delv’ and
‘delete’: they modify the pointers in the existing LST rather than
creating a new list. Caveat evaluator: Like other destructive list
functions, these functions cannot modify the binding of LST, and so
cannot be used to delete the first element of LST destructively.
-- Scheme Procedure: delq1! item lst
-- C Function: scm_delq1_x (item, lst)
Like ‘delq!’, but only deletes the first occurrence of ITEM from
LST. Tests for equality using ‘eq?’. See also ‘delv1!’ and
‘delete1!’.
-- Scheme Procedure: delv1! item lst
-- C Function: scm_delv1_x (item, lst)
Like ‘delv!’, but only deletes the first occurrence of ITEM from
LST. Tests for equality using ‘eqv?’. See also ‘delq1!’ and
‘delete1!’.
-- Scheme Procedure: delete1! item lst
-- C Function: scm_delete1_x (item, lst)
Like ‘delete!’, but only deletes the first occurrence of ITEM from
LST. Tests for equality using ‘equal?’. See also ‘delq1!’ and
‘delv1!’.
-- Scheme Procedure: filter pred lst
-- Scheme Procedure: filter! pred lst
Return a list containing all elements from LST which satisfy the
predicate PRED. The elements in the result list have the same
order as in LST. The order in which PRED is applied to the list
elements is not specified.
‘filter’ does not change LST, but the result may share a tail with
it. ‘filter!’ may modify LST to construct its return.
6.6.9.7 List Searching
......................
The following procedures search lists for particular elements. They use
different comparison predicates for comparing list elements with the
object to be searched. When they fail, they return ‘#f’, otherwise they
return the sublist whose car is equal to the search object, where
equality depends on the equality predicate used.
-- Scheme Procedure: memq x lst
-- C Function: scm_memq (x, lst)
Return the first sublist of LST whose car is ‘eq?’ to X where the
sublists of LST are the non-empty lists returned by ‘(list-tail LST
K)’ for K less than the length of LST. If X does not occur in LST,
then ‘#f’ (not the empty list) is returned.
-- Scheme Procedure: memv x lst
-- C Function: scm_memv (x, lst)
Return the first sublist of LST whose car is ‘eqv?’ to X where the
sublists of LST are the non-empty lists returned by ‘(list-tail LST
K)’ for K less than the length of LST. If X does not occur in LST,
then ‘#f’ (not the empty list) is returned.
-- Scheme Procedure: member x lst
-- C Function: scm_member (x, lst)
Return the first sublist of LST whose car is ‘equal?’ to X where
the sublists of LST are the non-empty lists returned by ‘(list-tail
LST K)’ for K less than the length of LST. If X does not occur in
LST, then ‘#f’ (not the empty list) is returned.
See also SRFI-1 which has an extended ‘member’ function (*note
SRFI-1 Searching::).
6.6.9.8 List Mapping
....................
List processing is very convenient in Scheme because the process of
iterating over the elements of a list can be highly abstracted. The
procedures in this section are the most basic iterating procedures for
lists. They take a procedure and one or more lists as arguments, and
apply the procedure to each element of the list. They differ in their
return value.
-- Scheme Procedure: map proc arg1 arg2 ...
-- Scheme Procedure: map-in-order proc arg1 arg2 ...
-- C Function: scm_map (proc, arg1, args)
Apply PROC to each element of the list ARG1 (if only two arguments
are given), or to the corresponding elements of the argument lists
(if more than two arguments are given). The result(s) of the
procedure applications are saved and returned in a list. For
‘map’, the order of procedure applications is not specified,
‘map-in-order’ applies the procedure from left to right to the list
elements.
-- Scheme Procedure: for-each proc arg1 arg2 ...
Like ‘map’, but the procedure is always applied from left to right,
and the result(s) of the procedure applications are thrown away.
The return value is not specified.
See also SRFI-1 which extends these functions to take lists of
unequal lengths (*note SRFI-1 Fold and Map::).
6.6.10 Vectors
--------------
Vectors are sequences of Scheme objects. Unlike lists, the length of a
vector, once the vector is created, cannot be changed. The advantage of
vectors over lists is that the time required to access one element of a
vector given its “position” (synonymous with “index”), a zero-origin
number, is constant, whereas lists have an access time linear to the
position of the accessed element in the list.
Vectors can contain any kind of Scheme object; it is even possible to
have different types of objects in the same vector. For vectors
containing vectors, you may wish to use *note Arrays:: instead. Note,
too, that vectors are a special case of one dimensional non-uniform
arrays and that array procedures operate happily on vectors.
Also see *note SRFI-43::, *note R6RS Support::, or *note R7RS
Support::, for more comprehensive vector libraries.
6.6.10.1 Read Syntax for Vectors
................................
Vectors can literally be entered in source code, just like strings,
characters or some of the other data types. The read syntax for vectors
is as follows: A sharp sign (‘#’), followed by an opening parentheses,
all elements of the vector in their respective read syntax, and finally
a closing parentheses. Like strings, vectors do not have to be quoted.
The following are examples of the read syntax for vectors; where the
first vector only contains numbers and the second three different object
types: a string, a symbol and a number in hexadecimal notation.
#(1 2 3)
#("Hello" foo #xdeadbeef)
6.6.10.2 Dynamic Vector Creation and Validation
...............................................
Instead of creating a vector implicitly by using the read syntax just
described, you can create a vector dynamically by calling one of the
‘vector’ and ‘list->vector’ primitives with the list of Scheme values
that you want to place into a vector. The size of the vector thus
created is determined implicitly by the number of arguments given.
-- Scheme Procedure: vector arg ...
-- Scheme Procedure: list->vector l
-- C Function: scm_vector (l)
Return a newly allocated vector composed of the given arguments.
Analogous to ‘list’.
(vector 'a 'b 'c) ⇒ #(a b c)
The inverse operation is ‘vector->list’:
-- Scheme Procedure: vector->list v
-- C Function: scm_vector_to_list (v)
Return a newly allocated list composed of the elements of V.
(vector->list #(dah dah didah)) ⇒ (dah dah didah)
(list->vector '(dididit dah)) ⇒ #(dididit dah)
To allocate a vector with an explicitly specified size, use
‘make-vector’. With this primitive you can also specify an initial
value for the vector elements (the same value for all elements, that
is):
-- Scheme Procedure: make-vector len [fill]
-- C Function: scm_make_vector (len, fill)
Return a newly allocated vector of LEN elements. If a second
argument is given, then each position is initialized to FILL.
Otherwise the initial contents of each position is unspecified.
-- C Function: SCM scm_c_make_vector (size_t k, SCM fill)
Like ‘scm_make_vector’, but the length is given as a ‘size_t’.
To check whether an arbitrary Scheme value _is_ a vector, use the
‘vector?’ primitive:
-- Scheme Procedure: vector? obj
-- C Function: scm_vector_p (obj)
Return ‘#t’ if OBJ is a vector, otherwise return ‘#f’.
-- C Function: int scm_is_vector (SCM obj)
Return non-zero when OBJ is a vector, otherwise return ‘zero’.
6.6.10.3 Accessing and Modifying Vector Contents
................................................
‘vector-length’ and ‘vector-ref’ return information about a given
vector, respectively its size and the elements that are contained in the
vector.
-- Scheme Procedure: vector-length vector
-- C Function: scm_vector_length (vector)
Return the number of elements in VECTOR as an exact integer.
-- C Function: size_t scm_c_vector_length (SCM vec)
Return the number of elements in VEC as a ‘size_t’.
-- Scheme Procedure: vector-ref vec k
-- C Function: scm_vector_ref (vec, k)
Return the contents of position K of VEC. K must be a valid index
of VEC.
(vector-ref #(1 1 2 3 5 8 13 21) 5) ⇒ 8
(vector-ref #(1 1 2 3 5 8 13 21)
(let ((i (round (* 2 (acos -1)))))
(if (inexact? i)
(inexact->exact i)
i))) ⇒ 13
-- C Function: SCM scm_c_vector_ref (SCM vec, size_t k)
Return the contents of position K (a ‘size_t’) of VEC.
A vector created by one of the dynamic vector constructor procedures
(*note Vector Creation::) can be modified using the following
procedures.
_NOTE:_ According to R5RS, it is an error to use any of these
procedures on a literally read vector, because such vectors should be
considered as constants. Currently, however, Guile does not detect this
error.
-- Scheme Procedure: vector-set! vec k obj
-- C Function: scm_vector_set_x (vec, k, obj)
Store OBJ in position K of VEC. K must be a valid index of VEC.
The value returned by ‘vector-set!’ is unspecified.
(let ((vec (vector 0 '(2 2 2 2) "Anna")))
(vector-set! vec 1 '("Sue" "Sue"))
vec) ⇒ #(0 ("Sue" "Sue") "Anna")
-- C Function: void scm_c_vector_set_x (SCM vec, size_t k, SCM obj)
Store OBJ in position K (a ‘size_t’) of VEC.
-- Scheme Procedure: vector-fill! vec fill [start [end]]
-- C Function: scm_vector_fill_x (vec, fill)
Store FILL in every position of VEC in the range [START ... END).
START defaults to 0 and END defaults to the length of VEC.
The value returned by ‘vector-fill!’ is unspecified.
-- Scheme Procedure: vector-copy vec [start [end]]
-- C Function: scm_vector_copy (vec)
Returns a freshly allocated vector containing the elements of VEC
in the range [START ... END). START defaults to 0 and END
defaults to the length of VEC.
-- Scheme Procedure: vector-copy! dst at src [start [end]]
Copy the block of elements from vector SRC in the range [START ...
END) into vector DST, starting at position AT. AT and START
default to 0 and END defaults to the length of SRC.
It is an error for DST to have a length less than AT + (END -
START).
The order in which elements are copied is unspecified, except that
if the source and destination overlap, copying takes place as if
the source is first copied into a temporary vector and then into
the destination.
The value returned by ‘vector-copy!’ is unspecified.
-- Scheme Procedure: vector-move-left! vec1 start1 end1 vec2 start2
-- C Function: scm_vector_move_left_x (vec1, start1, end1, vec2,
start2)
Copy elements from VEC1, positions START1 to END1, to VEC2 starting
at position START2. START1 and START2 are inclusive indices; END1
is exclusive.
‘vector-move-left!’ copies elements in leftmost order. Therefore,
in the case where VEC1 and VEC2 refer to the same vector,
‘vector-move-left!’ is usually appropriate when START1 is greater
than START2.
The value returned by ‘vector-move-left!’ is unspecified.
-- Scheme Procedure: vector-move-right! vec1 start1 end1 vec2 start2
-- C Function: scm_vector_move_right_x (vec1, start1, end1, vec2,
start2)
Copy elements from VEC1, positions START1 to END1, to VEC2 starting
at position START2. START1 and START2 are inclusive indices; END1
is exclusive.
‘vector-move-right!’ copies elements in rightmost order.
Therefore, in the case where VEC1 and VEC2 refer to the same
vector, ‘vector-move-right!’ is usually appropriate when START1 is
less than START2.
The value returned by ‘vector-move-right!’ is unspecified.
6.6.10.4 Vector Accessing from C
................................
A vector can be read and modified from C with the functions *note
‘scm_c_vector_ref’: x-scm_c_vector_ref. and *note ‘scm_c_vector_set_x’:
x-scm_c_vector_set_x. In addition to these functions, there are two
other ways to access vectors from C that might be more efficient in
certain situations: you can use the unsafe _vector macros_; or you can
use the general framework for accessing all kinds of arrays (*note
Accessing Arrays from C::), which is more verbose, but can deal
efficiently with all kinds of vectors (and arrays). For arrays of rank
1 whose backing store is a vector, you can use the ‘scm_vector_elements’
and ‘scm_vector_writable_elements’ functions as shortcuts.
-- C Macro: size_t SCM_SIMPLE_VECTOR_LENGTH (SCM vec)
Evaluates to the length of the vector VEC. No type checking is
done.
-- C Macro: SCM SCM_SIMPLE_VECTOR_REF (SCM vec, size_t idx)
Evaluates to the element at position IDX in the vector VEC. No
type or range checking is done.
-- C Macro: void SCM_SIMPLE_VECTOR_SET (SCM vec, size_t idx, SCM val)
Sets the element at position IDX in the vector VEC to VAL. No type
or range checking is done.
-- C Function: const SCM * scm_vector_elements (SCM array,
scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
Acquire a *note handle: Accessing Arrays from C. for ARRAY and
return a read-only pointer to its elements. ARRAY must be either a
vector, or an array of rank 1 whose backing store is a vector;
otherwise an error is signaled. The handle must eventually be
released with *note ‘scm_array_handle_release’:
x-scm_array_handle_release.
The variables pointed to by LENP and INCP are filled with the
number of elements of the array and the increment (number of
elements) between successive elements, respectively. Successive
elements of ARRAY need not be contiguous in their underlying “root
vector” returned here; hence the increment is not necessarily equal
to 1 and may well be negative too (*note Shared Arrays::).
The following example shows the typical way to use this function.
It creates a list of all elements of ARRAY (in reverse order).
scm_t_array_handle handle;
size_t i, len;
ssize_t inc;
const SCM *elt;
SCM list;
elt = scm_vector_elements (array, &handle, &len, &inc);
list = SCM_EOL;
for (i = 0; i < len; i++, elt += inc)
list = scm_cons (*elt, list);
scm_array_handle_release (&handle);
-- C Function: SCM * scm_vector_writable_elements (SCM array,
scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
Like ‘scm_vector_elements’ but the pointer can be used to modify
the array.
The following example shows the typical way to use this function.
It fills an array with ‘#t’.
scm_t_array_handle handle;
size_t i, len;
ssize_t inc;
SCM *elt;
elt = scm_vector_writable_elements (array, &handle, &len, &inc);
for (i = 0; i < len; i++, elt += inc)
*elt = SCM_BOOL_T;
scm_array_handle_release (&handle);
6.6.10.5 Uniform Numeric Vectors
................................
A uniform numeric vector is a vector whose elements are all of a single
numeric type. Guile offers uniform numeric vectors for signed and
unsigned 8-bit, 16-bit, 32-bit, and 64-bit integers, two sizes of
floating point values, and complex floating-point numbers of these two
sizes. *Note SRFI-4::, for more information.
For many purposes, bytevectors work just as well as uniform vectors,
and have the advantage that they integrate well with binary input and
output. *Note Bytevectors::, for more information on bytevectors.
6.6.11 Bit Vectors
------------------
Bit vectors are zero-origin, one-dimensional arrays of booleans. They
are displayed as a sequence of ‘0’s and ‘1’s prefixed by ‘#*’, e.g.,
(make-bitvector 8 #f) ⇒
#*00000000
Bit vectors are the special case of one dimensional bit arrays, and
can thus be used with the array procedures, *Note Arrays::.
-- Scheme Procedure: bitvector? obj
Return ‘#t’ when OBJ is a bitvector, else return ‘#f’.
-- Scheme Procedure: make-bitvector len [fill]
Create a new bitvector of length LEN and optionally initialize all
elements to FILL.
-- Scheme Procedure: bitvector bit ...
Create a new bitvector with the arguments as elements.
-- Scheme Procedure: bitvector-length vec
Return the length of the bitvector VEC.
-- Scheme Procedure: bitvector-bit-set? vec idx
-- Scheme Procedure: bitvector-bit-clear? vec idx
Return ‘#t’ if the bit at index IDX of the bitvector VEC is set
(for ‘bitvector-bit-set?’) or clear (for ‘bitvector-bit-clear?’).
-- Scheme Procedure: bitvector-set-bit! vec idx
-- Scheme Procedure: bitvector-clear-bit! vec idx
Set (for ‘bitvector-set-bit!’) or clear (for
‘bitvector-clear-bit!’) the bit at index IDX of the bitvector VEC.
-- Scheme Procedure: bitvector-set-all-bits! vec
-- Scheme Procedure: bitvector-clear-all-bits! vec
-- Scheme Procedure: bitvector-flip-all-bits! vec
Set, clear, or flip all bits of VEC.
-- Scheme Procedure: list->bitvector list
-- C Function: scm_list_to_bitvector (list)
Return a new bitvector initialized with the elements of LIST.
-- Scheme Procedure: bitvector->list vec
-- C Function: scm_bitvector_to_list (vec)
Return a new list initialized with the elements of the bitvector
VEC.
-- Scheme Procedure: bitvector-copy bitvector [start [end]]
-- C Function: scm_bitvector_copy (bitvector, start, end)
Returns a freshly allocated bitvector containing the elements of
BITVECTOR in the range [START ... END). START defaults to 0 and
END defaults to the length of BITVECTOR.
-- Scheme Procedure: bitvector-count bitvector
Return a count of how many entries in BITVECTOR are set.
(bitvector-count #*000111000) ⇒ 3
-- Scheme Procedure: bitvector-count-bits bitvector bits
Return a count of how many entries in BITVECTOR are set, with the
bitvector BITS selecting the entries to consider. BITVECTOR must
be at least as long as BITS.
For example,
(bitvector-count-bits #*01110111 #*11001101) ⇒ 3
-- Scheme Procedure: bitvector-position bitvector bool start
-- C Function: scm_bitvector_position (bitvector, bool, start)
Return the index of the first occurrence of BOOL in BITVECTOR,
starting from START. If there is no BOOL entry between START and
the end of BITVECTOR, then return ‘#f’. For example,
(bitvector-position #*000101 #t 0) ⇒ 3
(bitvector-position #*0001111 #f 3) ⇒ #f
-- Scheme Procedure: bitvector-set-bits! bitvector bits
Set entries of BITVECTOR to ‘#t’, with BITS selecting the bits to
set. The return value is unspecified. BITVECTOR must be at least
as long as BITS.
(define bv (bitvector-copy #*11000010))
(bitvector-set-bits! bv #*10010001)
bv
⇒ #*11010011
-- Scheme Procedure: bitvector-clear-bits! bitvector bits
Set entries of BITVECTOR to ‘#f’, with BITS selecting the bits to
clear. The return value is unspecified. BITVECTOR must be at
least as long as BITS.
(define bv (bitvector-copy #*11000010))
(bitvector-clear-bits! bv #*10010001)
bv
⇒ #*01000010
-- C Function: int scm_is_bitvector (SCM obj)
-- C Function: SCM scm_c_make_bitvector (size_t len, SCM fill)
-- C Function: int scm_bitvector_bit_is_set (SCM vec, size_t idx)
-- C Function: int scm_bitvector_bit_is_clear (SCM vec, size_t idx)
-- C Function: void scm_c_bitvector_set_bit_x (SCM vec, size_t idx)
-- C Function: void scm_c_bitvector_clear_bit_x (SCM vec, size_t idx)
-- C Function: void scm_c_bitvector_set_bits_x (SCM vec, SCM bits)
-- C Function: void scm_c_bitvector_clear_bits_x (SCM vec, SCM bits)
-- C Function: void scm_c_bitvector_set_all_bits_x (SCM vec)
-- C Function: void scm_c_bitvector_clear_all_bits_x (SCM vec)
-- C Function: void scm_c_bitvector_flip_all_bits_x (SCM vec)
-- C Function: size_t scm_c_bitvector_length (SCM bitvector)
-- C Function: size_t scm_c_bitvector_count (SCM bitvector)
-- C Function: size_t scm_c_bitvector_count_bits (SCM bitvector, SCM
bits)
C API for the corresponding Scheme bitvector interfaces.
-- C Function: const scm_t_uint32 * scm_bitvector_elements (SCM vec,
scm_t_array_handle *handle, size_t *offp, size_t *lenp,
ssize_t *incp)
Like ‘scm_vector_elements’ (*note Vector Accessing from C::), but
for bitvectors. The variable pointed to by OFFP is set to the
value returned by ‘scm_array_handle_bit_elements_offset’. See
‘scm_array_handle_bit_elements’ for how to use the returned pointer
and the offset.
-- C Function: scm_t_uint32 * scm_bitvector_writable_elements (SCM vec,
scm_t_array_handle *handle, size_t *offp, size_t *lenp,
ssize_t *incp)
Like ‘scm_bitvector_elements’, but the pointer is good for reading
and writing.
6.6.12 Bytevectors
------------------
A “bytevector” is a raw byte string. The ‘(rnrs bytevectors)’ module
provides the programming interface specified by the Revised^6 Report on
the Algorithmic Language Scheme (R6RS) (http://www.r6rs.org/). It
contains procedures to manipulate bytevectors and interpret their
contents in a number of ways: as signed or unsigned integer of various
sizes and endianness, as IEEE-754 floating point numbers, or as strings.
It is a useful tool to encode and decode binary data. The *note R7RS:
R7RS Support. offers its own set of bytevector procedures (*note
Bytevector Procedures in R7RS::).
The R6RS (Section 4.3.4) specifies an external representation for
bytevectors, whereby the octets (integers in the range 0–255) contained
in the bytevector are represented as a list prefixed by ‘#vu8’:
#vu8(1 53 204)
denotes a 3-byte bytevector containing the octets 1, 53, and 204.
Like string literals, booleans, etc., bytevectors are “self-quoting”,
i.e., they do not need to be quoted:
#vu8(1 53 204)
⇒ #vu8(1 53 204)
Bytevectors can be used with the binary input/output primitives
(*note Binary I/O::).
6.6.12.1 Endianness
...................
Some of the following procedures take an ENDIANNESS parameter. The
“endianness” is defined as the order of bytes in multi-byte numbers:
numbers encoded in “big endian” have their most significant bytes
written first, whereas numbers encoded in “little endian” have their
least significant bytes first(1).
Little-endian is the native endianness of the IA32 architecture and
its derivatives, while big-endian is native to SPARC and PowerPC, among
others. The ‘native-endianness’ procedure returns the native endianness
of the machine it runs on.
-- Scheme Procedure: native-endianness
-- C Function: scm_native_endianness ()
Return a value denoting the native endianness of the host machine.
-- Scheme Macro: endianness symbol
Return an object denoting the endianness specified by SYMBOL. If
SYMBOL is neither ‘big’ nor ‘little’ then an error is raised at
expand-time.
-- C Variable: scm_endianness_big
-- C Variable: scm_endianness_little
The objects denoting big- and little-endianness, respectively.
---------- Footnotes ----------
(1) Big-endian and little-endian are the most common “endiannesses”,
but others do exist. For instance, the GNU MP library allows “word
order” to be specified independently of “byte order” (*note (gmp)Integer
Import and Export::).
6.6.12.2 Manipulating Bytevectors
.................................
Bytevectors can be created, copied, and analyzed with the following
procedures and C functions.
-- Scheme Procedure: make-bytevector len [fill]
-- C Function: scm_make_bytevector (len, fill)
-- C Function: scm_c_make_bytevector (size_t len)
Return a new bytevector of LEN bytes. Optionally, if FILL is
given, fill it with FILL; FILL must be in the range [-128,255].
-- Scheme Procedure: bytevector? obj
-- C Function: scm_bytevector_p (obj)
Return true if OBJ is a bytevector.
-- C Function: int scm_is_bytevector (SCM obj)
Equivalent to ‘scm_is_true (scm_bytevector_p (obj))’.
-- Scheme Procedure: bytevector-length bv
-- C Function: scm_bytevector_length (bv)
Return the length in bytes of bytevector BV.
-- C Function: size_t scm_c_bytevector_length (SCM bv)
Likewise, return the length in bytes of bytevector BV.
-- Scheme Procedure: bytevector=? bv1 bv2
-- C Function: scm_bytevector_eq_p (bv1, bv2)
Return ‘#t’ if BV1 equals BV2—i.e., if they have the same length
and contents.
-- Scheme Procedure: bytevector-fill! bv fill [start [end]]
-- C Function: scm_bytevector_fill_x (bv, fill)
Fill positions [START ... END) of bytevector BV with byte FILL.
START defaults to 0 and END defaults to the length of BV.(1)
-- Scheme Procedure: bytevector-copy! source source-start target
target-start len
-- C Function: scm_bytevector_copy_x (source, source_start, target,
target_start, len)
Copy LEN bytes from SOURCE into TARGET, starting reading from
SOURCE-START (an index index within SOURCE) and writing at
TARGET-START.
It is permitted for the SOURCE and TARGET regions to overlap. In
that case, copying takes place as if the source is first copied
into a temporary bytevector and then into the destination.
-- Scheme Procedure: bytevector-copy bv
-- C Function: scm_bytevector_copy (bv)
Return a newly allocated copy of BV.
-- C Function: scm_t_uint8 scm_c_bytevector_ref (SCM bv, size_t index)
Return the byte at INDEX in bytevector BV.
-- C Function: void scm_c_bytevector_set_x (SCM bv, size_t index,
scm_t_uint8 value)
Set the byte at INDEX in BV to VALUE.
Low-level C macros are available. They do not perform any
type-checking; as such they should be used with care.
-- C Macro: size_t SCM_BYTEVECTOR_LENGTH (bv)
Return the length in bytes of bytevector BV.
-- C Macro: signed char * SCM_BYTEVECTOR_CONTENTS (bv)
Return a pointer to the contents of bytevector BV.
---------- Footnotes ----------
(1) R6RS only defines ‘(bytevector-fill! bv fill)’. Arguments START
and END are a Guile extension (cf. *note ‘vector-fill!’:
x-vector-fill!, *note ‘string-fill!’: x-string-fill!.).
6.6.12.3 Interpreting Bytevector Contents as Integers
.....................................................
The contents of a bytevector can be interpreted as a sequence of
integers of any given size, sign, and endianness.
(let ((bv (make-bytevector 4)))
(bytevector-u8-set! bv 0 #x12)
(bytevector-u8-set! bv 1 #x34)
(bytevector-u8-set! bv 2 #x56)
(bytevector-u8-set! bv 3 #x78)
(map (lambda (number)
(number->string number 16))
(list (bytevector-u8-ref bv 0)
(bytevector-u16-ref bv 0 (endianness big))
(bytevector-u32-ref bv 0 (endianness little)))))
⇒ ("12" "1234" "78563412")
The most generic procedures to interpret bytevector contents as
integers are described below.
-- Scheme Procedure: bytevector-uint-ref bv index endianness size
-- C Function: scm_bytevector_uint_ref (bv, index, endianness, size)
Return the SIZE-byte long unsigned integer at index INDEX in BV,
decoded according to ENDIANNESS.
-- Scheme Procedure: bytevector-sint-ref bv index endianness size
-- C Function: scm_bytevector_sint_ref (bv, index, endianness, size)
Return the SIZE-byte long signed integer at index INDEX in BV,
decoded according to ENDIANNESS.
-- Scheme Procedure: bytevector-uint-set! bv index value endianness
size
-- C Function: scm_bytevector_uint_set_x (bv, index, value, endianness,
size)
Set the SIZE-byte long unsigned integer at INDEX to VALUE, encoded
according to ENDIANNESS.
-- Scheme Procedure: bytevector-sint-set! bv index value endianness
size
-- C Function: scm_bytevector_sint_set_x (bv, index, value, endianness,
size)
Set the SIZE-byte long signed integer at INDEX to VALUE, encoded
according to ENDIANNESS.
The following procedures are similar to the ones above, but
specialized to a given integer size:
-- Scheme Procedure: bytevector-u8-ref bv index
-- Scheme Procedure: bytevector-s8-ref bv index
-- Scheme Procedure: bytevector-u16-ref bv index endianness
-- Scheme Procedure: bytevector-s16-ref bv index endianness
-- Scheme Procedure: bytevector-u32-ref bv index endianness
-- Scheme Procedure: bytevector-s32-ref bv index endianness
-- Scheme Procedure: bytevector-u64-ref bv index endianness
-- Scheme Procedure: bytevector-s64-ref bv index endianness
-- C Function: scm_bytevector_u8_ref (bv, index)
-- C Function: scm_bytevector_s8_ref (bv, index)
-- C Function: scm_bytevector_u16_ref (bv, index, endianness)
-- C Function: scm_bytevector_s16_ref (bv, index, endianness)
-- C Function: scm_bytevector_u32_ref (bv, index, endianness)
-- C Function: scm_bytevector_s32_ref (bv, index, endianness)
-- C Function: scm_bytevector_u64_ref (bv, index, endianness)
-- C Function: scm_bytevector_s64_ref (bv, index, endianness)
Return the unsigned N-bit (signed) integer (where N is 8, 16, 32 or
64) from BV at INDEX, decoded according to ENDIANNESS.
-- Scheme Procedure: bytevector-u8-set! bv index value
-- Scheme Procedure: bytevector-s8-set! bv index value
-- Scheme Procedure: bytevector-u16-set! bv index value endianness
-- Scheme Procedure: bytevector-s16-set! bv index value endianness
-- Scheme Procedure: bytevector-u32-set! bv index value endianness
-- Scheme Procedure: bytevector-s32-set! bv index value endianness
-- Scheme Procedure: bytevector-u64-set! bv index value endianness
-- Scheme Procedure: bytevector-s64-set! bv index value endianness
-- C Function: scm_bytevector_u8_set_x (bv, index, value)
-- C Function: scm_bytevector_s8_set_x (bv, index, value)
-- C Function: scm_bytevector_u16_set_x (bv, index, value, endianness)
-- C Function: scm_bytevector_s16_set_x (bv, index, value, endianness)
-- C Function: scm_bytevector_u32_set_x (bv, index, value, endianness)
-- C Function: scm_bytevector_s32_set_x (bv, index, value, endianness)
-- C Function: scm_bytevector_u64_set_x (bv, index, value, endianness)
-- C Function: scm_bytevector_s64_set_x (bv, index, value, endianness)
Store VALUE as an N-bit (signed) integer (where N is 8, 16, 32 or
64) in BV at INDEX, encoded according to ENDIANNESS.
Finally, a variant specialized for the host’s endianness is available
for each of these functions (with the exception of the ‘u8’ and ‘s8’
accessors, as endianness is about byte order and there is only 1 byte):
-- Scheme Procedure: bytevector-u16-native-ref bv index
-- Scheme Procedure: bytevector-s16-native-ref bv index
-- Scheme Procedure: bytevector-u32-native-ref bv index
-- Scheme Procedure: bytevector-s32-native-ref bv index
-- Scheme Procedure: bytevector-u64-native-ref bv index
-- Scheme Procedure: bytevector-s64-native-ref bv index
-- C Function: scm_bytevector_u16_native_ref (bv, index)
-- C Function: scm_bytevector_s16_native_ref (bv, index)
-- C Function: scm_bytevector_u32_native_ref (bv, index)
-- C Function: scm_bytevector_s32_native_ref (bv, index)
-- C Function: scm_bytevector_u64_native_ref (bv, index)
-- C Function: scm_bytevector_s64_native_ref (bv, index)
Return the unsigned N-bit (signed) integer (where N is 8, 16, 32 or
64) from BV at INDEX, decoded according to the host’s native
endianness.
-- Scheme Procedure: bytevector-u16-native-set! bv index value
-- Scheme Procedure: bytevector-s16-native-set! bv index value
-- Scheme Procedure: bytevector-u32-native-set! bv index value
-- Scheme Procedure: bytevector-s32-native-set! bv index value
-- Scheme Procedure: bytevector-u64-native-set! bv index value
-- Scheme Procedure: bytevector-s64-native-set! bv index value
-- C Function: scm_bytevector_u16_native_set_x (bv, index, value)
-- C Function: scm_bytevector_s16_native_set_x (bv, index, value)
-- C Function: scm_bytevector_u32_native_set_x (bv, index, value)
-- C Function: scm_bytevector_s32_native_set_x (bv, index, value)
-- C Function: scm_bytevector_u64_native_set_x (bv, index, value)
-- C Function: scm_bytevector_s64_native_set_x (bv, index, value)
Store VALUE as an N-bit (signed) integer (where N is 8, 16, 32 or
64) in BV at INDEX, encoded according to the host’s native
endianness.
6.6.12.4 Converting Bytevectors to/from Integer Lists
.....................................................
Bytevector contents can readily be converted to/from lists of signed or
unsigned integers:
(bytevector->sint-list (u8-list->bytevector (make-list 4 255))
(endianness little) 2)
⇒ (-1 -1)
-- Scheme Procedure: bytevector->u8-list bv
-- C Function: scm_bytevector_to_u8_list (bv)
Return a newly allocated list of unsigned 8-bit integers from the
contents of BV.
-- Scheme Procedure: u8-list->bytevector lst
-- C Function: scm_u8_list_to_bytevector (lst)
Return a newly allocated bytevector consisting of the unsigned
8-bit integers listed in LST.
-- Scheme Procedure: bytevector->uint-list bv endianness size
-- C Function: scm_bytevector_to_uint_list (bv, endianness, size)
Return a list of unsigned integers of SIZE bytes representing the
contents of BV, decoded according to ENDIANNESS.
-- Scheme Procedure: bytevector->sint-list bv endianness size
-- C Function: scm_bytevector_to_sint_list (bv, endianness, size)
Return a list of signed integers of SIZE bytes representing the
contents of BV, decoded according to ENDIANNESS.
-- Scheme Procedure: uint-list->bytevector lst endianness size
-- C Function: scm_uint_list_to_bytevector (lst, endianness, size)
Return a new bytevector containing the unsigned integers listed in
LST and encoded on SIZE bytes according to ENDIANNESS.
-- Scheme Procedure: sint-list->bytevector lst endianness size
-- C Function: scm_sint_list_to_bytevector (lst, endianness, size)
Return a new bytevector containing the signed integers listed in
LST and encoded on SIZE bytes according to ENDIANNESS.
6.6.12.5 Interpreting Bytevector Contents as Floating Point Numbers
...................................................................
Bytevector contents can also be accessed as IEEE-754 single- or
double-precision floating point numbers (respectively 32 and 64-bit
long) using the procedures described here.
-- Scheme Procedure: bytevector-ieee-single-ref bv index endianness
-- Scheme Procedure: bytevector-ieee-double-ref bv index endianness
-- C Function: scm_bytevector_ieee_single_ref (bv, index, endianness)
-- C Function: scm_bytevector_ieee_double_ref (bv, index, endianness)
Return the IEEE-754 single-precision floating point number from BV
at INDEX according to ENDIANNESS.
-- Scheme Procedure: bytevector-ieee-single-set! bv index value
endianness
-- Scheme Procedure: bytevector-ieee-double-set! bv index value
endianness
-- C Function: scm_bytevector_ieee_single_set_x (bv, index, value,
endianness)
-- C Function: scm_bytevector_ieee_double_set_x (bv, index, value,
endianness)
Store real number VALUE in BV at INDEX according to ENDIANNESS.
Specialized procedures are also available:
-- Scheme Procedure: bytevector-ieee-single-native-ref bv index
-- Scheme Procedure: bytevector-ieee-double-native-ref bv index
-- C Function: scm_bytevector_ieee_single_native_ref (bv, index)
-- C Function: scm_bytevector_ieee_double_native_ref (bv, index)
Return the IEEE-754 single-precision floating point number from BV
at INDEX according to the host’s native endianness.
-- Scheme Procedure: bytevector-ieee-single-native-set! bv index value
-- Scheme Procedure: bytevector-ieee-double-native-set! bv index value
-- C Function: scm_bytevector_ieee_single_native_set_x (bv, index,
value)
-- C Function: scm_bytevector_ieee_double_native_set_x (bv, index,
value)
Store real number VALUE in BV at INDEX according to the host’s
native endianness.
6.6.12.6 Interpreting Bytevector Contents as Unicode Strings
............................................................
Bytevector contents can also be interpreted as Unicode strings encoded
in one of the most commonly available encoding formats. *Note
Representing Strings as Bytes::, for a more generic interface.
(utf8->string (u8-list->bytevector '(99 97 102 101)))
⇒ "cafe"
(string->utf8 "café") ;; SMALL LATIN LETTER E WITH ACUTE ACCENT
⇒ #vu8(99 97 102 195 169)
-- Scheme Procedure: string-utf8-length str
-- C function: SCM scm_string_utf8_length (str)
-- C function: size_t scm_c_string_utf8_length (str)
Return the number of bytes in the UTF-8 representation of STR.
-- Scheme Procedure: string->utf8 str
-- Scheme Procedure: string->utf16 str [endianness]
-- Scheme Procedure: string->utf32 str [endianness]
-- C Function: scm_string_to_utf8 (str)
-- C Function: scm_string_to_utf16 (str, endianness)
-- C Function: scm_string_to_utf32 (str, endianness)
Return a newly allocated bytevector that contains the UTF-8,
UTF-16, or UTF-32 (aka. UCS-4) encoding of STR. For UTF-16 and
UTF-32, ENDIANNESS should be the symbol ‘big’ or ‘little’; when
omitted, it defaults to big endian.
-- Scheme Procedure: utf8->string utf
-- Scheme Procedure: utf16->string utf [endianness]
-- Scheme Procedure: utf32->string utf [endianness]
-- C Function: scm_utf8_to_string (utf)
-- C Function: scm_utf16_to_string (utf, endianness)
-- C Function: scm_utf32_to_string (utf, endianness)
Return a newly allocated string that contains from the UTF-8-,
UTF-16-, or UTF-32-decoded contents of bytevector UTF. For UTF-16
and UTF-32, ENDIANNESS should be the symbol ‘big’ or ‘little’; when
omitted, it defaults to big endian.
6.6.12.7 Accessing Bytevectors with the Array API
.................................................
As an extension to the R6RS, Guile allows bytevectors to be manipulated
with the “array” procedures (*note Arrays::). When using these APIs,
bytes are accessed one at a time as 8-bit unsigned integers:
(define bv #vu8(0 1 2 3))
(array? bv)
⇒ #t
(array-rank bv)
⇒ 1
(array-ref bv 2)
⇒ 2
;; Note the different argument order on array-set!.
(array-set! bv 77 2)
(array-ref bv 2)
⇒ 77
(array-type bv)
⇒ vu8
6.6.12.8 Accessing Bytevectors with the SRFI-4 API
..................................................
Bytevectors may also be accessed with the SRFI-4 API. *Note SRFI-4 and
Bytevectors::, for more information.
6.6.12.9 Bytevector Procedures in R7RS
......................................
The *note R7RS: R7RS Support. (Section 6.9) defines a set of bytevector
manipulation procedures, accessible with
(use-modules (scheme base))
Of these, *note ‘make-bytevector’: x-make-bytevector, *note
‘bytevector?’: x-bytevector?, *note ‘bytevector-length’:
x-bytevector-length, *note ‘bytevector-u8-ref’: x-bytevector-u8-ref. and
*note ‘bytevector-u8-set!’: x-bytevector-u8-set!. have the same
definition as in R6RS. The procedures listed below either have a
different definition in R7RS and R6RS, or are not defined in R6RS.
-- Scheme Procedure: bytevector arg ...
Return a newly allocated bytevector composed of the given
arguments. Analogous to ‘list’.
(bytevector 2 3 4) ⇒ #vu8(2 3 4)
See also *note ‘u8-list->bytevector’: x-u8-list->bytevector.
-- Scheme Procedure: bytevector-copy bv [start [end]]
Returns a newly allocated bytevector containing the elements of BV
in the range [START ... END). START defaults to 0 and END
defaults to the length of BV.
(define bv #vu8(0 1 2 3 4 5))
(bytevector-copy bv) ⇒ #vu8(0 1 2 3 4 5)
(bytevector-copy bv 2) ⇒ #vu8(2 3 4 5)
(bytevector-copy bv 2 4) ⇒ #vu8(2 3)
See also *note the R6RS version: x-r6:bytevector-copy.
-- Scheme Procedure: bytevector-copy! dst at src [start [end]]
Copy the block of elements from bytevector SRC in the range [START
... END) into bytevector DST, starting at position AT. START
defaults to 0 and END defaults to the length of SRC. It is an
error for DST to have a length less than AT + (END - START).
See also *note the R6RS version: x-r6:bytevector-copy!. With
(use-modules ((rnrs bytevectors) #:prefix r6:)
((scheme base) #:prefix r7:))
the following calls are equivalent:
(r6:bytevector-copy! source source-start target target-start len)
(r7:bytevector-copy! target target-start source source-start (+ source-start len))
-- Scheme Procedure: bytevector-append arg ...
Return a newly allocated bytevector whose characters form the
concatenation of the given bytevectors ARG ...
(bytevector-append #vu8(0 1 2) #vu8(3 4 5))
⇒ #vu8(0 1 2 3 4 5)
6.6.12.10 Bytevector Slices
...........................
As an extension to the R6RS specification, the ‘(rnrs bytevectors gnu)’
module provides the ‘bytevector-slice’ procedure, which returns a
bytevector aliasing part of an existing bytevector.
-- Scheme Procedure: bytevector-slice BV OFFSET [SIZE]
-- C Function: scm_bytevector_slice (BV, OFFSET, SIZE)
Return the slice of BV starting at OFFSET and counting SIZE bytes.
When SIZE is omitted, the slice covers all of BV starting from
OFFSET. The returned slice shares storage with BV: changes to the
slice are visible in BV and vice-versa.
When BV is actually a SRFI-4 uniform vector, its element type is
preserved unless OFFSET and SIZE are not aligned on its element
type size.
Here is an example showing how to use it:
(use-modules (rnrs bytevectors)
(rnrs bytevectors gnu))
(define bv (u8-list->bytevector (iota 10)))
(define slice (bytevector-slice bv 2 3))
slice
⇒ #vu8(2 3 4)
(bytevector-u8-set! slice 0 77)
slice
⇒ #vu8(77 3 4)
bv
⇒ #vu8(0 1 77 3 4 5 6 7 8 9)
6.6.13 Arrays
-------------
“Arrays” are a collection of cells organized into an arbitrary number of
dimensions. Each cell can be accessed in constant time by supplying an
index for each dimension.
In the current implementation, an array uses a vector of some kind
for the actual storage of its elements. Any kind of vector will do, so
you can have arrays of uniform numeric values, arrays of characters,
arrays of bits, and of course, arrays of arbitrary Scheme values. For
example, arrays with an underlying ‘c64vector’ might be nice for digital
signal processing, while arrays made from a ‘u8vector’ might be used to
hold gray-scale images.
The number of dimensions of an array is called its “rank”. Thus, a
matrix is an array of rank 2, while a vector has rank 1. When accessing
an array element, you have to specify one exact integer for each
dimension. These integers are called the “indices” of the element. An
array specifies the allowed range of indices for each dimension via an
inclusive lower and upper bound. These bounds can well be negative, but
the upper bound must be greater than or equal to the lower bound minus
one. When all lower bounds of an array are zero, it is called a
“zero-origin” array.
Arrays can be of rank 0, which could be interpreted as a scalar.
Thus, a zero-rank array can store exactly one object and the list of
indices of this element is the empty list.
Arrays contain zero elements when one of their dimensions has a zero
length. These empty arrays maintain information about their shape: a
matrix with zero columns and 3 rows is different from a matrix with 3
columns and zero rows, which again is different from a vector of length
zero.
The array procedures are all polymorphic, treating strings, uniform
numeric vectors, bytevectors, bit vectors and ordinary vectors as one
dimensional arrays.
6.6.13.1 Array Syntax
.....................
An array is displayed as ‘#’ followed by its rank, followed by a tag
that describes the underlying vector, optionally followed by information
about its shape, and finally followed by the cells, organized into
dimensions using parentheses.
In more words, the array tag is of the form
#<@lower><:len><@lower><:len>...
where ‘’ is a positive integer in decimal giving the rank of
the array. It is omitted when the rank is 1 and the array is non-shared
and has zero-origin (see below). For shared arrays and for a non-zero
origin, the rank is always printed even when it is 1 to distinguish them
from ordinary vectors.
The ‘’ part is the tag for a uniform numeric vector, like
‘u8’, ‘s16’, etc, ‘b’ for bitvectors, or ‘a’ for strings. It is empty
for ordinary vectors.
The ‘<@lower>’ part is a ‘@’ character followed by a signed integer
in decimal giving the lower bound of a dimension. There is one
‘<@lower>’ for each dimension. When all lower bounds are zero, all
‘<@lower>’ parts are omitted.
The ‘<:len>’ part is a ‘:’ character followed by an unsigned integer
in decimal giving the length of a dimension. Like for the lower bounds,
there is one ‘<:len>’ for each dimension, and the ‘<:len>’ part always
follows the ‘<@lower>’ part for a dimension. Lengths are only then
printed when they can’t be deduced from the nested lists of elements of
the array literal, which can happen when at least one length is zero.
As a special case, an array of rank 0 is printed as
‘#0()’, where ‘’ is the result of printing the
single element of the array.
Thus,
‘#(1 2 3)’
is an ordinary array of rank 1 with lower bound 0 in dimension 0.
(I.e., a regular vector.)
‘#@2(1 2 3)’
is an ordinary array of rank 1 with lower bound 2 in dimension 0.
‘#2((1 2 3) (4 5 6))’
is a non-uniform array of rank 2; a 2x3 matrix with index ranges
0..1 and 0..2.
‘#u8(0 1 2)’
is a uniform u8 array of rank 1.
‘#2u32@2@3((1 2) (2 3))’
is a uniform u32 array of rank 2 with index ranges 2..3 and 3..4.
‘#2()’
is a two-dimensional array with index ranges 0..-1 and 0..-1, i.e.
both dimensions have length zero.
‘#2:0:2()’
is a two-dimensional array with index ranges 0..-1 and 0..1, i.e.
the first dimension has length zero, but the second has length 2.
‘#0(12)’
is a rank-zero array with contents 12.
In addition, bytevectors are also arrays, but use a different syntax
(*note Bytevectors::):
‘#vu8(1 2 3)’
is a 3-byte long bytevector, with contents 1, 2, 3.
6.6.13.2 Array Procedures
.........................
When an array is created, the range of each dimension must be specified,
e.g., to create a 2x3 array with a zero-based index:
(make-array 'ho 2 3) ⇒ #2((ho ho ho) (ho ho ho))
The range of each dimension can also be given explicitly, e.g.,
another way to create the same array:
(make-array 'ho '(0 1) '(0 2)) ⇒ #2((ho ho ho) (ho ho ho))
The following procedures can be used with arrays (or vectors). An
argument shown as IDX... means one parameter for each dimension in the
array. A IDXLIST argument means a list of such values, one for each
dimension.
-- Scheme Procedure: array? obj
-- C Function: scm_array_p (obj, unused)
Return ‘#t’ if the OBJ is an array, and ‘#f’ if not.
The second argument to scm_array_p is there for historical reasons,
but it is not used. You should always pass ‘SCM_UNDEFINED’ as its
value.
-- Scheme Procedure: typed-array? obj type
-- C Function: scm_typed_array_p (obj, type)
Return ‘#t’ if the OBJ is an array of type TYPE, and ‘#f’ if not.
-- C Function: int scm_is_array (SCM obj)
Return ‘1’ if the OBJ is an array and ‘0’ if not.
-- C Function: int scm_is_typed_array (SCM obj, SCM type)
Return ‘0’ if the OBJ is an array of type TYPE, and ‘1’ if not.
-- Scheme Procedure: make-array fill bound ...
-- C Function: scm_make_array (fill, bounds)
Equivalent to ‘(make-typed-array #t FILL BOUND ...)’.
-- Scheme Procedure: make-typed-array type fill bound ...
-- C Function: scm_make_typed_array (type, fill, bounds)
Create and return an array that has as many dimensions as there are
BOUNDs and (maybe) fill it with FILL.
The underlying storage vector is created according to TYPE, which
must be a symbol whose name is the ‘vectag’ of the array as
explained above, or ‘#t’ for ordinary, non-specialized arrays.
For example, using the symbol ‘f64’ for TYPE will create an array
that uses a ‘f64vector’ for storing its elements, and ‘a’ will use
a string.
When FILL is not the special _unspecified_ value, the new array is
filled with FILL. Otherwise, the initial contents of the array is
unspecified. The special _unspecified_ value is stored in the
variable ‘*unspecified*’ so that for example ‘(make-typed-array
'u32 *unspecified* 4)’ creates a uninitialized ‘u32’ vector of
length 4.
Each BOUND may be a positive non-zero integer N, in which case the
index for that dimension can range from 0 through N-1; or an
explicit index range specifier in the form ‘(LOWER UPPER)’, where
both LOWER and UPPER are integers, possibly less than zero, and
possibly the same number (however, LOWER cannot be greater than
UPPER).
-- Scheme Procedure: list->array dimspec list
Equivalent to ‘(list->typed-array #t DIMSPEC LIST)’.
-- Scheme Procedure: list->typed-array type dimspec list
-- C Function: scm_list_to_typed_array (type, dimspec, list)
Return an array of the type indicated by TYPE with elements the
same as those of LIST.
The argument DIMSPEC determines the number of dimensions of the
array and their lower bounds. When DIMSPEC is an exact integer, it
gives the number of dimensions directly and all lower bounds are
zero. When it is a list of exact integers, then each element is
the lower index bound of a dimension, and there will be as many
dimensions as elements in the list.
-- Scheme Procedure: array-type array
-- C Function: scm_array_type (array)
Return the type of ARRAY. This is the ‘vectag’ used for printing
ARRAY (or ‘#t’ for ordinary arrays) and can be used with
‘make-typed-array’ to create an array of the same kind as ARRAY.
-- Scheme Procedure: array-ref array idx ...
-- C Function: scm_array_ref (array, idxlist)
Return the element at ‘(idx ...)’ in ARRAY.
(define a (make-array 999 '(1 2) '(3 4)))
(array-ref a 2 4) ⇒ 999
-- Scheme Procedure: array-in-bounds? array idx ...
-- C Function: scm_array_in_bounds_p (array, idxlist)
Return ‘#t’ if the given indices would be acceptable to
‘array-ref’.
(define a (make-array #f '(1 2) '(3 4)))
(array-in-bounds? a 2 3) ⇒ #t
(array-in-bounds? a 0 0) ⇒ #f
-- Scheme Procedure: array-set! array obj idx ...
-- C Function: scm_array_set_x (array, obj, idxlist)
Set the element at ‘(idx ...)’ in ARRAY to OBJ. The return value
is unspecified.
(define a (make-array #f '(0 1) '(0 1)))
(array-set! a #t 1 1)
a ⇒ #2((#f #f) (#f #t))
-- Scheme Procedure: array-shape array
-- Scheme Procedure: array-dimensions array
-- C Function: scm_array_dimensions (array)
Return a list of the bounds for each dimension of ARRAY.
‘array-shape’ gives ‘(LOWER UPPER)’ for each dimension.
‘array-dimensions’ instead returns just UPPER+1 for dimensions with
a 0 lower bound. Both are suitable as input to ‘make-array’.
For example,
(define a (make-array 'foo '(-1 3) 5))
(array-shape a) ⇒ ((-1 3) (0 4))
(array-dimensions a) ⇒ ((-1 3) 5)
-- Scheme Procedure: array-length array
-- C Function: scm_array_length (array)
-- C Function: size_t scm_c_array_length (array)
Return the length of an array: its first dimension. It is an error
to ask for the length of an array of rank 0.
-- Scheme Procedure: array-rank array
-- C Function: scm_array_rank (array)
Return the rank of ARRAY.
-- C Function: size_t scm_c_array_rank (SCM array)
Return the rank of ARRAY as a ‘size_t’.
-- Scheme Procedure: array->list array
-- C Function: scm_array_to_list (array)
Return a list consisting of all the elements, in order, of ARRAY.
-- Scheme Procedure: array-copy! src dst
-- Scheme Procedure: array-copy-in-order! src dst
-- C Function: scm_array_copy_x (src, dst)
Copy every element from vector or array SRC to the corresponding
element of DST. DST must have the same rank as SRC, and be at
least as large in each dimension. The return value is unspecified.
-- Scheme Procedure: array-fill! array fill
-- C Function: scm_array_fill_x (array, fill)
Store FILL in every element of ARRAY. The value returned is
unspecified.
-- Scheme Procedure: array-equal? array ...
Return ‘#t’ if all arguments are arrays with the same shape, the
same type, and have corresponding elements which are either
‘equal?’ or ‘array-equal?’. This function differs from ‘equal?’
(*note Equality::) in that all arguments must be arrays.
-- Scheme Procedure: array-map! dst proc src ...
-- Scheme Procedure: array-map-in-order! dst proc src ...
-- C Function: scm_array_map_x (dst, proc, srclist)
Set each element of the DST array to values obtained from calls to
PROC. The list of SRC arguments may be empty. The value returned
is unspecified.
Each call is ‘(PROC ELEM ...)’, where each ELEM is from the
corresponding SRC array, at the DST index. ‘array-map-in-order!’
makes the calls in row-major order, ‘array-map!’ makes them in an
unspecified order.
The SRC arrays must have the same number of dimensions as DST, and
must have a range for each dimension which covers the range in DST.
This ensures all DST indices are valid in each SRC.
-- Scheme Procedure: array-for-each proc src1 src2 ...
-- C Function: scm_array_for_each (proc, src1, srclist)
Apply PROC to each tuple of elements of SRC1 SRC2 ..., in row-major
order. The value returned is unspecified.
-- Scheme Procedure: array-index-map! dst proc
-- C Function: scm_array_index_map_x (dst, proc)
Set each element of the DST array to values returned by calls to
PROC. The value returned is unspecified.
Each call is ‘(PROC I1 ... IN)’, where I1...IN is the destination
index, one parameter for each dimension. The order in which the
calls are made is unspecified.
For example, to create a 4x4 matrix representing a cyclic group,
/ 0 1 2 3 \
| 1 2 3 0 |
| 2 3 0 1 |
\ 3 0 1 2 /
(define a (make-array #f 4 4))
(array-index-map! a (lambda (i j)
(modulo (+ i j) 4)))
An additional array function is available in the module ‘(ice-9
arrays)’. It can be used with:
(use-modules (ice-9 arrays))
-- Scheme Procedure: array-copy src
Return a new array with the same elements, type and shape as SRC.
However, the array increments may not be the same as those of SRC.
In the current implementation, the returned array will be in
row-major order, but that might change in the future. Use
‘array-copy!’ on an array of known order if that is a concern.
6.6.13.3 Shared Arrays
......................
-- Scheme Procedure: make-shared-array oldarray mapfunc bound ...
-- C Function: scm_make_shared_array (oldarray, mapfunc, boundlist)
Return a new array which shares the storage of OLDARRAY. Changes
made through either affect the same underlying storage. The BOUND
... arguments are the shape of the new array, the same as
‘make-array’ (*note Array Procedures::).
MAPFUNC translates coordinates from the new array to the OLDARRAY.
It’s called as ‘(MAPFUNC newidx1 ...)’ with one parameter for each
dimension of the new array, and should return a list of indices for
OLDARRAY, one for each dimension of OLDARRAY.
MAPFUNC must be affine linear, meaning that each OLDARRAY index
must be formed by adding integer multiples (possibly negative) of
some or all of NEWIDX1 etc, plus a possible integer offset. The
multiples and offset must be the same in each call.
One good use for a shared array is to restrict the range of some
dimensions, so as to apply say ‘array-for-each’ or ‘array-fill!’ to
only part of an array. The plain ‘list’ function can be used for
MAPFUNC in this case, making no changes to the index values. For
example,
(make-shared-array #2((a b c) (d e f) (g h i)) list 3 2)
⇒ #2((a b) (d e) (g h))
The new array can have fewer dimensions than OLDARRAY, for example
to take a column from an array.
(make-shared-array #2((a b c) (d e f) (g h i))
(lambda (i) (list i 2))
'(0 2))
⇒ #1(c f i)
A diagonal can be taken by using the single new array index for
both row and column in the old array. For example,
(make-shared-array #2((a b c) (d e f) (g h i))
(lambda (i) (list i i))
'(0 2))
⇒ #1(a e i)
Dimensions can be increased by for instance considering portions of
a one dimensional array as rows in a two dimensional array.
(‘array-contents’ below can do the opposite, flattening an array.)
(make-shared-array #1(a b c d e f g h i j k l)
(lambda (i j) (list (+ (* i 3) j)))
4 3)
⇒ #2((a b c) (d e f) (g h i) (j k l))
By negating an index the order that elements appear can be
reversed. The following just reverses the column order,
(make-shared-array #2((a b c) (d e f) (g h i))
(lambda (i j) (list i (- 2 j)))
3 3)
⇒ #2((c b a) (f e d) (i h g))
A fixed offset on indexes allows for instance a change from a 0
based to a 1 based array,
(define x #2((a b c) (d e f) (g h i)))
(define y (make-shared-array x
(lambda (i j) (list (1- i) (1- j)))
'(1 3) '(1 3)))
(array-ref x 0 0) ⇒ a
(array-ref y 1 1) ⇒ a
A multiple on an index allows every Nth element of an array to be
taken. The following is every third element,
(make-shared-array #1(a b c d e f g h i j k l)
(lambda (i) (list (* i 3)))
4)
⇒ #1(a d g j)
The above examples can be combined to make weird and wonderful
selections from an array, but it’s important to note that because
MAPFUNC must be affine linear, arbitrary permutations are not
possible.
In the current implementation, MAPFUNC is not called for every
access to the new array but only on some sample points to establish
a base and stride for new array indices in OLDARRAY data. A few
sample points are enough because MAPFUNC is linear.
-- Scheme Procedure: shared-array-increments array
-- C Function: scm_shared_array_increments (array)
For each dimension, return the distance between elements in the
root vector.
-- Scheme Procedure: shared-array-offset array
-- C Function: scm_shared_array_offset (array)
Return the root vector index of the first element in the array.
-- Scheme Procedure: shared-array-root array
-- C Function: scm_shared_array_root (array)
Return the root vector of a shared array.
-- Scheme Procedure: array-contents array [strict]
-- C Function: scm_array_contents (array, strict)
If ARRAY may be “unrolled” into a one dimensional shared array
without changing their order (last subscript changing fastest),
then ‘array-contents’ returns that shared array, otherwise it
returns ‘#f’. All arrays made by ‘make-array’ and
‘make-typed-array’ may be unrolled, some arrays made by
‘make-shared-array’ may not be.
If the optional argument STRICT is provided, a shared array will be
returned only if its elements are stored internally contiguous in
memory.
-- Scheme Procedure: transpose-array array dim1 dim2 ...
-- C Function: scm_transpose_array (array, dimlist)
Return an array sharing contents with ARRAY, but with dimensions
arranged in a different order. There must be one DIM argument for
each dimension of ARRAY. DIM1, DIM2, ... should be integers
between 0 and the rank of the array to be returned. Each integer
in that range must appear at least once in the argument list.
The values of DIM1, DIM2, ... correspond to dimensions in the array
to be returned, and their positions in the argument list to
dimensions of ARRAY. Several DIMs may have the same value, in
which case the returned array will have smaller rank than ARRAY.
(transpose-array '#2((a b) (c d)) 1 0) ⇒ #2((a c) (b d))
(transpose-array '#2((a b) (c d)) 0 0) ⇒ #1(a d)
(transpose-array '#3(((a b c) (d e f)) ((1 2 3) (4 5 6))) 1 1 0) ⇒
#2((a 4) (b 5) (c 6))
6.6.13.4 Arrays as arrays of arrays
...................................
One can see an array of rank n (an n-array) as an array of lower rank
where the elements are themselves arrays (‘cells’).
We speak of the first n-k dimensions of the array as the n-k-‘frame’
of the array, while the last k dimensions are the dimensions of the
k-‘cells’. For example, a 3-array can be seen as a 2-array of vectors
(1-arrays) or as a 1-array of matrices (2-arrays). In each case, the
vectors or matrices are the 1-cells or 2-cells of the array. This
terminology originates in the J language.
The more vague concept of a ‘slice’ refers to a subset of the array
where some indices are fixed and others are left free. As a Guile data
object, a cell is the same as a ‘prefix slice’ (the first n-k indices
into the original array are fixed), except that a 0-cell is not a shared
array of the original array, but a 0-slice (where all the indices into
the original array are fixed) is.
Before version 2.0, Guile had a feature called ‘enclosed arrays’ to
create special ‘array of arrays’ objects. The functions in this section
do not need special types; instead, the frame rank is stated in each
function call, either implicitly or explicitly.
-- Scheme Procedure: array-cell-ref array idx ...
-- C Function: scm_array_cell_ref (array, idxlist)
If the length of IDXLIST equals the rank n of ARRAY, return the
element at ‘(idx ...)’, just like ‘(array-ref array idx ...)’. If,
however, the length k of IDXLIST is smaller than n, then return the
(n-k)-cell of ARRAY given by IDXLIST, as a shared array.
For example:
(array-cell-ref #2((a b) (c d)) 0) ⇒ #(a b)
(array-cell-ref #2((a b) (c d)) 1) ⇒ #(c d)
(array-cell-ref #2((a b) (c d)) 1 1) ⇒ d
(array-cell-ref #2((a b) (c d))) ⇒ #2((a b) (c d))
‘(apply array-cell-ref array indices)’ is equivalent to
(let ((len (length indices)))
(if (= (array-rank a) len)
(apply array-ref a indices)
(apply make-shared-array a
(lambda t (append indices t))
(drop (array-dimensions a) len))))
-- Scheme Procedure: array-slice array idx ...
-- C Function: scm_array_slice (array, idxlist)
Like ‘(array-cell-ref array idx ...)’, but return a 0-rank shared
array into ARRAY if the length of IDXLIST matches the rank of
ARRAY. This can be useful when using ARRAY as a place to write to.
Compare:
(array-cell-ref #2((a b) (c d)) 1 1) ⇒ d
(array-slice #2((a b) (c d)) 1 1) ⇒ #0(d)
(define a (make-array 'a 2 2))
(array-fill! (array-slice a 1 1) 'b)
a ⇒ #2((a a) (a b)).
(array-fill! (array-cell-ref a 1 1) 'b) ⇒ error: not an array
‘(apply array-slice array indices)’ is equivalent to
(apply make-shared-array a
(lambda t (append indices t))
(drop (array-dimensions a) (length indices)))
-- Scheme Procedure: array-cell-set! array x idx ...
-- C Function: scm_array_cell_set_x (array, x, idxlist)
If the length of IDXLIST equals the rank n of ARRAY, set the
element at ‘(idx ...)’ of ARRAY to X, just like ‘(array-set! array
x idx ...)’. If, however, the length k of IDXLIST is smaller than
n, then copy the (n-k)-rank array X into the (n-k)-cell of ARRAY
given by IDXLIST. In this case, the last (n-k) dimensions of ARRAY
and the dimensions of X must match exactly.
This function returns the modified ARRAY.
For example:
(array-cell-set! (make-array 'a 2 2) b 1 1)
⇒ #2((a a) (a b))
(array-cell-set! (make-array 'a 2 2) #(x y) 1)
⇒ #2((a a) (x y))
Note that ‘array-cell-set!’ will expect elements, not arrays, when
the destination has rank 0. Use ‘array-slice’ for the opposite
behavior.
(array-cell-set! (make-array 'a 2 2) #0(b) 1 1)
⇒ #2((a a) (a #0(b)))
(let ((a (make-array 'a 2 2)))
(array-copy! #0(b) (array-slice a 1 1)) a)
⇒ #2((a a) (a b))
‘(apply array-cell-set! array x indices)’ is equivalent to
(let ((len (length indices)))
(if (= (array-rank array) len)
(apply array-set! array x indices)
(array-copy! x (apply array-cell-ref array indices)))
array)
-- Scheme Procedure: array-slice-for-each frame-rank op x ...
-- C Function: scm_array_slice_for_each (array, frame_rank, op, xlist)
Each X must be an array of rank ≥ FRAME-RANK, and the first
FRAME-RANK dimensions of each X must all be the same.
ARRAY-SLICE-FOR-EACH calls OP with each set of (rank(X) -
FRAME-RANK)-cells from X, in unspecified order.
ARRAY-SLICE-FOR-EACH allows you to loop over cells of any rank
without having to carry an index list or construct shared arrays
manually. The slices passed to OP are always shared arrays of X,
even if they are of rank 0, so it is possible to write to them.
This function returns an unspecified value.
For example, to sort the rows of rank-2 array ‘a’:
(array-slice-for-each 1 (lambda (x) (sort! x <)) a)
As another example, let ‘a’ be a rank-2 array where each row is a
2-element vector (x,y). Let’s compute the arguments of these
vectors and store them in rank-1 array ‘b’.
(array-slice-for-each 1
(lambda (a b)
(array-set! b (atan (array-ref a 1) (array-ref a 0))))
a b)
‘(apply array-slice-for-each frame-rank op x)’ is equivalent to
(let ((frame (take (array-dimensions (car x)) frank)))
(unless (every (lambda (x)
(equal? frame (take (array-dimensions x) frank)))
(cdr x))
(error))
(array-index-map!
(apply make-shared-array (make-array #t) (const '()) frame)
(lambda i (apply op (map (lambda (x) (apply array-slice x i)) x)))))
-- Scheme Procedure: array-slice-for-each-in-order frame-rank op x ...
-- C Function: scm_array_slice_for_each_in_order (array, frame_rank,
op, xlist)
Same as ‘array-slice-for-each’, but the arguments are traversed
sequentially and in row-major order.
6.6.13.5 Accessing Arrays from C
................................
For interworking with external C code, Guile provides an API to allow C
code to access the elements of a Scheme array. In particular, for
uniform numeric arrays, the API exposes the underlying uniform data as a
C array of numbers of the relevant type.
While pointers to the elements of an array are in use, the array
itself must be protected so that the pointer remains valid. Such a
protected array is said to be “reserved”. A reserved array can be read
but modifications to it that would cause the pointer to its elements to
become invalid are prevented. When you attempt such a modification, an
error is signalled.
(This is similar to locking the array while it is in use, but without
the danger of a deadlock. In a multi-threaded program, you will need
additional synchronization to avoid modifying reserved arrays.)
You must take care to always unreserve an array after reserving it,
even in the presence of non-local exits. If a non-local exit can happen
between these two calls, you should install a dynwind context that
releases the array when it is left (*note Dynamic Wind::).
In addition, array reserving and unreserving must be properly paired.
For instance, when reserving two or more arrays in a certain order, you
need to unreserve them in the opposite order.
Once you have reserved an array and have retrieved the pointer to its
elements, you must figure out the layout of the elements in memory.
Guile allows slices to be taken out of arrays without actually making a
copy, such as making an alias for the diagonal of a matrix that can be
treated as a vector. Arrays that result from such an operation are not
stored contiguously in memory and when working with their elements
directly, you need to take this into account.
The layout of array elements in memory can be defined via a _mapping
function_ that computes a scalar position from a vector of indices. The
scalar position then is the offset of the element with the given indices
from the start of the storage block of the array.
In Guile, this mapping function is restricted to be “affine”: all
mapping functions of Guile arrays can be written as ‘p = b + c[0]*i[0] +
c[1]*i[1] + ... + c[n-1]*i[n-1]’ where ‘i[k]’ is the kth index and ‘n’
is the rank of the array. For example, a matrix of size 3x3 would have
‘b == 0’, ‘c[0] == 3’ and ‘c[1] == 1’. When you transpose this matrix
(with ‘transpose-array’, say), you will get an array whose mapping
function has ‘b == 0’, ‘c[0] == 1’ and ‘c[1] == 3’.
The function ‘scm_array_handle_dims’ gives you (indirect) access to
the coefficients ‘c[k]’.
Note that there are no functions for accessing the elements of a
character array yet. Once the string implementation of Guile has been
changed to use Unicode, we will provide them.
-- C Type: scm_t_array_handle
This is a structure type that holds all information necessary to
manage the reservation of arrays as explained above. Structures of
this type must be allocated on the stack and must only be accessed
by the functions listed below.
-- C Function: void scm_array_get_handle (SCM array, scm_t_array_handle
*handle)
Reserve ARRAY, which must be an array, and prepare HANDLE to be
used with the functions below. You must eventually call
‘scm_array_handle_release’ on HANDLE, and do this in a properly
nested fashion, as explained above. The structure pointed to by
HANDLE does not need to be initialized before calling this
function.
-- C Function: void scm_array_handle_release (scm_t_array_handle
*handle)
End the array reservation represented by HANDLE. After a call to
this function, HANDLE might be used for another reservation.
-- C Function: size_t scm_array_handle_rank (scm_t_array_handle
*handle)
Return the rank of the array represented by HANDLE.
-- C Type: scm_t_array_dim
This structure type holds information about the layout of one
dimension of an array. It includes the following fields:
‘ssize_t lbnd’
‘ssize_t ubnd’
The lower and upper bounds (both inclusive) of the permissible
index range for the given dimension. Both values can be
negative, but LBND is always less than or equal to UBND.
‘ssize_t inc’
The distance from one element of this dimension to the next.
Note, too, that this can be negative.
-- C Function: const scm_t_array_dim * scm_array_handle_dims
(scm_t_array_handle *handle)
Return a pointer to a C vector of information about the dimensions
of the array represented by HANDLE. This pointer is valid as long
as the array remains reserved. As explained above, the
‘scm_t_array_dim’ structures returned by this function can be used
calculate the position of an element in the storage block of the
array from its indices.
This position can then be used as an index into the C array pointer
returned by the various ‘scm_array_handle__elements’
functions, or with ‘scm_array_handle_ref’ and
‘scm_array_handle_set’.
Here is how one can compute the position POS of an element given
its indices in the vector INDICES:
ssize_t indices[RANK];
scm_t_array_dim *dims;
ssize_t pos;
size_t i;
pos = 0;
for (i = 0; i < RANK; i++)
{
if (indices[i] < dims[i].lbnd || indices[i] > dims[i].ubnd)
out_of_range ();
pos += (indices[i] - dims[i].lbnd) * dims[i].inc;
}
-- C Function: ssize_t scm_array_handle_pos (scm_t_array_handle
*handle, SCM indices)
Compute the position corresponding to INDICES, a list of indices.
The position is computed as described above for
‘scm_array_handle_dims’. The number of the indices and their range
is checked and an appropriate error is signalled for invalid
indices.
-- C Function: SCM scm_array_handle_ref (scm_t_array_handle *handle,
ssize_t pos)
Return the element at position POS in the storage block of the
array represented by HANDLE. Any kind of array is acceptable. No
range checking is done on POS.
-- C Function: void scm_array_handle_set (scm_t_array_handle *handle,
ssize_t pos, SCM val)
Set the element at position POS in the storage block of the array
represented by HANDLE to VAL. Any kind of array is acceptable. No
range checking is done on POS. An error is signalled when the
array can not store VAL.
-- C Function: const SCM * scm_array_handle_elements
(scm_t_array_handle *handle)
Return a pointer to the elements of a ordinary array of general
Scheme values (i.e., a non-uniform array) for reading. This
pointer is valid as long as the array remains reserved.
-- C Function: SCM * scm_array_handle_writable_elements
(scm_t_array_handle *handle)
Like ‘scm_array_handle_elements’, but the pointer is good for
reading and writing.
-- C Function: const void * scm_array_handle_uniform_elements
(scm_t_array_handle *handle)
Return a pointer to the elements of a uniform numeric array for
reading. This pointer is valid as long as the array remains
reserved. The size of each element is given by
‘scm_array_handle_uniform_element_size’.
-- C Function: void * scm_array_handle_uniform_writable_elements
(scm_t_array_handle *handle)
Like ‘scm_array_handle_uniform_elements’, but the pointer is good
reading and writing.
-- C Function: size_t scm_array_handle_uniform_element_size
(scm_t_array_handle *handle)
Return the size of one element of the uniform numeric array
represented by HANDLE.
-- C Function: const scm_t_uint8 * scm_array_handle_u8_elements
(scm_t_array_handle *handle)
-- C Function: const scm_t_int8 * scm_array_handle_s8_elements
(scm_t_array_handle *handle)
-- C Function: const scm_t_uint16 * scm_array_handle_u16_elements
(scm_t_array_handle *handle)
-- C Function: const scm_t_int16 * scm_array_handle_s16_elements
(scm_t_array_handle *handle)
-- C Function: const scm_t_uint32 * scm_array_handle_u32_elements
(scm_t_array_handle *handle)
-- C Function: const scm_t_int32 * scm_array_handle_s32_elements
(scm_t_array_handle *handle)
-- C Function: const scm_t_uint64 * scm_array_handle_u64_elements
(scm_t_array_handle *handle)
-- C Function: const scm_t_int64 * scm_array_handle_s64_elements
(scm_t_array_handle *handle)
-- C Function: const float * scm_array_handle_f32_elements
(scm_t_array_handle *handle)
-- C Function: const double * scm_array_handle_f64_elements
(scm_t_array_handle *handle)
-- C Function: const float * scm_array_handle_c32_elements
(scm_t_array_handle *handle)
-- C Function: const double * scm_array_handle_c64_elements
(scm_t_array_handle *handle)
Return a pointer to the elements of a uniform numeric array of the
indicated kind for reading. This pointer is valid as long as the
array remains reserved.
The pointers for ‘c32’ and ‘c64’ uniform numeric arrays point to
pairs of floating point numbers. The even index holds the real
part, the odd index the imaginary part of the complex number.
-- C Function: scm_t_uint8 * scm_array_handle_u8_writable_elements
(scm_t_array_handle *handle)
-- C Function: scm_t_int8 * scm_array_handle_s8_writable_elements
(scm_t_array_handle *handle)
-- C Function: scm_t_uint16 * scm_array_handle_u16_writable_elements
(scm_t_array_handle *handle)
-- C Function: scm_t_int16 * scm_array_handle_s16_writable_elements
(scm_t_array_handle *handle)
-- C Function: scm_t_uint32 * scm_array_handle_u32_writable_elements
(scm_t_array_handle *handle)
-- C Function: scm_t_int32 * scm_array_handle_s32_writable_elements
(scm_t_array_handle *handle)
-- C Function: scm_t_uint64 * scm_array_handle_u64_writable_elements
(scm_t_array_handle *handle)
-- C Function: scm_t_int64 * scm_array_handle_s64_writable_elements
(scm_t_array_handle *handle)
-- C Function: float * scm_array_handle_f32_writable_elements
(scm_t_array_handle *handle)
-- C Function: double * scm_array_handle_f64_writable_elements
(scm_t_array_handle *handle)
-- C Function: float * scm_array_handle_c32_writable_elements
(scm_t_array_handle *handle)
-- C Function: double * scm_array_handle_c64_writable_elements
(scm_t_array_handle *handle)
Like ‘scm_array_handle__elements’, but the pointer is good
for reading and writing.
-- C Function: const scm_t_uint32 * scm_array_handle_bit_elements
(scm_t_array_handle *handle)
Return a pointer to the words that store the bits of the
represented array, which must be a bit array.
Unlike other arrays, bit arrays have an additional offset that must
be figured into index calculations. That offset is returned by
‘scm_array_handle_bit_elements_offset’.
To find a certain bit you first need to calculate its position as
explained above for ‘scm_array_handle_dims’ and then add the
offset. This gives the absolute position of the bit, which is
always a non-negative integer.
Each word of the bit array storage block contains exactly 32 bits,
with the least significant bit in that word having the lowest
absolute position number. The next word contains the next 32 bits.
Thus, the following code can be used to access a bit whose position
according to ‘scm_array_handle_dims’ is given in POS:
SCM bit_array;
scm_t_array_handle handle;
scm_t_uint32 *bits;
ssize_t pos;
size_t abs_pos;
size_t word_pos, mask;
scm_array_get_handle (&bit_array, &handle);
bits = scm_array_handle_bit_elements (&handle);
pos = ...
abs_pos = pos + scm_array_handle_bit_elements_offset (&handle);
word_pos = abs_pos / 32;
mask = 1L << (abs_pos % 32);
if (bits[word_pos] & mask)
/* bit is set. */
scm_array_handle_release (&handle);
-- C Function: scm_t_uint32 * scm_array_handle_bit_writable_elements
(scm_t_array_handle *handle)
Like ‘scm_array_handle_bit_elements’ but the pointer is good for
reading and writing. You must take care not to modify bits outside
of the allowed index range of the array, even for contiguous
arrays.
6.6.14 VLists
-------------
The ‘(ice-9 vlist)’ module provides an implementation of the “VList”
data structure designed by Phil Bagwell in 2002. VLists are immutable
lists, which can contain any Scheme object. They improve on standard
Scheme linked lists in several areas:
• Random access has typically constant-time complexity.
• Computing the length of a VList has time complexity logarithmic in
the number of elements.
• VLists use less storage space than standard lists.
• VList elements are stored in contiguous regions, which improves
memory locality and leads to more efficient use of hardware caches.
The idea behind VLists is to store vlist elements in increasingly
large contiguous blocks (implemented as vectors here). These blocks are
linked to one another using a pointer to the next block and an offset
within that block. The size of these blocks form a geometric series
with ratio ‘block-growth-factor’ (2 by default).
The VList structure also serves as the basis for the “VList-based
hash lists” or “vhashes”, an immutable dictionary type (*note
VHashes::).
However, the current implementation in ‘(ice-9 vlist)’ has several
noteworthy shortcomings:
• It is _not_ thread-safe. Although operations on vlists are all
“referentially transparent” (i.e., purely functional), adding
elements to a vlist with ‘vlist-cons’ mutates part of its internal
structure, which makes it non-thread-safe. This could be fixed,
but it would slow down ‘vlist-cons’.
• ‘vlist-cons’ always allocates at least as much memory as ‘cons’.
Again, Phil Bagwell describes how to fix it, but that would require
tuning the garbage collector in a way that may not be generally
beneficial.
• ‘vlist-cons’ is a Scheme procedure compiled to bytecode, and it
does not compete with the straightforward C implementation of
‘cons’, and with the fact that the VM has a special ‘cons’
instruction.
We hope to address these in the future.
The programming interface exported by ‘(ice-9 vlist)’ is defined
below. Most of it is the same as SRFI-1 with an added ‘vlist-’ prefix
to function names.
-- Scheme Procedure: vlist? obj
Return true if OBJ is a VList.
-- Scheme Variable: vlist-null
The empty VList. Note that it’s possible to create an empty VList
not ‘eq?’ to ‘vlist-null’; thus, callers should always use
‘vlist-null?’ when testing whether a VList is empty.
-- Scheme Procedure: vlist-null? vlist
Return true if VLIST is empty.
-- Scheme Procedure: vlist-cons item vlist
Return a new vlist with ITEM as its head and VLIST as its tail.
-- Scheme Procedure: vlist-head vlist
Return the head of VLIST.
-- Scheme Procedure: vlist-tail vlist
Return the tail of VLIST.
-- Scheme Variable: block-growth-factor
A fluid that defines the growth factor of VList blocks, 2 by
default.
The functions below provide the usual set of higher-level list
operations.
-- Scheme Procedure: vlist-fold proc init vlist
-- Scheme Procedure: vlist-fold-right proc init vlist
Fold over VLIST, calling PROC for each element, as for SRFI-1
‘fold’ and ‘fold-right’ (*note ‘fold’: SRFI-1.).
-- Scheme Procedure: vlist-ref vlist index
Return the element at index INDEX in VLIST. This is typically a
constant-time operation.
-- Scheme Procedure: vlist-length vlist
Return the length of VLIST. This is typically logarithmic in the
number of elements in VLIST.
-- Scheme Procedure: vlist-reverse vlist
Return a new VLIST whose content are those of VLIST in reverse
order.
-- Scheme Procedure: vlist-map proc vlist
Map PROC over the elements of VLIST and return a new vlist.
-- Scheme Procedure: vlist-for-each proc vlist
Call PROC on each element of VLIST. The result is unspecified.
-- Scheme Procedure: vlist-drop vlist count
Return a new vlist that does not contain the COUNT first elements
of VLIST. This is typically a constant-time operation.
-- Scheme Procedure: vlist-take vlist count
Return a new vlist that contains only the COUNT first elements of
VLIST.
-- Scheme Procedure: vlist-filter pred vlist
Return a new vlist containing all the elements from VLIST that
satisfy PRED.
-- Scheme Procedure: vlist-delete x vlist [equal?]
Return a new vlist corresponding to VLIST without the elements
EQUAL? to X.
-- Scheme Procedure: vlist-unfold p f g seed [tail-gen]
-- Scheme Procedure: vlist-unfold-right p f g seed [tail]
Return a new vlist, as for SRFI-1 ‘unfold’ and ‘unfold-right’
(*note ‘unfold’: SRFI-1.).
-- Scheme Procedure: vlist-append vlist ...
Append the given vlists and return the resulting vlist.
-- Scheme Procedure: list->vlist lst
Return a new vlist whose contents correspond to LST.
-- Scheme Procedure: vlist->list vlist
Return a new list whose contents match those of VLIST.
6.6.15 Record Overview
----------------------
“Records”, also called “structures”, are Scheme’s primary mechanism to
define new disjoint types. A “record type” defines a list of “fields”
that instances of the type consist of. This is like C’s ‘struct’.
Historically, Guile has offered several different ways to define
record types and to create records, offering different features, and
making different trade-offs. Over the years, each “standard” has also
come with its own new record interface, leading to a maze of record
APIs.
At the highest level is SRFI-9, a high-level record interface
implemented by most Scheme implementations (*note SRFI-9 Records::). It
defines a simple and efficient syntactic abstraction of record types and
their associated type predicate, fields, and field accessors. SRFI-9 is
suitable for most uses, and this is the recommended way to create record
types in Guile. Similar high-level record APIs include SRFI-35 (*note
SRFI-35::) and R6RS records (*note rnrs records syntactic::).
Then comes Guile’s historical “records” API (*note Records::).
Record types defined this way are first-class objects. Introspection
facilities are available, allowing users to query the list of fields or
the value of a specific field at run-time, without prior knowledge of
the type.
Finally, the common denominator of these interfaces is Guile’s
“structure” API (*note Structures::). Guile’s structures are the
low-level building block for all other record APIs. Application writers
will normally not need to use it.
Records created with these APIs may all be pattern-matched using
Guile’s standard pattern matcher (*note Pattern Matching::).
6.6.16 SRFI-9 Records
---------------------
SRFI-9 standardizes a syntax for defining new record types and creating
predicate, constructor, and field getter and setter functions. In Guile
this is the recommended option to create new record types (*note Record
Overview::). It can be used with:
(use-modules (srfi srfi-9))
-- Scheme Syntax: define-record-type type
(constructor fieldname ...)
predicate
(fieldname accessor [modifier]) ...
Create a new record type, and make various ‘define’s for using it.
This syntax can only occur at the top-level, not nested within some
other form.
TYPE is bound to the record type, which is as per the return from
the core ‘make-record-type’. TYPE also provides the name for the
record, as per ‘record-type-name’.
CONSTRUCTOR is bound to a function to be called as ‘(CONSTRUCTOR
fieldval ...)’ to create a new record of this type. The arguments
are initial values for the fields, one argument for each field, in
the order they appear in the ‘define-record-type’ form.
The FIELDNAMEs provide the names for the record fields, as per the
core ‘record-type-fields’ etc, and are referred to in the
subsequent accessor/modifier forms.
PREDICATE is bound to a function to be called as ‘(PREDICATE obj)’.
It returns ‘#t’ or ‘#f’ according to whether OBJ is a record of
this type.
Each ACCESSOR is bound to a function to be called ‘(ACCESSOR
record)’ to retrieve the respective field from a RECORD. Similarly
each MODIFIER is bound to a function to be called ‘(MODIFIER record
val)’ to set the respective field in a RECORD.
An example will illustrate typical usage,
(define-record-type
(make-employee name age salary)
employee?
(name employee-name)
(age employee-age set-employee-age!)
(salary employee-salary set-employee-salary!))
This creates a new employee data type, with name, age and salary
fields. Accessor functions are created for each field, but no modifier
function for the name (the intention in this example being that it’s
established only when an employee object is created). These can all
then be used as for example,
⇒ #>
(define fred (make-employee "Fred" 45 20000.00))
(employee? fred) ⇒ #t
(employee-age fred) ⇒ 45
(set-employee-salary! fred 25000.00) ;; pay rise
The functions created by ‘define-record-type’ are ordinary top-level
‘define’s. They can be redefined or ‘set!’ as desired, exported from a
module, etc.
Non-toplevel Record Definitions
...............................
The SRFI-9 specification explicitly disallows record definitions in a
non-toplevel context, such as inside ‘lambda’ body or inside a LET
block. However, Guile’s implementation does not enforce that
restriction.
Custom Printers
...............
You may use ‘set-record-type-printer!’ to customize the default printing
behavior of records. This is a Guile extension and is not part of
SRFI-9. It is located in the (srfi srfi-9 gnu) module.
-- Scheme Syntax: set-record-type-printer! type proc
Where TYPE corresponds to the first argument of
‘define-record-type’, and PROC is a procedure accepting two
arguments, the record to print, and an output port.
This example prints the employee’s name in brackets, for instance
‘[Fred]’.
(set-record-type-printer!
(lambda (record port)
(write-char #\[ port)
(display (employee-name record) port)
(write-char #\] port)))
Functional “Setters”
....................
When writing code in a functional style, it is desirable to never alter
the contents of records. For such code, a simple way to return new
record instances based on existing ones is highly desirable.
The ‘(srfi srfi-9 gnu)’ module extends SRFI-9 with facilities to
return new record instances based on existing ones, only with one or
more field values changed—“functional setters”. First, the
‘define-immutable-record-type’ works like ‘define-record-type’, except
that fields are immutable and setters are defined as functional setters.
-- Scheme Syntax: define-immutable-record-type type
(constructor fieldname ...)
predicate
(fieldname accessor [modifier]) ...
Define TYPE as a new record type, like ‘define-record-type’.
However, the record type is made _immutable_ (records may not be
mutated, even with ‘struct-set!’), and any MODIFIER is defined to
be a functional setter—a procedure that returns a new record
instance with the specified field changed, and leaves the original
unchanged (see example below.)
In addition, the generic ‘set-field’ and ‘set-fields’ macros may be
applied to any SRFI-9 record.
-- Scheme Syntax: set-field record (field sub-fields ...) value
Return a new record of RECORD’s type whose fields are equal to the
corresponding fields of RECORD except for the one specified by
FIELD.
FIELD must be the name of the getter corresponding to the field of
RECORD being “set”. Subsequent SUB-FIELDS must be record getters
designating sub-fields within that field value to be set (see
example below.)
-- Scheme Syntax: set-fields record ((field sub-fields ...) value) ...
Like ‘set-field’, but can be used to set more than one field at a
time. This expands to code that is more efficient than a series of
single ‘set-field’ calls.
To illustrate the use of functional setters, let’s assume these two
record type definitions:
(define-record-type
(address street city country)
address?
(street address-street)
(city address-city)
(country address-country))
(define-immutable-record-type
(person age email address)
person?
(age person-age set-person-age)
(email person-email set-person-email)
(address person-address set-person-address))
First, note that the ‘’ record type definition introduces named
functional setters. These may be used like this:
(define fsf-address
(address "Franklin Street" "Boston" "USA"))
(define rms
(person 30 "rms@gnu.org" fsf-address))
(and (equal? (set-person-age rms 60)
(person 60 "rms@gnu.org" fsf-address))
(= (person-age rms) 30))
⇒ #t
Here, the original ‘’ record, to which RMS is bound, is left
unchanged.
Now, suppose we want to change both the street and age of RMS. This
can be achieved using ‘set-fields’:
(set-fields rms
((person-age) 60)
((person-address address-street) "Temple Place"))
⇒ #< age: 60 email: "rms@gnu.org"
address: #< street: "Temple Place" city: "Boston" country: "USA">>
Notice how the above changed two fields of RMS, including the ‘street’
field of its ‘address’ field, in a concise way. Also note that
‘set-fields’ works equally well for types defined with just
‘define-record-type’.
6.6.17 Records
--------------
A “record type” is a first class object representing a user-defined data
type. A “record” is an instance of a record type.
Note that in many ways, this interface is too low-level for every-day
use. Most uses of records are better served by SRFI-9 records. *Note
SRFI-9 Records::.
-- Scheme Procedure: record? obj
Return ‘#t’ if OBJ is a record of any type and ‘#f’ otherwise.
Note that ‘record?’ may be true of any Scheme value; there is no
promise that records are disjoint with other Scheme types.
-- Scheme Procedure: make-record-type type-name field-names [print]
[#:parent=#f] [#:uid=#f] [#:extensible?=#f] [#:opaque?=#f]
[#:allow-duplicate-field-names?=#t]
Create and return a new “record-type descriptor”.
TYPE-NAME is a string naming the type. Currently it’s only used in
the printed representation of records, and in diagnostics.
FIELD-NAMES is a list of elements of the form ‘(immutable NAME)’,
‘(mutable NAME)’, or NAME, where NAME are symbols naming the fields
of a record of the type. Duplicates are not allowed among these
symbols, unless ALLOW-DUPLICATE-FIELD-NAMES? is true.
(make-record-type "employee" '(name age salary))
The optional PRINT argument is a function used by ‘display’,
‘write’, etc, for printing a record of the new type. It’s called
as ‘(PRINT record port)’ and should look at RECORD and write to
PORT.
Pass the ‘#:parent’ keyword to derive a record type from a
supertype. A derived record type has the fields from its parent
type, followed by fields declared in the ‘make-record-type’ call.
Record predicates and field accessors for instance of a parent type
will also work on any instance of a subtype.
Allowing record subtyping has a small amount of overhead. To avoid
this overhead, prevent extensibility by passing ‘#:extensible? #f’.
By default, record types in Guile are not extensible.
Generally speaking, calling ‘make-record-type’ returns a fresh
record type; it _generates_ new record types. However sometimes
you only want to define a record type if one hasn’t been defined
already. For a _nongenerative_ record type definition, pass a
symbol as the ‘#:uid’ keyword parameter. If a record with the
given UID was already defined, it will be returned instead. The
type name, fields, parent (if any), and so on for the
previously-defined type must be compatible.
R6RS defines a notion of “opaque” record types. Given an instance
of an opaque record type, one cannot obtain a run-time
representation of the record type. *Note rnrs records
procedural::, for full details. The ‘#:opaque?’ flag is used by
Guile’s R6RS layer to record this information. The default is
determined by whether the parent type, if any, was opaque.
Fields are mutable by default, meaning that ‘record-modifier’ will
return a procedure that can update a record in place. Specifying a
field using the form ‘(immutable NAME)’ instead marks a field as
immutable.
-- Scheme Procedure: record-constructor rtd
Return a procedure for constructing new members of the type
represented by RTD. The result will be a procedure accepting
exactly as many arguments as there are fields in the record type.
-- Scheme Procedure: record-predicate rtd
Return a procedure for testing membership in the type represented
by RTD. The returned procedure accepts exactly one argument and
returns a true value if the argument is a member of the indicated
record type; it returns a false value otherwise.
-- Scheme Procedure: record-accessor rtd field-name
Return a procedure for reading the value of a particular field of a
member of the type represented by RTD. The returned procedure
accepts exactly one argument which must be a record of the
appropriate type; it returns the current value of the field named
by the symbol FIELD-NAME in that record.
If FIELD-NAME is a symbol, it must be a member of the list of
field-names in the call to ‘make-record-type’ that created the type
represented by RTD. If multiple fields in RTD have the same name,
‘record-accessor’ returns the first one.
If FIELD-NAME is an integer, it should be an index into
‘(record-type-fields RTD)’. This allows accessing fields with
duplicate names.
-- Scheme Procedure: record-modifier rtd field-name
Return a procedure for writing the value of a particular field of a
member of the type represented by RTD. The returned procedure
accepts exactly two arguments: first, a record of the appropriate
type, and second, an arbitrary Scheme value; it modifies the field
named by the symbol FIELD-NAME in that record to contain the given
value. The returned value of the modifier procedure is
unspecified. The symbol FIELD-NAME is a field name or a field
index, as in ‘record-modifier’.
-- Scheme Procedure: record-type-descriptor record
Return a record-type descriptor representing the type of the given
record. That is, for example, if the returned descriptor were
passed to ‘record-predicate’, the resulting predicate would return
a true value when passed the given record. Note that it is not
necessarily the case that the returned descriptor is the one that
was passed to ‘record-constructor’ in the call that created the
constructor procedure that created the given record.
-- Scheme Procedure: record-type-name rtd
Return the type-name associated with the type represented by rtd.
The returned value is ‘eqv?’ to the TYPE-NAME argument given in the
call to ‘make-record-type’ that created the type represented by
RTD.
-- Scheme Procedure: record-type-fields rtd
Return a list of the symbols naming the fields in members of the
type represented by RTD. The returned value is ‘equal?’ to the
field-names argument given in the call to ‘make-record-type’ that
created the type represented by RTD.
6.6.18 Structures
-----------------
A “structure” is a first class data type which holds Scheme values or C
words in fields numbered 0 upwards. A “vtable” is a structure that
represents a structure type, giving field types and permissions, and an
optional print function for ‘write’ etc.
Structures are lower level than records (*note Records::). Usually,
when you need to represent structured data, you just want to use
records. But sometimes you need to implement new kinds of structured
data abstractions, and for that purpose structures are useful. Indeed,
records in Guile are implemented with structures.
6.6.18.1 Vtables
................
A vtable is a structure type, specifying its layout, and other
information. A vtable is actually itself a structure, but there’s no
need to worry about that initially (*note Vtable Contents::.)
-- Scheme Procedure: make-vtable fields [print]
Create a new vtable.
FIELDS is a string describing the fields in the structures to be
created. Each field is represented by two characters, a type
letter and a permissions letter, for example ‘"pw"’. The types are
as follows.
• ‘p’ – a Scheme value. “p” stands for “protected” meaning it’s
protected against garbage collection.
• ‘u’ – an arbitrary word of data (an ‘scm_t_bits’). At the
Scheme level it’s read and written as an unsigned integer.
“u” stands for “unboxed”, as it’s stored as a raw value
without additional type annotations.
It used to be that the second letter for each field was a
permission code, such as ‘w’ for writable or ‘r’ for read-only.
However over time structs have become more of a raw low-level
facility; access control is better implemented as a layer on top.
After all, ‘struct-set!’ is a cross-cutting operator that can
bypass abstractions made by higher-level record facilities; it’s
not generally safe (in the sense of abstraction-preserving) to
expose ‘struct-set!’ to “untrusted” code, even if the fields happen
to be writable. Additionally, permission checks added overhead to
every structure access in a way that couldn’t be optimized out,
hampering the ability of structs to act as a low-level building
block. For all of these reasons, all fields in Guile structs are
now writable; attempting to make a read-only field will now issue a
deprecation warning, and the field will be writable regardless.
(make-vtable "pw") ;; one scheme field
(make-vtable "pwuwuw") ;; one scheme and two unboxed fields
The optional PRINT argument is a function called by ‘display’ and
‘write’ (etc) to give a printed representation of a structure
created from this vtable. It’s called ‘(PRINT struct port)’ and
should look at STRUCT and write to PORT. The default print merely
gives a form like ‘#’ with a pair of machine
addresses.
The following print function for example shows the two fields of
its structure.
(make-vtable "pwpw"
(lambda (struct port)
(format port "#<~a and ~a>"
(struct-ref struct 0)
(struct-ref struct 1))))
6.6.18.2 Structure Basics
.........................
This section describes the basic procedures for working with structures.
‘make-struct/no-tail’ creates a structure, and ‘struct-ref’ and
‘struct-set!’ access its fields.
-- Scheme Procedure: make-struct/no-tail vtable init ...
Create a new structure, with layout per the given VTABLE (*note
Vtables::).
The optional INIT... arguments are initial values for the fields of
the structure. This is the only way to put values in read-only
fields. If there are fewer INIT arguments than fields then the
defaults are ‘#f’ for a Scheme field (type ‘p’) or 0 for an unboxed
field (type ‘u’).
The name is a bit strange, we admit. The reason for it is that
Guile used to have a ‘make-struct’ that took an additional
argument; while we deprecate that old interface,
‘make-struct/no-tail’ is the new name for this functionality.
For example,
(define v (make-vtable "pwpwpw"))
(define s (make-struct/no-tail v 123 "abc" 456))
(struct-ref s 0) ⇒ 123
(struct-ref s 1) ⇒ "abc"
-- C Function: SCM scm_make_struct (SCM vtable, SCM tail_size, SCM
init_list)
-- C Function: SCM scm_c_make_struct (SCM vtable, SCM tail_size, SCM
init, ...)
-- C Function: SCM scm_c_make_structv (SCM vtable, SCM tail_size,
size_t n_inits, scm_t_bits init[])
There are a few ways to make structures from C. ‘scm_make_struct’
takes a list, ‘scm_c_make_struct’ takes variable arguments
terminated with SCM_UNDEFINED, and ‘scm_c_make_structv’ takes a
packed array.
For all of these, TAIL_SIZE should be zero (as a SCM value).
-- Scheme Procedure: struct? obj
-- C Function: scm_struct_p (obj)
Return ‘#t’ if OBJ is a structure, or ‘#f’ if not.
-- Scheme Procedure: struct-ref struct n
-- C Function: scm_struct_ref (struct, n)
Return the contents of field number N in STRUCT. The first field
is number 0.
An error is thrown if N is out of range.
-- Scheme Procedure: struct-set! struct n value
-- C Function: scm_struct_set_x (struct, n, value)
Set field number N in STRUCT to VALUE. The first field is number
0.
An error is thrown if N is out of range, or if the field cannot be
written because it’s ‘r’ read-only.
Unboxed fields (those with type ‘u’) need to be accessed with special
procedures.
-- Scheme Procedure: struct-ref/unboxed struct n
-- Scheme Procedure: struct-set!/unboxed struct n value
-- C Function: scm_struct_ref_unboxed (struct, n)
-- C Function: scm_struct_set_x_unboxed (struct, n, value)
Like ‘struct-ref’ and ‘struct-set!’, except that these may only be
used on unboxed fields. ‘struct-ref/unboxed’ will always return a
positive integer. Likewise, ‘struct-set!/unboxed’ takes an
unsigned integer as the VALUE argument, and will signal an error
otherwise.
-- Scheme Procedure: struct-vtable struct
-- C Function: scm_struct_vtable (struct)
Return the vtable that describes STRUCT.
The vtable is effectively the type of the structure. See *note
Vtable Contents::, for more on vtables.
6.6.18.3 Vtable Contents
........................
A vtable is itself a structure. It has a specific set of fields
describing various aspects of its “instances”: the structures created
from a vtable. Some of the fields are internal to Guile, some of them
are part of the public interface, and there may be additional fields
added on by the user.
Every vtable has a field for the layout of their instances, a field
for the procedure used to print its instances, and a field for the name
of the vtable itself. Access to the layout and printer is exposed
directly via field indexes. Access to the vtable name is exposed via
accessor procedures.
-- Scheme Variable: vtable-index-layout
-- C Macro: scm_vtable_index_layout
The field number of the layout specification in a vtable. The
layout specification is a symbol like ‘pwpw’ formed from the fields
string passed to ‘make-vtable’, or created by ‘make-struct-layout’
(*note Meta-Vtables::).
(define v (make-vtable "pwpw" 0))
(struct-ref v vtable-index-layout) ⇒ pwpw
This field is read-only, since the layout of structures using a
vtable cannot be changed.
-- Scheme Variable: vtable-index-printer
-- C Macro: scm_vtable_index_printer
The field number of the printer function. This field contains ‘#f’
if the default print function should be used.
(define (my-print-func struct port)
...)
(define v (make-vtable "pwpw" my-print-func))
(struct-ref v vtable-index-printer) ⇒ my-print-func
This field is writable, allowing the print function to be changed
dynamically.
-- Scheme Procedure: struct-vtable-name vtable
-- Scheme Procedure: set-struct-vtable-name! vtable name
-- C Function: scm_struct_vtable_name (vtable)
-- C Function: scm_set_struct_vtable_name_x (vtable, name)
Get or set the name of VTABLE. NAME is a symbol and is used in the
default print function when printing structures created from
VTABLE.
(define v (make-vtable "pw"))
(set-struct-vtable-name! v 'my-name)
(define s (make-struct v 0))
(display s) ⊣ #
6.6.18.4 Meta-Vtables
.....................
As a structure, a vtable also has a vtable, which is also a structure.
Structures, their vtables, the vtables of the vtables, and so on form a
tree of structures. Making a new structure adds a leaf to the tree, and
if that structure is a vtable, it may be used to create other leaves.
If you traverse up the tree of vtables, via calling ‘struct-vtable’,
eventually you reach a root which is the vtable of itself:
scheme@(guile-user)> (current-module)
$1 = #
scheme@(guile-user)> (struct-vtable $1)
$2 = #
scheme@(guile-user)> (struct-vtable $2)
$3 = #< 12c30a0>
scheme@(guile-user)> (struct-vtable $3)
$4 = #< 12c3fa0>
scheme@(guile-user)> (struct-vtable $4)
$5 = #< 12c3fa0>
scheme@(guile-user)>
$6 = #< 12c3fa0>
In this example, we can say that ‘$1’ is an instance of ‘$2’, ‘$2’ is
an instance of ‘$3’, ‘$3’ is an instance of ‘$4’, and ‘$4’, strangely
enough, is an instance of itself. The value bound to ‘$4’ in this
console session also bound to ‘’ in the default
environment.
-- Scheme Variable:
A meta-vtable, useful for making new vtables.
All of these values are structures. All but ‘$1’ are vtables. As
‘$2’ is an instance of ‘$3’, and ‘$3’ is a vtable, we can say that ‘$3’
is a “meta-vtable”: a vtable that can create vtables.
With this definition, we can specify more precisely what a vtable is:
a vtable is a structure made from a meta-vtable. Making a structure
from a meta-vtable runs some special checks to ensure that the first
field of the structure is a valid layout. Additionally, if these checks
see that the layout of the child vtable contains all the required fields
of a vtable, in the correct order, then the child vtable will also be a
meta-table, inheriting a magical bit from the parent.
-- Scheme Procedure: struct-vtable? obj
-- C Function: scm_struct_vtable_p (obj)
Return ‘#t’ if OBJ is a vtable structure: an instance of a
meta-vtable.
‘’ is a root of the vtable tree. (Normally there is
only one root in a given Guile process, but due to some legacy
interfaces there may be more than one.)
The set of required fields of a vtable is the set of fields in the
‘’, and is bound to ‘standard-vtable-fields’ in the
default environment. It is possible to create a meta-vtable that with
additional fields in its layout, which can be used to create vtables
with additional data:
scheme@(guile-user)> (struct-ref $3 vtable-index-layout)
$6 = pwuhuhpwphuhuhpwpwpw
scheme@(guile-user)> (struct-ref $4 vtable-index-layout)
$7 = pwuhuhpwphuhuh
scheme@(guile-user)> standard-vtable-fields
$8 = "pwuhuhpwphuhuh"
scheme@(guile-user)> (struct-ref $2 vtable-offset-user)
$9 = module
In this continuation of our earlier example, ‘$2’ is a vtable that
has extra fields, because its vtable, ‘$3’, was made from a meta-vtable
with an extended layout. ‘vtable-offset-user’ is a convenient
definition that indicates the number of fields in
‘standard-vtable-fields’.
-- Scheme Variable: standard-vtable-fields
A string containing the ordered set of fields that a vtable must
have.
-- Scheme Variable: vtable-offset-user
The first index in a vtable that is available for a user.
-- Scheme Procedure: make-struct-layout fields
-- C Function: scm_make_struct_layout (fields)
Return a structure layout symbol, from a FIELDS string. FIELDS is
as described under ‘make-vtable’ (*note Vtables::). An invalid
FIELDS string is an error.
With these definitions, one can define ‘make-vtable’ in this way:
(define* (make-vtable fields #:optional printer)
(make-struct/no-tail
(make-struct-layout fields)
printer))
6.6.18.5 Vtable Example
.......................
Let us bring these points together with an example. Consider a simple
object system with single inheritance. Objects will be normal
structures, and classes will be vtables with three extra class fields:
the name of the class, the parent class, and the list of fields.
So, first we need a meta-vtable that allocates instances with these
extra class fields.
(define
(make-vtable
(string-append standard-vtable-fields "pwpwpw")
(lambda (x port)
(format port "< ~a>" (class-name x)))))
(define (class? x)
(and (struct? x)
(eq? (struct-vtable x) )))
To make a structure with a specific meta-vtable, we will use
‘make-struct/no-tail’, passing it the computed instance layout and
printer, as with ‘make-vtable’, and additionally the extra three class
fields.
(define (make-class name parent fields)
(let* ((fields (compute-fields parent fields))
(layout (compute-layout fields)))
(make-struct/no-tail
layout
(lambda (x port)
(print-instance x port))
name
parent
fields)))
Instances will store their associated data in slots in the structure:
as many slots as there are fields. The ‘compute-layout’ procedure below
can compute a layout, and ‘field-index’ returns the slot corresponding
to a field.
(define-syntax-rule (define-accessor name n)
(define (name obj)
(struct-ref obj n)))
;; Accessors for classes
(define-accessor class-name (+ vtable-offset-user 0))
(define-accessor class-parent (+ vtable-offset-user 1))
(define-accessor class-fields (+ vtable-offset-user 2))
(define (compute-fields parent fields)
(if parent
(append (class-fields parent) fields)
fields))
(define (compute-layout fields)
(make-struct-layout
(string-concatenate (make-list (length fields) "pw"))))
(define (field-index class field)
(list-index (class-fields class) field))
(define (print-instance x port)
(format port "<~a" (class-name (struct-vtable x)))
(for-each (lambda (field idx)
(format port " ~a: ~a" field (struct-ref x idx)))
(class-fields (struct-vtable x))
(iota (length (class-fields (struct-vtable x)))))
(format port ">"))
So, at this point we can actually make a few classes:
(define-syntax-rule (define-class name parent field ...)
(define name (make-class 'name parent '(field ...))))
(define-class #f
width height)
(define-class
x y)
And finally, make an instance:
(make-struct/no-tail 400 300 10 20)
⇒ < width: 400 height: 300 x: 10 y: 20>
And that’s that. Note that there are many possible optimizations and
feature enhancements that can be made to this object system, and the
included GOOPS system does make most of them. For more simple use
cases, the records facility is usually sufficient. But sometimes you
need to make new kinds of data abstractions, and for that purpose,
structs are here.
6.6.19 Dictionary Types
-----------------------
A “dictionary” object is a data structure used to index information in a
user-defined way. In standard Scheme, the main aggregate data types are
lists and vectors. Lists are not really indexed at all, and vectors are
indexed only by number (e.g. ‘(vector-ref foo 5)’). Often you will find
it useful to index your data on some other type; for example, in a
library catalog you might want to look up a book by the name of its
author. Dictionaries are used to help you organize information in such
a way.
An “association list” (or “alist” for short) is a list of key-value
pairs. Each pair represents a single quantity or object; the ‘car’ of
the pair is a key which is used to identify the object, and the ‘cdr’ is
the object’s value.
A “hash table” also permits you to index objects with arbitrary keys,
but in a way that makes looking up any one object extremely fast. A
well-designed hash system makes hash table lookups almost as fast as
conventional array or vector references.
Alists are popular among Lisp programmers because they use only the
language’s primitive operations (lists, “car”, “cdr” and the equality
primitives). No changes to the language core are necessary. Therefore,
with Scheme’s built-in list manipulation facilities, it is very
convenient to handle data stored in an association list. Also, alists
are highly portable and can be easily implemented on even the most
minimal Lisp systems.
However, alists are inefficient, especially for storing large
quantities of data. Because we want Guile to be useful for large
software systems as well as small ones, Guile provides a rich set of
tools for using either association lists or hash tables.
6.6.20 Association Lists
------------------------
An association list is a conventional data structure that is often used
to implement simple key-value databases. It consists of a list of
entries in which each entry is a pair. The “key” of each entry is the
‘car’ of the pair and the “value” of each entry is the ‘cdr’.
ASSOCIATION LIST ::= '( (KEY1 . VALUE1)
(KEY2 . VALUE2)
(KEY3 . VALUE3)
...
)
Association lists are also known, for short, as “alists”.
The structure of an association list is just one example of the
infinite number of possible structures that can be built using pairs and
lists. As such, the keys and values in an association list can be
manipulated using the general list structure procedures ‘cons’, ‘car’,
‘cdr’, ‘set-car!’, ‘set-cdr!’ and so on. However, because association
lists are so useful, Guile also provides specific procedures for
manipulating them.
6.6.20.1 Alist Key Equality
...........................
All of Guile’s dedicated association list procedures, apart from
‘acons’, come in three flavours, depending on the level of equality that
is required to decide whether an existing key in the association list is
the same as the key that the procedure call uses to identify the
required entry.
• Procedures with “assq” in their name use ‘eq?’ to determine key
equality.
• Procedures with “assv” in their name use ‘eqv?’ to determine key
equality.
• Procedures with “assoc” in their name use ‘equal?’ to determine key
equality.
‘acons’ is an exception because it is used to build association lists
which do not require their entries’ keys to be unique.
6.6.20.2 Adding or Setting Alist Entries
........................................
‘acons’ adds a new entry to an association list and returns the combined
association list. The combined alist is formed by consing the new entry
onto the head of the alist specified in the ‘acons’ procedure call. So
the specified alist is not modified, but its contents become shared with
the tail of the combined alist that ‘acons’ returns.
In the most common usage of ‘acons’, a variable holding the original
association list is updated with the combined alist:
(set! address-list (acons name address address-list))
In such cases, it doesn’t matter that the old and new values of
‘address-list’ share some of their contents, since the old value is
usually no longer independently accessible.
Note that ‘acons’ adds the specified new entry regardless of whether
the alist may already contain entries with keys that are, in some sense,
the same as that of the new entry. Thus ‘acons’ is ideal for building
alists where there is no concept of key uniqueness.
(set! task-list (acons 3 "pay gas bill" '()))
task-list
⇒
((3 . "pay gas bill"))
(set! task-list (acons 3 "tidy bedroom" task-list))
task-list
⇒
((3 . "tidy bedroom") (3 . "pay gas bill"))
‘assq-set!’, ‘assv-set!’ and ‘assoc-set!’ are used to add or replace
an entry in an association list where there _is_ a concept of key
uniqueness. If the specified association list already contains an entry
whose key is the same as that specified in the procedure call, the
existing entry is replaced by the new one. Otherwise, the new entry is
consed onto the head of the old association list to create the combined
alist. In all cases, these procedures return the combined alist.
‘assq-set!’ and friends _may_ destructively modify the structure of
the old association list in such a way that an existing variable is
correctly updated without having to ‘set!’ it to the value returned:
address-list
⇒
(("mary" . "34 Elm Road") ("james" . "16 Bow Street"))
(assoc-set! address-list "james" "1a London Road")
⇒
(("mary" . "34 Elm Road") ("james" . "1a London Road"))
address-list
⇒
(("mary" . "34 Elm Road") ("james" . "1a London Road"))
Or they may not:
(assoc-set! address-list "bob" "11 Newington Avenue")
⇒
(("bob" . "11 Newington Avenue") ("mary" . "34 Elm Road")
("james" . "1a London Road"))
address-list
⇒
(("mary" . "34 Elm Road") ("james" . "1a London Road"))
The only safe way to update an association list variable when adding
or replacing an entry like this is to ‘set!’ the variable to the
returned value:
(set! address-list
(assoc-set! address-list "bob" "11 Newington Avenue"))
address-list
⇒
(("bob" . "11 Newington Avenue") ("mary" . "34 Elm Road")
("james" . "1a London Road"))
Because of this slight inconvenience, you may find it more convenient
to use hash tables to store dictionary data. If your application will
not be modifying the contents of an alist very often, this may not make
much difference to you.
If you need to keep the old value of an association list in a form
independent from the list that results from modification by ‘acons’,
‘assq-set!’, ‘assv-set!’ or ‘assoc-set!’, use ‘list-copy’ to copy the
old association list before modifying it.
-- Scheme Procedure: acons key value alist
-- C Function: scm_acons (key, value, alist)
Add a new key-value pair to ALIST. A new pair is created whose car
is KEY and whose cdr is VALUE, and the pair is consed onto ALIST,
and the new list is returned. This function is _not_ destructive;
ALIST is not modified.
-- Scheme Procedure: assq-set! alist key val
-- Scheme Procedure: assv-set! alist key value
-- Scheme Procedure: assoc-set! alist key value
-- C Function: scm_assq_set_x (alist, key, val)
-- C Function: scm_assv_set_x (alist, key, val)
-- C Function: scm_assoc_set_x (alist, key, val)
Reassociate KEY in ALIST with VALUE: find any existing ALIST entry
for KEY and associate it with the new VALUE. If ALIST does not
contain an entry for KEY, add a new one. Return the (possibly new)
alist.
These functions do not attempt to verify the structure of ALIST,
and so may cause unusual results if passed an object that is not an
association list.
6.6.20.3 Retrieving Alist Entries
.................................
‘assq’, ‘assv’ and ‘assoc’ find the entry in an alist for a given key,
and return the ‘(KEY . VALUE)’ pair. ‘assq-ref’, ‘assv-ref’ and
‘assoc-ref’ do a similar lookup, but return just the VALUE.
-- Scheme Procedure: assq key alist
-- Scheme Procedure: assv key alist
-- Scheme Procedure: assoc key alist
-- C Function: scm_assq (key, alist)
-- C Function: scm_assv (key, alist)
-- C Function: scm_assoc (key, alist)
Return the first entry in ALIST with the given KEY. The return is
the pair ‘(KEY . VALUE)’ from ALIST. If there’s no matching entry
the return is ‘#f’.
‘assq’ compares keys with ‘eq?’, ‘assv’ uses ‘eqv?’ and ‘assoc’
uses ‘equal?’. See also SRFI-1 which has an extended ‘assoc’
(*note SRFI-1 Association Lists::).
-- Scheme Procedure: assq-ref alist key
-- Scheme Procedure: assv-ref alist key
-- Scheme Procedure: assoc-ref alist key
-- C Function: scm_assq_ref (alist, key)
-- C Function: scm_assv_ref (alist, key)
-- C Function: scm_assoc_ref (alist, key)
Return the value from the first entry in ALIST with the given KEY,
or ‘#f’ if there’s no such entry.
‘assq-ref’ compares keys with ‘eq?’, ‘assv-ref’ uses ‘eqv?’ and
‘assoc-ref’ uses ‘equal?’.
Notice these functions have the KEY argument last, like other
‘-ref’ functions, but this is opposite to what ‘assq’ etc above
use.
When the return is ‘#f’ it can be either KEY not found, or an entry
which happens to have value ‘#f’ in the ‘cdr’. Use ‘assq’ etc
above if you need to differentiate these cases.
6.6.20.4 Removing Alist Entries
...............................
To remove the element from an association list whose key matches a
specified key, use ‘assq-remove!’, ‘assv-remove!’ or ‘assoc-remove!’
(depending, as usual, on the level of equality required between the key
that you specify and the keys in the association list).
As with ‘assq-set!’ and friends, the specified alist may or may not
be modified destructively, and the only safe way to update a variable
containing the alist is to ‘set!’ it to the value that ‘assq-remove!’
and friends return.
address-list
⇒
(("bob" . "11 Newington Avenue") ("mary" . "34 Elm Road")
("james" . "1a London Road"))
(set! address-list (assoc-remove! address-list "mary"))
address-list
⇒
(("bob" . "11 Newington Avenue") ("james" . "1a London Road"))
Note that, when ‘assq/v/oc-remove!’ is used to modify an association
list that has been constructed only using the corresponding
‘assq/v/oc-set!’, there can be at most one matching entry in the alist,
so the question of multiple entries being removed in one go does not
arise. If ‘assq/v/oc-remove!’ is applied to an association list that
has been constructed using ‘acons’, or an ‘assq/v/oc-set!’ with a
different level of equality, or any mixture of these, it removes only
the first matching entry from the alist, even if the alist might contain
further matching entries. For example:
(define address-list '())
(set! address-list (assq-set! address-list "mary" "11 Elm Street"))
(set! address-list (assq-set! address-list "mary" "57 Pine Drive"))
address-list
⇒
(("mary" . "57 Pine Drive") ("mary" . "11 Elm Street"))
(set! address-list (assoc-remove! address-list "mary"))
address-list
⇒
(("mary" . "11 Elm Street"))
In this example, the two instances of the string "mary" are not the
same when compared using ‘eq?’, so the two ‘assq-set!’ calls add two
distinct entries to ‘address-list’. When compared using ‘equal?’, both
"mary"s in ‘address-list’ are the same as the "mary" in the
‘assoc-remove!’ call, but ‘assoc-remove!’ stops after removing the first
matching entry that it finds, and so one of the "mary" entries is left
in place.
-- Scheme Procedure: assq-remove! alist key
-- Scheme Procedure: assv-remove! alist key
-- Scheme Procedure: assoc-remove! alist key
-- C Function: scm_assq_remove_x (alist, key)
-- C Function: scm_assv_remove_x (alist, key)
-- C Function: scm_assoc_remove_x (alist, key)
Delete the first entry in ALIST associated with KEY, and return the
resulting alist.
6.6.20.5 Sloppy Alist Functions
...............................
‘sloppy-assq’, ‘sloppy-assv’ and ‘sloppy-assoc’ behave like the
corresponding non-‘sloppy-’ procedures, except that they return ‘#f’
when the specified association list is not well-formed, where the
non-‘sloppy-’ versions would signal an error.
Specifically, there are two conditions for which the non-‘sloppy-’
procedures signal an error, which the ‘sloppy-’ procedures handle
instead by returning ‘#f’. Firstly, if the specified alist as a whole
is not a proper list:
(assoc "mary" '((1 . 2) ("key" . "door") . "open sesame"))
⇒
ERROR: In procedure assoc in expression (assoc "mary" (quote #)):
ERROR: Wrong type argument in position 2 (expecting
association list): ((1 . 2) ("key" . "door") . "open sesame")
(sloppy-assoc "mary" '((1 . 2) ("key" . "door") . "open sesame"))
⇒
#f
Secondly, if one of the entries in the specified alist is not a pair:
(assoc 2 '((1 . 1) 2 (3 . 9)))
⇒
ERROR: In procedure assoc in expression (assoc 2 (quote #)):
ERROR: Wrong type argument in position 2 (expecting
association list): ((1 . 1) 2 (3 . 9))
(sloppy-assoc 2 '((1 . 1) 2 (3 . 9)))
⇒
#f
Unless you are explicitly working with badly formed association
lists, it is much safer to use the non-‘sloppy-’ procedures, because
they help to highlight coding and data errors that the ‘sloppy-’
versions would silently cover up.
-- Scheme Procedure: sloppy-assq key alist
-- C Function: scm_sloppy_assq (key, alist)
Behaves like ‘assq’ but does not do any error checking.
Recommended only for use in Guile internals.
-- Scheme Procedure: sloppy-assv key alist
-- C Function: scm_sloppy_assv (key, alist)
Behaves like ‘assv’ but does not do any error checking.
Recommended only for use in Guile internals.
-- Scheme Procedure: sloppy-assoc key alist
-- C Function: scm_sloppy_assoc (key, alist)
Behaves like ‘assoc’ but does not do any error checking.
Recommended only for use in Guile internals.
6.6.20.6 Alist Example
......................
Here is a longer example of how alists may be used in practice.
(define capitals '(("New York" . "Albany")
("Oregon" . "Salem")
("Florida" . "Miami")))
;; What's the capital of Oregon?
(assoc "Oregon" capitals) ⇒ ("Oregon" . "Salem")
(assoc-ref capitals "Oregon") ⇒ "Salem"
;; We left out South Dakota.
(set! capitals
(assoc-set! capitals "South Dakota" "Pierre"))
capitals
⇒ (("South Dakota" . "Pierre")
("New York" . "Albany")
("Oregon" . "Salem")
("Florida" . "Miami"))
;; And we got Florida wrong.
(set! capitals
(assoc-set! capitals "Florida" "Tallahassee"))
capitals
⇒ (("South Dakota" . "Pierre")
("New York" . "Albany")
("Oregon" . "Salem")
("Florida" . "Tallahassee"))
;; After Oregon secedes, we can remove it.
(set! capitals
(assoc-remove! capitals "Oregon"))
capitals
⇒ (("South Dakota" . "Pierre")
("New York" . "Albany")
("Florida" . "Tallahassee"))
6.6.21 VList-Based Hash Lists or “VHashes”
------------------------------------------
The ‘(ice-9 vlist)’ module provides an implementation of “VList-based
hash lists” (*note VLists::). VList-based hash lists, or “vhashes”, are
an immutable dictionary type similar to association lists that maps
“keys” to “values”. However, unlike association lists, accessing a
value given its key is typically a constant-time operation.
The VHash programming interface of ‘(ice-9 vlist)’ is mostly the same
as that of association lists found in SRFI-1, with procedure names
prefixed by ‘vhash-’ instead of ‘alist-’ (*note SRFI-1 Association
Lists::).
In addition, vhashes can be manipulated using VList operations:
(vlist-head (vhash-consq 'a 1 vlist-null))
⇒ (a . 1)
(define vh1 (vhash-consq 'b 2 (vhash-consq 'a 1 vlist-null)))
(define vh2 (vhash-consq 'c 3 (vlist-tail vh1)))
(vhash-assq 'a vh2)
⇒ (a . 1)
(vhash-assq 'b vh2)
⇒ #f
(vhash-assq 'c vh2)
⇒ (c . 3)
(vlist->list vh2)
⇒ ((c . 3) (a . 1))
However, keep in mind that procedures that construct new VLists
(‘vlist-map’, ‘vlist-filter’, etc.) return raw VLists, not vhashes:
(define vh (alist->vhash '((a . 1) (b . 2) (c . 3)) hashq))
(vhash-assq 'a vh)
⇒ (a . 1)
(define vl
;; This will create a raw vlist.
(vlist-filter (lambda (key+value) (odd? (cdr key+value))) vh))
(vhash-assq 'a vl)
⇒ ERROR: Wrong type argument in position 2
(vlist->list vl)
⇒ ((a . 1) (c . 3))
-- Scheme Procedure: vhash? obj
Return true if OBJ is a vhash.
-- Scheme Procedure: vhash-cons key value vhash [hash-proc]
-- Scheme Procedure: vhash-consq key value vhash
-- Scheme Procedure: vhash-consv key value vhash
Return a new hash list based on VHASH where KEY is associated with
VALUE, using HASH-PROC to compute the hash of KEY. VHASH must be
either ‘vlist-null’ or a vhash returned by a previous call to
‘vhash-cons’. HASH-PROC defaults to ‘hash’ (*note ‘hash’
procedure: Hash Table Reference.). With ‘vhash-consq’, the ‘hashq’
hash function is used; with ‘vhash-consv’ the ‘hashv’ hash function
is used.
All ‘vhash-cons’ calls made to construct a vhash should use the
same HASH-PROC. Failing to do that, the result is undefined.
-- Scheme Procedure: vhash-assoc key vhash [equal? [hash-proc]]
-- Scheme Procedure: vhash-assq key vhash
-- Scheme Procedure: vhash-assv key vhash
Return the first key/value pair from VHASH whose key is equal to
KEY according to the EQUAL? equality predicate (which defaults to
‘equal?’), and using HASH-PROC (which defaults to ‘hash’) to
compute the hash of KEY. The second form uses ‘eq?’ as the
equality predicate and ‘hashq’ as the hash function; the last form
uses ‘eqv?’ and ‘hashv’.
Note that it is important to consistently use the same hash
function for HASH-PROC as was passed to ‘vhash-cons’. Failing to
do that, the result is unpredictable.
-- Scheme Procedure: vhash-delete key vhash [equal? [hash-proc]]
-- Scheme Procedure: vhash-delq key vhash
-- Scheme Procedure: vhash-delv key vhash
Remove all associations from VHASH with KEY, comparing keys with
EQUAL? (which defaults to ‘equal?’), and computing the hash of KEY
using HASH-PROC (which defaults to ‘hash’). The second form uses
‘eq?’ as the equality predicate and ‘hashq’ as the hash function;
the last one uses ‘eqv?’ and ‘hashv’.
Again the choice of HASH-PROC must be consistent with previous
calls to ‘vhash-cons’.
-- Scheme Procedure: vhash-fold proc init vhash
-- Scheme Procedure: vhash-fold-right proc init vhash
Fold over the key/value elements of VHASH in the given direction,
with each call to PROC having the form ‘(PROC key value result)’,
where RESULT is the result of the previous call to PROC and INIT
the value of RESULT for the first call to PROC.
-- Scheme Procedure: vhash-fold* proc init key vhash [equal? [hash]]
-- Scheme Procedure: vhash-foldq* proc init key vhash
-- Scheme Procedure: vhash-foldv* proc init key vhash
Fold over all the values associated with KEY in VHASH, with each
call to PROC having the form ‘(proc value result)’, where RESULT is
the result of the previous call to PROC and INIT the value of
RESULT for the first call to PROC.
Keys in VHASH are hashed using HASH are compared using EQUAL?. The
second form uses ‘eq?’ as the equality predicate and ‘hashq’ as the
hash function; the third one uses ‘eqv?’ and ‘hashv’.
Example:
(define vh
(alist->vhash '((a . 1) (a . 2) (z . 0) (a . 3))))
(vhash-fold* cons '() 'a vh)
⇒ (3 2 1)
(vhash-fold* cons '() 'z vh)
⇒ (0)
-- Scheme Procedure: alist->vhash alist [hash-proc]
Return the vhash corresponding to ALIST, an association list, using
HASH-PROC to compute key hashes. When omitted, HASH-PROC defaults
to ‘hash’.
6.6.22 Hash Tables
------------------
Hash tables are dictionaries which offer similar functionality as
association lists: They provide a mapping from keys to values. The
difference is that association lists need time linear in the size of
elements when searching for entries, whereas hash tables can normally
search in constant time. The drawback is that hash tables require a
little bit more memory, and that you can not use the normal list
procedures (*note Lists::) for working with them.
6.6.22.1 Hash Table Examples
............................
For demonstration purposes, this section gives a few usage examples of
some hash table procedures, together with some explanation what they do.
First we start by creating a new hash table with 31 slots, and
populate it with two key/value pairs.
(define h (make-hash-table 31))
;; This is an opaque object
h
⇒
#
;; Inserting into a hash table can be done with hashq-set!
(hashq-set! h 'foo "bar")
⇒
"bar"
(hashq-set! h 'braz "zonk")
⇒
"zonk"
;; Or with hash-create-handle!
(hashq-create-handle! h 'frob #f)
⇒
(frob . #f)
You can get the value for a given key with the procedure ‘hashq-ref’,
but the problem with this procedure is that you cannot reliably
determine whether a key does exists in the table. The reason is that
the procedure returns ‘#f’ if the key is not in the table, but it will
return the same value if the key is in the table and just happens to
have the value ‘#f’, as you can see in the following examples.
(hashq-ref h 'foo)
⇒
"bar"
(hashq-ref h 'frob)
⇒
#f
(hashq-ref h 'not-there)
⇒
#f
It is often better is to use the procedure ‘hashq-get-handle’, which
makes a distinction between the two cases. Just like ‘assq’, this
procedure returns a key/value-pair on success, and ‘#f’ if the key is
not found.
(hashq-get-handle h 'foo)
⇒
(foo . "bar")
(hashq-get-handle h 'not-there)
⇒
#f
Interesting results can be computed by using ‘hash-fold’ to work
through each element. This example will count the total number of
elements:
(hash-fold (lambda (key value seed) (+ 1 seed)) 0 h)
⇒
3
The same thing can be done with the procedure ‘hash-count’, which can
also count the number of elements matching a particular predicate. For
example, count the number of elements with string values:
(hash-count (lambda (key value) (string? value)) h)
⇒
2
Counting all the elements is a simple task using ‘const’:
(hash-count (const #t) h)
⇒
3
6.6.22.2 Hash Table Reference
.............................
Like the association list functions, the hash table functions come in
several varieties, according to the equality test used for the keys.
Plain ‘hash-’ functions use ‘equal?’, ‘hashq-’ functions use ‘eq?’,
‘hashv-’ functions use ‘eqv?’, and the ‘hashx-’ functions use an
application supplied test.
A single ‘make-hash-table’ creates a hash table suitable for use with
any set of functions, but it’s imperative that just one set is then used
consistently, or results will be unpredictable.
Hash tables are implemented as a vector indexed by a hash value
formed from the key, with an association list of key/value pairs for
each bucket in case distinct keys hash together. Direct access to the
pairs in those lists is provided by the ‘-handle-’ functions.
When the number of entries in a hash table goes above a threshold,
the vector is made larger and the entries are rehashed, to prevent the
bucket lists from becoming too long and slowing down accesses. When the
number of entries goes below a threshold, the vector is shrunk to save
space.
For the ‘hashx-’ “extended” routines, an application supplies a HASH
function producing an integer index like ‘hashq’ etc below, and an ASSOC
alist search function like ‘assq’ etc (*note Retrieving Alist
Entries::). Here’s an example of such functions implementing
case-insensitive hashing of string keys,
(use-modules (srfi srfi-1)
(srfi srfi-13))
(define (my-hash str size)
(remainder (string-hash-ci str) size))
(define (my-assoc str alist)
(find (lambda (pair) (string-ci=? str (car pair))) alist))
(define my-table (make-hash-table))
(hashx-set! my-hash my-assoc my-table "foo" 123)
(hashx-ref my-hash my-assoc my-table "FOO")
⇒ 123
In a ‘hashx-’ HASH function the aim is to spread keys across the
vector, so bucket lists don’t become long. But the actual values are
arbitrary as long as they’re in the range 0 to SIZE-1. Helpful
functions for forming a hash value, in addition to ‘hashq’ etc below,
include ‘symbol-hash’ (*note Symbol Keys::), ‘string-hash’ and
‘string-hash-ci’ (*note String Comparison::), and ‘char-set-hash’ (*note
Character Set Predicates/Comparison::).
-- Scheme Procedure: make-hash-table [size]
Create a new hash table object, with an optional minimum vector
SIZE.
When SIZE is given, the table vector will still grow and shrink
automatically, as described above, but with SIZE as a minimum. If
an application knows roughly how many entries the table will hold
then it can use SIZE to avoid rehashing when initial entries are
added.
-- Scheme Procedure: alist->hash-table alist
-- Scheme Procedure: alist->hashq-table alist
-- Scheme Procedure: alist->hashv-table alist
-- Scheme Procedure: alist->hashx-table hash assoc alist
Convert ALIST into a hash table. When keys are repeated in ALIST,
the leftmost association takes precedence.
(use-modules (ice-9 hash-table))
(alist->hash-table '((foo . 1) (bar . 2)))
When converting to an extended hash table, custom HASH and ASSOC
procedures must be provided.
(alist->hashx-table hash assoc '((foo . 1) (bar . 2)))
-- Scheme Procedure: hash-table? obj
-- C Function: scm_hash_table_p (obj)
Return ‘#t’ if OBJ is a abstract hash table object.
-- Scheme Procedure: hash-clear! table
-- C Function: scm_hash_clear_x (table)
Remove all items from TABLE (without triggering a resize).
-- Scheme Procedure: hash-ref table key [dflt]
-- Scheme Procedure: hashq-ref table key [dflt]
-- Scheme Procedure: hashv-ref table key [dflt]
-- Scheme Procedure: hashx-ref hash assoc table key [dflt]
-- C Function: scm_hash_ref (table, key, dflt)
-- C Function: scm_hashq_ref (table, key, dflt)
-- C Function: scm_hashv_ref (table, key, dflt)
-- C Function: scm_hashx_ref (hash, assoc, table, key, dflt)
Lookup KEY in the given hash TABLE, and return the associated
value. If KEY is not found, return DFLT, or ‘#f’ if DFLT is not
given.
-- Scheme Procedure: hash-set! table key val
-- Scheme Procedure: hashq-set! table key val
-- Scheme Procedure: hashv-set! table key val
-- Scheme Procedure: hashx-set! hash assoc table key val
-- C Function: scm_hash_set_x (table, key, val)
-- C Function: scm_hashq_set_x (table, key, val)
-- C Function: scm_hashv_set_x (table, key, val)
-- C Function: scm_hashx_set_x (hash, assoc, table, key, val)
Associate VAL with KEY in the given hash TABLE. If KEY is already
present then it’s associated value is changed. If it’s not present
then a new entry is created.
-- Scheme Procedure: hash-remove! table key
-- Scheme Procedure: hashq-remove! table key
-- Scheme Procedure: hashv-remove! table key
-- Scheme Procedure: hashx-remove! hash assoc table key
-- C Function: scm_hash_remove_x (table, key)
-- C Function: scm_hashq_remove_x (table, key)
-- C Function: scm_hashv_remove_x (table, key)
-- C Function: scm_hashx_remove_x (hash, assoc, table, key)
Remove any association for KEY in the given hash TABLE. If KEY is
not in TABLE then nothing is done.
-- Scheme Procedure: hash key size
-- Scheme Procedure: hashq key size
-- Scheme Procedure: hashv key size
-- C Function: scm_hash (key, size)
-- C Function: scm_hashq (key, size)
-- C Function: scm_hashv (key, size)
Return a hash value for KEY. This is a number in the range 0 to
SIZE-1, which is suitable for use in a hash table of the given
SIZE.
Note that ‘hashq’ and ‘hashv’ may use internal addresses of
objects, so if an object is garbage collected and re-created it can
have a different hash value, even when the two are notionally
‘eq?’. For instance with symbols,
(hashq 'something 123) ⇒ 19
(gc)
(hashq 'something 123) ⇒ 62
In normal use this is not a problem, since an object entered into a
hash table won’t be garbage collected until removed. It’s only if
hashing calculations are somehow separated from normal references
that its lifetime needs to be considered.
-- Scheme Procedure: hash-get-handle table key
-- Scheme Procedure: hashq-get-handle table key
-- Scheme Procedure: hashv-get-handle table key
-- Scheme Procedure: hashx-get-handle hash assoc table key
-- C Function: scm_hash_get_handle (table, key)
-- C Function: scm_hashq_get_handle (table, key)
-- C Function: scm_hashv_get_handle (table, key)
-- C Function: scm_hashx_get_handle (hash, assoc, table, key)
Return the ‘(KEY . VALUE)’ pair for KEY in the given hash TABLE, or
‘#f’ if KEY is not in TABLE.
-- Scheme Procedure: hash-create-handle! table key init
-- Scheme Procedure: hashq-create-handle! table key init
-- Scheme Procedure: hashv-create-handle! table key init
-- Scheme Procedure: hashx-create-handle! hash assoc table key init
-- C Function: scm_hash_create_handle_x (table, key, init)
-- C Function: scm_hashq_create_handle_x (table, key, init)
-- C Function: scm_hashv_create_handle_x (table, key, init)
-- C Function: scm_hashx_create_handle_x (hash, assoc, table, key,
init)
Return the ‘(KEY . VALUE)’ pair for KEY in the given hash TABLE.
If KEY is not in TABLE then create an entry for it with INIT as the
value, and return that pair.
-- Scheme Procedure: hash-map->list proc table
-- Scheme Procedure: hash-for-each proc table
-- C Function: scm_hash_map_to_list (proc, table)
-- C Function: scm_hash_for_each (proc, table)
Apply PROC to the entries in the given hash TABLE. Each call is
‘(PROC KEY VALUE)’. ‘hash-map->list’ returns a list of the results
from these calls, ‘hash-for-each’ discards the results and returns
an unspecified value.
Calls are made over the table entries in an unspecified order, and
for ‘hash-map->list’ the order of the values in the returned list
is unspecified. Results will be unpredictable if TABLE is modified
while iterating.
For example the following returns a new alist comprising all the
entries from ‘mytable’, in no particular order.
(hash-map->list cons mytable)
-- Scheme Procedure: hash-for-each-handle proc table
-- C Function: scm_hash_for_each_handle (proc, table)
Apply PROC to the entries in the given hash TABLE. Each call is
‘(PROC HANDLE)’, where HANDLE is a ‘(KEY . VALUE)’ pair. Return an
unspecified value.
‘hash-for-each-handle’ differs from ‘hash-for-each’ only in the
argument list of PROC.
-- Scheme Procedure: hash-fold proc init table
-- C Function: scm_hash_fold (proc, init, table)
Accumulate a result by applying PROC to the elements of the given
hash TABLE. Each call is ‘(PROC KEY VALUE PRIOR-RESULT)’, where
KEY and VALUE are from the TABLE and PRIOR-RESULT is the return
from the previous PROC call. For the first call, PRIOR-RESULT is
the given INIT value.
Calls are made over the table entries in an unspecified order.
Results will be unpredictable if TABLE is modified while
‘hash-fold’ is running.
For example, the following returns a count of how many keys in
‘mytable’ are strings.
(hash-fold (lambda (key value prior)
(if (string? key) (1+ prior) prior))
0 mytable)
-- Scheme Procedure: hash-count pred table
-- C Function: scm_hash_count (pred, table)
Return the number of elements in the given hash TABLE that cause
‘(PRED KEY VALUE)’ to return true. To quickly determine the total
number of elements, use ‘(const #t)’ for PRED.
6.6.23 Other Types
------------------
Procedures are documented in their own section. *Note Procedures::.
Variable objects are documented as part of the description of Guile’s
module system: see *note Variables::.
*Note Scheduling::, for discussion of threads, mutexes, and so on.
Ports are described in the section on I/O: see *note Input and
Output::.
Regular expressions are described in their own section: see *note
Regular Expressions::.
There are quite a number of additional data types documented in this
manual; if you feel a link is missing here, please file a bug.
6.7 Procedures
==============
6.7.1 Lambda: Basic Procedure Creation
--------------------------------------
A ‘lambda’ expression evaluates to a procedure. The environment which
is in effect when a ‘lambda’ expression is evaluated is enclosed in the
newly created procedure, this is referred to as a “closure” (*note About
Closure::).
When a procedure created by ‘lambda’ is called with some actual
arguments, the environment enclosed in the procedure is extended by
binding the variables named in the formal argument list to new locations
and storing the actual arguments into these locations. Then the body of
the ‘lambda’ expression is evaluated sequentially. The result of the
last expression in the procedure body is then the result of the
procedure invocation.
The following examples will show how procedures can be created using
‘lambda’, and what you can do with these procedures.
(lambda (x) (+ x x)) ⇒ a procedure
((lambda (x) (+ x x)) 4) ⇒ 8
The fact that the environment in effect when creating a procedure is
enclosed in the procedure is shown with this example:
(define add4
(let ((x 4))
(lambda (y) (+ x y))))
(add4 6) ⇒ 10
-- syntax: lambda formals body
FORMALS should be a formal argument list as described in the
following table.
‘(VARIABLE1 ...)’
The procedure takes a fixed number of arguments; when the
procedure is called, the arguments will be stored into the
newly created location for the formal variables.
‘VARIABLE’
The procedure takes any number of arguments; when the
procedure is called, the sequence of actual arguments will be
converted into a list and stored into the newly created
location for the formal variable.
‘(VARIABLE1 ... VARIABLEN . VARIABLEN+1)’
If a space-delimited period precedes the last variable, then
the procedure takes N or more variables where N is the number
of formal arguments before the period. There must be at least
one argument before the period. The first N actual arguments
will be stored into the newly allocated locations for the
first N formal arguments and the sequence of the remaining
actual arguments is converted into a list and the stored into
the location for the last formal argument. If there are
exactly N actual arguments, the empty list is stored into the
location of the last formal argument.
The list in VARIABLE or VARIABLEN+1 is always newly created and the
procedure can modify it if desired. This is the case even when the
procedure is invoked via ‘apply’, the required part of the list
argument there will be copied (*note Procedures for On the Fly
Evaluation: Fly Evaluation.).
BODY is a sequence of Scheme expressions which are evaluated in
order when the procedure is invoked.
6.7.2 Primitive Procedures
--------------------------
Procedures written in C can be registered for use from Scheme, provided
they take only arguments of type ‘SCM’ and return ‘SCM’ values.
‘scm_c_define_gsubr’ is likely to be the most useful mechanism,
combining the process of registration (‘scm_c_make_gsubr’) and
definition (‘scm_define’).
-- Function: SCM scm_c_make_gsubr (const char *name, int req, int opt,
int rst, fcn)
Register a C procedure FCN as a “subr” — a primitive subroutine
that can be called from Scheme. It will be associated with the
given NAME but no environment binding will be created. The
arguments REQ, OPT and RST specify the number of required, optional
and “rest” arguments respectively. The total number of these
arguments should match the actual number of arguments to FCN, but
may not exceed 10. The number of rest arguments should be 0 or 1.
‘scm_c_make_gsubr’ returns a value of type ‘SCM’ which is a
“handle” for the procedure.
-- Function: SCM scm_c_define_gsubr (const char *name, int req, int
opt, int rst, fcn)
Register a C procedure FCN, as for ‘scm_c_make_gsubr’ above, and
additionally create a top-level Scheme binding for the procedure in
the “current environment” using ‘scm_define’. ‘scm_c_define_gsubr’
returns a handle for the procedure in the same way as
‘scm_c_make_gsubr’, which is usually not further required.
6.7.3 Compiled Procedures
-------------------------
The evaluation strategy given in *note Lambda:: describes how procedures
are “interpreted”. Interpretation operates directly on expanded Scheme
source code, recursively calling the evaluator to obtain the value of
nested expressions.
Most procedures are compiled, however. This means that Guile has
done some pre-computation on the procedure, to determine what it will
need to do each time the procedure runs. Compiled procedures run faster
than interpreted procedures.
Loading files is the normal way that compiled procedures come to
being. If Guile sees that a file is uncompiled, or that its compiled
file is out of date, it will attempt to compile the file when it is
loaded, and save the result to disk. Procedures can be compiled at
runtime as well. *Note Read/Load/Eval/Compile::, for more information
on runtime compilation.
Compiled procedures, also known as “programs”, respond to all
procedures that operate on procedures: you can pass a program to
‘procedure?’, ‘procedure-name’, and so on (*note Procedure
Properties::). In addition, there are a few more accessors for
low-level details on programs.
Most people won’t need to use the routines described in this section,
but it’s good to have them documented. You’ll have to include the
appropriate module first, though:
(use-modules (system vm program))
-- Scheme Procedure: program? obj
-- C Function: scm_program_p (obj)
Returns ‘#t’ if OBJ is a compiled procedure, or ‘#f’ otherwise.
-- Scheme Procedure: program-code program
-- C Function: scm_program_code (program)
Returns the address of the program’s entry, as an integer. This
address is mostly useful to procedures in ‘(system vm debug)’.
-- Scheme Procedure: program-num-free-variable program
-- C Function: scm_program_num_free_variables (program)
Return the number of free variables captured by this program.
-- Scheme Procedure: program-free-variable-ref program n
-- C Function: scm_program_free_variable-ref (program, n)
-- Scheme Procedure: program-free-variable-set! program n val
-- C Function: scm_program_free_variable_set_x (program, n, val)
Accessors for a program’s free variables. Some of the values
captured are actually in variable “boxes”. *Note Variables and the
VM::, for more information.
Users must not modify the returned value unless they think they’re
really clever.
-- Scheme Procedure: program-bindings program
-- Scheme Procedure: make-binding name boxed? index start end
-- Scheme Procedure: binding:name binding
-- Scheme Procedure: binding:boxed? binding
-- Scheme Procedure: binding:index binding
-- Scheme Procedure: binding:start binding
-- Scheme Procedure: binding:end binding
Bindings annotations for programs, along with their accessors.
Bindings declare names and liveness extents for block-local
variables. The best way to see what these are is to play around
with them at a REPL. *Note VM Concepts::, for more information.
Note that bindings information is stored in a program as part of
its metadata thunk, so including it in the generated object code
does not impose a runtime performance penalty.
-- Scheme Procedure: program-sources program
-- Scheme Procedure: source:addr source
-- Scheme Procedure: source:line source
-- Scheme Procedure: source:column source
-- Scheme Procedure: source:file source
Source location annotations for programs, along with their
accessors.
Source location information propagates through the compiler and
ends up being serialized to the program’s metadata. This
information is keyed by the offset of the instruction pointer
within the object code of the program. Specifically, it is keyed
on the ‘ip’ _just following_ an instruction, so that backtraces can
find the source location of a call that is in progress.
-- Scheme Procedure: program-arities program
-- C Function: scm_program_arities (program)
-- Scheme Procedure: program-arity program ip
-- Scheme Procedure: arity:start arity
-- Scheme Procedure: arity:end arity
-- Scheme Procedure: arity:nreq arity
-- Scheme Procedure: arity:nopt arity
-- Scheme Procedure: arity:rest? arity
-- Scheme Procedure: arity:kw arity
-- Scheme Procedure: arity:allow-other-keys? arity
Accessors for a representation of the “arity” of a program.
The normal case is that a procedure has one arity. For example,
‘(lambda (x) x)’, takes one required argument, and that’s it. One
could access that number of required arguments via ‘(arity:nreq
(program-arities (lambda (x) x)))’. Similarly, ‘arity:nopt’ gets
the number of optional arguments, and ‘arity:rest?’ returns a true
value if the procedure has a rest arg.
‘arity:kw’ returns a list of ‘(KW . IDX)’ pairs, if the procedure
has keyword arguments. The IDX refers to the IDXth local variable;
*Note Variables and the VM::, for more information. Finally
‘arity:allow-other-keys?’ returns a true value if other keys are
allowed. *Note Optional Arguments::, for more information.
So what about ‘arity:start’ and ‘arity:end’, then? They return the
range of bytes in the program’s bytecode for which a given arity is
valid. You see, a procedure can actually have more than one arity.
The question, “what is a procedure’s arity” only really makes sense
at certain points in the program, delimited by these ‘arity:start’
and ‘arity:end’ values.
-- Scheme Procedure: program-arguments-alist program [ip]
Return an association list describing the arguments that PROGRAM
accepts, or ‘#f’ if the information cannot be obtained.
The alist keys that are currently defined are ‘required’,
‘optional’, ‘keyword’, ‘allow-other-keys?’, and ‘rest’. For
example:
(program-arguments-alist
(lambda* (a b #:optional c #:key (d 1) #:rest e)
#t)) ⇒
((required . (a b))
(optional . (c))
(keyword . ((#:d . 4)))
(allow-other-keys? . #f)
(rest . d))
-- Scheme Procedure: program-lambda-list program [ip]
Return a representation of the arguments of PROGRAM as a lambda
list, or ‘#f’ if this information is not available.
For example:
(program-lambda-list
(lambda* (a b #:optional c #:key (d 1) #:rest e)
#t)) ⇒
6.7.4 Optional Arguments
------------------------
Scheme procedures, as defined in R5RS, can either handle a fixed number
of actual arguments, or a fixed number of actual arguments followed by
arbitrarily many additional arguments. Writing procedures of variable
arity can be useful, but unfortunately, the syntactic means for handling
argument lists of varying length is a bit inconvenient. It is possible
to give names to the fixed number of arguments, but the remaining
(optional) arguments can be only referenced as a list of values (*note
Lambda::).
For this reason, Guile provides an extension to ‘lambda’, ‘lambda*’,
which allows the user to define procedures with optional and keyword
arguments. In addition, Guile’s virtual machine has low-level support
for optional and keyword argument dispatch. Calls to procedures with
optional and keyword arguments can be made cheaply, without allocating a
rest list.
6.7.4.1 lambda* and define*.
............................
‘lambda*’ is like ‘lambda’, except with some extensions to allow
optional and keyword arguments.
-- library syntax: lambda* ([var...]
[#:optional vardef...]
[#:key vardef... [#:allow-other-keys]]
[#:rest var | . var])
body1 body2 ...
Create a procedure which takes optional and/or keyword arguments
specified with ‘#:optional’ and ‘#:key’. For example,
(lambda* (a b #:optional c d . e) '())
is a procedure with fixed arguments A and B, optional arguments C
and D, and rest argument E. If the optional arguments are omitted
in a call, the variables for them are bound to ‘#f’.
Likewise, ‘define*’ is syntactic sugar for defining procedures
using ‘lambda*’.
‘lambda*’ can also make procedures with keyword arguments. For
example, a procedure defined like this:
(define* (sir-yes-sir #:key action how-high)
(list action how-high))
can be called as ‘(sir-yes-sir #:action 'jump)’, ‘(sir-yes-sir
#:how-high 13)’, ‘(sir-yes-sir #:action 'lay-down #:how-high 0)’,
or just ‘(sir-yes-sir)’. Whichever arguments are given as keywords
are bound to values (and those not given are ‘#f’).
Optional and keyword arguments can also have default values to take
when not present in a call, by giving a two-element list of
variable name and expression. For example in
(define* (frob foo #:optional (bar 42) #:key (baz 73))
(list foo bar baz))
FOO is a fixed argument, BAR is an optional argument with default
value 42, and baz is a keyword argument with default value 73.
Default value expressions are not evaluated unless they are needed,
and until the procedure is called.
Normally it’s an error if a call has keywords other than those
specified by ‘#:key’, but adding ‘#:allow-other-keys’ to the
definition (after the keyword argument declarations) will ignore
unknown keywords.
If a call has a keyword given twice, the last value is used. For
example,
(define* (flips #:key (heads 0) (tails 0))
(display (list heads tails)))
(flips #:heads 37 #:tails 42 #:heads 99)
⊣ (99 42)
‘#:rest’ is a synonym for the dotted syntax rest argument. The
argument lists ‘(a . b)’ and ‘(a #:rest b)’ are equivalent in all
respects. This is provided for more similarity to DSSSL,
MIT-Scheme and Kawa among others, as well as for refugees from
other Lisp dialects.
When ‘#:key’ is used together with a rest argument, the keyword
parameters in a call all remain in the rest list. This is the same
as Common Lisp. For example,
((lambda* (#:key (x 0) #:allow-other-keys #:rest r)
(display r))
#:x 123 #:y 456)
⊣ (#:x 123 #:y 456)
‘#:optional’ and ‘#:key’ establish their bindings successively,
from left to right. This means default expressions can refer back
to prior parameters, for example
(lambda* (start #:optional (end (+ 10 start)))
(do ((i start (1+ i)))
((> i end))
(display i)))
The exception to this left-to-right scoping rule is the rest
argument. If there is a rest argument, it is bound after the
optional arguments, but before the keyword arguments.
6.7.4.2 (ice-9 optargs)
.......................
Before Guile 2.0, ‘lambda*’ and ‘define*’ were implemented using macros
that processed rest list arguments. This was not optimal, as calling
procedures with optional arguments had to allocate rest lists at every
procedure invocation. Guile 2.0 improved this situation by bringing
optional and keyword arguments into Guile’s core.
However there are occasions in which you have a list and want to
parse it for optional or keyword arguments. Guile’s ‘(ice-9 optargs)’
provides some macros to help with that task.
The syntax ‘let-optional’ and ‘let-optional*’ are for destructuring
rest argument lists and giving names to the various list elements.
‘let-optional’ binds all variables simultaneously, while ‘let-optional*’
binds them sequentially, consistent with ‘let’ and ‘let*’ (*note Local
Bindings::).
-- library syntax: let-optional rest-arg (binding ...) body1 body2 ...
-- library syntax: let-optional* rest-arg (binding ...) body1 body2 ...
These two macros give you an optional argument interface that is
very “Schemey” and introduces no fancy syntax. They are compatible
with the scsh macros of the same name, but are slightly extended.
Each of BINDING may be of one of the forms VAR or ‘(VAR
DEFAULT-VALUE)’. REST-ARG should be the rest-argument of the
procedures these are used from. The items in REST-ARG are
sequentially bound to the variable names are given. When REST-ARG
runs out, the remaining vars are bound either to the default values
or ‘#f’ if no default value was specified. REST-ARG remains bound
to whatever may have been left of REST-ARG.
After binding the variables, the expressions BODY1 BODY2 ... are
evaluated in order.
Similarly, ‘let-keywords’ and ‘let-keywords*’ extract values from
keyword style argument lists, binding local variables to those values or
to defaults.
-- library syntax: let-keywords args allow-other-keys? (binding ...)
body1 body2 ...
-- library syntax: let-keywords* args allow-other-keys? (binding ...)
body1 body2 ...
ARGS is evaluated and should give a list of the form ‘(#:keyword1
value1 #:keyword2 value2 ...)’. The BINDINGs are variables and
default expressions, with the variables to be set (by name) from
the keyword values. The BODY1 BODY2 ... forms are then evaluated
and the last is the result. An example will make the syntax
clearest,
(define args '(#:xyzzy "hello" #:foo "world"))
(let-keywords args #t
((foo "default for foo")
(bar (string-append "default" "for" "bar")))
(display foo)
(display ", ")
(display bar))
⊣ world, defaultforbar
The binding for ‘foo’ comes from the ‘#:foo’ keyword in ‘args’.
But the binding for ‘bar’ is the default in the ‘let-keywords’,
since there’s no ‘#:bar’ in the args.
ALLOW-OTHER-KEYS? is evaluated and controls whether unknown
keywords are allowed in the ARGS list. When true other keys are
ignored (such as ‘#:xyzzy’ in the example), when ‘#f’ an error is
thrown for anything unknown.
‘(ice-9 optargs)’ also provides some more ‘define*’ sugar, which is
not so useful with modern Guile coding, but still supported:
‘define*-public’ is the ‘lambda*’ version of ‘define-public’;
‘defmacro*’ and ‘defmacro*-public’ exist for defining macros with the
improved argument list handling possibilities. The ‘-public’ versions
not only define the procedures/macros, but also export them from the
current module.
-- library syntax: define*-public formals body1 body2 ...
Like a mix of ‘define*’ and ‘define-public’.
-- library syntax: defmacro* name formals body1 body2 ...
-- library syntax: defmacro*-public name formals body1 body2 ...
These are just like ‘defmacro’ and ‘defmacro-public’ except that
they take ‘lambda*’-style extended parameter lists, where
‘#:optional’, ‘#:key’, ‘#:allow-other-keys’ and ‘#:rest’ are
allowed with the usual semantics. Here is an example of a macro
with an optional argument:
(defmacro* transmogrify (a #:optional b)
(a 1))
6.7.5 Case-lambda
-----------------
R5RS’s rest arguments are indeed useful and very general, but they often
aren’t the most appropriate or efficient means to get the job done. For
example, ‘lambda*’ is a much better solution to the optional argument
problem than ‘lambda’ with rest arguments.
Likewise, ‘case-lambda’ works well for when you want one procedure to
do double duty (or triple, or ...), without the penalty of consing a
rest list.
For example:
(define (make-accum n)
(case-lambda
(() n)
((m) (set! n (+ n m)) n)))
(define a (make-accum 20))
(a) ⇒ 20
(a 10) ⇒ 30
(a) ⇒ 30
The value returned by a ‘case-lambda’ form is a procedure which
matches the number of actual arguments against the formals in the
various clauses, in order. The first matching clause is selected, the
corresponding values from the actual parameter list are bound to the
variable names in the clauses and the body of the clause is evaluated.
If no clause matches, an error is signalled.
The syntax of the ‘case-lambda’ form is defined in the following EBNF
grammar. “Formals” means a formal argument list just like with ‘lambda’
(*note Lambda::).
--> (case-lambda *)
--> (case-lambda *)
--> ( *)
--> (*)
| (* . )
|
Rest lists can be useful with ‘case-lambda’:
(define plus
(case-lambda
"Return the sum of all arguments."
(() 0)
((a) a)
((a b) (+ a b))
((a b . rest) (apply plus (+ a b) rest))))
(plus 1 2 3) ⇒ 6
Also, for completeness. Guile defines ‘case-lambda*’ as well, which
is like ‘case-lambda’, except with ‘lambda*’ clauses. A ‘case-lambda*’
clause matches if the arguments fill the required arguments, but are not
too many for the optional and/or rest arguments.
Keyword arguments are possible with ‘case-lambda*’ as well, but they
do not contribute to the “matching” behavior, and their interactions
with required, optional, and rest arguments can be surprising.
For the purposes of ‘case-lambda*’ (and of ‘case-lambda’, as a
special case), a clause “matches” if it has enough required arguments,
and not too many positional arguments. The required arguments are any
arguments before the ‘#:optional’, ‘#:key’, and ‘#:rest’ arguments.
“Positional” arguments are the required arguments, together with the
optional arguments.
In the absence of ‘#:key’ or ‘#:rest’ arguments, it’s easy to see how
there could be too many positional arguments: you pass 5 arguments to a
function that only takes 4 arguments, including optional arguments. If
there is a ‘#:rest’ argument, there can never be too many positional
arguments: any application with enough required arguments for a clause
will match that clause, even if there are also ‘#:key’ arguments.
Otherwise, for applications to a clause with ‘#:key’ arguments (and
without a ‘#:rest’ argument), a clause will match there only if there
are enough required arguments and if the next argument after binding
required and optional arguments, if any, is a keyword. For efficiency
reasons, Guile is currently unable to include keyword arguments in the
matching algorithm. Clauses match on positional arguments only, not by
comparing a given keyword to the available set of keyword arguments that
a function has.
Some examples follow.
(define f
(case-lambda*
((a #:optional b) 'clause-1)
((a #:optional b #:key c) 'clause-2)
((a #:key d) 'clause-3)
((#:key e #:rest f) 'clause-4)))
(f) ⇒ clause-4
(f 1) ⇒ clause-1
(f) ⇒ clause-4
(f #:e 10) clause-1
(f 1 #:foo) clause-1
(f 1 #:c 2) clause-2
(f #:a #:b #:c #:d #:e) clause-4
;; clause-2 will match anything that clause-3 would match.
(f 1 #:d 2) ⇒ error: bad keyword args in clause 2
Don’t forget that the clauses are matched in order, and the first
matching clause will be taken. This can result in a keyword being bound
to a required argument, as in the case of ‘f #:e 10’.
6.7.6 Higher-Order Functions
----------------------------
As a functional programming language, Scheme allows the definition of
“higher-order functions”, i.e., functions that take functions as
arguments and/or return functions. Utilities to derive procedures from
other procedures are provided and described below.
-- Scheme Procedure: const value
Return a procedure that accepts any number of arguments and returns
VALUE.
(procedure? (const 3)) ⇒ #t
((const 'hello)) ⇒ hello
((const 'hello) 'world) ⇒ hello
-- Scheme Procedure: negate proc
Return a procedure with the same arity as PROC that returns the
‘not’ of PROC’s result.
(procedure? (negate number?)) ⇒ #t
((negate odd?) 2) ⇒ #t
((negate real?) 'dream) ⇒ #t
((negate string-prefix?) "GNU" "GNU Guile")
⇒ #f
(filter (negate number?) '(a 2 "b"))
⇒ (a "b")
-- Scheme Procedure: compose proc1 proc2 ...
Compose PROC1 with the procedures PROC2 ... such that the last PROC
argument is applied first and PROC1 last, and return the resulting
procedure. The given procedures must have compatible arity.
(procedure? (compose 1+ 1-)) ⇒ #t
((compose sqrt 1+ 1+) 2) ⇒ 2.0
((compose 1+ sqrt) 3) ⇒ 2.73205080756888
(eq? (compose 1+) 1+) ⇒ #t
((compose zip unzip2) '((1 2) (a b)))
⇒ ((1 2) (a b))
-- Scheme Procedure: identity x
Return X.
-- Scheme Procedure: and=> value proc
When VALUE is ‘#f’, return ‘#f’. Otherwise, return ‘(PROC VALUE)’.
6.7.7 Procedure Properties and Meta-information
-----------------------------------------------
In addition to the information that is strictly necessary to run,
procedures may have other associated information. For example, the name
of a procedure is information not for the procedure, but about the
procedure. This meta-information can be accessed via the procedure
properties interface.
The first group of procedures in this meta-interface are predicates
to test whether a Scheme object is a procedure, or a special procedure,
respectively. ‘procedure?’ is the most general predicates, it returns
‘#t’ for any kind of procedure.
-- Scheme Procedure: procedure? obj
-- C Function: scm_procedure_p (obj)
Return ‘#t’ if OBJ is a procedure.
-- Scheme Procedure: thunk? obj
-- C Function: scm_thunk_p (obj)
Return ‘#t’ if OBJ is a procedure that can be called with zero
arguments.
Procedure properties are general properties associated with
procedures. These can be the name of a procedure or other relevant
information, such as debug hints.
-- Scheme Procedure: procedure-name proc
-- C Function: scm_procedure_name (proc)
Return the name of the procedure PROC
-- Scheme Procedure: procedure-source proc
-- C Function: scm_procedure_source (proc)
Return the source of the procedure PROC. Returns ‘#f’ if the
source code is not available.
-- Scheme Procedure: procedure-properties proc
-- C Function: scm_procedure_properties (proc)
Return the properties associated with PROC, as an association list.
-- Scheme Procedure: procedure-property proc key
-- C Function: scm_procedure_property (proc, key)
Return the property of PROC with name KEY.
-- Scheme Procedure: set-procedure-properties! proc alist
-- C Function: scm_set_procedure_properties_x (proc, alist)
Set PROC’s property list to ALIST.
-- Scheme Procedure: set-procedure-property! proc key value
-- C Function: scm_set_procedure_property_x (proc, key, value)
In PROC’s property list, set the property named KEY to VALUE.
Documentation for a procedure can be accessed with the procedure
‘procedure-documentation’.
-- Scheme Procedure: procedure-documentation proc
-- C Function: scm_procedure_documentation (proc)
Return the documentation string associated with ‘proc’. By
convention, if a procedure contains more than one expression and
the first expression is a string constant, that string is assumed
to contain documentation for that procedure.
6.7.8 Procedures with Setters
-----------------------------
A “procedure with setter” is a special kind of procedure which normally
behaves like any accessor procedure, that is a procedure which accesses
a data structure. The difference is that this kind of procedure has a
so-called “setter” attached, which is a procedure for storing something
into a data structure.
Procedures with setters are treated specially when the procedure
appears in the special form ‘set!’. How it works is best shown by
example.
Suppose we have a procedure called ‘foo-ref’, which accepts two
arguments, a value of type ‘foo’ and an integer. The procedure returns
the value stored at the given index in the ‘foo’ object. Let ‘f’ be a
variable containing such a ‘foo’ data structure.(1)
(foo-ref f 0) ⇒ bar
(foo-ref f 1) ⇒ braz
Also suppose that a corresponding setter procedure called ‘foo-set!’
does exist.
(foo-set! f 0 'bla)
(foo-ref f 0) ⇒ bla
Now we could create a new procedure called ‘foo’, which is a
procedure with setter, by calling ‘make-procedure-with-setter’ with the
accessor and setter procedures ‘foo-ref’ and ‘foo-set!’. Let us call
this new procedure ‘foo’.
(define foo (make-procedure-with-setter foo-ref foo-set!))
‘foo’ can from now on be used to either read from the data structure
stored in ‘f’, or to write into the structure.
(set! (foo f 0) 'dum)
(foo f 0) ⇒ dum
-- Scheme Procedure: make-procedure-with-setter procedure setter
-- C Function: scm_make_procedure_with_setter (procedure, setter)
Create a new procedure which behaves like PROCEDURE, but with the
associated setter SETTER.
-- Scheme Procedure: procedure-with-setter? obj
-- C Function: scm_procedure_with_setter_p (obj)
Return ‘#t’ if OBJ is a procedure with an associated setter
procedure.
-- Scheme Procedure: procedure proc
-- C Function: scm_procedure (proc)
Return the procedure of PROC, which must be an applicable struct.
-- Scheme Procedure: setter proc
Return the setter of PROC, which must be either a procedure with
setter or an operator struct.
---------- Footnotes ----------
(1) Working definitions would be:
(define foo-ref vector-ref)
(define foo-set! vector-set!)
(define f (make-vector 2 #f))
6.7.9 Inlinable Procedures
--------------------------
You can define an “inlinable procedure” by using ‘define-inlinable’
instead of ‘define’. An inlinable procedure behaves the same as a
regular procedure, but direct calls will result in the procedure body
being inlined into the caller.
Bear in mind that starting from version 2.0.3, Guile has a partial
evaluator that can inline the body of inner procedures when deemed
appropriate:
scheme@(guile-user)> ,optimize (define (foo x)
(define (bar) (+ x 3))
(* (bar) 2))
$1 = (define foo
(lambda (#{x 94}#) (* (+ #{x 94}# 3) 2)))
The partial evaluator does not inline top-level bindings, though, so
this is a situation where you may find it interesting to use
‘define-inlinable’.
Procedures defined with ‘define-inlinable’ are _always_ inlined, at
all direct call sites. This eliminates function call overhead at the
expense of an increase in code size. Additionally, the caller will not
transparently use the new definition if the inline procedure is
redefined. It is not possible to trace an inlined procedures or install
a breakpoint in it (*note Traps::). For these reasons, you should not
make a procedure inlinable unless it demonstrably improves performance
in a crucial way.
In general, only small procedures should be considered for inlining,
as making large procedures inlinable will probably result in an increase
in code size. Additionally, the elimination of the call overhead rarely
matters for large procedures.
-- Scheme Syntax: define-inlinable (name parameter ...) body1 body2 ...
Define NAME as a procedure with parameters PARAMETERs and bodies
BODY1, BODY2, ....
6.8 Macros
==========
At its best, programming in Lisp is an iterative process of building up
a language appropriate to the problem at hand, and then solving the
problem in that language. Defining new procedures is part of that, but
Lisp also allows the user to extend its syntax, with its famous
“macros”.
Macros are syntactic extensions which cause the expression that they
appear in to be transformed in some way _before_ being evaluated. In
expressions that are intended for macro transformation, the identifier
that names the relevant macro must appear as the first element, like
this:
(MACRO-NAME MACRO-ARGS ...)
Macro expansion is a separate phase of evaluation, run before code is
interpreted or compiled. A macro is a program that runs on programs,
translating an embedded language into core Scheme(1).
---------- Footnotes ----------
(1) These days such embedded languages are often referred to as
“embedded domain-specific languages”, or EDSLs.
6.8.1 Defining Macros
---------------------
A macro is a binding between a keyword and a syntax transformer. Since
it’s difficult to discuss ‘define-syntax’ without discussing the format
of transformers, consider the following example macro definition:
(define-syntax when
(syntax-rules ()
((when condition exp ...)
(if condition
(begin exp ...)))))
(when #t
(display "hey ho\n")
(display "let's go\n"))
⊣ hey ho
⊣ let's go
In this example, the ‘when’ binding is bound with ‘define-syntax’.
Syntax transformers are discussed in more depth in *note Syntax Rules::
and *note Syntax Case::.
-- Syntax: define-syntax keyword transformer
Bind KEYWORD to the syntax transformer obtained by evaluating
TRANSFORMER.
After a macro has been defined, further instances of KEYWORD in
Scheme source code will invoke the syntax transformer defined by
TRANSFORMER.
One can also establish local syntactic bindings with ‘let-syntax’.
-- Syntax: let-syntax ((keyword transformer) ...) exp1 exp2 ...
Bind each KEYWORD to its corresponding TRANSFORMER while expanding
EXP1 EXP2 ....
A ‘let-syntax’ binding only exists at expansion-time.
(let-syntax ((unless
(syntax-rules ()
((unless condition exp ...)
(if (not condition)
(begin exp ...))))))
(unless #t
(primitive-exit 1))
"rock rock rock")
⇒ "rock rock rock"
A ‘define-syntax’ form is valid anywhere a definition may appear: at
the top-level, or locally. Just as a local ‘define’ expands out to an
instance of ‘letrec’, a local ‘define-syntax’ expands out to
‘letrec-syntax’.
-- Syntax: letrec-syntax ((keyword transformer) ...) exp1 exp2 ...
Bind each KEYWORD to its corresponding TRANSFORMER while expanding
EXP1 EXP2 ....
In the spirit of ‘letrec’ versus ‘let’, an expansion produced by
TRANSFORMER may reference a KEYWORD bound by the same
LETREC-SYNTAX.
(letrec-syntax ((my-or
(syntax-rules ()
((my-or)
#t)
((my-or exp)
exp)
((my-or exp rest ...)
(let ((t exp))
(if t
t
(my-or rest ...)))))))
(my-or #f "rockaway beach"))
⇒ "rockaway beach"
6.8.2 Syntax-rules Macros
-------------------------
‘syntax-rules’ macros are simple, pattern-driven syntax transformers,
with a beauty worthy of Scheme.
-- Syntax: syntax-rules literals (pattern template) ...
Create a syntax transformer that will rewrite an expression using
the rules embodied in the PATTERN and TEMPLATE clauses.
A ‘syntax-rules’ macro consists of three parts: the literals (if
any), the patterns, and as many templates as there are patterns.
When the syntax expander sees the invocation of a ‘syntax-rules’
macro, it matches the expression against the patterns, in order, and
rewrites the expression using the template from the first matching
pattern. If no pattern matches, a syntax error is signalled.
6.8.2.1 Patterns
................
We have already seen some examples of patterns in the previous section:
‘(unless condition exp ...)’, ‘(my-or exp)’, and so on. A pattern is
structured like the expression that it is to match. It can have nested
structure as well, like ‘(let ((var val) ...) exp exp* ...)’. Broadly
speaking, patterns are made of lists, improper lists, vectors,
identifiers, and datums. Users can match a sequence of patterns using
the ellipsis (‘...’).
Identifiers in a pattern are called “literals” if they are present in
the ‘syntax-rules’ literals list, and “pattern variables” otherwise.
When building up the macro output, the expander replaces instances of a
pattern variable in the template with the matched subexpression.
(define-syntax kwote
(syntax-rules ()
((kwote exp)
(quote exp))))
(kwote (foo . bar))
⇒ (foo . bar)
An improper list of patterns matches as rest arguments do:
(define-syntax let1
(syntax-rules ()
((_ (var val) . exps)
(let ((var val)) . exps))))
However this definition of ‘let1’ probably isn’t what you want, as
the tail pattern EXPS will match non-lists, like ‘(let1 (foo 'bar) .
baz)’. So often instead of using improper lists as patterns, ellipsized
patterns are better. Instances of a pattern variable in the template
must be followed by an ellipsis.
(define-syntax let1
(syntax-rules ()
((_ (var val) exp ...)
(let ((var val)) exp ...))))
This ‘let1’ probably still doesn’t do what we want, because the body
matches sequences of zero expressions, like ‘(let1 (foo 'bar))’. In
this case we need to assert we have at least one body expression. A
common idiom for this is to name the ellipsized pattern variable with an
asterisk:
(define-syntax let1
(syntax-rules ()
((_ (var val) exp exp* ...)
(let ((var val)) exp exp* ...))))
A vector of patterns matches a vector whose contents match the
patterns, including ellipsizing and tail patterns.
(define-syntax letv
(syntax-rules ()
((_ #((var val) ...) exp exp* ...)
(let ((var val) ...) exp exp* ...))))
(letv #((foo 'bar)) foo)
⇒ bar
Literals are used to match specific datums in an expression, like the
use of ‘=>’ and ‘else’ in ‘cond’ expressions.
(define-syntax cond1
(syntax-rules (=> else)
((cond1 test => fun)
(let ((exp test))
(if exp (fun exp) #f)))
((cond1 test exp exp* ...)
(if test (begin exp exp* ...)))
((cond1 else exp exp* ...)
(begin exp exp* ...))))
(define (square x) (* x x))
(cond1 10 => square)
⇒ 100
(let ((=> #t))
(cond1 10 => square))
⇒ #
A literal matches an input expression if the input expression is an
identifier with the same name as the literal, and both are unbound(1).
Although literals can be unbound, usually they are bound to allow
them to be imported, exported, and renamed. *Note Modules::, for more
information on imports and exports. In Guile there are a few standard
auxiliary syntax definitions, as specified by R6RS and R7RS:
-- Scheme Syntax: else
-- Scheme Syntax: =>
-- Scheme Syntax: _
-- Scheme Syntax: ...
Auxiliary syntax definitions.
These are defined as if with a macro that never matches, e.g.:
(define-syntax else (syntax-rules ()))
If a pattern is not a list, vector, or an identifier, it matches as a
literal, with ‘equal?’.
(define-syntax define-matcher-macro
(syntax-rules ()
((_ name lit)
(define-syntax name
(syntax-rules ()
((_ lit) #t)
((_ else) #f))))))
(define-matcher-macro is-literal-foo? "foo")
(is-literal-foo? "foo")
⇒ #t
(is-literal-foo? "bar")
⇒ #f
(let ((foo "foo"))
(is-literal-foo? foo))
⇒ #f
The last example indicates that matching happens at expansion-time,
not at run-time.
Syntax-rules macros are always used as ‘(MACRO . ARGS)’, and the
MACRO will always be a symbol. Correspondingly, a ‘syntax-rules’
pattern must be a list (proper or improper), and the first pattern in
that list must be an identifier. Incidentally it can be any identifier
– it doesn’t have to actually be the name of the macro. Thus the
following three are equivalent:
(define-syntax when
(syntax-rules ()
((when c e ...)
(if c (begin e ...)))))
(define-syntax when
(syntax-rules ()
((_ c e ...)
(if c (begin e ...)))))
(define-syntax when
(syntax-rules ()
((something-else-entirely c e ...)
(if c (begin e ...)))))
For clarity, use one of the first two variants. Also note that since
the pattern variable will always match the macro itself (e.g., ‘cond1’),
it is actually left unbound in the template.
6.8.2.2 Hygiene
...............
‘syntax-rules’ macros have a magical property: they preserve referential
transparency. When you read a macro definition, any free bindings in
that macro are resolved relative to the macro definition; and when you
read a macro instantiation, all free bindings in that expression are
resolved relative to the expression.
This property is sometimes known as “hygiene”, and it does aid in
code cleanliness. In your macro definitions, you can feel free to
introduce temporary variables, without worrying about inadvertently
introducing bindings into the macro expansion.
Consider the definition of ‘my-or’ from the previous section:
(define-syntax my-or
(syntax-rules ()
((my-or)
#t)
((my-or exp)
exp)
((my-or exp rest ...)
(let ((t exp))
(if t
t
(my-or rest ...))))))
A naive expansion of ‘(let ((t #t)) (my-or #f t))’ would yield:
(let ((t #t))
(let ((t #f))
(if t t t)))
⇒ #f
Which clearly is not what we want. Somehow the ‘t’ in the definition is
distinct from the ‘t’ at the site of use; and it is indeed this
distinction that is maintained by the syntax expander, when expanding
hygienic macros.
This discussion is mostly relevant in the context of traditional Lisp
macros (*note Defmacros::), which do not preserve referential
transparency. Hygiene adds to the expressive power of Scheme.
6.8.2.3 Shorthands
..................
One often ends up writing simple one-clause ‘syntax-rules’ macros.
There is a convenient shorthand for this idiom, in the form of
‘define-syntax-rule’.
-- Syntax: define-syntax-rule (keyword . pattern) [docstring] template
Define KEYWORD as a new ‘syntax-rules’ macro with one clause.
Cast into this form, our ‘when’ example is significantly shorter:
(define-syntax-rule (when c e ...)
(if c (begin e ...)))
6.8.2.4 Reporting Syntax Errors in Macros
.........................................
-- Syntax: syntax-error message [arg ...]
Report an error at macro-expansion time. MESSAGE must be a string
literal, and the optional ARG operands can be arbitrary expressions
providing additional information.
‘syntax-error’ is intended to be used within ‘syntax-rules’
templates. For example:
(define-syntax simple-let
(syntax-rules ()
((_ (head ... ((x . y) val) . tail)
body1 body2 ...)
(syntax-error
"expected an identifier but got"
(x . y)))
((_ ((name val) ...) body1 body2 ...)
((lambda (name ...) body1 body2 ...)
val ...))))
6.8.2.5 Specifying a Custom Ellipsis Identifier
...............................................
When writing macros that generate macro definitions, it is convenient to
use a different ellipsis identifier at each level. Guile allows the
desired ellipsis identifier to be specified as the first operand to
‘syntax-rules’, as specified by SRFI-46 and R7RS. For example:
(define-syntax define-quotation-macros
(syntax-rules ()
((_ (macro-name head-symbol) ...)
(begin (define-syntax macro-name
(syntax-rules ::: ()
((_ x :::)
(quote (head-symbol x :::)))))
...))))
(define-quotation-macros (quote-a a) (quote-b b) (quote-c c))
(quote-a 1 2 3) ⇒ (a 1 2 3)
6.8.2.6 Further Information
...........................
For a formal definition of ‘syntax-rules’ and its pattern language, see
*Note Macros: (r5rs)Macros.
‘syntax-rules’ macros are simple and clean, but do they have
limitations. They do not lend themselves to expressive error messages:
patterns either match or they don’t. Their ability to generate code is
limited to template-driven expansion; often one needs to define a number
of helper macros to get real work done. Sometimes one wants to
introduce a binding into the lexical context of the generated code; this
is impossible with ‘syntax-rules’. Relatedly, they cannot
programmatically generate identifiers.
The solution to all of these problems is to use ‘syntax-case’ if you
need its features. But if for some reason you’re stuck with
‘syntax-rules’, you might enjoy Joe Marshall’s ‘syntax-rules’ Primer for
the Merely Eccentric
(http://sites.google.com/site/evalapply/eccentric.txt).
---------- Footnotes ----------
(1) Language lawyers probably see the need here for use of
‘literal-identifier=?’ rather than ‘free-identifier=?’, and would
probably be correct. Patches accepted.
6.8.3 Support for the ‘syntax-case’ System
------------------------------------------
‘syntax-case’ macros are procedural syntax transformers, with a power
worthy of Scheme.
-- Syntax: syntax-case syntax literals (pattern [guard] exp) ...
Match the syntax object SYNTAX against the given patterns, in
order. If a PATTERN matches, return the result of evaluating the
associated EXP.
Compare the following definitions of ‘when’:
(define-syntax when
(syntax-rules ()
((_ test e e* ...)
(if test (begin e e* ...)))))
(define-syntax when
(lambda (x)
(syntax-case x ()
((_ test e e* ...)
#'(if test (begin e e* ...))))))
Clearly, the ‘syntax-case’ definition is similar to its
‘syntax-rules’ counterpart, and equally clearly there are some
differences. The ‘syntax-case’ definition is wrapped in a ‘lambda’, a
function of one argument; that argument is passed to the ‘syntax-case’
invocation; and the “return value” of the macro has a ‘#'’ prefix.
All of these differences stem from the fact that ‘syntax-case’ does
not define a syntax transformer itself – instead, ‘syntax-case’
expressions provide a way to destructure a “syntax object”, and to
rebuild syntax objects as output.
So the ‘lambda’ wrapper is simply a leaky implementation detail, that
syntax transformers are just functions that transform syntax to syntax.
This should not be surprising, given that we have already described
macros as “programs that write programs”. ‘syntax-case’ is simply a way
to take apart and put together program text, and to be a valid syntax
transformer it needs to be wrapped in a procedure.
Unlike traditional Lisp macros (*note Defmacros::), ‘syntax-case’
macros transform syntax objects, not raw Scheme forms. Recall the naive
expansion of ‘my-or’ given in the previous section:
(let ((t #t))
(my-or #f t))
;; naive expansion:
(let ((t #t))
(let ((t #f))
(if t t t)))
Raw Scheme forms simply don’t have enough information to distinguish
the first two ‘t’ instances in ‘(if t t t)’ from the third ‘t’. So
instead of representing identifiers as symbols, the syntax expander
represents identifiers as annotated syntax objects, attaching such
information to those syntax objects as is needed to maintain referential
transparency.
-- Syntax: syntax form
Create a syntax object wrapping FORM within the current lexical
context.
Syntax objects are typically created internally to the process of
expansion, but it is possible to create them outside of syntax
expansion:
(syntax (foo bar baz))
⇒ #
However it is more common, and useful, to create syntax objects when
building output from a ‘syntax-case’ expression.
(define-syntax add1
(lambda (x)
(syntax-case x ()
((_ exp)
(syntax (+ exp 1))))))
It is not strictly necessary for a ‘syntax-case’ expression to return
a syntax object, because ‘syntax-case’ expressions can be used in helper
functions, or otherwise used outside of syntax expansion itself.
However a syntax transformer procedure must return a syntax object, so
most uses of ‘syntax-case’ do end up returning syntax objects.
Here in this case, the form that built the return value was ‘(syntax
(+ exp 1))’. The interesting thing about this is that within a ‘syntax’
expression, any appearance of a pattern variable is substituted into the
resulting syntax object, carrying with it all relevant metadata from the
source expression, such as lexical identity and source location.
Indeed, a pattern variable may only be referenced from inside a
‘syntax’ form. The syntax expander would raise an error when defining
‘add1’ if it found EXP referenced outside a ‘syntax’ form.
Since ‘syntax’ appears frequently in macro-heavy code, it has a
special reader macro: ‘#'’. ‘#'foo’ is transformed by the reader into
‘(syntax foo)’, just as ‘'foo’ is transformed into ‘(quote foo)’.
The pattern language used by ‘syntax-case’ is conveniently the same
language used by ‘syntax-rules’. Given this, Guile actually defines
‘syntax-rules’ in terms of ‘syntax-case’:
(define-syntax syntax-rules
(lambda (x)
(syntax-case x ()
((_ (k ...) ((keyword . pattern) template) ...)
#'(lambda (x)
(syntax-case x (k ...)
((dummy . pattern) #'template)
...))))))
And that’s that.
6.8.3.1 Why ‘syntax-case’?
..........................
The examples we have shown thus far could just as well have been
expressed with ‘syntax-rules’, and have just shown that ‘syntax-case’ is
more verbose, which is true. But there is a difference: ‘syntax-case’
creates _procedural_ macros, giving the full power of Scheme to the
macro expander. This has many practical applications.
A common desire is to be able to match a form only if it is an
identifier. This is impossible with ‘syntax-rules’, given the datum
matching forms. But with ‘syntax-case’ it is easy:
-- Scheme Procedure: identifier? syntax-object
Returns ‘#t’ if SYNTAX-OBJECT is an identifier, or ‘#f’ otherwise.
;; relying on previous add1 definition
(define-syntax add1!
(lambda (x)
(syntax-case x ()
((_ var) (identifier? #'var)
#'(set! var (add1 var))))))
(define foo 0)
(add1! foo)
foo ⇒ 1
(add1! "not-an-identifier") ⇒ error
With ‘syntax-rules’, the error for ‘(add1! "not-an-identifier")’
would be something like “invalid ‘set!’”. With ‘syntax-case’, it will
say something like “invalid ‘add1!’”, because we attach the “guard
clause” to the pattern: ‘(identifier? #'var)’. This becomes more
important with more complicated macros. It is necessary to use
‘identifier?’, because to the expander, an identifier is more than a
bare symbol.
Note that even in the guard clause, we reference the VAR pattern
variable within a ‘syntax’ form, via ‘#'var’.
Another common desire is to introduce bindings into the lexical
context of the output expression. One example would be in the so-called
“anaphoric macros”, like ‘aif’. Anaphoric macros bind some expression
to a well-known identifier, often ‘it’, within their bodies. For
example, in ‘(aif (foo) (bar it))’, ‘it’ would be bound to the result of
‘(foo)’.
To begin with, we should mention a solution that doesn’t work:
;; doesn't work
(define-syntax aif
(lambda (x)
(syntax-case x ()
((_ test then else)
#'(let ((it test))
(if it then else))))))
The reason that this doesn’t work is that, by default, the expander
will preserve referential transparency; the THEN and ELSE expressions
won’t have access to the binding of ‘it’.
But they can, if we explicitly introduce a binding via
‘datum->syntax’.
-- Scheme Procedure: datum->syntax template-id datum [#:source=#f]
Create a syntax object that wraps DATUM, within the lexical context
corresponding to the identifier TEMPLATE-ID. If TEMPLATE-ID is
false, the datum will have no lexical context information.
Syntax objects have an associated source location. Internally this
is represented as a 3-element vector of filename, line, and column.
Usually this location ultimately is provided by ‘read-syntax’;
*Note Annotated Scheme Read::.
If a syntax object is passed as SOURCE, the resulting syntax object
will have the source location of SOURCE. Otherwise if SOURCE is a
3-element source location vector, that vector will be the source
location of the resulting syntax object. If SOURCE is a source
properties alist, those will be parsed and set as the source
location of the resulting syntax object. Otherwise if SOURCE is
false, the source properties are looked up from ‘(source-properties
DATUM)’. *Note Source Properties::.
For completeness, we should mention that it is possible to strip the
metadata from a syntax object, returning a raw Scheme datum:
-- Scheme Procedure: syntax->datum syntax-object
Strip the metadata from SYNTAX-OBJECT, returning its contents as a
raw Scheme datum.
In this case we want to introduce ‘it’ in the context of the whole
expression, so we can create a syntax object as ‘(datum->syntax x 'it)’,
where ‘x’ is the whole expression, as passed to the transformer
procedure.
Here’s another solution that doesn’t work:
;; doesn't work either
(define-syntax aif
(lambda (x)
(syntax-case x ()
((_ test then else)
(let ((it (datum->syntax x 'it)))
#'(let ((it test))
(if it then else)))))))
The reason that this one doesn’t work is that there are really two
environments at work here – the environment of pattern variables, as
bound by ‘syntax-case’, and the environment of lexical variables, as
bound by normal Scheme. The outer let form establishes a binding in the
environment of lexical variables, but the inner let form is inside a
syntax form, where only pattern variables will be substituted. Here we
need to introduce a piece of the lexical environment into the pattern
variable environment, and we can do so using ‘syntax-case’ itself:
;; works, but is obtuse
(define-syntax aif
(lambda (x)
(syntax-case x ()
((_ test then else)
;; invoking syntax-case on the generated
;; syntax object to expose it to `syntax'
(syntax-case (datum->syntax x 'it) ()
(it
#'(let ((it test))
(if it then else))))))))
(aif (getuid) (display it) (display "none")) (newline)
⊣ 500
However there are easier ways to write this. ‘with-syntax’ is often
convenient:
-- Syntax: with-syntax ((pat val) ...) exp ...
Bind patterns PAT from their corresponding values VAL, within the
lexical context of EXP ....
;; better
(define-syntax aif
(lambda (x)
(syntax-case x ()
((_ test then else)
(with-syntax ((it (datum->syntax x 'it)))
#'(let ((it test))
(if it then else)))))))
As you might imagine, ‘with-syntax’ is defined in terms of
‘syntax-case’. But even that might be off-putting to you if you are an
old Lisp macro hacker, used to building macro output with ‘quasiquote’.
The issue is that ‘with-syntax’ creates a separation between the point
of definition of a value and its point of substitution.
So for cases in which a ‘quasiquote’ style makes more sense,
‘syntax-case’ also defines ‘quasisyntax’, and the related ‘unsyntax’ and
‘unsyntax-splicing’, abbreviated by the reader as ‘#`’, ‘#,’, and ‘#,@’,
respectively.
For example, to define a macro that inserts a compile-time timestamp
into a source file, one may write:
(define-syntax display-compile-timestamp
(lambda (x)
(syntax-case x ()
((_)
#`(begin
(display "The compile timestamp was: ")
(display #,(current-time))
(newline))))))
Readers interested in further information on ‘syntax-case’ macros
should see R. Kent Dybvig’s excellent ‘The Scheme Programming Language’,
either edition 3 or 4, in the chapter on syntax. Dybvig was the primary
author of the ‘syntax-case’ system. The book itself is available online
at )
Guile’s perspective is that defmacros have had a good run, but that
modern macros should be written with ‘syntax-rules’ or ‘syntax-case’.
There are still many uses of defmacros within Guile itself, but we will
be phasing them out over time. Of course we won’t take away ‘defmacro’
or ‘define-macro’ themselves, as there is lots of code out there that
uses them.
6.8.6 Identifier Macros
-----------------------
When the syntax expander sees a form in which the first element is a
macro, the whole form gets passed to the macro’s syntax transformer.
One may visualize this as:
(define-syntax foo foo-transformer)
(foo ARG...)
;; expands via
(foo-transformer #'(foo ARG...))
If, on the other hand, a macro is referenced in some other part of a
form, the syntax transformer is invoked with only the macro reference,
not the whole form.
(define-syntax foo foo-transformer)
foo
;; expands via
(foo-transformer #'foo)
This allows bare identifier references to be replaced
programmatically via a macro. ‘syntax-rules’ provides some syntax to
effect this transformation more easily.
-- Syntax: identifier-syntax exp
Returns a macro transformer that will replace occurrences of the
macro with EXP.
For example, if you are importing external code written in terms of
‘fx+’, the fixnum addition operator, but Guile doesn’t have ‘fx+’, you
may use the following to replace ‘fx+’ with ‘+’:
(define-syntax fx+ (identifier-syntax +))
There is also special support for recognizing identifiers on the
left-hand side of a ‘set!’ expression, as in the following:
(define-syntax foo foo-transformer)
(set! foo VAL)
;; expands via
(foo-transformer #'(set! foo VAL))
;; if foo-transformer is a "variable transformer"
As the example notes, the transformer procedure must be explicitly
marked as being a “variable transformer”, as most macros aren’t written
to discriminate on the form in the operator position.
-- Scheme Procedure: make-variable-transformer transformer
Mark the TRANSFORMER procedure as being a “variable transformer”.
In practice this means that, when bound to a syntactic keyword, it
may detect references to that keyword on the left-hand-side of a
‘set!’.
(define bar 10)
(define-syntax bar-alias
(make-variable-transformer
(lambda (x)
(syntax-case x (set!)
((set! var val) #'(set! bar val))
((var arg ...) #'(bar arg ...))
(var (identifier? #'var) #'bar)))))
bar-alias ⇒ 10
(set! bar-alias 20)
bar ⇒ 20
(set! bar 30)
bar-alias ⇒ 30
There is an extension to identifier-syntax which allows it to handle
the ‘set!’ case as well:
-- Syntax: identifier-syntax (var exp1) ((set! var val) exp2)
Create a variable transformer. The first clause is used for
references to the variable in operator or operand position, and the
second for appearances of the variable on the left-hand-side of an
assignment.
For example, the previous ‘bar-alias’ example could be expressed
more succinctly like this:
(define-syntax bar-alias
(identifier-syntax
(var bar)
((set! var val) (set! bar val))))
As before, the templates in ‘identifier-syntax’ forms do not need
wrapping in ‘#'’ syntax forms.
6.8.7 Syntax Parameters
-----------------------
Syntax parameters(1) are a mechanism for rebinding a macro definition
within the dynamic extent of a macro expansion. This provides a
convenient solution to one of the most common types of unhygienic macro:
those that introduce a unhygienic binding each time the macro is used.
Examples include a ‘lambda’ form with a ‘return’ keyword, or class
macros that introduce a special ‘self’ binding.
With syntax parameters, instead of introducing the binding
unhygienically each time, we instead create one binding for the keyword,
which we can then adjust later when we want the keyword to have a
different meaning. As no new bindings are introduced, hygiene is
preserved. This is similar to the dynamic binding mechanisms we have at
run-time (*note parameters: SRFI-39.), except that the dynamic binding
only occurs during macro expansion. The code after macro expansion
remains lexically scoped.
-- Syntax: define-syntax-parameter keyword transformer
Binds KEYWORD to the value obtained by evaluating TRANSFORMER. The
TRANSFORMER provides the default expansion for the syntax
parameter, and in the absence of ‘syntax-parameterize’, is
functionally equivalent to ‘define-syntax’. Usually, you will just
want to have the TRANSFORMER throw a syntax error indicating that
the KEYWORD is supposed to be used in conjunction with another
macro, for example:
(define-syntax-parameter return
(lambda (stx)
(syntax-violation 'return "return used outside of a lambda^" stx)))
-- Syntax: syntax-parameterize ((keyword transformer) ...) exp ...
Adjusts KEYWORD ... to use the values obtained by evaluating their
TRANSFORMER ..., in the expansion of the EXP ... forms. Each
KEYWORD must be bound to a syntax-parameter. ‘syntax-parameterize’
differs from ‘let-syntax’, in that the binding is not shadowed, but
adjusted, and so uses of the keyword in the expansion of EXP ...
use the new transformers. This is somewhat similar to how
‘parameterize’ adjusts the values of regular parameters, rather
than creating new bindings.
(define-syntax lambda^
(syntax-rules ()
[(lambda^ argument-list body body* ...)
(lambda argument-list
(call-with-current-continuation
(lambda (escape)
;; In the body we adjust the 'return' keyword so that calls
;; to 'return' are replaced with calls to the escape
;; continuation.
(syntax-parameterize ([return (syntax-rules ()
[(return vals (... ...))
(escape vals (... ...))])])
body body* ...))))]))
;; Now we can write functions that return early. Here, 'product' will
;; return immediately if it sees any 0 element.
(define product
(lambda^ (list)
(fold (lambda (n o)
(if (zero? n)
(return 0)
(* n o)))
1
list)))
---------- Footnotes ----------
(1) Described in the paper ‘Keeping it Clean with Syntax Parameters’
by Barzilay, Culpepper and Flatt.
6.8.8 Eval-when
---------------
As ‘syntax-case’ macros have the whole power of Scheme available to
them, they present a problem regarding time: when a macro runs, what
parts of the program are available for the macro to use?
The default answer to this question is that when you import a module
(via ‘define-module’ or ‘use-modules’), that module will be loaded up at
expansion-time, as well as at run-time. Additionally, top-level
syntactic definitions within one compilation unit made by
‘define-syntax’ are also evaluated at expansion time, in the order that
they appear in the compilation unit (file).
But if a syntactic definition needs to call out to a normal procedure
at expansion-time, it might well need need special declarations to
indicate that the procedure should be made available at expansion-time.
For example, the following code tries to embed a compilation
timestamp in the compiled bytecode using a macro that expands to the
date as a string literal. It will work at a REPL, but not in a file, as
it cannot be byte-compiled:
(use-modules (srfi srfi-19))
(define start-date (date->string (current-date)))
(define-syntax *compilation-date*
(lambda (sintax)
start-date))
(display *compilation-date*)
(newline)
It works at a REPL because the expressions are evaluated one-by-one,
in order, but if placed in a file, the expressions are expanded
one-by-one, but not evaluated until the compiled file is loaded.
The fix is to use ‘eval-when’.
(use-modules (srfi srfi-19))
(eval-when (expand load eval)
(define start-date (date->string (current-date))))
(define-syntax *compilation-date*
(lambda (sintax)
start-date))
(display *compilation-date*)
(newline)
-- Syntax: eval-when conditions exp...
Evaluate EXP... under the given CONDITIONS. Valid conditions
include:
‘expand’
Evaluate during macro expansion, whether compiling or not.
‘load’
Evaluate during the evaluation phase of compiled code, e.g.
when loading a compiled module or running compiled code at the
REPL.
‘eval’
Evaluate during the evaluation phase of non-compiled code.
‘compile’
Evaluate during macro expansion, but only when compiling.
In other words, when using the primitive evaluator, ‘eval-when’
expressions with ‘expand’ are run during macro expansion, and those
with ‘eval’ are run during the evaluation phase.
When using the compiler, ‘eval-when’ expressions with either
‘expand’ or ‘compile’ are run during macro expansion, and those
with ‘load’ are run during the evaluation phase.
When in doubt, use the three conditions ‘(expand load eval)’, as in
the example above. Other uses of ‘eval-when’ may void your
warranty or poison your cat.
6.8.9 Macro Expansion
---------------------
Usually, macros are expanded on behalf of the user as needed. Macro
expansion is an integral part of ‘eval’ and ‘compile’. Users can also
expand macros at the REPL prompt via the ‘expand’ REPL command; *Note
Compile Commands::.
Macros can also be expanded programmatically, via ‘macroexpand’, but
the details get a bit hairy for two reasons.
The first complication is that the result of macro-expansion isn’t
Scheme: it’s Tree-IL, Guile’s high-level intermediate language. *Note
Tree-IL::. As “hygienic macros” can produce identifiers that are
distinct but have the same name, the output format needs to be able to
represent distinctions between variable identities and names. Again,
*Note Tree-IL::, for all the details. The easiest thing is to just run
‘tree-il->scheme’ on the result of macro-expansion:
(macroexpand '(+ 1 2))
⇒
#
(use-modules (language tree-il))
(tree-il->scheme (macroexpand '(+ 1 2)))
⇒
(+ 1 2)
The second complication involves ‘eval-when’. As an example, what
would it mean to macro-expand the definition of a macro?
(macroexpand '(define-syntax qux (identifier-syntax 'bar)))
⇒
?
The answer is that it depends who is macro-expanding, and why. Do
you define the macro in the current environment? Residualize a macro
definition? Both? Neither? The default is to expand in “eval” mode,
which means an ‘eval-when’ clauses will only proceed when ‘eval’ (or
‘expand’) is in its condition set. Top-level macros will be ‘eval’’d in
the top-level environment.
In this way ‘(macroexpand FOO)’ is equivalent to ‘(macroexpand FOO 'e
'(eval))’. The second argument is the mode (‘'e’ for “eval”) and the
third is the eval-syntax-expanders-when parameter (only ‘eval’ in this
default setting).
But if you are compiling the macro definition, probably you want to
reify the macro definition itself. In that case you pass ‘'c’ as the
second argument to ‘macroexpand’. But probably you want the macro
definition to be present at compile time as well, so you pass ‘'(compile
load eval)’ as the ESEW parameter. In fact ‘(compile FOO #:to
'tree-il)’ is entirely equivalent to ‘(macroexpand FOO 'c '(compile load
eval))’; *Note The Scheme Compiler::.
It’s a terrible interface; we know. The macroexpander is somewhat
tricksy regarding modes, so unless you are building a macro-expanding
tool, we suggest to avoid invoking it directly.
6.8.10 Hygiene and the Top-Level
--------------------------------
Consider the following macro.
(define-syntax-rule (defconst name val)
(begin
(define t val)
(define-syntax-rule (name) t)))
If we use it to make a couple of bindings:
(defconst foo 42)
(defconst bar 37)
The expansion would look something like this:
(begin
(define t 42)
(define-syntax-rule (foo) t))
(begin
(define t 37)
(define-syntax-rule (bar) t))
As the two ‘t’ bindings were introduced by the macro, they should be
introduced hygienically – and indeed they are, inside a lexical contour
(a ‘let’ or some other lexical scope). The ‘t’ reference in ‘foo’ is
distinct to the reference in ‘bar’.
At the top-level things are more complicated. Before Guile 2.2, a
use of ‘defconst’ at the top-level would not introduce a fresh binding
for ‘t’. This was consistent with a weaselly interpretation of the
Scheme standard, in which all possible bindings may be assumed to exist,
at the top-level, and in which we merely take advantage of toplevel
‘define’ of an existing binding being equivalent to ‘set!’. But it’s
not a good reason.
The solution is to create fresh names for all bindings introduced by
macros – not just bindings in lexical contours, but also bindings
introduced at the top-level.
However, the obvious strategy of just giving random names to
introduced toplevel identifiers poses a problem for separate
compilation. Consider without loss of generality a ‘defconst’ of ‘foo’
in module ‘a’ that introduces the fresh top-level name ‘t-1’. If we
then compile a module ‘b’ that uses ‘foo’, there is now a reference to
‘t-1’ in module ‘b’. If module ‘a’ is then expanded again, for whatever
reason, for example in a simple recompilation, the introduced ‘t’ gets a
fresh name; say, ‘t-2’. Now module ‘b’ has broken because module ‘a’ no
longer has a binding for ‘t-1’.
If introduced top-level identifiers “escape” a module, in whatever
way, they then form part of the binary interface (ABI) of a module. It
is unacceptable from an engineering point of view to allow the ABI to
change randomly. (It also poses practical problems in meeting the
recompilation conditions of the Lesser GPL license, for such modules.)
For this reason many people prefer to never use identifier-introducing
macros at the top-level, instead making those macros receive the names
for their introduced identifiers as part of their arguments, or to
construct them programmatically and use ‘datum->syntax’. But this
approach requires omniscience as to the implementation of all macros one
might use, and also limits the expressive power of Scheme macros.
There is no perfect solution to this issue. Guile does a terrible
thing here. When it goes to introduce a top-level identifier, Guile
gives the identifier a pseudo-fresh name: a name that depends on the
hash of the source expression in which the name occurs. The result in
this case is that the introduced definitions expand as:
(begin
(define t-1dc5e42de7c1050c 42)
(define-syntax-rule (foo) t-1dc5e42de7c1050c))
(begin
(define t-10cb8ce9fdddd6e9 37)
(define-syntax-rule (bar) t-10cb8ce9fdddd6e9))
However, note that as the hash depends solely on the expression
introducing the definition, we also have:
(defconst baz 42)
⇒ (begin
(define t-1dc5e42de7c1050c 42)
(define-syntax-rule (baz) t-1dc5e42de7c1050c))
Note that the introduced binding has the same name! This is because
the source expression, ‘(define t 42)’, was the same. Probably you will
never see an error in this area, but it is important to understand the
components of the interface of a module, and that interface may include
macro-introduced identifiers.
6.8.11 Internal Macros
----------------------
-- Scheme Procedure: make-syntax-transformer name type binding
Construct a syntax transformer object. This is part of Guile’s
low-level support for syntax-case.
-- Scheme Procedure: macro? obj
-- C Function: scm_macro_p (obj)
Return ‘#t’ if OBJ is a syntax transformer, or ‘#f’ otherwise.
Note that it’s a bit difficult to actually get a macro as a
first-class object; simply naming it (like ‘case’) will produce a
syntax error. But it is possible to get these objects using
‘module-ref’:
(macro? (module-ref (current-module) 'case))
⇒ #t
-- Scheme Procedure: macro-type m
-- C Function: scm_macro_type (m)
Return the TYPE that was given when M was constructed, via
‘make-syntax-transformer’.
-- Scheme Procedure: macro-name m
-- C Function: scm_macro_name (m)
Return the name of the macro M.
-- Scheme Procedure: macro-binding m
-- C Function: scm_macro_binding (m)
Return the binding of the macro M.
-- Scheme Procedure: macro-transformer m
-- C Function: scm_macro_transformer (m)
Return the transformer of the macro M. This will return a
procedure, for which one may ask the docstring. That’s the whole
reason this section is documented. Actually a part of the result
of ‘macro-binding’.
6.9 General Utility Functions
=============================
This chapter contains information about procedures which are not cleanly
tied to a specific data type. Because of their wide range of
applications, they are collected in a “utility” chapter.
6.9.1 Equality
--------------
There are three kinds of core equality predicates in Scheme, described
below. The same kinds of comparisons arise in other functions, like
‘memq’ and friends (*note List Searching::).
For all three tests, objects of different types are never equal. So
for instance a list and a vector are not ‘equal?’, even if their
contents are the same. Exact and inexact numbers are considered
different types too, and are hence not equal even if their values are
the same.
‘eq?’ tests just for the same object (essentially a pointer
comparison). This is fast, and can be used when searching for a
particular object, or when working with symbols or keywords (which are
always unique objects).
‘eqv?’ extends ‘eq?’ to look at the value of numbers and characters.
It can for instance be used somewhat like ‘=’ (*note Comparison::) but
without an error if one operand isn’t a number.
‘equal?’ goes further, it looks (recursively) into the contents of
lists, vectors, etc. This is good for instance on lists that have been
read or calculated in various places and are the same, just not made up
of the same pairs. Such lists look the same (when printed), and
‘equal?’ will consider them the same.
-- Scheme Procedure: eq? ...
-- C Function: scm_eq_p (x, y)
The Scheme procedure returns ‘#t’ if all of its arguments are the
same object, except for numbers and characters. The C function
does the same but takes exactly two arguments. For example,
(define x (vector 1 2 3))
(define y (vector 1 2 3))
(eq? x x) ⇒ #t
(eq? x y) ⇒ #f
Numbers and characters are not equal to any other object, but the
problem is they’re not necessarily ‘eq?’ to themselves either.
This is even so when the number comes directly from a variable,
(let ((n (+ 2 3)))
(eq? n n)) ⇒ *unspecified*
Generally ‘eqv?’ below should be used when comparing numbers or
characters. ‘=’ (*note Comparison::) or ‘char=?’ (*note
Characters::) can be used too.
It’s worth noting that end-of-list ‘()’, ‘#t’, ‘#f’, a symbol of a
given name, and a keyword of a given name, are unique objects.
There’s just one of each, so for instance no matter how ‘()’ arises
in a program, it’s the same object and can be compared with ‘eq?’,
(define x (cdr '(123)))
(define y (cdr '(456)))
(eq? x y) ⇒ #t
(define x (string->symbol "foo"))
(eq? x 'foo) ⇒ #t
-- C Function: int scm_is_eq (SCM x, SCM y)
Return ‘1’ when X and Y are equal in the sense of ‘eq?’, otherwise
return ‘0’.
The ‘==’ operator should not be used on ‘SCM’ values, an ‘SCM’ is a
C type which cannot necessarily be compared using ‘==’ (*note The
SCM Type::).
-- Scheme Procedure: eqv? ...
-- C Function: scm_eqv_p (x, y)
The Scheme procedure returns ‘#t’ if all of its arguments are the
same object, or for characters and numbers the same value. The C
function is similar but takes exactly two arguments.
On objects except characters and numbers, ‘eqv?’ is the same as
‘eq?’ above. ‘(eqv? x y)’ is true if X and Y are the same object.
If X and Y are numbers or characters, ‘eqv?’ compares their type
and value. An exact number is not ‘eqv?’ to an inexact number
(even if their value is the same).
(eqv? 3 (+ 1 2)) ⇒ #t
(eqv? 1 1.0) ⇒ #f
-- Scheme Procedure: equal? ...
-- C Function: scm_equal_p (x, y)
The Scheme procedure returns ‘#t’ if all of its arguments are the
same type, and their contents or value are equal. The C function
is similar, but takes exactly two arguments.
For a pair, string, vector, array or structure, ‘equal?’ compares
the contents, and does so using the same ‘equal?’ recursively, so a
deep structure can be traversed.
(equal? (list 1 2 3) (list 1 2 3)) ⇒ #t
(equal? (list 1 2 3) (vector 1 2 3)) ⇒ #f
For other objects, ‘equal?’ compares as per ‘eqv?’ above, which
means characters and numbers are compared by type and value (and
like ‘eqv?’, exact and inexact numbers are not ‘equal?’, even if
their value is the same).
(equal? 3 (+ 1 2)) ⇒ #t
(equal? 1 1.0) ⇒ #f
Hash tables are currently only compared as per ‘eq?’, so two
different tables are not ‘equal?’, even if their contents are the
same.
‘equal?’ does not support circular data structures, it may go into
an infinite loop if asked to compare two circular lists or similar.
GOOPS object types (*note GOOPS::), including foreign object types
(*note Defining New Foreign Object Types::), can have an ‘equal?’
implementation specialized on two values of the same type. If
‘equal?’ is called on two GOOPS objects of the same type, ‘equal?’
will dispatch out to a generic function. This lets an application
traverse the contents or control what is considered ‘equal?’ for
two objects of such a type. If there’s no such handler, the
default is to just compare as per ‘eq?’.
6.9.2 Object Properties
-----------------------
It’s often useful to associate a piece of additional information with a
Scheme object even though that object does not have a dedicated slot
available in which the additional information could be stored. Object
properties allow you to do just that.
Guile’s representation of an object property is a
procedure-with-setter (*note Procedures with Setters::) that can be used
with the generalized form of ‘set!’ to set and retrieve that property
for any Scheme object. So, setting a property looks like this:
(set! (my-property obj1) value-for-obj1)
(set! (my-property obj2) value-for-obj2)
And retrieving values of the same property looks like this:
(my-property obj1)
⇒
value-for-obj1
(my-property obj2)
⇒
value-for-obj2
To create an object property in the first place, use the
‘make-object-property’ procedure:
(define my-property (make-object-property))
-- Scheme Procedure: make-object-property
Create and return an object property. An object property is a
procedure-with-setter that can be called in two ways. ‘(set!
(PROPERTY OBJ) VAL)’ sets OBJ’s PROPERTY to VAL. ‘(PROPERTY OBJ)’
returns the current setting of OBJ’s PROPERTY.
A single object property created by ‘make-object-property’ can
associate distinct property values with all Scheme values that are
distinguishable by ‘eq?’ (ruling out numeric values).
Internally, object properties are implemented using a weak key hash
table. This means that, as long as a Scheme value with property values
is protected from garbage collection, its property values are also
protected. When the Scheme value is collected, its entry in the
property table is removed and so the (ex-) property values are no longer
protected by the table.
Guile also implements a more traditional Lispy interface to
properties, in which each object has an list of key-value pairs
associated with it. Properties in that list are keyed by symbols. This
is a legacy interface; you should use weak hash tables or object
properties instead.
-- Scheme Procedure: object-properties obj
-- C Function: scm_object_properties (obj)
Return OBJ’s property list.
-- Scheme Procedure: set-object-properties! obj alist
-- C Function: scm_set_object_properties_x (obj, alist)
Set OBJ’s property list to ALIST.
-- Scheme Procedure: object-property obj key
-- C Function: scm_object_property (obj, key)
Return the property of OBJ with name KEY.
-- Scheme Procedure: set-object-property! obj key value
-- C Function: scm_set_object_property_x (obj, key, value)
In OBJ’s property list, set the property named KEY to VALUE.
6.9.3 Sorting
-------------
Sorting is very important in computer programs. Therefore, Guile comes
with several sorting procedures built-in. As always, procedures with
names ending in ‘!’ are side-effecting, that means that they may modify
their parameters in order to produce their results.
The first group of procedures can be used to merge two lists (which
must be already sorted on their own) and produce sorted lists containing
all elements of the input lists.
-- Scheme Procedure: merge alist blist less
-- C Function: scm_merge (alist, blist, less)
Merge two already sorted lists into one. Given two lists ALIST and
BLIST, such that ‘(sorted? alist less?)’ and ‘(sorted? blist
less?)’, return a new list in which the elements of ALIST and BLIST
have been stably interleaved so that ‘(sorted? (merge alist blist
less?) less?)’. Note: this does _not_ accept vectors.
-- Scheme Procedure: merge! alist blist less
-- C Function: scm_merge_x (alist, blist, less)
Takes two lists ALIST and BLIST such that ‘(sorted? alist less?)’
and ‘(sorted? blist less?)’ and returns a new list in which the
elements of ALIST and BLIST have been stably interleaved so that
‘(sorted? (merge alist blist less?) less?)’. This is the
destructive variant of ‘merge’ Note: this does _not_ accept
vectors.
The following procedures can operate on sequences which are either
vectors or list. According to the given arguments, they return sorted
vectors or lists, respectively. The first of the following procedures
determines whether a sequence is already sorted, the other sort a given
sequence. The variants with names starting with ‘stable-’ are special
in that they maintain a special property of the input sequences: If two
or more elements are the same according to the comparison predicate,
they are left in the same order as they appeared in the input.
-- Scheme Procedure: sorted? items less
-- C Function: scm_sorted_p (items, less)
Return ‘#t’ if ITEMS is a list or vector such that, for each
element X and the next element Y of ITEMS, ‘(LESS Y X)’ returns
‘#f’. Otherwise return ‘#f’.
-- Scheme Procedure: sort items less
-- C Function: scm_sort (items, less)
Sort the sequence ITEMS, which may be a list or a vector. LESS is
used for comparing the sequence elements. This is not a stable
sort.
-- Scheme Procedure: sort! items less
-- C Function: scm_sort_x (items, less)
Sort the sequence ITEMS, which may be a list or a vector. LESS is
used for comparing the sequence elements. The sorting is
destructive, that means that the input sequence is modified to
produce the sorted result. This is not a stable sort.
-- Scheme Procedure: stable-sort items less
-- C Function: scm_stable_sort (items, less)
Sort the sequence ITEMS, which may be a list or a vector. LESS is
used for comparing the sequence elements. This is a stable sort.
-- Scheme Procedure: stable-sort! items less
-- C Function: scm_stable_sort_x (items, less)
Sort the sequence ITEMS, which may be a list or a vector. LESS is
used for comparing the sequence elements. The sorting is
destructive, that means that the input sequence is modified to
produce the sorted result. This is a stable sort.
The procedures in the last group only accept lists or vectors as
input, as their names indicate.
-- Scheme Procedure: sort-list items less
-- C Function: scm_sort_list (items, less)
Sort the list ITEMS, using LESS for comparing the list elements.
This is a stable sort.
-- Scheme Procedure: sort-list! items less
-- C Function: scm_sort_list_x (items, less)
Sort the list ITEMS, using LESS for comparing the list elements.
The sorting is destructive, that means that the input list is
modified to produce the sorted result. This is a stable sort.
-- Scheme Procedure: restricted-vector-sort! vec less startpos endpos
-- C Function: scm_restricted_vector_sort_x (vec, less, startpos,
endpos)
Sort the vector VEC, using LESS for comparing the vector elements.
STARTPOS (inclusively) and ENDPOS (exclusively) delimit the range
of the vector which gets sorted. The return value is not
specified.
6.9.4 Copying Deep Structures
-----------------------------
The procedures for copying lists (*note Lists::) only produce a flat
copy of the input list, and currently Guile does not even contain
procedures for copying vectors. The ‘(ice-9 copy-tree)’ module contains
a ‘copy-tree’ function that can be used for this purpose, as it does not
only copy the spine of a list, but also copies any pairs in the cars of
the input lists.
(use-modules (ice-9 copy-tree))
-- Scheme Procedure: copy-tree obj
-- C Function: scm_copy_tree (obj)
Recursively copy the data tree that is bound to OBJ, and return the
new data structure. ‘copy-tree’ recurses down the contents of both
pairs and vectors (since both cons cells and vector cells may point
to arbitrary objects), and stops recursing when it hits any other
object.
6.9.5 General String Conversion
-------------------------------
When debugging Scheme programs, but also for providing a human-friendly
interface, a procedure for converting any Scheme object into string
format is very useful. Conversion from/to strings can of course be done
with specialized procedures when the data type of the object to convert
is known, but with this procedure, it is often more comfortable.
‘object->string’ converts an object by using a print procedure for
writing to a string port, and then returning the resulting string.
Converting an object back from the string is only possible if the object
type has a read syntax and the read syntax is preserved by the printing
procedure.
-- Scheme Procedure: object->string obj [printer]
-- C Function: scm_object_to_string (obj, printer)
Return a Scheme string obtained by printing OBJ. Printing function
can be specified by the optional second argument PRINTER (default:
‘write’).
6.9.6 Hooks
-----------
A hook is a list of procedures to be called at well defined points in
time. Typically, an application provides a hook H and promises its
users that it will call all of the procedures in H at a defined point in
the application’s processing. By adding its own procedure to H, an
application user can tap into or even influence the progress of the
application.
Guile itself provides several such hooks for debugging and
customization purposes: these are listed in a subsection below.
When an application first creates a hook, it needs to know how many
arguments will be passed to the hook’s procedures when the hook is run.
The chosen number of arguments (which may be none) is declared when the
hook is created, and all the procedures that are added to that hook must
be capable of accepting that number of arguments.
A hook is created using ‘make-hook’. A procedure can be added to or
removed from a hook using ‘add-hook!’ or ‘remove-hook!’, and all of a
hook’s procedures can be removed together using ‘reset-hook!’. When an
application wants to run a hook, it does so using ‘run-hook’.
6.9.6.1 Hook Usage by Example
.............................
Hook usage is shown by some examples in this section. First, we will
define a hook of arity 2 — that is, the procedures stored in the hook
will have to accept two arguments.
(define hook (make-hook 2))
hook
⇒ #
Now we are ready to add some procedures to the newly created hook
with ‘add-hook!’. In the following example, two procedures are added,
which print different messages and do different things with their
arguments.
(add-hook! hook (lambda (x y)
(display "Foo: ")
(display (+ x y))
(newline)))
(add-hook! hook (lambda (x y)
(display "Bar: ")
(display (* x y))
(newline)))
Once the procedures have been added, we can invoke the hook using
‘run-hook’.
(run-hook hook 3 4)
⊣ Bar: 12
⊣ Foo: 7
Note that the procedures are called in the reverse of the order with
which they were added. This is because the default behaviour of
‘add-hook!’ is to add its procedure to the _front_ of the hook’s
procedure list. You can force ‘add-hook!’ to add its procedure to the
_end_ of the list instead by providing a third ‘#t’ argument on the
second call to ‘add-hook!’.
(add-hook! hook (lambda (x y)
(display "Foo: ")
(display (+ x y))
(newline)))
(add-hook! hook (lambda (x y)
(display "Bar: ")
(display (* x y))
(newline))
#t) ; <- Change here!
(run-hook hook 3 4)
⊣ Foo: 7
⊣ Bar: 12
6.9.6.2 Hook Reference
......................
When you create a hook with ‘make-hook’, you must specify the arity of
the procedures which can be added to the hook. If the arity is not
given explicitly as an argument to ‘make-hook’, it defaults to zero.
All procedures of a given hook must have the same arity, and when the
procedures are invoked using ‘run-hook’, the number of arguments passed
must match the arity specified at hook creation time.
The order in which procedures are added to a hook matters. If the
third parameter to ‘add-hook!’ is omitted or is equal to ‘#f’, the
procedure is added in front of the procedures which might already be on
that hook, otherwise the procedure is added at the end. The procedures
are always called from the front to the end of the list when they are
invoked via ‘run-hook’.
The ordering of the list of procedures returned by ‘hook->list’
matches the order in which those procedures would be called if the hook
was run using ‘run-hook’.
Note that the C functions in the following entries are for handling
“Scheme-level” hooks in C. There are also “C-level” hooks which have
their own interface (*note C Hooks::).
-- Scheme Procedure: make-hook [n_args]
-- C Function: scm_make_hook (n_args)
Create a hook for storing procedure of arity N_ARGS. N_ARGS
defaults to zero. The returned value is a hook object to be used
with the other hook procedures.
-- Scheme Procedure: hook? x
-- C Function: scm_hook_p (x)
Return ‘#t’ if X is a hook, ‘#f’ otherwise.
-- Scheme Procedure: hook-empty? hook
-- C Function: scm_hook_empty_p (hook)
Return ‘#t’ if HOOK is an empty hook, ‘#f’ otherwise.
-- Scheme Procedure: add-hook! hook proc [append_p]
-- C Function: scm_add_hook_x (hook, proc, append_p)
Add the procedure PROC to the hook HOOK. The procedure is added to
the end if APPEND_P is true, otherwise it is added to the front.
The return value of this procedure is not specified.
-- Scheme Procedure: remove-hook! hook proc
-- C Function: scm_remove_hook_x (hook, proc)
Remove the procedure PROC from the hook HOOK. The return value of
this procedure is not specified.
-- Scheme Procedure: reset-hook! hook
-- C Function: scm_reset_hook_x (hook)
Remove all procedures from the hook HOOK. The return value of this
procedure is not specified.
-- Scheme Procedure: hook->list hook
-- C Function: scm_hook_to_list (hook)
Convert the procedure list of HOOK to a list.
-- Scheme Procedure: run-hook hook arg ...
-- C Function: scm_run_hook (hook, args)
Apply all procedures from the hook HOOK to the arguments ARG ....
The order of the procedure application is first to last. The
return value of this procedure is not specified.
If, in C code, you are certain that you have a hook object and well
formed argument list for that hook, you can also use ‘scm_c_run_hook’,
which is identical to ‘scm_run_hook’ but does no type checking.
-- C Function: void scm_c_run_hook (SCM hook, SCM args)
The same as ‘scm_run_hook’ but without any type checking to confirm
that HOOK is actually a hook object and that ARGS is a well-formed
list matching the arity of the hook.
For C code, ‘SCM_HOOKP’ is a faster alternative to ‘scm_hook_p’:
-- C Macro: int SCM_HOOKP (x)
Return 1 if X is a Scheme-level hook, 0 otherwise.
6.9.6.3 Hooks For C Code.
.........................
The hooks already described are intended to be populated by Scheme-level
procedures. In addition to this, the Guile library provides an
independent set of interfaces for the creation and manipulation of hooks
that are designed to be populated by functions implemented in C.
The original motivation here was to provide a kind of hook that could
safely be invoked at various points during garbage collection.
Scheme-level hooks are unsuitable for this purpose as running them could
itself require memory allocation, which would then invoke garbage
collection recursively ... However, it is also the case that these hooks
are easier to work with than the Scheme-level ones if you only want to
register C functions with them. So if that is mainly what your code
needs to do, you may prefer to use this interface.
To create a C hook, you should allocate storage for a structure of
type ‘scm_t_c_hook’ and then initialize it using ‘scm_c_hook_init’.
-- C Type: scm_t_c_hook
Data type for a C hook. The internals of this type should be
treated as opaque.
-- C Enum: scm_t_c_hook_type
Enumeration of possible hook types, which are:
‘SCM_C_HOOK_NORMAL’
Type of hook for which all the registered functions will
always be called.
‘SCM_C_HOOK_OR’
Type of hook for which the sequence of registered functions
will be called only until one of them returns C true (a
non-NULL pointer).
‘SCM_C_HOOK_AND’
Type of hook for which the sequence of registered functions
will be called only until one of them returns C false (a NULL
pointer).
-- C Function: void scm_c_hook_init (scm_t_c_hook *hook, void
*hook_data, scm_t_c_hook_type type)
Initialize the C hook at memory pointed to by HOOK. TYPE should be
one of the values of the ‘scm_t_c_hook_type’ enumeration, and
controls how the hook functions will be called. HOOK_DATA is a
closure parameter that will be passed to all registered hook
functions when they are called.
To add or remove a C function from a C hook, use ‘scm_c_hook_add’ or
‘scm_c_hook_remove’. A hook function must expect three ‘void *’
parameters which are, respectively:
HOOK_DATA
The hook closure data that was specified at the time the hook was
initialized by ‘scm_c_hook_init’.
FUNC_DATA
The function closure data that was specified at the time that that
function was registered with the hook by ‘scm_c_hook_add’.
DATA
The call closure data specified by the ‘scm_c_hook_run’ call that
runs the hook.
-- C Type: scm_t_c_hook_function
Function type for a C hook function: takes three ‘void *’
parameters and returns a ‘void *’ result.
-- C Function: void scm_c_hook_add (scm_t_c_hook *hook,
scm_t_c_hook_function func, void *func_data, int appendp)
Add function FUNC, with function closure data FUNC_DATA, to the C
hook HOOK. The new function is appended to the hook’s list of
functions if APPENDP is non-zero, otherwise prepended.
-- C Function: void scm_c_hook_remove (scm_t_c_hook *hook,
scm_t_c_hook_function func, void *func_data)
Remove function FUNC, with function closure data FUNC_DATA, from
the C hook HOOK. ‘scm_c_hook_remove’ checks both FUNC and
FUNC_DATA so as to allow for the same FUNC being registered
multiple times with different closure data.
Finally, to invoke a C hook, call the ‘scm_c_hook_run’ function
specifying the hook and the call closure data for this run:
-- C Function: void * scm_c_hook_run (scm_t_c_hook *hook, void *data)
Run the C hook HOOK will call closure data DATA. Subject to the
variations for hook types ‘SCM_C_HOOK_OR’ and ‘SCM_C_HOOK_AND’,
‘scm_c_hook_run’ calls HOOK’s registered functions in turn, passing
them the hook’s closure data, each function’s closure data, and the
call closure data.
‘scm_c_hook_run’’s return value is the return value of the last
function to be called.
6.9.6.4 Hooks for Garbage Collection
....................................
Whenever Guile performs a garbage collection, it calls the following
hooks in the order shown.
-- C Hook: scm_before_gc_c_hook
C hook called at the very start of a garbage collection, after
setting ‘scm_gc_running_p’ to 1, but before entering the GC
critical section.
If garbage collection is blocked because ‘scm_block_gc’ is
non-zero, GC exits early soon after calling this hook, and no
further hooks will be called.
-- C Hook: scm_before_mark_c_hook
C hook called before beginning the mark phase of garbage
collection, after the GC thread has entered a critical section.
-- C Hook: scm_before_sweep_c_hook
C hook called before beginning the sweep phase of garbage
collection. This is the same as at the end of the mark phase,
since nothing else happens between marking and sweeping.
-- C Hook: scm_after_sweep_c_hook
C hook called after the end of the sweep phase of garbage
collection, but while the GC thread is still inside its critical
section.
-- C Hook: scm_after_gc_c_hook
C hook called at the very end of a garbage collection, after the GC
thread has left its critical section.
-- Scheme Hook: after-gc-hook
Scheme hook with arity 0. This hook is run asynchronously (*note
Asyncs::) soon after the GC has completed and any other events that
were deferred during garbage collection have been processed. (Also
accessible from C with the name ‘scm_after_gc_hook’.)
All the C hooks listed here have type ‘SCM_C_HOOK_NORMAL’, are
initialized with hook closure data NULL, are invoked by ‘scm_c_hook_run’
with call closure data NULL.
The Scheme hook ‘after-gc-hook’ is particularly useful in conjunction
with guardians (*note Guardians::). Typically, if you are using a
guardian, you want to call the guardian after garbage collection to see
if any of the objects added to the guardian have been collected. By
adding a thunk that performs this call to ‘after-gc-hook’, you can
ensure that your guardian is tested after every garbage collection
cycle.
6.9.6.5 Hooks into the Guile REPL
.................................
6.10 Definitions and Variable Bindings
======================================
Scheme supports the definition of variables in different contexts.
Variables can be defined at the top level, so that they are visible in
the entire program, and variables can be defined locally to procedures
and expressions. This is important for modularity and data abstraction.
6.10.1 Top Level Variable Definitions
-------------------------------------
At the top level of a program (i.e., not nested within any other
expression), a definition of the form
(define a VALUE)
defines a variable called ‘a’ and sets it to the value VALUE.
If the variable already exists in the current module, because it has
already been created by a previous ‘define’ expression with the same
name, its value is simply changed to the new VALUE. In this case, then,
the above form is completely equivalent to
(set! a VALUE)
This equivalence means that ‘define’ can be used interchangeably with
‘set!’ to change the value of variables at the top level of the REPL or
a Scheme source file. It is useful during interactive development when
reloading a Scheme file that you have modified, because it allows the
‘define’ expressions in that file to work as expected both the first
time that the file is loaded and on subsequent occasions.
Note, though, that ‘define’ and ‘set!’ are not always equivalent.
For example, a ‘set!’ is not allowed if the named variable does not
already exist, and the two expressions can behave differently in the
case where there are imported variables visible from another module.
-- Scheme Syntax: define name value
Create a top level variable named NAME with value VALUE. If the
named variable already exists, just change its value. The return
value of a ‘define’ expression is unspecified.
The C API equivalents of ‘define’ are ‘scm_define’ and
‘scm_c_define’, which differ from each other in whether the variable
name is specified as a ‘SCM’ symbol or as a null-terminated C string.
-- C Function: scm_define (sym, value)
-- C Function: scm_c_define (const char *name, value)
C equivalents of ‘define’, with variable name specified either by
SYM, a symbol, or by NAME, a null-terminated C string. Both
variants return the new or preexisting variable object.
‘define’ (when it occurs at top level), ‘scm_define’ and
‘scm_c_define’ all create or set the value of a variable in the top
level environment of the current module. If there was not already a
variable with the specified name belonging to the current module, but a
similarly named variable from another module was visible through having
been imported, the newly created variable in the current module will
shadow the imported variable, such that the imported variable is no
longer visible.
Attention: Scheme definitions inside local binding constructs (*note
Local Bindings::) act differently (*note Internal Definitions::).
Many people end up in a development style of adding and changing
definitions at runtime, building out their program without restarting
it. (You can do this using ‘reload-module’, the ‘reload’ REPL command,
the ‘load’ procedure, or even just pasting code into a REPL.) If you are
one of these people, you will find that sometimes there are some
variables that you _don’t_ want to redefine all the time. For these,
use ‘define-once’.
-- Scheme Syntax: define-once name value
Create a top level variable named NAME with value VALUE, but only
if NAME is not already bound in the current module.
Old Lispers probably know ‘define-once’ under its Lisp name,
‘defvar’.
6.10.2 Local Variable Bindings
------------------------------
As opposed to definitions at the top level, which creates bindings that
are visible to all code in a module, it is also possible to define
variables which are only visible in a well-defined part of the program.
Normally, this part of a program will be a procedure or a subexpression
of a procedure.
With the constructs for local binding (‘let’, ‘let*’, ‘letrec’, and
‘letrec*’), the Scheme language has a block structure like most other
programming languages since the days of ALGOL 60. Readers familiar to
languages like C or Java should already be used to this concept, but the
family of ‘let’ expressions has a few properties which are well worth
knowing.
The most basic local binding construct is ‘let’.
-- syntax: let bindings body
BINDINGS has the form
((VARIABLE1 INIT1) ...)
that is zero or more two-element lists of a variable and an
arbitrary expression each. All VARIABLE names must be distinct.
A ‘let’ expression is evaluated as follows.
• All INIT expressions are evaluated.
• New storage is allocated for the VARIABLES.
• The values of the INIT expressions are stored into the
variables.
• The expressions in BODY are evaluated in order, and the value
of the last expression is returned as the value of the ‘let’
expression.
The INIT expressions are not allowed to refer to any of the
VARIABLES.
The other binding constructs are variations on the same theme: making
new values, binding them to variables, and executing a body in that new,
extended lexical context.
-- syntax: let* bindings body
Similar to ‘let’, but the variable bindings are performed
sequentially, that means that all INIT expression are allowed to
use the variables defined on their left in the binding list.
A ‘let*’ expression can always be expressed with nested ‘let’
expressions.
(let* ((a 1) (b a))
b)
≡
(let ((a 1))
(let ((b a))
b))
-- syntax: letrec bindings body
Similar to ‘let’, but it is possible to refer to the VARIABLE from
lambda expression created in any of the INITS. That is, procedures
created in the INIT expression can recursively refer to the defined
variables.
(letrec ((even? (lambda (n)
(if (zero? n)
#t
(odd? (- n 1)))))
(odd? (lambda (n)
(if (zero? n)
#f
(even? (- n 1))))))
(even? 88))
⇒
#t
Note that while the INIT expressions may refer to the new
variables, they may not access their values. For example, making
the ‘even?’ function above creates a closure (*note About
Closure::) referencing the ‘odd?’ variable. But ‘odd?’ can’t be
called until after execution has entered the body.
-- syntax: letrec* bindings body
Similar to ‘letrec’, except the INIT expressions are bound to their
variables in order.
‘letrec*’ thus relaxes the letrec restriction, in that later INIT
expressions may refer to the values of previously bound variables.
(letrec ((a 42)
(b (+ a 10))) ;; Illegal access
(* a b))
;; The behavior of the expression above is unspecified
(letrec* ((a 42)
(b (+ a 10)))
(* a b))
⇒ 2184
There is also an alternative form of the ‘let’ form, which is used
for expressing iteration. Because of the use as a looping construct,
this form (the “named let”) is documented in the section about iteration
(*note Iteration: while do.)
6.10.3 Internal definitions
---------------------------
A ‘define’ form which appears inside the body of a ‘lambda’, ‘let’,
‘let*’, ‘letrec’, ‘letrec*’ or equivalent expression is called an
“internal definition”. An internal definition differs from a top level
definition (*note Top Level::), because the definition is only visible
inside the complete body of the enclosing form. Let us examine the
following example.
(let ((frumble "froz"))
(define banana (lambda () (apple 'peach)))
(define apple (lambda (x) x))
(banana))
⇒
peach
Here the enclosing form is a ‘let’, so the ‘define’s in the
‘let’-body are internal definitions. Because the scope of the internal
definitions is the *complete* body of the ‘let’-expression, the
‘lambda’-expression which gets bound to the variable ‘banana’ may refer
to the variable ‘apple’, even though its definition appears lexically
_after_ the definition of ‘banana’. This is because a sequence of
internal definition acts as if it were a ‘letrec*’ expression.
(let ()
(define a 1)
(define b 2)
(+ a b))
is equivalent to
(let ()
(letrec* ((a 1) (b 2))
(+ a b)))
Internal definitions may be mixed with non-definition expressions.
If an expression precedes a definition, it is treated as if it were a
definition of an unreferenced variable. So this:
(let ()
(define a 1)
(foo)
(define b 2)
(+ a b))
is equivalent to
(let ()
(letrec* ((a 1) (_ (begin (foo) #f)) (b 2))
(+ a b)))
Another noteworthy difference to top level definitions is that within
one group of internal definitions all variable names must be distinct.
Whereas on the top level a second define for a given variable acts like
a ‘set!’, for internal definitions, duplicate bound identifiers signals
an error.
As a historical note, it used to be that internal bindings were
expanded in terms of ‘letrec’, not ‘letrec*’. This was the situation
for the R5RS report and before. However with the R6RS, it was
recognized that sequential definition was a more intuitive expansion, as
in the following case:
(let ()
(define a 1)
(define b (+ a a))
(+ a b))
Guile decided to follow the R6RS in this regard, and now expands
internal definitions using ‘letrec*’. Relatedly, it used to be that
internal definitions had to precede all expressions in the body; this
restriction was relaxed in Guile 3.0.
6.10.4 Querying variable bindings
---------------------------------
Guile provides a procedure for checking whether a symbol is bound in the
top level environment.
-- Scheme Procedure: defined? sym [module]
-- C Function: scm_defined_p (sym, module)
Return ‘#t’ if SYM is defined in the module MODULE or the current
module when MODULE is not specified; otherwise return ‘#f’.
6.10.5 Binding multiple return values
-------------------------------------
-- Syntax: define-values formals expression
The EXPRESSION is evaluated, and the FORMALS are bound to the
return values in the same way that the formals in a ‘lambda’
expression are matched to the arguments in a procedure call.
(define-values (q r) (floor/ 10 3))
(list q r) ⇒ (3 1)
(define-values (x . y) (values 1 2 3))
x ⇒ 1
y ⇒ (2 3)
(define-values x (values 1 2 3))
x ⇒ (1 2 3)
6.11 Controlling the Flow of Program Execution
==============================================
See *note Control Flow:: for a discussion of how the more general
control flow of Scheme affects C code.
6.11.1 Sequencing and Splicing
------------------------------
As an expression, the ‘begin’ syntax is used to evaluate a sequence of
sub-expressions in order. Consider the conditional expression below:
(if (> x 0)
(begin (display "greater") (newline)))
If the test is true, we want to display “greater” to the current
output port, then display a newline. We use ‘begin’ to form a compound
expression out of this sequence of sub-expressions.
-- syntax: begin expr ...
The expression(s) are evaluated in left-to-right order and the
value of the last expression is returned as the value of the
‘begin’-expression. This expression type is used when the
expressions before the last one are evaluated for their side
effects.
The ‘begin’ syntax has another role in definition context (*note
Internal Definitions::). A ‘begin’ form in a definition context
“splices” its subforms into its place. For example, consider the
following procedure:
(define (make-seal)
(define-sealant seal open)
(values seal open))
Let us assume the existence of a ‘define-sealant’ macro that expands
out to some definitions wrapped in a ‘begin’, like so:
(define (make-seal)
(begin
(define seal-tag
(list 'seal))
(define (seal x)
(cons seal-tag x))
(define (sealed? x)
(and (pair? x) (eq? (car x) seal-tag)))
(define (open x)
(if (sealed? x)
(cdr x)
(error "Expected a sealed value:" x))))
(values seal open))
Here, because the ‘begin’ is in definition context, its subforms are
“spliced” into the place of the ‘begin’. This allows the definitions
created by the macro to be visible to the following expression, the
‘values’ form.
It is a fine point, but splicing and sequencing are different. It
can make sense to splice zero forms, because it can make sense to have
zero internal definitions before the expressions in a procedure or
lexical binding form. However it does not make sense to have a sequence
of zero expressions, because in that case it would not be clear what the
value of the sequence would be, because in a sequence of zero
expressions, there can be no last value. Sequencing zero expressions is
an error.
It would be more elegant in some ways to eliminate splicing from the
Scheme language, and without macros (*note Macros::), that would be a
good idea. But it is useful to be able to write macros that expand out
to multiple definitions, as in ‘define-sealant’ above, so Scheme abuses
the ‘begin’ form for these two tasks.
6.11.2 Simple Conditional Evaluation
------------------------------------
Guile provides three syntactic constructs for conditional evaluation.
‘if’ is the normal if-then-else expression (with an optional else
branch), ‘cond’ is a conditional expression with multiple branches and
‘case’ branches if an expression has one of a set of constant values.
-- syntax: if test consequent [alternate]
All arguments may be arbitrary expressions. First, TEST is
evaluated. If it returns a true value, the expression CONSEQUENT
is evaluated and ALTERNATE is ignored. If TEST evaluates to ‘#f’,
ALTERNATE is evaluated instead. The values of the evaluated branch
(CONSEQUENT or ALTERNATE) are returned as the values of the ‘if’
expression.
When ALTERNATE is omitted and the TEST evaluates to ‘#f’, the value
of the expression is not specified.
When you go to write an ‘if’ without an alternate (a “one-armed
‘if’”), part of what you are expressing is that you don’t care about the
return value (or values) of the expression. As such, you are more
interested in the _effect_ of evaluating the consequent expression. (By
convention, we use the word “statement” to refer to an expression that
is evaluated for effect, not for value).
In such a case, it is considered more clear to express these
intentions with these special forms, ‘when’ and ‘unless’. As an added
bonus, these forms accept multiple statements to evaluate, which are
implicitly wrapped in a ‘begin’.
-- Scheme Syntax: when test statement1 statement2 ...
-- Scheme Syntax: unless test statement1 statement2 ...
The actual definitions of these forms are in many ways their most
clear documentation:
(define-syntax-rule (when test stmt stmt* ...)
(if test (begin stmt stmt* ...)))
(define-syntax-rule (unless test stmt stmt* ...)
(if (not test) (begin stmt stmt* ...)))
That is to say, ‘when’ evaluates its consequent statements in order
if TEST is true. ‘unless’ is the opposite: it evaluates the
statements if TEST is false.
-- syntax: cond clause1 clause2 ...
Each ‘cond’-clause must look like this:
(TEST EXPRESSION ...)
where TEST and EXPRESSION are arbitrary expressions, or like this
(TEST => EXPRESSION)
where EXPRESSION must evaluate to a procedure.
The TESTs of the clauses are evaluated in order and as soon as one
of them evaluates to a true value, the corresponding EXPRESSIONs
are evaluated in order and the last value is returned as the value
of the ‘cond’-expression. For the ‘=>’ clause type, EXPRESSION is
evaluated and the resulting procedure is applied to the value of
TEST. The result of this procedure application is then the result
of the ‘cond’-expression.
One additional ‘cond’-clause is available as an extension to
standard Scheme:
(TEST GUARD => EXPRESSION)
where GUARD and EXPRESSION must evaluate to procedures. For this
clause type, TEST may return multiple values, and ‘cond’ ignores
its boolean state; instead, ‘cond’ evaluates GUARD and applies the
resulting procedure to the value(s) of TEST, as if GUARD were the
CONSUMER argument of ‘call-with-values’. If the result of that
procedure call is a true value, it evaluates EXPRESSION and applies
the resulting procedure to the value(s) of TEST, in the same manner
as the GUARD was called.
The TEST of the last CLAUSE may be the symbol ‘else’. Then, if
none of the preceding TESTs is true, the EXPRESSIONs following the
‘else’ are evaluated to produce the result of the
‘cond’-expression.
-- syntax: case key clause1 clause2 ...
KEY may be any expression, and the CLAUSEs must have the form
((DATUM1 ...) EXPR1 EXPR2 ...)
or
((DATUM1 ...) => EXPRESSION)
and the last CLAUSE may have the form
(else EXPR1 EXPR2 ...)
or
(else => EXPRESSION)
All DATUMs must be distinct. First, KEY is evaluated. The result
of this evaluation is compared against all DATUM values using
‘eqv?’. When this comparison succeeds, the expression(s) following
the DATUM are evaluated from left to right, returning the value of
the last expression as the result of the ‘case’ expression.
If the KEY matches no DATUM and there is an ‘else’-clause, the
expressions following the ‘else’ are evaluated. If there is no
such clause, the result of the expression is unspecified.
For the ‘=>’ clause types, EXPRESSION is evaluated and the
resulting procedure is applied to the value of KEY. The result of
this procedure application is then the result of the
‘case’-expression.
6.11.3 Conditional Evaluation of a Sequence of Expressions
----------------------------------------------------------
‘and’ and ‘or’ evaluate all their arguments in order, similar to
‘begin’, but evaluation stops as soon as one of the expressions
evaluates to false or true, respectively.
-- syntax: and expr ...
Evaluate the EXPRs from left to right and stop evaluation as soon
as one expression evaluates to ‘#f’; the remaining expressions are
not evaluated. The value of the last evaluated expression is
returned. If no expression evaluates to ‘#f’, the value of the
last expression is returned.
If used without expressions, ‘#t’ is returned.
-- syntax: or expr ...
Evaluate the EXPRs from left to right and stop evaluation as soon
as one expression evaluates to a true value (that is, a value
different from ‘#f’); the remaining expressions are not evaluated.
The value of the last evaluated expression is returned. If all
expressions evaluate to ‘#f’, ‘#f’ is returned.
If used without expressions, ‘#f’ is returned.
6.11.4 Iteration mechanisms
---------------------------
Scheme has only few iteration mechanisms, mainly because iteration in
Scheme programs is normally expressed using recursion. Nevertheless,
R5RS defines a construct for programming loops, calling ‘do’. In
addition, Guile has an explicit looping syntax called ‘while’.
-- syntax: do ((variable init [step]) ...) (test expr ...) body ...
Bind VARIABLEs and evaluate BODY until TEST is true. The return
value is the last EXPR after TEST, if given. A simple example will
illustrate the basic form,
(do ((i 1 (1+ i)))
((> i 4))
(display i))
⊣ 1234
Or with two variables and a final return value,
(do ((i 1 (1+ i))
(p 3 (* 3 p)))
((> i 4)
p)
(format #t "3**~s is ~s\n" i p))
⊣
3**1 is 3
3**2 is 9
3**3 is 27
3**4 is 81
⇒
243
The VARIABLE bindings are established like a ‘let’, in that the
expressions are all evaluated and then all bindings made. When
iterating, the optional STEP expressions are evaluated with the
previous bindings in scope, then new bindings all made.
The TEST expression is a termination condition. Looping stops when
the TEST is true. It’s evaluated before running the BODY each
time, so if it’s true the first time then BODY is not run at all.
The optional EXPRs after the TEST are evaluated at the end of
looping, with the final VARIABLE bindings available. The last EXPR
gives the return value, or if there are no EXPRs the return value
is unspecified.
Each iteration establishes bindings to fresh locations for the
VARIABLEs, like a new ‘let’ for each iteration. This is done for
VARIABLEs without STEP expressions too. The following illustrates
this, showing how a new ‘i’ is captured by the ‘lambda’ in each
iteration (*note The Concept of Closure: About Closure.).
(define lst '())
(do ((i 1 (1+ i)))
((> i 4))
(set! lst (cons (lambda () i) lst)))
(map (lambda (proc) (proc)) lst)
⇒
(4 3 2 1)
-- syntax: while cond body ...
Run a loop executing the BODY forms while COND is true. COND is
tested at the start of each iteration, so if it’s ‘#f’ the first
time then BODY is not executed at all.
Within ‘while’, two extra bindings are provided, they can be used
from both COND and BODY.
-- Scheme Procedure: break break-arg ...
Break out of the ‘while’ form.
-- Scheme Procedure: continue
Abandon the current iteration, go back to the start and test
COND again, etc.
If the loop terminates normally, by the COND evaluating to ‘#f’,
then the ‘while’ expression as a whole evaluates to ‘#f’. If it
terminates by a call to ‘break’ with some number of arguments,
those arguments are returned from the ‘while’ expression, as
multiple values. Otherwise if it terminates by a call to ‘break’
with no arguments, then return value is ‘#t’.
(while #f (error "not reached")) ⇒ #f
(while #t (break)) ⇒ #t
(while #t (break 1 2 3)) ⇒ 1 2 3
Each ‘while’ form gets its own ‘break’ and ‘continue’ procedures,
operating on that ‘while’. This means when loops are nested the
outer ‘break’ can be used to escape all the way out. For example,
(while (test1)
(let ((outer-break break))
(while (test2)
(if (something)
(outer-break #f))
...)))
Note that each ‘break’ and ‘continue’ procedure can only be used
within the dynamic extent of its ‘while’. Outside the ‘while’
their behaviour is unspecified.
Another very common way of expressing iteration in Scheme programs is
the use of the so-called “named let”.
Named let is a variant of ‘let’ which creates a procedure and calls
it in one step. Because of the newly created procedure, named let is
more powerful than ‘do’–it can be used for iteration, but also for
arbitrary recursion.
-- syntax: let variable bindings body
For the definition of BINDINGS see the documentation about ‘let’
(*note Local Bindings::).
Named ‘let’ works as follows:
• A new procedure which accepts as many arguments as are in
BINDINGS is created and bound locally (using ‘let’) to
VARIABLE. The new procedure’s formal argument names are the
name of the VARIABLES.
• The BODY expressions are inserted into the newly created
procedure.
• The procedure is called with the INIT expressions as the
formal arguments.
The next example implements a loop which iterates (by recursion)
1000 times.
(let lp ((x 1000))
(if (positive? x)
(lp (- x 1))
x))
⇒
0
6.11.5 Prompts
--------------
Prompts are control-flow barriers between different parts of a program.
In the same way that a user sees a shell prompt (e.g., the Bash prompt)
as a barrier between the operating system and her programs, Scheme
prompts allow the Scheme programmer to treat parts of programs as if
they were running in different operating systems.
We use this roundabout explanation because, unless you’re a
functional programming junkie, you probably haven’t heard the term,
“delimited, composable continuation”. That’s OK; it’s a relatively
recent topic, but a very useful one to know about.
6.11.5.1 Prompt Primitives
..........................
Guile’s primitive delimited control operators are ‘call-with-prompt’ and
‘abort-to-prompt’.
-- Scheme Procedure: call-with-prompt tag thunk handler
Set up a prompt, and call THUNK within that prompt.
During the dynamic extent of the call to THUNK, a prompt named TAG
will be present in the dynamic context, such that if a user calls
‘abort-to-prompt’ (see below) with that tag, control rewinds back
to the prompt, and the HANDLER is run.
HANDLER must be a procedure. The first argument to HANDLER will be
the state of the computation begun when THUNK was called, and
ending with the call to ‘abort-to-prompt’. The remaining arguments
to HANDLER are those passed to ‘abort-to-prompt’.
-- Scheme Procedure: make-prompt-tag [stem]
Make a new prompt tag. A prompt tag is simply a unique object.
Currently, a prompt tag is a fresh pair. This may change in some
future Guile version.
-- Scheme Procedure: default-prompt-tag
Return the default prompt tag. Having a distinguished default
prompt tag allows some useful prompt and abort idioms, discussed in
the next section. Note that ‘default-prompt-tag’ is actually a
parameter, and so may be dynamically rebound using ‘parameterize’.
*Note Parameters::.
-- Scheme Procedure: abort-to-prompt tag val1 val2 ...
Unwind the dynamic and control context to the nearest prompt named
TAG, also passing the given values.
C programmers may recognize ‘call-with-prompt’ and ‘abort-to-prompt’
as a fancy kind of ‘setjmp’ and ‘longjmp’, respectively. Prompts are
indeed quite useful as non-local escape mechanisms. Guile’s
‘with-exception-handler’ and ‘raise-exception’ are implemented in terms
of prompts. Prompts are more convenient than ‘longjmp’, in that one has
the opportunity to pass multiple values to the jump target.
Also unlike ‘longjmp’, the prompt handler is given the full state of
the process that was aborted, as the first argument to the prompt’s
handler. That state is the “continuation” of the computation wrapped by
the prompt. It is a “delimited continuation”, because it is not the
whole continuation of the program; rather, just the computation
initiated by the call to ‘call-with-prompt’.
The continuation is a procedure, and may be reinstated simply by
invoking it, with any number of values. Here’s where things get
interesting, and complicated as well. Besides being described as
delimited, continuations reified by prompts are also “composable”,
because invoking a prompt-saved continuation composes that continuation
with the current one.
Imagine you have saved a continuation via call-with-prompt:
(define cont
(call-with-prompt
;; tag
'foo
;; thunk
(lambda ()
(+ 34 (abort-to-prompt 'foo)))
;; handler
(lambda (k) k)))
The resulting continuation is the addition of 34. It’s as if you had
written:
(define cont
(lambda (x)
(+ 34 x)))
So, if we call ‘cont’ with one numeric value, we get that number,
incremented by 34:
(cont 8)
⇒ 42
(* 2 (cont 8))
⇒ 84
The last example illustrates what we mean when we say, "composes with
the current continuation". We mean that there is a current continuation
– some remaining things to compute, like ‘(lambda (x) (* x 2))’ – and
that calling the saved continuation doesn’t wipe out the current
continuation, it composes the saved continuation with the current one.
We’re belaboring the point here because traditional Scheme
continuations, as discussed in the next section, aren’t composable, and
are actually less expressive than continuations captured by prompts.
But there’s a place for them both.
Before moving on, we should mention that if the handler of a prompt
is a ‘lambda’ expression, and the first argument isn’t referenced, an
abort to that prompt will not cause a continuation to be reified. This
can be an important efficiency consideration to keep in mind.
One example where this optimization matters is “escape
continuations”. Escape continuations are delimited continuations whose
only use is to make a non-local exit—i.e., to escape from the current
continuation. A common use of escape continuations is when handling an
exception (*note Exceptions::).
The constructs below are syntactic sugar atop prompts to simplify the
use of escape continuations.
-- Scheme Procedure: call-with-escape-continuation proc
-- Scheme Procedure: call/ec proc
Call PROC with an escape continuation.
In the example below, the RETURN continuation is used to escape the
continuation of the call to ‘fold’.
(use-modules (ice-9 control)
(srfi srfi-1))
(define (prefix x lst)
;; Return all the elements before the first occurrence
;; of X in LST.
(call/ec
(lambda (return)
(fold (lambda (element prefix)
(if (equal? element x)
(return (reverse prefix)) ; escape `fold'
(cons element prefix)))
'()
lst))))
(prefix 'a '(0 1 2 a 3 4 5))
⇒ (0 1 2)
-- Scheme Syntax: let-escape-continuation k body ...
-- Scheme Syntax: let/ec k body ...
Bind K within BODY to an escape continuation.
This is equivalent to ‘(call/ec (lambda (K) BODY ...))’.
Additionally there is another helper primitive exported by ‘(ice-9
control)’, so load up that module for ‘suspendable-continuation?’:
(use-modules (ice-9 control))
-- Scheme Procedure: suspendable-continuation? tag
Return ‘#t’ if a call to ‘abort-to-prompt’ with the prompt tag TAG
would produce a delimited continuation that could be resumed later.
Almost all continuations have this property. The exception is
where some code between the ‘call-with-prompt’ and the
‘abort-to-prompt’ recursed through C for some reason, the
‘abort-to-prompt’ will succeed but any attempt to resume the
continuation (by calling it) would fail. This is because composing
a saved continuation with the current continuation involves
relocating the stack frames that were saved from the old stack onto
a (possibly) new position on the new stack, and Guile can only do
this for stack frames that it created for Scheme code, not stack
frames created by the C compiler. It’s a bit gnarly but if you
stick with Scheme, you won’t have any problem.
If no prompt is found with the given tag, this procedure just
returns ‘#f’.
6.11.5.2 Shift, Reset, and All That
...................................
There is a whole zoo of delimited control operators, and as it does not
seem to be a bounded set, Guile implements support for them in a
separate module:
(use-modules (ice-9 control))
Firstly, we have a helpful abbreviation for the ‘call-with-prompt’
operator.
-- Scheme Syntax: % expr
-- Scheme Syntax: % expr handler
-- Scheme Syntax: % tag expr handler
Evaluate EXPR in a prompt, optionally specifying a tag and a
handler. If no tag is given, the default prompt tag is used.
If no handler is given, a default handler is installed. The
default handler accepts a procedure of one argument, which will be
called on the captured continuation, within a prompt.
Sometimes it’s easier just to show code, as in this case:
(define (default-prompt-handler k proc)
(% (default-prompt-tag)
(proc k)
default-prompt-handler))
The ‘%’ symbol is chosen because it looks like a prompt.
Likewise there is an abbreviation for ‘abort-to-prompt’, which
assumes the default prompt tag:
-- Scheme Procedure: abort val1 val2 ...
Abort to the default prompt tag, passing VAL1 VAL2 ... to the
handler.
As mentioned before, ‘(ice-9 control)’ also provides other delimited
control operators. This section is a bit technical, and first-time
users of delimited continuations should probably come back to it after
some practice with ‘%’.
Still here? So, when one implements a delimited control operator
like ‘call-with-prompt’, one needs to make two decisions. Firstly, does
the handler run within or outside the prompt? Having the handler run
within the prompt allows an abort inside the handler to return to the
same prompt handler, which is often useful. However it prevents tail
calls from the handler, so it is less general.
Similarly, does invoking a captured continuation reinstate a prompt?
Again we have the tradeoff of convenience versus proper tail calls.
These decisions are captured in the Felleisen “F” operator. If
neither the continuations nor the handlers implicitly add a prompt, the
operator is known as “–F–”. This is the case for Guile’s
‘call-with-prompt’ and ‘abort-to-prompt’.
If both continuation and handler implicitly add prompts, then the
operator is “+F+”. ‘shift’ and ‘reset’ are such operators.
-- Scheme Syntax: reset body1 body2 ...
Establish a prompt, and evaluate BODY1 BODY2 ... within that
prompt.
The prompt handler is designed to work with ‘shift’, described
below.
-- Scheme Syntax: shift cont body1 body2 ...
Abort to the nearest ‘reset’, and evaluate BODY1 BODY2 ... in a
context in which the captured continuation is bound to CONT.
As mentioned above, taken together, the BODY1 BODY2 ... expressions
and the invocations of CONT implicitly establish a prompt.
Interested readers are invited to explore Oleg Kiselyov’s wonderful
web site at
-- Scheme Procedure: read-bytevector! bv [port [start [end]]]
Reads the next END - START bytes, or as many as are available
before the end of file, from the binary input port into the
bytevector BV in left-to-right order beginning at the START
position. If END is not supplied, reads until the end of BV has
been reached. If START is not supplied, reads beginning at
position 0.
Returns the number of bytes read. If no bytes are available, an
end-of-file object is returned.
(define in (open-input-bytevector #vu8(1 2 3)))
(define bv (make-bytevector 5 0))
(read-bytevector! bv in 1 3) ⇒ 2
bv ⇒ #vu8(0 1 2 0 0 0)
-- Scheme Procedure: read-bytevector k in
Reads the next K bytes, or as many as are available before the end
of file if that is less than K, from the binary input port IN into
a newly allocated bytevector in left-to-right order, and returns
the bytevector. If no bytes are available before the end of file,
an end-of-file object is returned.
(define bv #vu8(1 2 3))
(read-bytevector 2 (open-input-bytevector bv)) ⇒ #vu8(1 2)
(read-bytevector 10 (open-input-bytevector bv)) ⇒ #vu8(1 2 3)
-- Scheme Procedure: write-bytevector bv [port [start [end]]]
Writes the bytes of bytevector BV from START to END in
left-to-right order to the binary output PORT. START defaults to 0
and END defaults to the length of BV.
(define out (open-output-bytevector))
(write-bytevector #vu8(0 1 2 3 4) out 2 4)
(get-output-bytevector out) ⇒ #vu8(2 3)
6.12.3 Encoding
---------------
Textual input and output on Guile ports is layered on top of binary
operations. To this end, each port has an associated character encoding
that controls how bytes read from the port are converted to characters,
and how characters written to the port are converted to bytes.
-- Scheme Procedure: port-encoding port
-- C Function: scm_port_encoding (port)
Returns, as a string, the character encoding that PORT uses to
interpret its input and output.
-- Scheme Procedure: set-port-encoding! port enc
-- C Function: scm_set_port_encoding_x (port, enc)
Sets the character encoding that will be used to interpret I/O to
PORT. ENC is a string containing the name of an encoding. Valid
encoding names are those defined by IANA
(http://www.iana.org/assignments/character-sets), for example
‘"UTF-8"’ or ‘"ISO-8859-1"’.
When ports are created, they are assigned an encoding. The usual
process to determine the initial encoding for a port is to take the
value of the ‘%default-port-encoding’ fluid.
-- Scheme Variable: %default-port-encoding
A fluid containing name of the encoding to be used by default for
newly created ports (*note Fluids and Dynamic States::). As a
special case, the value ‘#f’ is equivalent to ‘"ISO-8859-1"’.
The ‘%default-port-encoding’ itself defaults to the encoding
appropriate for the current locale, if ‘setlocale’ has been called.
*Note Locales::, for more on locales and when you might need to call
‘setlocale’ explicitly.
Some port types have other ways of determining their initial locales.
String ports, for example, default to the UTF-8 encoding, in order to be
able to represent all characters regardless of the current locale. File
ports can optionally sniff their file for a ‘coding:’ declaration; *Note
File Ports::. Binary ports might be initialized to the ISO-8859-1
encoding in which each codepoint between 0 and 255 corresponds to a byte
with that value.
Currently, the ports only work with _non-modal_ encodings. Most
encodings are non-modal, meaning that the conversion of bytes to a
string doesn’t depend on its context: the same byte sequence will always
return the same string. A couple of modal encodings are in common use,
like ISO-2022-JP and ISO-2022-KR, and they are not yet supported.
Each port also has an associated conversion strategy, which
determines what to do when a Guile character can’t be converted to the
port’s encoded character representation for output. There are three
possible strategies: to raise an error, to replace the character with a
hex escape, or to replace the character with a substitute character.
Port conversion strategies are also used when decoding characters from
an input port.
-- Scheme Procedure: port-conversion-strategy port
-- C Function: scm_port_conversion_strategy (port)
Returns the behavior of the port when outputting a character that
is not representable in the port’s current encoding.
If PORT is ‘#f’, then the current default behavior will be
returned. New ports will have this default behavior when they are
created.
-- Scheme Procedure: set-port-conversion-strategy! port sym
-- C Function: scm_set_port_conversion_strategy_x (port, sym)
Sets the behavior of Guile when outputting a character that is not
representable in the port’s current encoding, or when Guile
encounters a decoding error when trying to read a character. SYM
can be either ‘error’, ‘substitute’, or ‘escape’.
If PORT is an open port, the conversion error behavior is set for
that port. If it is ‘#f’, it is set as the default behavior for
any future ports that get created in this thread.
As with port encodings, there is a fluid which determines the initial
conversion strategy for a port.
-- Scheme Variable: %default-port-conversion-strategy
The fluid that defines the conversion strategy for newly created
ports, and also for other conversion routines such as
‘scm_to_stringn’, ‘scm_from_stringn’, ‘string->pointer’, and
‘pointer->string’.
Its value must be one of the symbols described above, with the same
semantics: ‘error’, ‘substitute’, or ‘escape’.
When Guile starts, its value is ‘substitute’.
Note that ‘(set-port-conversion-strategy! #f SYM)’ is equivalent to
‘(fluid-set! %default-port-conversion-strategy SYM)’.
As mentioned above, for an output port there are three possible port
conversion strategies. The ‘error’ strategy will throw an error when a
nonconvertible character is encountered. The ‘substitute’ strategy will
replace nonconvertible characters with a question mark (‘?’). Finally
the ‘escape’ strategy will print nonconvertible characters as a hex
escape, using the escaping that is recognized by Guile’s string syntax.
Note that if the port’s encoding is a Unicode encoding, like ‘UTF-8’,
then encoding errors are impossible.
For an input port, the ‘error’ strategy will cause Guile to throw an
error if it encounters an invalid encoding, such as might happen if you
tried to read ‘ISO-8859-1’ as ‘UTF-8’. The error is thrown before
advancing the read position. The ‘substitute’ strategy will replace the
bad bytes with a U+FFFD replacement character, in accordance with
Unicode recommendations. When reading from an input port, the ‘escape’
strategy is treated as if it were ‘error’.
6.12.4 Textual I/O
------------------
This section describes Guile’s core textual I/O operations on characters
and strings. *Note Binary I/O::, for input and output of bytes and
bytevectors. *Note Encoding::, for more on how characters relate to
bytes. To read general S-expressions from ports, *Note Scheme Read::.
*Note Scheme Write::, for interfaces that write generic Scheme datums.
To use these routines, first include the textual I/O module:
(use-modules (ice-9 textual-ports))
Note that although this module’s name suggests that textual ports are
some different kind of port, that’s not the case: all ports in Guile are
both binary and textual ports.
-- Scheme Procedure: get-char input-port
Reads from INPUT-PORT, blocking as necessary, until a complete
character is available from INPUT-PORT, or until an end of file is
reached.
If a complete character is available before the next end of file,
‘get-char’ returns that character and updates the input port to
point past the character. If an end of file is reached before any
character is read, ‘get-char’ returns the end-of-file object.
-- Scheme Procedure: lookahead-char input-port
The ‘lookahead-char’ procedure is like ‘get-char’, but it does not
update INPUT-PORT to point past the character.
In the same way that it’s possible to "unget" a byte or bytes, it’s
possible to "unget" the bytes corresponding to an encoded character.
-- Scheme Procedure: unget-char port char
Place character CHAR in PORT so that it will be read by the next
read operation. If called multiple times, the unread characters
will be read again in last-in first-out order.
-- Scheme Procedure: unget-string port str
Place the string STR in PORT so that its characters will be read
from left-to-right as the next characters from PORT during
subsequent read operations. If called multiple times, the unread
characters will be read again in last-in first-out order.
Reading in a character at a time can be inefficient. If it’s
possible to perform I/O over multiple characters at a time, via strings,
that might be faster.
-- Scheme Procedure: get-string-n input-port count
The ‘get-string-n’ procedure reads from INPUT-PORT, blocking as
necessary, until COUNT characters are available, or until an end of
file is reached. COUNT must be an exact, non-negative integer,
representing the number of characters to be read.
If COUNT characters are available before end of file,
‘get-string-n’ returns a string consisting of those COUNT
characters. If fewer characters are available before an end of
file, but one or more characters can be read, ‘get-string-n’
returns a string containing those characters. In either case, the
input port is updated to point just past the characters read. If
no characters can be read before an end of file, the end-of-file
object is returned.
-- Scheme Procedure: get-string-n! input-port string start count
The ‘get-string-n!’ procedure reads from INPUT-PORT in the same
manner as ‘get-string-n’. START and COUNT must be exact,
non-negative integer objects, with COUNT representing the number of
characters to be read. STRING must be a string with at least
$START + COUNT$ characters.
If COUNT characters are available before an end of file, they are
written into STRING starting at index START, and COUNT is returned.
If fewer characters are available before an end of file, but one or
more can be read, those characters are written into STRING starting
at index START and the number of characters actually read is
returned as an exact integer object. If no characters can be read
before an end of file, the end-of-file object is returned.
-- Scheme Procedure: get-string-all input-port
Reads from INPUT-PORT until an end of file, decoding characters in
the same manner as ‘get-string-n’ and ‘get-string-n!’.
If characters are available before the end of file, a string
containing all the characters decoded from that data are returned.
If no character precedes the end of file, the end-of-file object is
returned.
-- Scheme Procedure: get-line input-port
Reads from INPUT-PORT up to and including the linefeed character or
end of file, decoding characters in the same manner as
‘get-string-n’ and ‘get-string-n!’.
If a linefeed character is read, a string containing all of the
text up to (but not including) the linefeed character is returned,
and the port is updated to point just past the linefeed character.
If an end of file is encountered before any linefeed character is
read, but some characters have been read and decoded as characters,
a string containing those characters is returned. If an end of
file is encountered before any characters are read, the end-of-file
object is returned.
Finally, there are just two core procedures to write characters to a
port.
-- Scheme Procedure: put-char port char
Writes CHAR to the port. The ‘put-char’ procedure returns an
unspecified value.
-- Scheme Procedure: put-string port string
-- Scheme Procedure: put-string port string start
-- Scheme Procedure: put-string port string start count
Write the COUNT characters of STRING starting at index START to the
port.
START and COUNT must be non-negative exact integer objects. STRING
must have a length of at least START + COUNT. START defaults to 0.
COUNT defaults to ‘(string-length STRING)’ - START$.
Calling ‘put-string’ is equivalent in all respects to calling
‘put-char’ on the relevant sequence of characters, except that it
will attempt to write multiple characters to the port at a time,
even if the port is unbuffered.
The ‘put-string’ procedure returns an unspecified value.
Textual ports have a textual position associated with them: a line
and a column. Reading in characters or writing them out advances the
line and the column appropriately.
-- Scheme Procedure: port-column port
-- Scheme Procedure: port-line port
-- C Function: scm_port_column (port)
-- C Function: scm_port_line (port)
Return the current column number or line number of PORT.
Port lines and positions are represented as 0-origin integers, which
is to say that the the first character of the first line is line 0,
column 0. However, when you display a line number, for example in an
error message, we recommend you add 1 to get 1-origin integers. This is
because lines numbers traditionally start with 1, and that is what
non-programmers will find most natural.
-- Scheme Procedure: set-port-column! port column
-- Scheme Procedure: set-port-line! port line
-- C Function: scm_set_port_column_x (port, column)
-- C Function: scm_set_port_line_x (port, line)
Set the current column or line number of PORT.
6.12.5 Simple Textual Output
----------------------------
Guile exports a simple formatted output function, ‘simple-format’. For
a more capable formatted output facility, *Note Formatted Output::.
-- Scheme Procedure: simple-format destination message . args
-- C Function: scm_simple_format (destination, message, args)
Write MESSAGE to DESTINATION, defaulting to the current output
port. MESSAGE can contain ‘~A’ and ‘~S’ escapes. When printed,
the escapes are replaced with corresponding members of ARGS: ‘~A’
formats using ‘display’ and ‘~S’ formats using ‘write’. If
DESTINATION is ‘#t’, then use the current output port, if
DESTINATION is ‘#f’, then return a string containing the formatted
text. Does not add a trailing newline.
Somewhat confusingly, Guile binds the ‘format’ identifier to
‘simple-format’ at startup. Once ‘(ice-9 format)’ loads, it actually
replaces the core ‘format’ binding, so depending on whether you or a
module you use has loaded ‘(ice-9 format)’, you may be using the simple
or the more capable version.
6.12.6 Buffering
----------------
Every port has associated input and output buffers. You can think of
ports as being backed by some mutable store, and that store might be far
away. For example, ports backed by file descriptors have to go all the
way to the kernel to read and write their data. To avoid this
round-trip cost, Guile usually reads in data from the mutable store in
chunks, and then services small requests like ‘get-char’ out of that
intermediate buffer. Similarly, small writes like ‘write-char’ first go
to a buffer, and are sent to the store when the buffer is full (or when
port is flushed). Buffered ports speed up your program by reducing the
number of round-trips to the mutable store, and they do so in a way that
is mostly transparent to the user.
There are two major ways, however, in which buffering affects program
semantics. Building correct, performant programs requires understanding
these situations.
The first case is in random-access read/write ports (*note Random
Access::). These ports, usually backed by a file, logically operate
over the same mutable store when both reading and writing. So, if you
read a character, causing the buffer to fill, then write a character,
the bytes you filled in your read buffer are now invalid. Every time
you switch between reading and writing, Guile has to flush any pending
buffer. If this happens frequently, the cost can be high. In that case
you should reduce the amount that you buffer, in both directions.
Similarly, Guile has to flush buffers before seeking. None of these
considerations apply to sockets, which don’t logically read from and
write to the same mutable store, and are not seekable. Note also that
sockets are unbuffered by default. *Note Network Sockets and
Communication::.
The second case is the more pernicious one. If you write data to a
buffered port, it probably doesn’t go out to the mutable store directly.
(This “probably” introduces some indeterminism in your program: what
goes to the store, and when, depends on how full the buffer is. It is
something that the user needs to explicitly be aware of.) The data is
written to the store later – when the buffer fills up due to another
write, or when ‘force-output’ is called, or when ‘close-port’ is called,
or when the program exits, or even when the garbage collector runs. The
salient point is, _the errors are signalled then too_. Buffered writes
defer error detection (and defer the side effects to the mutable store),
perhaps indefinitely if the port type does not need to be closed at GC.
One common heuristic that works well for textual ports is to flush
output when a newline (‘\n’) is written. This “line buffering” mode is
on by default for TTY ports. Most other ports are “block buffered”,
meaning that once the output buffer reaches the block size, which
depends on the port and its configuration, the output is flushed as a
block, without regard to what is in the block. Likewise reads are read
in at the block size, though if there are fewer bytes available to read,
the buffer may not be entirely filled.
Note that binary reads or writes that are larger than the buffer size
go directly to the mutable store without passing through the buffers.
If your access pattern involves many big reads or writes, buffering
might not matter so much to you.
To control the buffering behavior of a port, use ‘setvbuf’.
-- Scheme Procedure: setvbuf port mode [size]
-- C Function: scm_setvbuf (port, mode, size)
Set the buffering mode for PORT. MODE can be one of the following
symbols:
‘none’
non-buffered
‘line’
line buffered
‘block’
block buffered, using a newly allocated buffer of SIZE bytes.
If SIZE is omitted, a default size will be used.
Another way to set the buffering, for file ports, is to open the file
with ‘0’ or ‘l’ as part of the mode string, for unbuffered or
line-buffered ports, respectively. *Note File Ports::, for more.
Any buffered output data will be written out when the port is closed.
To make sure to flush it at specific points in your program, use
‘force-otput’.
-- Scheme Procedure: force-output [port]
-- C Function: scm_force_output (port)
Flush the specified output port, or the current output port if PORT
is omitted. The current output buffer contents, if any, are passed
to the underlying port implementation.
The return value is unspecified.
-- Scheme Procedure: flush-all-ports
-- C Function: scm_flush_all_ports ()
Equivalent to calling ‘force-output’ on all open output ports. The
return value is unspecified.
Similarly, sometimes you might want to switch from using Guile’s
ports to working directly on file descriptors. In that case, for input
ports use ‘drain-input’ to get any buffered input from that port.
-- Scheme Procedure: drain-input port
-- C Function: scm_drain_input (port)
This procedure clears a port’s input buffers, similar to the way
that force-output clears the output buffer. The contents of the
buffers are returned as a single string, e.g.,
(define p (open-input-file ...))
(drain-input p) => empty string, nothing buffered yet.
(unread-char (read-char p) p)
(drain-input p) => initial chars from p, up to the buffer size.
All of these considerations are very similar to those of streams in
the C library, although Guile’s ports are not built on top of C streams.
Still, it is useful to read what other systems do. *Note
(libc)Streams::, for more discussion on C streams.
6.12.7 Random Access
--------------------
-- Scheme Procedure: seek fd_port offset whence
-- C Function: scm_seek (fd_port, offset, whence)
Sets the current position of FD_PORT to the integer OFFSET. For a
file port, OFFSET is expressed as a number of bytes; for other
types of ports, such as string ports, OFFSET is an abstract
representation of the position within the port’s data, not
necessarily expressed as a number of bytes. OFFSET is interpreted
according to the value of WHENCE.
One of the following variables should be supplied for WHENCE:
-- Variable: SEEK_SET
Seek from the beginning of the file.
-- Variable: SEEK_CUR
Seek from the current position.
-- Variable: SEEK_END
Seek from the end of the file.
If FD_PORT is a file descriptor, the underlying system call is
‘lseek’. PORT may be a string port.
The value returned is the new position in FD_PORT. This means that
the current position of a port can be obtained using:
(seek port 0 SEEK_CUR)
-- Scheme Procedure: ftell fd_port
-- C Function: scm_ftell (fd_port)
Return an integer representing the current position of FD_PORT,
measured from the beginning. Equivalent to:
(seek port 0 SEEK_CUR)
-- Scheme Procedure: truncate-file file [length]
-- C Function: scm_truncate_file (file, length)
Truncate FILE to LENGTH bytes. FILE can be a filename string, a
port object, or an integer file descriptor. The return value is
unspecified.
For a port or file descriptor LENGTH can be omitted, in which case
the file is truncated at the current position (per ‘ftell’ above).
On most systems a file can be extended by giving a length greater
than the current size, but this is not mandatory in the POSIX
standard.
6.12.8 Line Oriented and Delimited Text
---------------------------------------
The delimited-I/O module can be accessed with:
(use-modules (ice-9 rdelim))
It can be used to read or write lines of text, or read text delimited
by a specified set of characters.
-- Scheme Procedure: read-line [port] [handle-delim]
Return a line of text from PORT if specified, otherwise from the
value returned by ‘(current-input-port)’. Under Unix, a line of
text is terminated by the first end-of-line character or by
end-of-file.
If HANDLE-DELIM is specified, it should be one of the following
symbols:
‘trim’
Discard the terminating delimiter. This is the default, but
it will be impossible to tell whether the read terminated with
a delimiter or end-of-file.
‘concat’
Append the terminating delimiter (if any) to the returned
string.
‘peek’
Push the terminating delimiter (if any) back on to the port.
‘split’
Return a pair containing the string read from the port and the
terminating delimiter or end-of-file object.
-- Scheme Procedure: read-line! buf [port]
Read a line of text into the supplied string BUF and return the
number of characters added to BUF. If BUF is filled, then ‘#f’ is
returned. Read from PORT if specified, otherwise from the value
returned by ‘(current-input-port)’.
-- Scheme Procedure: read-delimited delims [port] [handle-delim]
Read text until one of the characters in the string DELIMS is found
or end-of-file is reached. Read from PORT if supplied, otherwise
from the value returned by ‘(current-input-port)’. HANDLE-DELIM
takes the same values as described for ‘read-line’.
-- Scheme Procedure: read-delimited! delims buf [port] [handle-delim]
[start] [end]
Read text into the supplied string BUF.
If a delimiter was found, return the number of characters written,
except if HANDLE-DELIM is ‘split’, in which case the return value
is a pair, as noted above.
As a special case, if PORT was already at end-of-stream, the EOF
object is returned. Also, if no characters were written because
the buffer was full, ‘#f’ is returned.
It’s something of a wacky interface, to be honest.
-- Scheme Procedure: %read-delimited! delims str gobble [port [start
[end]]]
-- C Function: scm_read_delimited_x (delims, str, gobble, port, start,
end)
Read characters from PORT into STR until one of the characters in
the DELIMS string is encountered. If GOBBLE is true, discard the
delimiter character; otherwise, leave it in the input stream for
the next read. If PORT is not specified, use the value of
‘(current-input-port)’. If START or END are specified, store data
only into the substring of STR bounded by START and END (which
default to the beginning and end of the string, respectively).
Return a pair consisting of the delimiter that terminated the
string and the number of characters read. If reading stopped at
the end of file, the delimiter returned is the EOF-OBJECT; if the
string was filled without encountering a delimiter, this value is
‘#f’.
-- Scheme Procedure: %read-line [port]
-- C Function: scm_read_line (port)
Read a newline-terminated line from PORT, allocating storage as
necessary. The newline terminator (if any) is removed from the
string, and a pair consisting of the line and its delimiter is
returned. The delimiter may be either a newline or the EOF-OBJECT;
if ‘%read-line’ is called at the end of file, it returns the pair
‘(# . #)’.
-- Scheme Procedure: write-line obj [port]
-- C Function: scm_write_line (obj, port)
Display OBJ and a newline character to PORT. If PORT is not
specified, ‘(current-output-port)’ is used. This procedure is
equivalent to:
(display obj [port])
(newline [port])
6.12.9 Default Ports for Input, Output and Errors
-------------------------------------------------
-- Scheme Procedure: current-input-port
-- C Function: scm_current_input_port ()
Return the current input port. This is the default port used by
many input procedures.
Initially this is the “standard input” in Unix and C terminology.
When the standard input is a tty the port is unbuffered, otherwise
it’s fully buffered.
Unbuffered input is good if an application runs an interactive
subprocess, since any type-ahead input won’t go into Guile’s buffer
and be unavailable to the subprocess.
Note that Guile buffering is completely separate from the tty “line
discipline”. In the usual cooked mode on a tty Guile only sees a
line of input once the user presses .
-- Scheme Procedure: current-output-port
-- C Function: scm_current_output_port ()
Return the current output port. This is the default port used by
many output procedures.
Initially this is the “standard output” in Unix and C terminology.
When the standard output is a tty this port is unbuffered,
otherwise it’s fully buffered.
Unbuffered output to a tty is good for ensuring progress output or
a prompt is seen. But an application which always prints whole
lines could change to line buffered, or an application with a lot
of output could go fully buffered and perhaps make explicit
‘force-output’ calls (*note Buffering::) at selected points.
-- Scheme Procedure: current-error-port
-- C Function: scm_current_error_port ()
Return the port to which errors and warnings should be sent.
Initially this is the “standard error” in Unix and C terminology.
When the standard error is a tty this port is unbuffered, otherwise
it’s fully buffered.
-- Scheme Procedure: set-current-input-port port
-- Scheme Procedure: set-current-output-port port
-- Scheme Procedure: set-current-error-port port
-- C Function: scm_set_current_input_port (port)
-- C Function: scm_set_current_output_port (port)
-- C Function: scm_set_current_error_port (port)
Change the ports returned by ‘current-input-port’,
‘current-output-port’ and ‘current-error-port’, respectively, so
that they use the supplied PORT for input or output.
-- Scheme Procedure: with-input-from-port port thunk
-- Scheme Procedure: with-output-to-port port thunk
-- Scheme Procedure: with-error-to-port port thunk
Call THUNK in a dynamic environment in which ‘current-input-port’,
‘current-output-port’ or ‘current-error-port’ is rebound to the
given PORT.
-- C Function: void scm_dynwind_current_input_port (SCM port)
-- C Function: void scm_dynwind_current_output_port (SCM port)
-- C Function: void scm_dynwind_current_error_port (SCM port)
These functions must be used inside a pair of calls to
‘scm_dynwind_begin’ and ‘scm_dynwind_end’ (*note Dynamic Wind::).
During the dynwind context, the indicated port is set to PORT.
More precisely, the current port is swapped with a ‘backup’ value
whenever the dynwind context is entered or left. The backup value
is initialized with the PORT argument.
6.12.10 Types of Port
---------------------
6.12.10.1 File Ports
....................
The following procedures are used to open file ports. See also *note
open: Ports and File Descriptors, for an interface to the Unix ‘open’
system call.
All file access uses the “LFS” large file support functions when
available, so files bigger than 2 Gbytes (2^31 bytes) can be read and
written on a 32-bit system.
Most systems have limits on how many files can be open, so it’s
strongly recommended that file ports be closed explicitly when no longer
required (*note Ports::).
-- Scheme Procedure: open-file filename mode [#:guess-encoding=#f]
[#:encoding=#f]
-- C Function: scm_open_file_with_encoding (filename, mode,
guess_encoding, encoding)
-- C Function: scm_open_file (filename, mode)
Open the file whose name is FILENAME, and return a port
representing that file. The attributes of the port are determined
by the MODE string. The way in which this is interpreted is
similar to C stdio. The first character must be one of the
following:
‘r’
Open an existing file for input.
‘w’
Open a file for output, creating it if it doesn’t already
exist or removing its contents if it does.
‘a’
Open a file for output, creating it if it doesn’t already
exist. All writes to the port will go to the end of the file.
The "append mode" can be turned off while the port is in use
*note fcntl: Ports and File Descriptors.
The following additional characters can be appended:
‘b’
Open the underlying file in binary mode, if supported by the
system. Also, open the file using the binary-compatible
character encoding "ISO-8859-1", ignoring the default port
encoding.
‘+’
Open the port for both input and output. E.g., ‘r+’: open an
existing file for both input and output.
‘e’
Mark the underlying file descriptor as close-on-exec, as per
the ‘O_CLOEXEC’ flag.
‘0’
Create an "unbuffered" port. In this case input and output
operations are passed directly to the underlying port
implementation without additional buffering. This is likely
to slow down I/O operations. The buffering mode can be
changed while a port is in use (*note Buffering::).
‘l’
Add line-buffering to the port. The port output buffer will
be automatically flushed whenever a newline character is
written.
‘b’
Use binary mode, ensuring that each byte in the file will be
read as one Scheme character.
To provide this property, the file will be opened with the
8-bit character encoding "ISO-8859-1", ignoring the default
port encoding. *Note Ports::, for more information on port
encodings.
Note that while it is possible to read and write binary data
as characters or strings, it is usually better to treat bytes
as octets, and byte sequences as bytevectors. *Note Binary
I/O::, for more.
This option had another historical meaning, for DOS
compatibility: in the default (textual) mode, DOS reads a
CR-LF sequence as one LF byte. The ‘b’ flag prevents this
from happening, adding ‘O_BINARY’ to the underlying ‘open’
call. Still, the flag is generally useful because of its port
encoding ramifications.
Unless binary mode is requested, the character encoding of the new
port is determined as follows: First, if GUESS-ENCODING is true,
the ‘file-encoding’ procedure is used to guess the encoding of the
file (*note Character Encoding of Source Files::). If
GUESS-ENCODING is false or if ‘file-encoding’ fails, ENCODING is
used unless it is also false. As a last resort, the default port
encoding is used. *Note Ports::, for more information on port
encodings. It is an error to pass a non-false GUESS-ENCODING or
ENCODING if binary mode is requested.
If a file cannot be opened with the access requested, ‘open-file’
throws an exception.
-- Scheme Procedure: open-input-file filename [#:guess-encoding=#f]
[#:encoding=#f] [#:binary=#f]
Open FILENAME for input. If BINARY is true, open the port in
binary mode, otherwise use text mode. ENCODING and GUESS-ENCODING
determine the character encoding as described above for
‘open-file’. Equivalent to
(open-file FILENAME
(if BINARY "rb" "r")
#:guess-encoding GUESS-ENCODING
#:encoding ENCODING)
-- Scheme Procedure: open-output-file filename [#:encoding=#f]
[#:binary=#f]
Open FILENAME for output. If BINARY is true, open the port in
binary mode, otherwise use text mode. ENCODING specifies the
character encoding as described above for ‘open-file’. Equivalent
to
(open-file FILENAME
(if BINARY "wb" "w")
#:encoding ENCODING)
-- Scheme Procedure: call-with-input-file filename proc
[#:guess-encoding=#f] [#:encoding=#f] [#:binary=#f]
-- Scheme Procedure: call-with-output-file filename proc
[#:encoding=#f] [#:binary=#f]
Open FILENAME for input or output, and call ‘(PROC port)’ with the
resulting port. Return the value returned by PROC. FILENAME is
opened as per ‘open-input-file’ or ‘open-output-file’ respectively,
and an error is signaled if it cannot be opened.
When PROC returns, the port is closed. If PROC does not return
(e.g. if it throws an error), then the port might not be closed
automatically, though it will be garbage collected in the usual way
if not otherwise referenced.
-- Scheme Procedure: with-input-from-file filename thunk
[#:guess-encoding=#f] [#:encoding=#f] [#:binary=#f]
-- Scheme Procedure: with-output-to-file filename thunk [#:encoding=#f]
[#:binary=#f]
-- Scheme Procedure: with-error-to-file filename thunk [#:encoding=#f]
[#:binary=#f]
Open FILENAME and call ‘(THUNK)’ with the new port setup as
respectively the ‘current-input-port’, ‘current-output-port’, or
‘current-error-port’. Return the value returned by THUNK.
FILENAME is opened as per ‘open-input-file’ or ‘open-output-file’
respectively, and an error is signaled if it cannot be opened.
When THUNK returns, the port is closed and the previous setting of
the respective current port is restored.
The current port setting is managed with ‘dynamic-wind’, so the
previous value is restored no matter how THUNK exits (eg. an
exception), and if THUNK is re-entered (via a captured
continuation) then it’s set again to the FILENAME port.
The port is closed when THUNK returns normally, but not when exited
via an exception or new continuation. This ensures it’s still
ready for use if THUNK is re-entered by a captured continuation.
Of course the port is always garbage collected and closed in the
usual way when no longer referenced anywhere.
-- Scheme Procedure: port-mode port
-- C Function: scm_port_mode (port)
Return the port modes associated with the open port PORT. These
will not necessarily be identical to the modes used when the port
was opened, since modes such as "append" which are used only during
port creation are not retained.
-- Scheme Procedure: port-filename port
-- C Function: scm_port_filename (port)
Return the filename associated with PORT, or ‘#f’ if no filename is
associated with the port.
PORT must be open; ‘port-filename’ cannot be used once the port is
closed.
-- Scheme Procedure: set-port-filename! port filename
-- C Function: scm_set_port_filename_x (port, filename)
Change the filename associated with PORT, using the current input
port if none is specified. Note that this does not change the
port’s source of data, but only the value that is returned by
‘port-filename’ and reported in diagnostic output.
-- Scheme Procedure: file-port? obj
-- C Function: scm_file_port_p (obj)
Determine whether OBJ is a port that is related to a file.
6.12.10.2 Bytevector Ports
..........................
-- Scheme Procedure: open-bytevector-input-port bv [transcoder]
-- C Function: scm_open_bytevector_input_port (bv, transcoder)
Return an input port whose contents are drawn from bytevector BV
(*note Bytevectors::).
The TRANSCODER argument is currently not supported.
-- Scheme Procedure: open-bytevector-output-port [transcoder]
-- C Function: scm_open_bytevector_output_port (transcoder)
Return two values: a binary output port and a procedure. The
latter should be called with zero arguments to obtain a bytevector
containing the data accumulated by the port, as illustrated below.
(call-with-values
(lambda ()
(open-bytevector-output-port))
(lambda (port get-bytevector)
(display "hello" port)
(get-bytevector)))
⇒ #vu8(104 101 108 108 111)
The TRANSCODER argument is currently not supported.
-- Scheme Procedure: call-with-output-bytevector proc
Call the one-argument procedure PROC with a newly created
bytevector output port. When the function returns, the bytevector
composed of the characters written into the port is returned. PROC
should not close the port.
-- Scheme Procedure: call-with-input-bytevector bytevector proc
Call the one-argument procedure PROC with a newly created input
port from which BYTEVECTOR’s contents may be read. The values
yielded by the PROC is returned.
6.12.10.3 String Ports
......................
-- Scheme Procedure: call-with-output-string proc
-- C Function: scm_call_with_output_string (proc)
Calls the one-argument procedure PROC with a newly created output
port. When the function returns, the string composed of the
characters written into the port is returned. PROC should not
close the port.
-- Scheme Procedure: call-with-input-string string proc
-- C Function: scm_call_with_input_string (string, proc)
Calls the one-argument procedure PROC with a newly created input
port from which STRING’s contents may be read. The value yielded
by the PROC is returned.
-- Scheme Procedure: with-output-to-string thunk
Calls the zero-argument procedure THUNK with the current output
port set temporarily to a new string port. It returns a string
composed of the characters written to the current output.
-- Scheme Procedure: with-input-from-string string thunk
Calls the zero-argument procedure THUNK with the current input port
set temporarily to a string port opened on the specified STRING.
The value yielded by THUNK is returned.
-- Scheme Procedure: open-input-string str
-- C Function: scm_open_input_string (str)
Take a string and return an input port that delivers characters
from the string. The port can be closed by ‘close-input-port’,
though its storage will be reclaimed by the garbage collector if it
becomes inaccessible.
-- Scheme Procedure: open-output-string
-- C Function: scm_open_output_string ()
Return an output port that will accumulate characters for retrieval
by ‘get-output-string’. The port can be closed by the procedure
‘close-output-port’, though its storage will be reclaimed by the
garbage collector if it becomes inaccessible.
-- Scheme Procedure: get-output-string port
-- C Function: scm_get_output_string (port)
Given an output port created by ‘open-output-string’, return a
string consisting of the characters that have been output to the
port so far.
‘get-output-string’ must be used before closing PORT, once closed
the string cannot be obtained.
With string ports, the port-encoding is treated differently than
other types of ports. When string ports are created, they do not
inherit a character encoding from the current locale. They are given a
default locale that allows them to handle all valid string characters.
Typically one should not modify a string port’s character encoding away
from its default. *Note Encoding::.
6.12.10.4 Custom Ports
......................
Custom ports allow the user to provide input and handle output via
user-supplied procedures. Guile currently only provides custom binary
ports, not textual ports; for custom textual ports, *Note Soft Ports::.
We should add the R6RS custom textual port interfaces though.
Contributions are appreciated.
-- Scheme Procedure: make-custom-binary-input-port id read!
get-position set-position! close
Return a new custom binary input port(1) named ID (a string) whose
input is drained by invoking READ! and passing it a bytevector, an
index where bytes should be written, and the number of bytes to
read. The ‘read!’ procedure must return an integer indicating the
number of bytes read, or ‘0’ to indicate the end-of-file.
Optionally, if GET-POSITION is not ‘#f’, it must be a thunk that
will be called when ‘port-position’ is invoked on the custom binary
port and should return an integer indicating the position within
the underlying data stream; if GET-POSITION was not supplied, the
returned port does not support ‘port-position’.
Likewise, if SET-POSITION! is not ‘#f’, it should be a one-argument
procedure. When ‘set-port-position!’ is invoked on the custom
binary input port, SET-POSITION! is passed an integer indicating
the position of the next byte is to read.
Finally, if CLOSE is not ‘#f’, it must be a thunk. It is invoked
when the custom binary input port is closed.
The returned port is fully buffered by default, but its buffering
mode can be changed using ‘setvbuf’ (*note Buffering::).
Using a custom binary input port, the ‘open-bytevector-input-port’
procedure (*note Bytevector Ports::) could be implemented as
follows:
(define (open-bytevector-input-port source)
(define position 0)
(define length (bytevector-length source))
(define (read! bv start count)
(let ((count (min count (- length position))))
(bytevector-copy! source position
bv start count)
(set! position (+ position count))
count))
(define (get-position) position)
(define (set-position! new-position)
(set! position new-position))
(make-custom-binary-input-port "the port" read!
get-position set-position!
#f))
(read (open-bytevector-input-port (string->utf8 "hello")))
⇒ hello
-- Scheme Procedure: make-custom-binary-output-port id write!
get-position set-position! close
Return a new custom binary output port named ID (a string) whose
output is sunk by invoking WRITE! and passing it a bytevector, an
index where bytes should be read from this bytevector, and the
number of bytes to be “written”. The ‘write!’ procedure must
return an integer indicating the number of bytes actually written;
when it is passed ‘0’ as the number of bytes to write, it should
behave as though an end-of-file was sent to the byte sink.
The other arguments are as for ‘make-custom-binary-input-port’.
-- Scheme Procedure: make-custom-binary-input/output-port id read!
write! get-position set-position! close
Return a new custom binary input/output port named ID (a string).
The various arguments are the same as for The other arguments are
as for ‘make-custom-binary-input-port’ and
‘make-custom-binary-output-port’. If buffering is enabled on the
port, as is the case by default, input will be buffered in both
directions; *Note Buffering::. If the SET-POSITION! function is
provided and not ‘#f’, then the port will also be marked as
random-access, causing the buffer to be flushed between reads and
writes.
---------- Footnotes ----------
(1) This is similar in spirit to Guile’s “soft ports” (*note Soft
Ports::).
6.12.10.5 Soft Ports
....................
A “soft port” is a port based on a vector of procedures capable of
accepting or delivering characters. It allows emulation of I/O ports.
-- Scheme Procedure: make-soft-port pv modes
Return a port capable of receiving or delivering characters as
specified by the MODES string (*note open-file: File Ports.). PV
must be a vector of length 5 or 6. Its components are as follows:
0. procedure accepting one character for output
1. procedure accepting a string for output
2. thunk for flushing output
3. thunk for getting one character
4. thunk for closing port (not by garbage collection)
5. (if present and not ‘#f’) thunk for computing the number of
characters that can be read from the port without blocking.
For an output-only port only elements 0, 1, 2, and 4 need be
procedures. For an input-only port only elements 3 and 4 need be
procedures. Thunks 2 and 4 can instead be ‘#f’ if there is no
useful operation for them to perform.
If thunk 3 returns ‘#f’ or an ‘eof-object’ (*note eof-object?:
(r5rs)Input.) it indicates that the port has reached end-of-file.
For example:
(define stdout (current-output-port))
(define p (make-soft-port
(vector
(lambda (c) (write c stdout))
(lambda (s) (display s stdout))
(lambda () (display "." stdout))
(lambda () (char-upcase (read-char)))
(lambda () (display "@" stdout)))
"rw"))
(write p p) ⇒ #
6.12.10.6 Void Ports
....................
This kind of port causes any data to be discarded when written to, and
always returns the end-of-file object when read from.
-- Scheme Procedure: %make-void-port mode
-- C Function: scm_sys_make_void_port (mode)
Create and return a new void port. A void port acts like
‘/dev/null’. The MODE argument specifies the input/output modes
for this port: see the documentation for ‘open-file’ in *note File
Ports::.
6.12.11 Venerable Port Interfaces
---------------------------------
Over the 25 years or so that Guile has been around, its port system has
evolved, adding many useful features. At the same time there have been
four major Scheme standards released in those 25 years, which also
evolve the common Scheme understanding of what a port interface should
be. Alas, it would be too much to ask for all of these evolutionary
branches to be consistent. Some of Guile’s original interfaces don’t
mesh with the later Scheme standards, and yet Guile can’t just drop old
interfaces. Sadly as well, the R6RS and R7RS standards both part from a
base of R5RS, but end up in different and somewhat incompatible designs.
Guile’s approach is to pick a set of port primitives that make sense
together. We document that set of primitives, design our internal
interfaces around them, and recommend them to users. As the R6RS I/O
system is the most capable standard that Scheme has yet produced in this
domain, we mostly recommend that; ‘(ice-9 binary-ports)’ and ‘(ice-9
textual-ports)’ are wholly modelled on ‘(rnrs io ports)’. Guile does
not wholly copy R6RS, however; *Note R6RS Incompatibilities::.
At the same time, we have many venerable port interfaces, lore handed
down to us from our hacker ancestors. Most of these interfaces even
predate the expectation that Scheme should have modules, so they are
present in the default environment. In Guile we support them as well
and we have no plans to remove them, but again we don’t recommend them
for new users.
-- Scheme Procedure: char-ready? [port]
Return ‘#t’ if a character is ready on input PORT and return ‘#f’
otherwise. If ‘char-ready?’ returns ‘#t’ then the next ‘read-char’
operation on PORT is guaranteed not to hang. If PORT is a file
port at end of file then ‘char-ready?’ returns ‘#t’.
‘char-ready?’ exists to make it possible for a program to accept
characters from interactive ports without getting stuck waiting for
input. Any input editors associated with such ports must make sure
that characters whose existence has been asserted by ‘char-ready?’
cannot be rubbed out. If ‘char-ready?’ were to return ‘#f’ at end
of file, a port at end of file would be indistinguishable from an
interactive port that has no ready characters.
Note that ‘char-ready?’ only works reliably for terminals and
sockets with one-byte encodings. Under the hood it will return
‘#t’ if the port has any input buffered, or if the file descriptor
that backs the port polls as readable, indicating that Guile can
fetch more bytes from the kernel. However being able to fetch one
byte doesn’t mean that a full character is available; *Note
Encoding::. Also, on many systems it’s possible for a file
descriptor to poll as readable, but then block when it comes time
to read bytes. Note also that on Linux kernels, all file ports
backed by files always poll as readable. For non-file ports, this
procedure always returns ‘#t’, except for soft ports, which have a
‘char-ready?’ handler. *Note Soft Ports::.
In short, this is a legacy procedure whose semantics are hard to
provide. However it is a useful check to see if any input is
buffered. *Note Non-Blocking I/O::.
-- Scheme Procedure: read-char [port]
The same as ‘get-char’, except that PORT defaults to the current
input port. *Note Textual I/O::.
-- Scheme Procedure: peek-char [port]
The same as ‘lookahead-char’, except that PORT defaults to the
current input port. *Note Textual I/O::.
-- Scheme Procedure: unread-char cobj [port]
The same as ‘unget-char’, except that PORT defaults to the current
input port, and the arguments are swapped. *Note Textual I/O::.
-- Scheme Procedure: unread-string str port
-- C Function: scm_unread_string (str, port)
The same as ‘unget-string’, except that PORT defaults to the
current input port, and the arguments are swapped. *Note Textual
I/O::.
-- Scheme Procedure: newline [port]
Send a newline to PORT. If PORT is omitted, send to the current
output port. Equivalent to ‘(put-char port #\newline)’.
-- Scheme Procedure: write-char chr [port]
The same as ‘put-char’, except that PORT defaults to the current
input port, and the arguments are swapped. *Note Textual I/O::.
6.12.12 Using Ports from C
--------------------------
Guile’s C interfaces provides some niceties for sending and receiving
bytes and characters in a way that works better with C.
-- C Function: size_t scm_c_read (SCM port, void *buffer, size_t size)
Read up to SIZE bytes from PORT and store them in BUFFER. The
return value is the number of bytes actually read, which can be
less than SIZE if end-of-file has been reached.
Note that as this is a binary input procedure, this function does
not update ‘port-line’ and ‘port-column’ (*note Textual I/O::).
-- C Function: void scm_c_write (SCM port, const void *buffer, size_t
size)
Write SIZE bytes at BUFFER to PORT.
Note that as this is a binary output procedure, this function does
not update ‘port-line’ and ‘port-column’ (*note Textual I/O::).
-- C Function: size_t scm_c_read_bytes (SCM port, SCM bv, size_t start,
size_t count)
-- C Function: void scm_c_write_bytes (SCM port, SCM bv, size_t start,
size_t count)
Like ‘scm_c_read’ and ‘scm_c_write’, but reading into or writing
from the bytevector BV. COUNT indicates the byte index at which to
start in the bytevector, and the read or write will continue for
COUNT bytes.
-- C Function: void scm_unget_bytes (const unsigned char *buf, size_t
len, SCM port)
-- C Function: void scm_unget_byte (int c, SCM port)
-- C Function: void scm_ungetc (scm_t_wchar c, SCM port)
Like ‘unget-bytevector’, ‘unget-byte’, and ‘unget-char’,
respectively. *Note Textual I/O::.
-- C Function: void scm_c_put_latin1_chars (SCM port, const scm_t_uint8
*buf, size_t len)
-- C Function: void scm_c_put_utf32_chars (SCM port, const scm_t_uint32
*buf, size_t len);
Write a string to PORT. In the first case, the ‘scm_t_uint8*’
buffer is a string in the latin-1 encoding. In the second, the
‘scm_t_uint32*’ buffer is a string in the UTF-32 encoding. These
routines will update the port’s line and column.
6.12.13 Implementing New Port Types in C
----------------------------------------
This section describes how to implement a new port type in C. Although
ports support many operations, as a data structure they present an
opaque interface to the user. To the port implementor, you have two
pieces of information to work with: the port type, and the port’s
“stream”. The port type is an opaque pointer allocated when defining
your port type. It is your key into the port API, and it helps you
identify which ports are actually yours. The “stream” is a pointer you
control, and which you set when you create a port. Get a stream from a
port using the ‘SCM_STREAM’ macro. Note that your port methods are only
ever called with ports of your type.
A port type is created by calling ‘scm_make_port_type’. Once you
have your port type, you can create ports with ‘scm_c_make_port’, or
‘scm_c_make_port_with_encoding’.
-- Function: scm_t_port_type* scm_make_port_type (char *name, size_t
(*read) (SCM port, SCM dst, size_t start, size_t count),
size_t (*write) (SCM port, SCM src, size_t start, size_t
count))
Define a new port type. The NAME, READ and WRITE parameters are
initial values for those port type fields, as described below. The
other fields are initialized with default values and can be changed
later.
-- Function: SCM scm_c_make_port_with_encoding (scm_t_port_type *type,
unsigned long mode_bits, SCM encoding, SCM
conversion_strategy, scm_t_bits stream)
-- Function: SCM scm_c_make_port (scm_t_port_type *type, unsigned long
mode_bits, scm_t_bits stream)
Make a port with the given TYPE. The STREAM indicates the private
data associated with the port, which your port implementation may
later retrieve with ‘SCM_STREAM’. The mode bits should include one
or more of the flags ‘SCM_RDNG’ or ‘SCM_WRTNG’, indicating that the
port is an input and/or an output port, respectively. The mode
bits may also include ‘SCM_BUF0’ or ‘SCM_BUFLINE’, indicating that
the port should be unbuffered or line-buffered, respectively. The
default is that the port will be block-buffered. *Note
Buffering::.
As you would imagine, ENCODING and CONVERSION_STRATEGY specify the
port’s initial textual encoding and conversion strategy. Both are
symbols. ‘scm_c_make_port’ is the same as
‘scm_c_make_port_with_encoding’, except it uses the default port
encoding and conversion strategy.
The port type has a number of associate procedures and properties
which collectively implement the port’s behavior. Creating a new port
type mostly involves writing these procedures.
‘name’
A pointer to a NUL terminated string: the name of the port type.
This property is initialized via the first argument to
‘scm_make_port_type’.
‘read’
A port’s ‘read’ implementation fills read buffers. It should copy
bytes to the supplied bytevector ‘dst’, starting at offset ‘start’
and continuing for ‘count’ bytes, returning the number of bytes
read.
‘write’
A port’s ‘write’ implementation flushes write buffers to the
mutable store. It should write out bytes from the supplied
bytevector ‘src’, starting at offset ‘start’ and continuing for
‘count’ bytes, and return the number of bytes that were written.
‘read_wait_fd’
‘write_wait_fd’
If a port’s ‘read’ or ‘write’ function returns ‘(size_t) -1’, that
indicates that reading or writing would block. In that case to
preserve the illusion of a blocking read or write operation,
Guile’s C port run-time will ‘poll’ on the file descriptor returned
by either the port’s ‘read_wait_fd’ or ‘write_wait_fd’ function.
Set using
-- Function: void scm_set_port_read_wait_fd (scm_t_port_type
*type, int (*wait_fd) (SCM port))
-- Function: void scm_set_port_write_wait_fd (scm_t_port_type
*type, int (*wait_fd) (SCM port))
Only a port type which implements the ‘read_wait_fd’ or
‘write_wait_fd’ port methods can usefully return ‘(size_t) -1’ from
a read or write function. *Note Non-Blocking I/O::, for more on
non-blocking I/O in Guile.
‘print’
Called when ‘write’ is called on the port, to print a port
description. For example, for a file port it may produce something
like: ‘#’. Set using
-- Function: void scm_set_port_print (scm_t_port_type *type, int
(*print) (SCM port, SCM dest_port, scm_print_state
*pstate))
The first argument PORT is the port being printed, the second
argument DEST_PORT is where its description should go.
‘close’
Called when the port is closed. It should free any resources used
by the port. Set using
-- Function: void scm_set_port_close (scm_t_port_type *type, void
(*close) (SCM port))
By default, ports that are garbage collected just go away without
closing. If your port type needs to release some external resource
like a file descriptor, or needs to make sure that its internal
buffers are flushed even if the port is collected while it was
open, then mark the port type as needing a close on GC.
-- Function: void scm_set_port_needs_close_on_gc (scm_t_port_type
*type, int needs_close_p)
‘seek’
Set the current position of the port. Guile will flush read and/or
write buffers before seeking, as appropriate.
-- Function: void scm_set_port_seek (scm_t_port_type *type,
scm_t_off (*seek) (SCM port, scm_t_off offset, int
whence))
‘truncate’
Truncate the port data to be specified length. Guile will flush
buffers before hand, as appropriate. Set using
-- Function: void scm_set_port_truncate (scm_t_port_type *type,
void (*truncate) (SCM port, scm_t_off length))
‘random_access_p’
Determine whether this port is a random-access port.
Seeking on a random-access port with buffered input, or switching
to writing after reading, will cause the buffered input to be
discarded and Guile will seek the port back the buffered number of
bytes. Likewise seeking on a random-access port with buffered
output, or switching to reading after writing, will flush pending
bytes with a call to the ‘write’ procedure. *Note Buffering::.
Indicate to Guile that your port needs this behavior by returning a
nonzero value from your ‘random_access_p’ function. The default
implementation of this function returns nonzero if the port type
supplies a seek implementation.
-- Function: void scm_set_port_random_access_p (scm_t_port_type
*type, int (*random_access_p) (SCM port));
‘get_natural_buffer_sizes’
Guile will internally attach buffers to ports. An input port
always has a read buffer and an output port always has a write
buffer. *Note Buffering::. A port buffer consists of a
bytevector, along with some cursors into that bytevector denoting
where to get and put data.
Port implementations generally don’t have to be concerned with
buffering: a port type’s ‘read’ or ‘write’ function will receive
the buffer’s bytevector as an argument, along with an offset and a
length into that bytevector, and should then either fill or empty
that bytevector. However in some cases, port implementations may
be able to provide an appropriate default buffer size to Guile.
-- Function: void scm_set_port_get_natural_buffer_sizes
(scm_t_port_type *type, void (*get_natural_buffer_sizes)
(SCM, size_t *read_buf_size, size_t *write_buf_size))
Fill in READ_BUF_SIZE and WRITE_BUF_SIZE with an appropriate
buffer size for this port, if one is known.
File ports implement a ‘get_natural_buffer_sizes’ to let the
operating system inform Guile about the appropriate buffer sizes
for the particular file opened by the port.
Note that calls to all of these methods can proceed in parallel and
concurrently and from any thread up until the point that the port is
closed. The call to ‘close’ will happen when no other method is
running, and no method will be called after the ‘close’ method is
called. If your port implementation needs mutual exclusion to prevent
concurrency, it is responsible for locking appropriately.
6.12.14 Non-Blocking I/O
------------------------
Most ports in Guile are “blocking”: when you try to read a character
from a port, Guile will block on the read until a character is ready, or
end-of-stream is detected. Likewise whenever Guile goes to write
(possibly buffered) data to an output port, Guile will block until all
the data is written.
Interacting with ports in blocking mode is very convenient: you can
write straightforward, sequential algorithms whose code flow reflects
the flow of data. However, blocking I/O has two main limitations.
The first is that it’s easy to get into a situation where code is
waiting on data. Time spent waiting on data when code could be doing
something else is wasteful and prevents your program from reaching its
peak throughput. If you implement a web server that sequentially
handles requests from clients, it’s very easy for the server to end up
waiting on a client to finish its HTTP request, or waiting on it to
consume the response. The end result is that you are able to serve
fewer requests per second than you’d like to serve.
The second limitation is related: a blocking parser over
user-controlled input is a denial-of-service vulnerability. Indeed the
so-called “slow loris” attack of the early 2010s was just that: an
attack on common web servers that drip-fed HTTP requests, one character
at a time. All it took was a handful of slow loris connections to
occupy an entire web server.
In Guile we would like to preserve the ability to write
straightforward blocking networking processes of all kinds, but under
the hood to allow those processes to suspend their requests if they
would block.
To do this, the first piece is to allow Guile ports to declare
themselves as being nonblocking. This is currently supported only for
file ports, which also includes sockets, terminals, or any other port
that is backed by a file descriptor. To do that, we use an arcane UNIX
incantation:
(let ((flags (fcntl socket F_GETFL)))
(fcntl socket F_SETFL (logior O_NONBLOCK flags)))
Now the file descriptor is open in non-blocking mode. If Guile tries
to read or write from this file and the read or write returns a result
indicating that more data can only be had by doing a blocking read or
write, Guile will block by polling on the socket’s ‘read-wait-fd’ or
‘write-wait-fd’, to preserve the illusion of a blocking read or write.
*Note I/O Extensions:: for more on those internal interfaces.
So far we have just reproduced the status quo: the file descriptor is
non-blocking, but the operations on the port do block. To go farther,
it would be nice if we could suspend the “thread” using delimited
continuations, and only resume the thread once the file descriptor is
readable or writable. (*Note Prompts::).
But here we run into a difficulty. The ports code is implemented in
C, which means that although we can suspend the computation to some
outer prompt, we can’t resume it because Guile can’t resume delimited
continuations that capture the C stack.
To overcome this difficulty we have created a compatible but entirely
parallel implementation of port operations. To use this implementation,
do the following:
(use-modules (ice-9 suspendable-ports))
(install-suspendable-ports!)
This will replace the core I/O primitives like ‘get-char’ and
‘put-bytevector’ with new versions that are exactly the same as the ones
in the standard library, but with two differences. One is that when a
read or a write would block, the suspendable port operations call out
the value of the ‘current-read-waiter’ or ‘current-write-waiter’
parameter, as appropriate. *Note Parameters::. The default read and
write waiters do the same thing that the C read and write waiters do,
which is to poll. User code can parameterize the waiters, though,
enabling the computation to suspend and allow the program to process
other I/O operations. Because the new suspendable ports implementation
is written in Scheme, that suspended computation can resume again later
when it is able to make progress. Success!
The other main difference is that because the new ports
implementation is written in Scheme, it is slower than C, currently by a
factor of 3 or 4, though it depends on many factors. For this reason we
have to keep the C implementations as the default ones. One day when
Guile’s compiler is better, we can close this gap and have only one port
operation implementation again.
Note that Guile does not currently include an implementation of the
facility to suspend the current thread and schedule other threads in the
meantime. Before adding such a thing, we want to make sure that we’re
providing the right primitives that can be used to build schedulers and
other user-space concurrency patterns, and that the patterns that we
settle on are the right patterns. In the meantime, have a look at 8sync
() for a prototype of an asynchronous
I/O and concurrency facility.
-- Scheme Procedure: install-suspendable-ports!
Replace the core ports implementation with suspendable ports, as
described above. This will mutate the values of the bindings like
‘get-char’, ‘put-u8’, and so on in place.
-- Scheme Procedure: uninstall-suspendable-ports!
Restore the original core ports implementation, un-doing the effect
of ‘install-suspendable-ports!’.
-- Scheme Parameter: current-read-waiter
-- Scheme Parameter: current-write-waiter
Parameters whose values are procedures of one argument, called when
a suspendable port operation would block on a port while reading or
writing, respectively. The default values of these parameters do a
blocking ‘poll’ on the port’s file descriptor. The procedures are
passed the port in question as their one argument.
6.12.15 Handling of Unicode Byte Order Marks
--------------------------------------------
This section documents the finer points of Guile’s handling of Unicode
byte order marks (BOMs). A byte order mark (U+FEFF) is typically found
at the start of a UTF-16 or UTF-32 stream, to allow readers to reliably
determine the byte order. Occasionally, a BOM is found at the start of
a UTF-8 stream, but this is much less common and not generally
recommended.
Guile attempts to handle BOMs automatically, and in accordance with
the recommendations of the Unicode Standard, when the port encoding is
set to ‘UTF-8’, ‘UTF-16’, or ‘UTF-32’. In brief, Guile automatically
writes a BOM at the start of a UTF-16 or UTF-32 stream, and
automatically consumes one from the start of a UTF-8, UTF-16, or UTF-32
stream.
As specified in the Unicode Standard, a BOM is only handled specially
at the start of a stream, and only if the port encoding is set to
‘UTF-8’, ‘UTF-16’ or ‘UTF-32’. If the port encoding is set to
‘UTF-16BE’, ‘UTF-16LE’, ‘UTF-32BE’, or ‘UTF-32LE’, then BOMs are _not_
handled specially, and none of the special handling described in this
section applies.
• To ensure that Guile will properly detect the byte order of a
UTF-16 or UTF-32 stream, you must perform a textual read before any
writes, seeks, or binary I/O. Guile will not attempt to read a BOM
unless a read is explicitly requested at the start of the stream.
• If a textual write is performed before the first read, then an
arbitrary byte order will be chosen. Currently, big endian is the
default on all platforms, but that may change in the future. If
you wish to explicitly control the byte order of an output stream,
set the port encoding to ‘UTF-16BE’, ‘UTF-16LE’, ‘UTF-32BE’, or
‘UTF-32LE’, and explicitly write a BOM (‘#\xFEFF’) if desired.
• If ‘set-port-encoding!’ is called in the middle of a stream, Guile
treats this as a new logical “start of stream” for purposes of BOM
handling, and will forget about any BOMs that had previously been
seen. Therefore, it may choose a different byte order than had
been used previously. This is intended to support multiple logical
text streams embedded within a larger binary stream.
• Binary I/O operations are not guaranteed to update Guile’s notion
of whether the port is at the “start of the stream”, nor are they
guaranteed to produce or consume BOMs.
• For ports that support seeking (e.g. normal files), the input and
output streams are considered linked: if the user reads first, then
a BOM will be consumed (if appropriate), but later writes will
_not_ produce a BOM. Similarly, if the user writes first, then
later reads will _not_ consume a BOM.
• For ports that are not random access (e.g. pipes, sockets, and
terminals), the input and output streams are considered
_independent_ for purposes of BOM handling: the first read will
consume a BOM (if appropriate), and the first write will _also_
produce a BOM (if appropriate). However, the input and output
streams will always use the same byte order.
• Seeks to the beginning of a file will set the “start of stream”
flags. Therefore, a subsequent textual read or write will consume
or produce a BOM. However, unlike ‘set-port-encoding!’, if a byte
order had already been chosen for the port, it will remain in
effect after a seek, and cannot be changed by the presence of a
BOM. Seeks anywhere other than the beginning of a file clear the
“start of stream” flags.
6.13 Regular Expressions
========================
A “regular expression” (or “regexp”) is a pattern that describes a whole
class of strings. A full description of regular expressions and their
syntax is beyond the scope of this manual.
If your system does not include a POSIX regular expression library,
and you have not linked Guile with a third-party regexp library such as
Rx, these functions will not be available. You can tell whether your
Guile installation includes regular expression support by checking
whether ‘(provided? 'regex)’ returns true.
The following regexp and string matching features are provided by the
‘(ice-9 regex)’ module. Before using the described functions, you
should load this module by executing ‘(use-modules (ice-9 regex))’.
6.13.1 Regexp Functions
-----------------------
By default, Guile supports POSIX extended regular expressions. That
means that the characters ‘(’, ‘)’, ‘+’ and ‘?’ are special, and must be
escaped if you wish to match the literal characters and there is no
support for “non-greedy” variants of ‘*’, ‘+’ or ‘?’.
This regular expression interface was modeled after that implemented
by SCSH, the Scheme Shell. It is intended to be upwardly compatible
with SCSH regular expressions.
Zero bytes (‘#\nul’) cannot be used in regex patterns or input
strings, since the underlying C functions treat that as the end of
string. If there’s a zero byte an error is thrown.
Internally, patterns and input strings are converted to the current
locale’s encoding, and then passed to the C library’s regular expression
routines (*note (libc)Regular Expressions::). The returned match
structures always point to characters in the strings, not to individual
bytes, even in the case of multi-byte encodings. This ensures that the
match structures are correct when performing matching with characters
that have a multi-byte representation in the locale encoding. Note,
however, that using characters which cannot be represented in the locale
encoding can lead to surprising results.
-- Scheme Procedure: string-match pattern str [start]
Compile the string PATTERN into a regular expression and compare it
with STR. The optional numeric argument START specifies the
position of STR at which to begin matching.
‘string-match’ returns a “match structure” which describes what, if
anything, was matched by the regular expression. *Note Match
Structures::. If STR does not match PATTERN at all, ‘string-match’
returns ‘#f’.
Two examples of a match follow. In the first example, the pattern
matches the four digits in the match string. In the second, the pattern
matches nothing.
(string-match "[0-9][0-9][0-9][0-9]" "blah2002")
⇒ #("blah2002" (4 . 8))
(string-match "[A-Za-z]" "123456")
⇒ #f
Each time ‘string-match’ is called, it must compile its PATTERN
argument into a regular expression structure. This operation is
expensive, which makes ‘string-match’ inefficient if the same regular
expression is used several times (for example, in a loop). For better
performance, you can compile a regular expression in advance and then
match strings against the compiled regexp.
-- Scheme Procedure: make-regexp pat flag...
-- C Function: scm_make_regexp (pat, flaglst)
Compile the regular expression described by PAT, and return the
compiled regexp structure. If PAT does not describe a legal
regular expression, ‘make-regexp’ throws a
‘regular-expression-syntax’ error.
The FLAG arguments change the behavior of the compiled regular
expression. The following values may be supplied:
-- Variable: regexp/icase
Consider uppercase and lowercase letters to be the same when
matching.
-- Variable: regexp/newline
If a newline appears in the target string, then permit the ‘^’
and ‘$’ operators to match immediately after or immediately
before the newline, respectively. Also, the ‘.’ and ‘[^...]’
operators will never match a newline character. The intent of
this flag is to treat the target string as a buffer containing
many lines of text, and the regular expression as a pattern
that may match a single one of those lines.
-- Variable: regexp/basic
Compile a basic (“obsolete”) regexp instead of the extended
(“modern”) regexps that are the default. Basic regexps do not
consider ‘|’, ‘+’ or ‘?’ to be special characters, and require
the ‘{...}’ and ‘(...)’ metacharacters to be backslash-escaped
(*note Backslash Escapes::). There are several other
differences between basic and extended regular expressions,
but these are the most significant.
-- Variable: regexp/extended
Compile an extended regular expression rather than a basic
regexp. This is the default behavior; this flag will not
usually be needed. If a call to ‘make-regexp’ includes both
‘regexp/basic’ and ‘regexp/extended’ flags, the one which
comes last will override the earlier one.
-- Scheme Procedure: regexp-exec rx str [start [flags]]
-- C Function: scm_regexp_exec (rx, str, start, flags)
Match the compiled regular expression RX against ‘str’. If the
optional integer START argument is provided, begin matching from
that position in the string. Return a match structure describing
the results of the match, or ‘#f’ if no match could be found.
The FLAGS argument changes the matching behavior. The following
flag values may be supplied, use ‘logior’ (*note Bitwise
Operations::) to combine them,
-- Variable: regexp/notbol
Consider that the START offset into STR is not the beginning
of a line and should not match operator ‘^’.
If RX was created with the ‘regexp/newline’ option above, ‘^’
will still match after a newline in STR.
-- Variable: regexp/noteol
Consider that the end of STR is not the end of a line and
should not match operator ‘$’.
If RX was created with the ‘regexp/newline’ option above, ‘$’
will still match before a newline in STR.
;; Regexp to match uppercase letters
(define r (make-regexp "[A-Z]*"))
;; Regexp to match letters, ignoring case
(define ri (make-regexp "[A-Z]*" regexp/icase))
;; Search for bob using regexp r
(match:substring (regexp-exec r "bob"))
⇒ "" ; no match
;; Search for bob using regexp ri
(match:substring (regexp-exec ri "Bob"))
⇒ "Bob" ; matched case insensitive
-- Scheme Procedure: regexp? obj
-- C Function: scm_regexp_p (obj)
Return ‘#t’ if OBJ is a compiled regular expression, or ‘#f’
otherwise.
-- Scheme Procedure: list-matches regexp str [flags]
Return a list of match structures which are the non-overlapping
matches of REGEXP in STR. REGEXP can be either a pattern string or
a compiled regexp. The FLAGS argument is as per ‘regexp-exec’
above.
(map match:substring (list-matches "[a-z]+" "abc 42 def 78"))
⇒ ("abc" "def")
-- Scheme Procedure: fold-matches regexp str init proc [flags]
Apply PROC to the non-overlapping matches of REGEXP in STR, to
build a result. REGEXP can be either a pattern string or a
compiled regexp. The FLAGS argument is as per ‘regexp-exec’ above.
PROC is called as ‘(PROC match prev)’ where MATCH is a match
structure and PREV is the previous return from PROC. For the first
call PREV is the given INIT parameter. ‘fold-matches’ returns the
final value from PROC.
For example to count matches,
(fold-matches "[a-z][0-9]" "abc x1 def y2" 0
(lambda (match count)
(1+ count)))
⇒ 2
Regular expressions are commonly used to find patterns in one string
and replace them with the contents of another string. The following
functions are convenient ways to do this.
-- Scheme Procedure: regexp-substitute port match item ...
Write to PORT selected parts of the match structure MATCH. Or if
PORT is ‘#f’ then form a string from those parts and return that.
Each ITEM specifies a part to be written, and may be one of the
following,
• A string. String arguments are written out verbatim.
• An integer. The submatch with that number is written
(‘match:substring’). Zero is the entire match.
• The symbol ‘pre’. The portion of the matched string preceding
the regexp match is written (‘match:prefix’).
• The symbol ‘post’. The portion of the matched string
following the regexp match is written (‘match:suffix’).
For example, changing a match and retaining the text before and
after,
(regexp-substitute #f (string-match "[0-9]+" "number 25 is good")
'pre "37" 'post)
⇒ "number 37 is good"
Or matching a YYYYMMDD format date such as ‘20020828’ and
re-ordering and hyphenating the fields.
(define date-regex
"([0-9][0-9][0-9][0-9])([0-9][0-9])([0-9][0-9])")
(define s "Date 20020429 12am.")
(regexp-substitute #f (string-match date-regex s)
'pre 2 "-" 3 "-" 1 'post " (" 0 ")")
⇒ "Date 04-29-2002 12am. (20020429)"
-- Scheme Procedure: regexp-substitute/global port regexp target
item...
Write to PORT selected parts of matches of REGEXP in TARGET. If
PORT is ‘#f’ then form a string from those parts and return that.
REGEXP can be a string or a compiled regex.
This is similar to ‘regexp-substitute’, but allows global
substitutions on TARGET. Each ITEM behaves as per
‘regexp-substitute’, with the following differences,
• A function. Called as ‘(ITEM match)’ with the match structure
for the REGEXP match, it should return a string to be written
to PORT.
• The symbol ‘post’. This doesn’t output anything, but instead
causes ‘regexp-substitute/global’ to recurse on the unmatched
portion of TARGET.
This _must_ be supplied to perform a global search and replace
on TARGET; without it ‘regexp-substitute/global’ returns after
a single match and output.
For example, to collapse runs of tabs and spaces to a single hyphen
each,
(regexp-substitute/global #f "[ \t]+" "this is the text"
'pre "-" 'post)
⇒ "this-is-the-text"
Or using a function to reverse the letters in each word,
(regexp-substitute/global #f "[a-z]+" "to do and not-do"
'pre (lambda (m) (string-reverse (match:substring m))) 'post)
⇒ "ot od dna ton-od"
Without the ‘post’ symbol, just one regexp match is made. For
example the following is the date example from ‘regexp-substitute’
above, without the need for the separate ‘string-match’ call.
(define date-regex
"([0-9][0-9][0-9][0-9])([0-9][0-9])([0-9][0-9])")
(define s "Date 20020429 12am.")
(regexp-substitute/global #f date-regex s
'pre 2 "-" 3 "-" 1 'post " (" 0 ")")
⇒ "Date 04-29-2002 12am. (20020429)"
6.13.2 Match Structures
-----------------------
A “match structure” is the object returned by ‘string-match’ and
‘regexp-exec’. It describes which portion of a string, if any, matched
the given regular expression. Match structures include: a reference to
the string that was checked for matches; the starting and ending
positions of the regexp match; and, if the regexp included any
parenthesized subexpressions, the starting and ending positions of each
submatch.
In each of the regexp match functions described below, the ‘match’
argument must be a match structure returned by a previous call to
‘string-match’ or ‘regexp-exec’. Most of these functions return some
information about the original target string that was matched against a
regular expression; we will call that string TARGET for easy reference.
-- Scheme Procedure: regexp-match? obj
Return ‘#t’ if OBJ is a match structure returned by a previous call
to ‘regexp-exec’, or ‘#f’ otherwise.
-- Scheme Procedure: match:substring match [n]
Return the portion of TARGET matched by subexpression number N.
Submatch 0 (the default) represents the entire regexp match. If
the regular expression as a whole matched, but the subexpression
number N did not match, return ‘#f’.
(define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
(match:substring s)
⇒ "2002"
;; match starting at offset 6 in the string
(match:substring
(string-match "[0-9][0-9][0-9][0-9]" "blah987654" 6))
⇒ "7654"
-- Scheme Procedure: match:start match [n]
Return the starting position of submatch number N.
In the following example, the result is 4, since the match starts at
character index 4:
(define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
(match:start s)
⇒ 4
-- Scheme Procedure: match:end match [n]
Return the ending position of submatch number N.
In the following example, the result is 8, since the match runs
between characters 4 and 8 (i.e. the “2002”).
(define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
(match:end s)
⇒ 8
-- Scheme Procedure: match:prefix match
Return the unmatched portion of TARGET preceding the regexp match.
(define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
(match:prefix s)
⇒ "blah"
-- Scheme Procedure: match:suffix match
Return the unmatched portion of TARGET following the regexp match.
(define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
(match:suffix s)
⇒ "foo"
-- Scheme Procedure: match:count match
Return the number of parenthesized subexpressions from MATCH. Note
that the entire regular expression match itself counts as a
subexpression, and failed submatches are included in the count.
-- Scheme Procedure: match:string match
Return the original TARGET string.
(define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
(match:string s)
⇒ "blah2002foo"
6.13.3 Backslash Escapes
------------------------
Sometimes you will want a regexp to match characters like ‘*’ or ‘$’
exactly. For example, to check whether a particular string represents a
menu entry from an Info node, it would be useful to match it against a
regexp like ‘^* [^:]*::’. However, this won’t work; because the
asterisk is a metacharacter, it won’t match the ‘*’ at the beginning of
the string. In this case, we want to make the first asterisk un-magic.
You can do this by preceding the metacharacter with a backslash
character ‘\’. (This is also called “quoting” the metacharacter, and is
known as a “backslash escape”.) When Guile sees a backslash in a
regular expression, it considers the following glyph to be an ordinary
character, no matter what special meaning it would ordinarily have.
Therefore, we can make the above example work by changing the regexp to
‘^\* [^:]*::’. The ‘\*’ sequence tells the regular expression engine to
match only a single asterisk in the target string.
Since the backslash is itself a metacharacter, you may force a regexp
to match a backslash in the target string by preceding the backslash
with itself. For example, to find variable references in a TeX program,
you might want to find occurrences of the string ‘\let\’ followed by any
number of alphabetic characters. The regular expression
‘\\let\\[A-Za-z]*’ would do this: the double backslashes in the regexp
each match a single backslash in the target string.
-- Scheme Procedure: regexp-quote str
Quote each special character found in STR with a backslash, and
return the resulting string.
*Very important:* Using backslash escapes in Guile source code (as in
Emacs Lisp or C) can be tricky, because the backslash character has
special meaning for the Guile reader. For example, if Guile encounters
the character sequence ‘\n’ in the middle of a string while processing
Scheme code, it replaces those characters with a newline character.
Similarly, the character sequence ‘\t’ is replaced by a horizontal tab.
Several of these “escape sequences” are processed by the Guile reader
before your code is executed. Unrecognized escape sequences are
ignored: if the characters ‘\*’ appear in a string, they will be
translated to the single character ‘*’.
This translation is obviously undesirable for regular expressions,
since we want to be able to include backslashes in a string in order to
escape regexp metacharacters. Therefore, to make sure that a backslash
is preserved in a string in your Guile program, you must use _two_
consecutive backslashes:
(define Info-menu-entry-pattern (make-regexp "^\\* [^:]*"))
The string in this example is preprocessed by the Guile reader before
any code is executed. The resulting argument to ‘make-regexp’ is the
string ‘^\* [^:]*’, which is what we really want.
This also means that in order to write a regular expression that
matches a single backslash character, the regular expression string in
the source code must include _four_ backslashes. Each consecutive pair
of backslashes gets translated by the Guile reader to a single
backslash, and the resulting double-backslash is interpreted by the
regexp engine as matching a single backslash character. Hence:
(define tex-variable-pattern (make-regexp "\\\\let\\\\=[A-Za-z]*"))
The reason for the unwieldiness of this syntax is historical. Both
regular expression pattern matchers and Unix string processing systems
have traditionally used backslashes with the special meanings described
above. The POSIX regular expression specification and ANSI C standard
both require these semantics. Attempting to abandon either convention
would cause other kinds of compatibility problems, possibly more severe
ones. Therefore, without extending the Scheme reader to support strings
with different quoting conventions (an ungainly and confusing extension
when implemented in other languages), we must adhere to this cumbersome
escape syntax.
6.14 LALR(1) Parsing
====================
The ‘(system base lalr)’ module provides the ‘lalr-scm’ LALR(1) parser
generator by Dominique Boucher (https://github.com/schemeway/lalr-scm/).
‘lalr-scm’ uses the same algorithm as GNU Bison (*note Introduction to
Bison: (bison)Introduction.). Parsers are defined using the
‘lalr-parser’ macro.
-- Scheme Syntax: lalr-parser [OPTIONS] TOKENS RULES...
Generate an LALR(1) syntax analyzer. TOKENS is a list of symbols
representing the terminal symbols of the grammar. RULES are the
grammar production rules.
Each rule has the form ‘(NON-TERMINAL (RHS ...) : ACTION ...)’,
where NON-TERMINAL is the name of the rule, RHS are the right-hand
sides, i.e., the production rule, and ACTION is a semantic action
associated with the rule.
The generated parser is a two-argument procedure that takes a
“tokenizer” and a “syntax error procedure”. The tokenizer should
be a thunk that returns lexical tokens as produced by
‘make-lexical-token’. The syntax error procedure may be called
with at least an error message (a string), and optionally the
lexical token that caused the error.
Please refer to the ‘lalr-scm’ documentation for details.
6.15 PEG Parsing
================
Parsing Expression Grammars (PEGs) are a way of specifying formal
languages for text processing. They can be used either for matching
(like regular expressions) or for building recursive descent parsers
(like lex/yacc). Guile uses a superset of PEG syntax that allows more
control over what information is preserved during parsing.
Wikipedia has a clear and concise introduction to PEGs if you want to
familiarize yourself with the syntax:
.
The ‘(ice-9 peg)’ module works by compiling PEGs down to lambda
expressions. These can either be stored in variables at compile-time by
the define macros (‘define-peg-pattern’ and
‘define-peg-string-patterns’) or calculated explicitly at runtime with
the compile functions (‘compile-peg-pattern’ and ‘peg-string-compile’).
They can then be used for either parsing (‘match-pattern’) or
searching (‘search-for-pattern’). For convenience, ‘search-for-pattern’
also takes pattern literals in case you want to inline a simple search
(people often use regular expressions this way).
The rest of this documentation consists of a syntax reference, an API
reference, and a tutorial.
6.15.1 PEG Syntax Reference
---------------------------
Normal PEG Syntax:
..................
-- PEG Pattern: sequence a b
Parses A. If this succeeds, continues to parse B from the end of
the text parsed as A. Succeeds if both A and B succeed.
‘"a b"’
‘(and a b)’
-- PEG Pattern: ordered choice a b
Parses A. If this fails, backtracks and parses B. Succeeds if
either A or B succeeds.
‘"a/b"’
‘(or a b)’
-- PEG Pattern: zero or more a
Parses A as many times in a row as it can, starting each A at the
end of the text parsed by the previous A. Always succeeds.
‘"a*"’
‘(* a)’
-- PEG Pattern: one or more a
Parses A as many times in a row as it can, starting each A at the
end of the text parsed by the previous A. Succeeds if at least one
A was parsed.
‘"a+"’
‘(+ a)’
-- PEG Pattern: optional a
Tries to parse A. Succeeds if A succeeds.
‘"a?"’
‘(? a)’
-- PEG Pattern: followed by a
Makes sure it is possible to parse A, but does not actually parse
it. Succeeds if A would succeed.
‘"&a"’
‘(followed-by a)’
-- PEG Pattern: not followed by a
Makes sure it is impossible to parse A, but does not actually parse
it. Succeeds if A would fail.
‘"!a"’
‘(not-followed-by a)’
-- PEG Pattern: string literal ``abc''
Parses the string "ABC". Succeeds if that parsing succeeds.
‘"'abc'"’
‘"abc"’
-- PEG Pattern: any character
Parses any single character. Succeeds unless there is no more text
to be parsed.
‘"."’
‘peg-any’
-- PEG Pattern: character class a b
Alternative syntax for “Ordered Choice A B” if A and B are
characters.
‘"[ab]"’
‘(or "a" "b")’
-- PEG Pattern: range of characters a z
Parses any character falling between A and Z.
‘"[a-z]"’
‘(range #\a #\z)’
Example:
"(a !b / c &d*) 'e'+"
Would be:
(and
(or
(and a (not-followed-by b))
(and c (followed-by (* d))))
(+ "e"))
Extended Syntax
...............
There is some extra syntax for S-expressions.
-- PEG Pattern: ignore a
Ignore the text matching A
-- PEG Pattern: capture a
Capture the text matching A.
-- PEG Pattern: peg a
Embed the PEG pattern A using string syntax.
Example:
"!a / 'b'"
Is equivalent to
(or (peg "!a") "b")
and
(or (not-followed-by a) "b")
6.15.2 PEG API Reference
------------------------
Define Macros
.............
The most straightforward way to define a PEG is by using one of the
define macros (both of these macroexpand into ‘define’ expressions).
These macros bind parsing functions to variables. These parsing
functions may be invoked by ‘match-pattern’ or ‘search-for-pattern’,
which return a PEG match record. Raw data can be retrieved from this
record with the PEG match deconstructor functions. More complicated
(and perhaps enlightening) examples can be found in the tutorial.
-- Scheme Macro: define-peg-string-patterns peg-string
Defines all the nonterminals in the PEG PEG-STRING. More
precisely, ‘define-peg-string-patterns’ takes a superset of PEGs.
A normal PEG has a ‘<-’ between the nonterminal and the pattern.
‘define-peg-string-patterns’ uses this symbol to determine what
information it should propagate up the parse tree. The normal ‘<-’
propagates the matched text up the parse tree, ‘<--’ propagates the
matched text up the parse tree tagged with the name of the
nonterminal, and ‘<’ discards that matched text and propagates
nothing up the parse tree. Also, nonterminals may consist of any
alphanumeric character or a “-” character (in normal PEGs
nonterminals can only be alphabetic).
For example, if we:
(define-peg-string-patterns
"as <- 'a'+
bs <- 'b'+
as-or-bs <- as/bs")
(define-peg-string-patterns
"as-tag <-- 'a'+
bs-tag <-- 'b'+
as-or-bs-tag <-- as-tag/bs-tag")
Then:
(match-pattern as-or-bs "aabbcc") ⇒
#
(match-pattern as-or-bs-tag "aabbcc") ⇒
#
Note that in doing this, we have bound 6 variables at the toplevel
(AS, BS, AS-OR-BS, AS-TAG, BS-TAG, and AS-OR-BS-TAG).
-- Scheme Macro: define-peg-pattern name capture-type peg-sexp
Defines a single nonterminal NAME. CAPTURE-TYPE determines how
much information is passed up the parse tree. PEG-SEXP is a PEG in
S-expression form.
Possible values for capture-type:
‘all’
passes the matched text up the parse tree tagged with the name
of the nonterminal.
‘body’
passes the matched text up the parse tree.
‘none’
passes nothing up the parse tree.
For Example, if we:
(define-peg-pattern as body (+ "a"))
(define-peg-pattern bs body (+ "b"))
(define-peg-pattern as-or-bs body (or as bs))
(define-peg-pattern as-tag all (+ "a"))
(define-peg-pattern bs-tag all (+ "b"))
(define-peg-pattern as-or-bs-tag all (or as-tag bs-tag))
Then:
(match-pattern as-or-bs "aabbcc") ⇒
#
(match-pattern as-or-bs-tag "aabbcc") ⇒
#
Note that in doing this, we have bound 6 variables at the toplevel
(AS, BS, AS-OR-BS, AS-TAG, BS-TAG, and AS-OR-BS-TAG).
Compile Functions
.................
It is sometimes useful to be able to compile anonymous PEG patterns at
runtime. These functions let you do that using either syntax.
-- Scheme Procedure: peg-string-compile peg-string capture-type
Compiles the PEG pattern in PEG-STRING propagating according to
CAPTURE-TYPE (capture-type can be any of the values from
‘define-peg-pattern’).
-- Scheme Procedure: compile-peg-pattern peg-sexp capture-type
Compiles the PEG pattern in PEG-SEXP propagating according to
CAPTURE-TYPE (capture-type can be any of the values from
‘define-peg-pattern’).
The functions return syntax objects, which can be useful if you want
to use them in macros. If all you want is to define a new nonterminal,
you can do the following:
(define exp '(+ "a"))
(define as (compile (compile-peg-pattern exp 'body)))
You can use this nonterminal with all of the regular PEG functions:
(match-pattern as "aaaaa") ⇒
#
Parsing & Matching Functions
............................
For our purposes, “parsing” means parsing a string into a tree starting
from the first character, while “matching” means searching through the
string for a substring. In practice, the only difference between the
two functions is that ‘match-pattern’ gives up if it can’t find a valid
substring starting at index 0 and ‘search-for-pattern’ keeps looking.
They are both equally capable of “parsing” and “matching” given those
constraints.
-- Scheme Procedure: match-pattern nonterm string
Parses STRING using the PEG stored in NONTERM. If no match was
found, ‘match-pattern’ returns false. If a match was found, a PEG
match record is returned.
The ‘capture-type’ argument to ‘define-peg-pattern’ allows you to
choose what information to hold on to while parsing. The options
are:
‘all’
tag the matched text with the nonterminal
‘body’
just the matched text
‘none’
nothing
(define-peg-pattern as all (+ "a"))
(match-pattern as "aabbcc") ⇒
#
(define-peg-pattern as body (+ "a"))
(match-pattern as "aabbcc") ⇒
#
(define-peg-pattern as none (+ "a"))
(match-pattern as "aabbcc") ⇒
#
(define-peg-pattern bs body (+ "b"))
(match-pattern bs "aabbcc") ⇒
#f
-- Scheme Macro: search-for-pattern nonterm-or-peg string
Searches through STRING looking for a matching subexpression.
NONTERM-OR-PEG can either be a nonterminal or a literal PEG
pattern. When a literal PEG pattern is provided,
‘search-for-pattern’ works very similarly to the regular expression
searches many hackers are used to. If no match was found,
‘search-for-pattern’ returns false. If a match was found, a PEG
match record is returned.
(define-peg-pattern as body (+ "a"))
(search-for-pattern as "aabbcc") ⇒
#
(search-for-pattern (+ "a") "aabbcc") ⇒
#
(search-for-pattern "'a'+" "aabbcc") ⇒
#
(define-peg-pattern as all (+ "a"))
(search-for-pattern as "aabbcc") ⇒
#
(define-peg-pattern bs body (+ "b"))
(search-for-pattern bs "aabbcc") ⇒
#
(search-for-pattern (+ "b") "aabbcc") ⇒
#
(search-for-pattern "'b'+" "aabbcc") ⇒
#
(define-peg-pattern zs body (+ "z"))
(search-for-pattern zs "aabbcc") ⇒
#f
(search-for-pattern (+ "z") "aabbcc") ⇒
#f
(search-for-pattern "'z'+" "aabbcc") ⇒
#f
PEG Match Records
.................
The ‘match-pattern’ and ‘search-for-pattern’ functions both return PEG
match records. Actual information can be extracted from these with the
following functions.
-- Scheme Procedure: peg:string match-record
Returns the original string that was parsed in the creation of
‘match-record’.
-- Scheme Procedure: peg:start match-record
Returns the index of the first parsed character in the original
string (from ‘peg:string’). If this is the same as ‘peg:end’,
nothing was parsed.
-- Scheme Procedure: peg:end match-record
Returns one more than the index of the last parsed character in the
original string (from ‘peg:string’). If this is the same as
‘peg:start’, nothing was parsed.
-- Scheme Procedure: peg:substring match-record
Returns the substring parsed by ‘match-record’. This is equivalent
to ‘(substring (peg:string match-record) (peg:start match-record)
(peg:end match-record))’.
-- Scheme Procedure: peg:tree match-record
Returns the tree parsed by ‘match-record’.
-- Scheme Procedure: peg-record? match-record
Returns true if ‘match-record’ is a PEG match record, or false
otherwise.
Example:
(define-peg-pattern bs all (peg "'b'+"))
(search-for-pattern bs "aabbcc") ⇒
#
(let ((pm (search-for-pattern bs "aabbcc")))
`((string ,(peg:string pm))
(start ,(peg:start pm))
(end ,(peg:end pm))
(substring ,(peg:substring pm))
(tree ,(peg:tree pm))
(record? ,(peg-record? pm)))) ⇒
((string "aabbcc")
(start 2)
(end 4)
(substring "bb")
(tree (bs "bb"))
(record? #t))
Miscellaneous
.............
-- Scheme Procedure: context-flatten tst lst
Takes a predicate TST and a list LST. Flattens LST until all
elements are either atoms or satisfy TST. If LST itself satisfies
TST, ‘(list lst)’ is returned (this is a flat list whose only
element satisfies TST).
(context-flatten (lambda (x) (and (number? (car x)) (= (car x) 1))) '(2 2 (1 1 (2 2)) (2 2 (1 1)))) ⇒
(2 2 (1 1 (2 2)) 2 2 (1 1))
(context-flatten (lambda (x) (and (number? (car x)) (= (car x) 1))) '(1 1 (1 1 (2 2)) (2 2 (1 1)))) ⇒
((1 1 (1 1 (2 2)) (2 2 (1 1))))
If you’re wondering why this is here, take a look at the tutorial.
-- Scheme Procedure: keyword-flatten terms lst
A less general form of ‘context-flatten’. Takes a list of terminal
atoms ‘terms’ and flattens LST until all elements are either atoms,
or lists which have an atom from ‘terms’ as their first element.
(keyword-flatten '(a b) '(c a b (a c) (b c) (c (b a) (c a)))) ⇒
(c a b (a c) (b c) c (b a) c a)
If you’re wondering why this is here, take a look at the tutorial.
6.15.3 PEG Tutorial
-------------------
Parsing /etc/passwd
...................
This example will show how to parse /etc/passwd using PEGs.
First we define an example /etc/passwd file:
(define *etc-passwd*
"root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/bin/sh
bin:x:2:2:bin:/bin:/bin/sh
sys:x:3:3:sys:/dev:/bin/sh
nobody:x:65534:65534:nobody:/nonexistent:/bin/sh
messagebus:x:103:107::/var/run/dbus:/bin/false
")
As a first pass at this, we might want to have all the entries in
/etc/passwd in a list.
Doing this with string-based PEG syntax would look like this:
(define-peg-string-patterns
"passwd <- entry* !.
entry <-- (! NL .)* NL*
NL < '\n'")
A ‘passwd’ file is 0 or more entries (‘entry*’) until the end of the
file (‘!.’ (‘.’ is any character, so ‘!.’ means “not anything”)). We
want to capture the data in the nonterminal ‘passwd’, but not tag it
with the name, so we use ‘<-’.
An entry is a series of 0 or more characters that aren’t newlines
(‘(! NL .)*’) followed by 0 or more newlines (‘NL*’). We want to tag
all the entries with ‘entry’, so we use ‘<--’.
A newline is just a literal newline (‘'\n'’). We don’t want a bunch
of newlines cluttering up the output, so we use ‘<’ to throw away the
captured data.
Here is the same PEG defined using S-expressions:
(define-peg-pattern passwd body (and (* entry) (not-followed-by peg-any)))
(define-peg-pattern entry all (and (* (and (not-followed-by NL) peg-any))
(* NL)))
(define-peg-pattern NL none "\n")
Obviously this is much more verbose. On the other hand, it’s more
explicit, and thus easier to build automatically. However, there are
some tricks that make S-expressions easier to use in some cases. One is
the ‘ignore’ keyword; the string syntax has no way to say “throw away
this text” except breaking it out into a separate nonterminal. For
instance, to throw away the newlines we had to define ‘NL’. In the
S-expression syntax, we could have simply written ‘(ignore "\n")’.
Also, for the cases where string syntax is really much cleaner, the
‘peg’ keyword can be used to embed string syntax in S-expression syntax.
For instance, we could have written:
(define-peg-pattern passwd body (peg "entry* !."))
However we define it, parsing ‘*etc-passwd*’ with the ‘passwd’
nonterminal yields the same results:
(peg:tree (match-pattern passwd *etc-passwd*)) ⇒
((entry "root:x:0:0:root:/root:/bin/bash")
(entry "daemon:x:1:1:daemon:/usr/sbin:/bin/sh")
(entry "bin:x:2:2:bin:/bin:/bin/sh")
(entry "sys:x:3:3:sys:/dev:/bin/sh")
(entry "nobody:x:65534:65534:nobody:/nonexistent:/bin/sh")
(entry "messagebus:x:103:107::/var/run/dbus:/bin/false"))
However, here is something to be wary of:
(peg:tree (match-pattern passwd "one entry")) ⇒
(entry "one entry")
By default, the parse trees generated by PEGs are compressed as much
as possible without losing information. It may not look like this is
what you want at first, but uncompressed parse trees are an enormous
headache (there’s no easy way to predict how deep particular lists will
nest, there are empty lists littered everywhere, etc. etc.). One
side-effect of this, however, is that sometimes the compressor is too
aggressive. No information is discarded when ‘((entry "one entry"))’ is
compressed to ‘(entry "one entry")’, but in this particular case it
probably isn’t what we want.
There are two functions for easily dealing with this:
‘keyword-flatten’ and ‘context-flatten’. The ‘keyword-flatten’ function
takes a list of keywords and a list to flatten, then tries to coerce the
list such that the first element of all sublists is one of the keywords.
The ‘context-flatten’ function is similar, but instead of a list of
keywords it takes a predicate that should indicate whether a given
sublist is good enough (refer to the API reference for more details).
What we want here is ‘keyword-flatten’.
(keyword-flatten '(entry) (peg:tree (match-pattern passwd *etc-passwd*))) ⇒
((entry "root:x:0:0:root:/root:/bin/bash")
(entry "daemon:x:1:1:daemon:/usr/sbin:/bin/sh")
(entry "bin:x:2:2:bin:/bin:/bin/sh")
(entry "sys:x:3:3:sys:/dev:/bin/sh")
(entry "nobody:x:65534:65534:nobody:/nonexistent:/bin/sh")
(entry "messagebus:x:103:107::/var/run/dbus:/bin/false"))
(keyword-flatten '(entry) (peg:tree (match-pattern passwd "one entry"))) ⇒
((entry "one entry"))
Of course, this is a somewhat contrived example. In practice we
would probably just tag the ‘passwd’ nonterminal to remove the ambiguity
(using either the ‘all’ keyword for S-expressions or the ‘<--’ symbol
for strings)..
(define-peg-pattern tag-passwd all (peg "entry* !."))
(peg:tree (match-pattern tag-passwd *etc-passwd*)) ⇒
(tag-passwd
(entry "root:x:0:0:root:/root:/bin/bash")
(entry "daemon:x:1:1:daemon:/usr/sbin:/bin/sh")
(entry "bin:x:2:2:bin:/bin:/bin/sh")
(entry "sys:x:3:3:sys:/dev:/bin/sh")
(entry "nobody:x:65534:65534:nobody:/nonexistent:/bin/sh")
(entry "messagebus:x:103:107::/var/run/dbus:/bin/false"))
(peg:tree (match-pattern tag-passwd "one entry"))
(tag-passwd
(entry "one entry"))
If you’re ever uncertain about the potential results of parsing
something, remember the two absolute rules:
1. No parsing information will ever be discarded.
2. There will never be any lists with fewer than 2 elements.
For the purposes of (1), "parsing information" means things tagged
with the ‘any’ keyword or the ‘<--’ symbol. Plain strings will be
concatenated.
Let’s extend this example a bit more and actually pull some useful
information out of the passwd file:
(define-peg-string-patterns
"passwd <-- entry* !.
entry <-- login C pass C uid C gid C nameORcomment C homedir C shell NL*
login <-- text
pass <-- text
uid <-- [0-9]*
gid <-- [0-9]*
nameORcomment <-- text
homedir <-- path
shell <-- path
path <-- (SLASH pathELEMENT)*
pathELEMENT <-- (!NL !C !'/' .)*
text <- (!NL !C .)*
C < ':'
NL < '\n'
SLASH < '/'")
This produces rather pretty parse trees:
(passwd
(entry (login "root")
(pass "x")
(uid "0")
(gid "0")
(nameORcomment "root")
(homedir (path (pathELEMENT "root")))
(shell (path (pathELEMENT "bin") (pathELEMENT "bash"))))
(entry (login "daemon")
(pass "x")
(uid "1")
(gid "1")
(nameORcomment "daemon")
(homedir
(path (pathELEMENT "usr") (pathELEMENT "sbin")))
(shell (path (pathELEMENT "bin") (pathELEMENT "sh"))))
(entry (login "bin")
(pass "x")
(uid "2")
(gid "2")
(nameORcomment "bin")
(homedir (path (pathELEMENT "bin")))
(shell (path (pathELEMENT "bin") (pathELEMENT "sh"))))
(entry (login "sys")
(pass "x")
(uid "3")
(gid "3")
(nameORcomment "sys")
(homedir (path (pathELEMENT "dev")))
(shell (path (pathELEMENT "bin") (pathELEMENT "sh"))))
(entry (login "nobody")
(pass "x")
(uid "65534")
(gid "65534")
(nameORcomment "nobody")
(homedir (path (pathELEMENT "nonexistent")))
(shell (path (pathELEMENT "bin") (pathELEMENT "sh"))))
(entry (login "messagebus")
(pass "x")
(uid "103")
(gid "107")
nameORcomment
(homedir
(path (pathELEMENT "var")
(pathELEMENT "run")
(pathELEMENT "dbus")))
(shell (path (pathELEMENT "bin") (pathELEMENT "false")))))
Notice that when there’s no entry in a field (e.g. ‘nameORcomment’
for messagebus) the symbol is inserted. This is the “don’t throw away
any information” rule—we succesfully matched a ‘nameORcomment’ of 0
characters (since we used ‘*’ when defining it). This is usually what
you want, because it allows you to e.g. use ‘list-ref’ to pull out
elements (since they all have known offsets).
If you’d prefer not to have symbols for empty matches, you can
replace the ‘*’ with a ‘+’ and add a ‘?’ after the ‘nameORcomment’ in
‘entry’. Then it will try to parse 1 or more characters, fail
(inserting nothing into the parse tree), but continue because it didn’t
have to match the nameORcomment to continue.
Embedding Arithmetic Expressions
................................
We can parse simple mathematical expressions with the following PEG:
(define-peg-string-patterns
"expr <- sum
sum <-- (product ('+' / '-') sum) / product
product <-- (value ('*' / '/') product) / value
value <-- number / '(' expr ')'
number <-- [0-9]+")
Then:
(peg:tree (match-pattern expr "1+1/2*3+(1+1)/2")) ⇒
(sum (product (value (number "1")))
"+"
(sum (product
(value (number "1"))
"/"
(product
(value (number "2"))
"*"
(product (value (number "3")))))
"+"
(sum (product
(value "("
(sum (product (value (number "1")))
"+"
(sum (product (value (number "1")))))
")")
"/"
(product (value (number "2")))))))
There is very little wasted effort in this PEG. The ‘number’
nonterminal has to be tagged because otherwise the numbers might run
together with the arithmetic expressions during the string concatenation
stage of parse-tree compression (the parser will see “1” followed by “/”
and decide to call it “1/”). When in doubt, tag.
It is very easy to turn these parse trees into lisp expressions:
(define (parse-sum sum left . rest)
(if (null? rest)
(apply parse-product left)
(list (string->symbol (car rest))
(apply parse-product left)
(apply parse-sum (cadr rest)))))
(define (parse-product product left . rest)
(if (null? rest)
(apply parse-value left)
(list (string->symbol (car rest))
(apply parse-value left)
(apply parse-product (cadr rest)))))
(define (parse-value value first . rest)
(if (null? rest)
(string->number (cadr first))
(apply parse-sum (car rest))))
(define parse-expr parse-sum)
(Notice all these functions look very similar; for a more complicated
PEG, it would be worth abstracting.)
Then:
(apply parse-expr (peg:tree (match-pattern expr "1+1/2*3+(1+1)/2"))) ⇒
(+ 1 (+ (/ 1 (* 2 3)) (/ (+ 1 1) 2)))
But wait! The associativity is wrong! Where it says ‘(/ 1 (* 2
3))’, it should say ‘(* (/ 1 2) 3)’.
It’s tempting to try replacing e.g. ‘"sum <-- (product ('+' / '-')
sum) / product"’ with ‘"sum <-- (sum ('+' / '-') product) / product"’,
but this is a Bad Idea. PEGs don’t support left recursion. To see why,
imagine what the parser will do here. When it tries to parse ‘sum’, it
first has to try and parse ‘sum’. But to do that, it first has to try
and parse ‘sum’. This will continue until the stack gets blown off.
So how does one parse left-associative binary operators with PEGs?
Honestly, this is one of their major shortcomings. There’s no
general-purpose way of doing this, but here the repetition operators are
a good choice:
(use-modules (srfi srfi-1))
(define-peg-string-patterns
"expr <- sum
sum <-- (product ('+' / '-'))* product
product <-- (value ('*' / '/'))* value
value <-- number / '(' expr ')'
number <-- [0-9]+")
;; take a deep breath...
(define (make-left-parser next-func)
(lambda (sum first . rest) ;; general form, comments below assume
;; that we're dealing with a sum expression
(if (null? rest) ;; form (sum (product ...))
(apply next-func first)
(if (string? (cadr first));; form (sum ((product ...) "+") (product ...))
(list (string->symbol (cadr first))
(apply next-func (car first))
(apply next-func (car rest)))
;; form (sum (((product ...) "+") ((product ...) "+")) (product ...))
(car
(reduce ;; walk through the list and build a left-associative tree
(lambda (l r)
(list (list (cadr r) (car r) (apply next-func (car l)))
(string->symbol (cadr l))))
'ignore
(append ;; make a list of all the products
;; the first one should be pre-parsed
(list (list (apply next-func (caar first))
(string->symbol (cadar first))))
(cdr first)
;; the last one has to be added in
(list (append rest '("done"))))))))))
(define (parse-value value first . rest)
(if (null? rest)
(string->number (cadr first))
(apply parse-sum (car rest))))
(define parse-product (make-left-parser parse-value))
(define parse-sum (make-left-parser parse-product))
(define parse-expr parse-sum)
Then:
(apply parse-expr (peg:tree (match-pattern expr "1+1/2*3+(1+1)/2"))) ⇒
(+ (+ 1 (* (/ 1 2) 3)) (/ (+ 1 1) 2))
As you can see, this is much uglier (it could be made prettier by
using ‘context-flatten’, but the way it’s written above makes it clear
how we deal with the three ways the zero-or-more ‘*’ expression can
parse). Fortunately, most of the time we can get away with only using
right-associativity.
Simplified Functions
....................
For a more tantalizing example, consider the following grammar that
parses (highly) simplified C functions:
(define-peg-string-patterns
"cfunc <-- cSP ctype cSP cname cSP cargs cLB cSP cbody cRB
ctype <-- cidentifier
cname <-- cidentifier
cargs <-- cLP (! (cSP cRP) carg cSP (cCOMMA / cRP) cSP)* cSP
carg <-- cSP ctype cSP cname
cbody <-- cstatement *
cidentifier <- [a-zA-z][a-zA-Z0-9_]*
cstatement <-- (!';'.)*cSC cSP
cSC < ';'
cCOMMA < ','
cLP < '('
cRP < ')'
cLB < '{'
cRB < '}'
cSP < [ \t\n]*")
Then:
(match-pattern cfunc "int square(int a) { return a*a;}") ⇒
(32
(cfunc (ctype "int")
(cname "square")
(cargs (carg (ctype "int") (cname "a")))
(cbody (cstatement "return a*a"))))
And:
(match-pattern cfunc "int mod(int a, int b) { int c = a/b;return a-b*c; }") ⇒
(52
(cfunc (ctype "int")
(cname "mod")
(cargs (carg (ctype "int") (cname "a"))
(carg (ctype "int") (cname "b")))
(cbody (cstatement "int c = a/b")
(cstatement "return a- b*c"))))
By wrapping all the ‘carg’ nonterminals in a ‘cargs’ nonterminal, we
were able to remove any ambiguity in the parsing structure and avoid
having to call ‘context-flatten’ on the output of ‘match-pattern’. We
used the same trick with the ‘cstatement’ nonterminals, wrapping them in
a ‘cbody’ nonterminal.
The whitespace nonterminal ‘cSP’ used here is a (very) useful
instantiation of a common pattern for matching syntactically irrelevant
information. Since it’s tagged with ‘<’ and ends with ‘*’ it won’t
clutter up the parse trees (all the empty lists will be discarded during
the compression step) and it will never cause parsing to fail.
6.15.4 PEG Internals
--------------------
A PEG parser takes a string as input and attempts to parse it as a given
nonterminal. The key idea of the PEG implementation is that every
nonterminal is just a function that takes a string as an argument and
attempts to parse that string as its nonterminal. The functions always
start from the beginning, but a parse is considered successful if there
is material left over at the end.
This makes it easy to model different PEG parsing operations. For
instance, consider the PEG grammar ‘"ab"’, which could also be written
‘(and "a" "b")’. It matches the string “ab”. Here’s how that might be
implemented in the PEG style:
(define (match-and-a-b str)
(match-a str)
(match-b str))
As you can see, the use of functions provides an easy way to model
sequencing. In a similar way, one could model ‘(or a b)’ with something
like the following:
(define (match-or-a-b str)
(or (match-a str) (match-b str)))
Here the semantics of a PEG ‘or’ expression map naturally onto
Scheme’s ‘or’ operator. This function will attempt to run ‘(match-a
str)’, and return its result if it succeeds. Otherwise it will run
‘(match-b str)’.
Of course, the code above wouldn’t quite work. We need some way for
the parsing functions to communicate. The actual interface used is
below.
Parsing Function Interface
..........................
A parsing function takes three arguments - a string, the length of that
string, and the position in that string it should start parsing at. In
effect, the parsing functions pass around substrings in pieces - the
first argument is a buffer of characters, and the second two give a
range within that buffer that the parsing function should look at.
Parsing functions return either #f, if they failed to match their
nonterminal, or a list whose first element must be an integer
representing the final position in the string they matched and whose cdr
can be any other data the function wishes to return, or ’() if it
doesn’t have any more data.
The one caveat is that if the extra data it returns is a list, any
adjacent strings in that list will be appended by ‘match-pattern’. For
instance, if a parsing function returns ‘(13 ("a" "b" "c"))’,
‘match-pattern’ will take ‘(13 ("abc"))’ as its value.
For example, here is a function to match “ab” using the actual
interface.
(define (match-a-b str len pos)
(and (<= (+ pos 2) len)
(string= str "ab" pos (+ pos 2))
(list (+ pos 2) '()))) ; we return no extra information
The above function can be used to match a string by running
‘(match-pattern match-a-b "ab")’.
Code Generators and Extensible Syntax
.....................................
PEG expressions, such as those in a ‘define-peg-pattern’ form, are
interpreted internally in two steps.
First, any string PEG is expanded into an s-expression PEG by the
code in the ‘(ice-9 peg string-peg)’ module.
Then, the s-expression PEG that results is compiled into a parsing
function by the ‘(ice-9 peg codegen)’ module. In particular, the
function ‘compile-peg-pattern’ is called on the s-expression. It then
decides what to do based on the form it is passed.
The PEG syntax can be expanded by providing ‘compile-peg-pattern’
more options for what to do with its forms. The extended syntax will be
associated with a symbol, for instance ‘my-parsing-form’, and will be
called on all PEG expressions of the form
(my-parsing-form ...)
The parsing function should take two arguments. The first will be a
syntax object containing a list with all of the arguments to the form
(but not the form’s name), and the second will be the ‘capture-type’
argument that is passed to ‘define-peg-pattern’.
New functions can be registered by calling ‘(add-peg-compiler! symbol
function)’, where ‘symbol’ is the symbol that will indicate a form of
this type and ‘function’ is the code generating function described
above. The function ‘add-peg-compiler!’ is exported from the ‘(ice-9
peg codegen)’ module.
6.16 Reading and Evaluating Scheme Code
=======================================
This chapter describes Guile functions that are concerned with reading,
loading, evaluating, and compiling Scheme code at run time.
6.16.1 Scheme Syntax: Standard and Guile Extensions
---------------------------------------------------
6.16.1.1 Expression Syntax
..........................
An expression to be evaluated takes one of the following forms.
SYMBOL
A symbol is evaluated by dereferencing. A binding of that symbol
is sought and the value there used. For example,
(define x 123)
x ⇒ 123
(PROC ARGS...)
A parenthesised expression is a function call. PROC and each
argument are evaluated, then the function (which PROC evaluated to)
is called with those arguments.
The order in which PROC and the arguments are evaluated is
unspecified, so be careful when using expressions with side
effects.
(max 1 2 3) ⇒ 3
(define (get-some-proc) min)
((get-some-proc) 1 2 3) ⇒ 1
The same sort of parenthesised form is used for a macro invocation,
but in that case the arguments are not evaluated. See the
descriptions of macros for more on this (*note Macros::, and *note
Syntax Rules::).
CONSTANT
Number, string, character and boolean constants evaluate “to
themselves”, so can appear as literals.
123 ⇒ 123
99.9 ⇒ 99.9
"hello" ⇒ "hello"
#\z ⇒ #\z
#t ⇒ #t
Note that an application must not attempt to modify literal
strings, since they may be in read-only memory.
(quote DATA)
’DATA
Quoting is used to obtain a literal symbol (instead of a variable
reference), a literal list (instead of a function call), or a
literal vector. ’ is simply a shorthand for a ‘quote’ form. For
example,
'x ⇒ x
'(1 2 3) ⇒ (1 2 3)
'#(1 (2 3) 4) ⇒ #(1 (2 3) 4)
(quote x) ⇒ x
(quote (1 2 3)) ⇒ (1 2 3)
(quote #(1 (2 3) 4)) ⇒ #(1 (2 3) 4)
Note that an application must not attempt to modify literal lists
or vectors obtained from a ‘quote’ form, since they may be in
read-only memory.
(quasiquote DATA)
‘DATA
Backquote quasi-quotation is like ‘quote’, but selected
sub-expressions are evaluated. This is a convenient way to
construct a list or vector structure most of which is constant, but
at certain points should have expressions substituted.
The same effect can always be had with suitable ‘list’, ‘cons’ or
‘vector’ calls, but quasi-quoting is often easier.
(unquote EXPR)
,EXPR
Within the quasiquote DATA, ‘unquote’ or ‘,’ indicates an
expression to be evaluated and inserted. The comma syntax ‘,’
is simply a shorthand for an ‘unquote’ form. For example,
`(1 2 (* 9 9) 3 4) ⇒ (1 2 (* 9 9) 3 4)
`(1 2 ,(* 9 9) 3 4) ⇒ (1 2 81 3 4)
`(1 (unquote (+ 1 1)) 3) ⇒ (1 2 3)
`#(1 ,(/ 12 2)) ⇒ #(1 6)
(unquote-splicing EXPR)
,@EXPR
Within the quasiquote DATA, ‘unquote-splicing’ or ‘,@’
indicates an expression to be evaluated and the elements of
the returned list inserted. EXPR must evaluate to a list.
The “comma-at” syntax ‘,@’ is simply a shorthand for an
‘unquote-splicing’ form.
(define x '(2 3))
`(1 ,x 4) ⇒ (1 (2 3) 4)
`(1 ,@x 4) ⇒ (1 2 3 4)
`(1 (unquote-splicing (map 1+ x))) ⇒ (1 3 4)
`#(9 ,@x 9) ⇒ #(9 2 3 9)
Notice ‘,@’ differs from plain ‘,’ in the way one level of
nesting is stripped. For ‘,@’ the elements of a returned list
are inserted, whereas with ‘,’ it would be the list itself
inserted.
6.16.1.2 Comments
.................
Comments in Scheme source files are written by starting them with a
semicolon character (‘;’). The comment then reaches up to the end of
the line. Comments can begin at any column, and the may be inserted on
the same line as Scheme code.
; Comment
;; Comment too
(define x 1) ; Comment after expression
(let ((y 1))
;; Display something.
(display y)
;;; Comment at left margin.
(display (+ y 1)))
It is common to use a single semicolon for comments following
expressions on a line, to use two semicolons for comments which are
indented like code, and three semicolons for comments which start at
column 0, even if they are inside an indented code block. This
convention is used when indenting code in Emacs’ Scheme mode.
6.16.1.3 Block Comments
.......................
In addition to the standard line comments defined by R5RS, Guile has
another comment type for multiline comments, called “block comments”.
This type of comment begins with the character sequence ‘#!’ and ends
with the characters ‘!#’.
These comments are compatible with the block comments in the Scheme
Shell ‘scsh’ (*note The Scheme shell (scsh)::). The characters ‘#!’
were chosen because they are the magic characters used in shell scripts
for indicating that the name of the program for executing the script
follows on the same line.
Thus a Guile script often starts like this.
#! /usr/local/bin/guile -s
!#
More details on Guile scripting can be found in the scripting section
(*note Guile Scripting::).
Similarly, Guile (starting from version 2.0) supports nested block
comments as specified by R6RS and SRFI-30
(http://srfi.schemers.org/srfi-30/srfi-30.html):
(+ 1 #| this is a #| nested |# block comment |# 2)
⇒ 3
For backward compatibility, this syntax can be overridden with
‘read-hash-extend’ (*note ‘read-hash-extend’: Reader Extensions.).
There is one special case where the contents of a comment can
actually affect the interpretation of code. When a character encoding
declaration, such as ‘coding: utf-8’ appears in one of the first few
lines of a source file, it indicates to Guile’s default reader that this
source code file is not ASCII. For details see *note Character Encoding
of Source Files::.
6.16.1.4 Case Sensitivity
.........................
Scheme as defined in R5RS is not case sensitive when reading symbols.
Guile, on the contrary is case sensitive by default, so the identifiers
guile-whuzzy
Guile-Whuzzy
are the same in R5RS Scheme, but are different in Guile.
It is possible to turn off case sensitivity in Guile by setting the
reader option ‘case-insensitive’. For more information on reader
options, *Note Scheme Read::.
(read-enable 'case-insensitive)
It is also possible to disable (or enable) case sensitivity within a
single file by placing the reader directives ‘#!fold-case’ (or
‘#!no-fold-case’) within the file itself.
6.16.1.5 Keyword Syntax
.......................
6.16.1.6 Reader Extensions
..........................
-- Scheme Procedure: read-hash-extend chr proc
-- C Function: scm_read_hash_extend (chr, proc)
Install the procedure PROC for reading expressions starting with
the character sequence ‘#’ and CHR. PROC will be called with two
arguments: the character CHR and the port to read further data
from. The object returned will be the return value of ‘read’.
Passing ‘#f’ for PROC will remove a previous setting.
6.16.2 Reading Scheme Code
--------------------------
-- Scheme Procedure: read [port]
-- C Function: scm_read (port)
Read an s-expression from the input port PORT, or from the current
input port if PORT is not specified. Any whitespace before the
next token is discarded.
The behaviour of Guile’s Scheme reader can be modified by
manipulating its read options.
-- Scheme Procedure: read-options [setting]
Display the current settings of the global read options. If
SETTING is omitted, only a short form of the current read options
is printed. Otherwise if SETTING is the symbol ‘help’, a complete
options description is displayed.
The set of available options, and their default values, may be had by
invoking ‘read-options’ at the prompt.
scheme@(guile-user)> (read-options)
(square-brackets keywords #f positions)
scheme@(guile-user)> (read-options 'help)
positions yes Record positions of source code expressions.
case-insensitive no Convert symbols to lower case.
keywords #f Style of keyword recognition: #f, 'prefix or 'postfix.
r6rs-hex-escapes no Use R6RS variable-length character and string hex escapes.
square-brackets yes Treat `[' and `]' as parentheses, for R6RS compatibility.
hungry-eol-escapes no In strings, consume leading whitespace after an
escaped end-of-line.
curly-infix no Support SRFI-105 curly infix expressions.
r7rs-symbols no Support R7RS |...| symbol notation.
Note that Guile also includes a preliminary mechanism for setting
read options on a per-port basis. For instance, the ‘case-insensitive’
read option is set (or unset) on the port when the reader encounters the
‘#!fold-case’ or ‘#!no-fold-case’ reader directives. Similarly, the
‘#!curly-infix’ reader directive sets the ‘curly-infix’ read option on
the port, and ‘#!curly-infix-and-bracket-lists’ sets ‘curly-infix’ and
unsets ‘square-brackets’ on the port (*note SRFI-105::). There is
currently no other way to access or set the per-port read options.
The boolean options may be toggled with ‘read-enable’ and
‘read-disable’. The non-boolean ‘keywords’ option must be set using
‘read-set!’.
-- Scheme Procedure: read-enable option-name
-- Scheme Procedure: read-disable option-name
-- Scheme Syntax: read-set! option-name value
Modify the read options. ‘read-enable’ should be used with boolean
options and switches them on, ‘read-disable’ switches them off.
‘read-set!’ can be used to set an option to a specific value. Due
to historical oddities, it is a macro that expects an unquoted
option name.
For example, to make ‘read’ fold all symbols to their lower case
(perhaps for compatibility with older Scheme code), you can enter:
(read-enable 'case-insensitive)
For more information on the effect of the ‘r6rs-hex-escapes’ and
‘hungry-eol-escapes’ options, see (*note String Syntax::).
For more information on the ‘r7rs-symbols’ option, see (*note Symbol
Read Syntax::).
6.16.3 Reading Scheme Code, For the Compiler
--------------------------------------------
When something goes wrong with a Scheme program, the user will want to
know how to fix it. This starts with identifying where the error
occured: we want to associate a source location with each component part
of source code, and propagate that source location information through
to the compiler or interpreter.
For that, Guile provides ‘read-syntax’.
-- Scheme Procedure: read-syntax [port]
Read an s-expression from the input port PORT, or from the current
input port if PORT is not specified.
If, after skipping white space and comments, no more bytes are
available from PORT, return the end-of-file object. *Note Binary
I/O::. Otherwise, return an annotated datum. An annotated datum
is a syntax object which associates a source location with a datum.
For example:
(call-with-input-string " foo" read-syntax)
; ⇒ #
(call-with-input-string "(foo)" read-syntax)
; ⇒
; #)>
As the second example shows, all fields of pairs and vectors are
also annotated, recursively.
Most users are familiar with syntax objects in the context of macros,
which use syntax objects to associate scope information with
identifiers. *Note Macros::. Here we use syntax objects to associate
source location information with any datum, but without attaching scope
information. The Scheme compiler (‘compile’) and the interpreter
(‘eval’) can accept syntax objects directly as input, allowing them to
associate source information with resulting code. *Note Compilation::,
and *Note Fly Evaluation::.
Note that there is a legacy interface for getting source locations
into the Scheme compiler or interpreter, which is to use a side table
that associates “source properties” with each subdatum returned by
‘read’, instead of wrapping the datums directly as in ‘read-syntax’.
This has the disadvantage of not being able to annotate all kinds of
datums. *Note Source Properties::, for more information.
6.16.4 Writing Scheme Values
----------------------------
Any scheme value may be written to a port. Not all values may be read
back in (*note Scheme Read::), however.
-- Scheme Procedure: write obj [port]
Send a representation of OBJ to PORT or to the current output port
if not given.
The output is designed to be machine readable, and can be read back
with ‘read’ (*note Scheme Read::). Strings are printed in double
quotes, with escapes if necessary, and characters are printed in
‘#\’ notation.
-- Scheme Procedure: display obj [port]
Send a representation of OBJ to PORT or to the current output port
if not given.
The output is designed for human readability, it differs from
‘write’ in that strings are printed without double quotes and
escapes, and characters are printed as per ‘write-char’, not in
‘#\’ form.
As was the case with the Scheme reader, there are a few options that
affect the behavior of the Scheme printer.
-- Scheme Procedure: print-options [setting]
Display the current settings of the read options. If SETTING is
omitted, only a short form of the current read options is printed.
Otherwise if SETTING is the symbol ‘help’, a complete options
description is displayed.
The set of available options, and their default values, may be had by
invoking ‘print-options’ at the prompt.
scheme@(guile-user)> (print-options)
(quote-keywordish-symbols reader highlight-suffix "}" highlight-prefix "{")
scheme@(guile-user)> (print-options 'help)
highlight-prefix { The string to print before highlighted values.
highlight-suffix } The string to print after highlighted values.
quote-keywordish-symbols reader How to print symbols that have a colon
as their first or last character. The
value '#f' does not quote the colons;
'#t' quotes them; 'reader' quotes them
when the reader option 'keywords' is
not '#f'.
escape-newlines yes Render newlines as \n when printing
using `write'.
r7rs-symbols no Escape symbols using R7RS |...| symbol
notation.
These options may be modified with the print-set! syntax.
-- Scheme Syntax: print-set! option-name value
Modify the print options. Due to historical oddities, ‘print-set!’
is a macro that expects an unquoted option name.
6.16.5 Procedures for On the Fly Evaluation
-------------------------------------------
Scheme has the lovely property that its expressions may be represented
as data. The ‘eval’ procedure takes a Scheme datum and evaluates it as
code.
-- Scheme Procedure: eval exp module_or_state
-- C Function: scm_eval (exp, module_or_state)
Evaluate EXP, a list representing a Scheme expression, in the
top-level environment specified by MODULE_OR_STATE. While EXP is
evaluated (using ‘primitive-eval’), MODULE_OR_STATE is made the
current module. The current module is reset to its previous value
when ‘eval’ returns. XXX - dynamic states. Example: (eval ’(+ 1
2) (interaction-environment))
-- Scheme Procedure: interaction-environment
-- C Function: scm_interaction_environment ()
Return a specifier for the environment that contains
implementation–defined bindings, typically a superset of those
listed in the report. The intent is that this procedure will
return the environment in which the implementation would evaluate
expressions dynamically typed by the user.
*Note Environments::, for other environments.
One does not always receive code as Scheme data, of course, and this
is especially the case for Guile’s other language implementations (*note
Other Languages::). For the case in which all you have is a string, we
have ‘eval-string’. There is a legacy version of this procedure in the
default environment, but you really want the one from ‘(ice-9
eval-string)’, so load it up:
(use-modules (ice-9 eval-string))
-- Scheme Procedure: eval-string string [#:module=#f] [#:file=#f]
[#:line=#f] [#:column=#f] [#:lang=(current-language)]
[#:compile?=#f]
Parse STRING according to the current language, normally Scheme.
Evaluate or compile the expressions it contains, in order,
returning the last expression.
If the MODULE keyword argument is set, save a module excursion
(*note Module System Reflection::) and set the current module to
MODULE before evaluation.
The FILE, LINE, and COLUMN keyword arguments can be used to
indicate that the source string begins at a particular source
location.
Finally, LANG is a language, defaulting to the current language,
and the expression is compiled if COMPILE? is true or there is no
evaluator for the given language.
-- C Function: scm_eval_string (string)
-- C Function: scm_eval_string_in_module (string, module)
These C bindings call ‘eval-string’ from ‘(ice-9 eval-string)’,
evaluating within MODULE or the current module.
-- C Function: SCM scm_c_eval_string (const char *string)
‘scm_eval_string’, but taking a C string in locale encoding instead
of an ‘SCM’.
-- Scheme Procedure: apply proc arg ... arglst
-- C Function: scm_apply_0 (proc, arglst)
-- C Function: scm_apply_1 (proc, arg1, arglst)
-- C Function: scm_apply_2 (proc, arg1, arg2, arglst)
-- C Function: scm_apply_3 (proc, arg1, arg2, arg3, arglst)
-- C Function: scm_apply (proc, arg, rest)
Call PROC with arguments ARG ... and the elements of the ARGLST
list.
‘scm_apply’ takes parameters corresponding to a Scheme level
‘(lambda (proc arg1 . rest) ...)’. So ARG1 and all but the last
element of the REST list make up ARG ..., and the last element of
REST is the ARGLST list. Or if REST is the empty list ‘SCM_EOL’
then there’s no ARG ..., and (ARG1) is the ARGLST.
ARGLST is not modified, but the REST list passed to ‘scm_apply’ is
modified.
-- C Function: scm_call_0 (proc)
-- C Function: scm_call_1 (proc, arg1)
-- C Function: scm_call_2 (proc, arg1, arg2)
-- C Function: scm_call_3 (proc, arg1, arg2, arg3)
-- C Function: scm_call_4 (proc, arg1, arg2, arg3, arg4)
-- C Function: scm_call_5 (proc, arg1, arg2, arg3, arg4, arg5)
-- C Function: scm_call_6 (proc, arg1, arg2, arg3, arg4, arg5, arg6)
-- C Function: scm_call_7 (proc, arg1, arg2, arg3, arg4, arg5, arg6,
arg7)
-- C Function: scm_call_8 (proc, arg1, arg2, arg3, arg4, arg5, arg6,
arg7, arg8)
-- C Function: scm_call_9 (proc, arg1, arg2, arg3, arg4, arg5, arg6,
arg7, arg8, arg9)
Call PROC with the given arguments.
-- C Function: scm_call (proc, ...)
Call PROC with any number of arguments. The argument list must be
terminated by ‘SCM_UNDEFINED’. For example:
scm_call (scm_c_public_ref ("guile", "+"),
scm_from_int (1),
scm_from_int (2),
SCM_UNDEFINED);
-- C Function: scm_call_n (proc, argv, nargs)
Call PROC with the array of arguments ARGV, as a ‘SCM*’. The
length of the arguments should be passed in NARGS, as a ‘size_t’.
-- Scheme Procedure: primitive-eval exp
-- C Function: scm_primitive_eval (exp)
Evaluate EXP in the top-level environment specified by the current
module.
6.16.6 Compiling Scheme Code
----------------------------
The ‘eval’ procedure directly interprets the S-expression representation
of Scheme. An alternate strategy for evaluation is to determine ahead
of time what computations will be necessary to evaluate the expression,
and then use that recipe to produce the desired results. This is known
as “compilation”.
While it is possible to compile simple Scheme expressions such as ‘(+
2 2)’ or even ‘"Hello world!"’, compilation is most interesting in the
context of procedures. Compiling a lambda expression produces a
compiled procedure, which is just like a normal procedure except
typically much faster, because it can bypass the generic interpreter.
Functions from system modules in a Guile installation are normally
compiled already, so they load and run quickly.
Note that well-written Scheme programs will not typically call the
procedures in this section, for the same reason that it is often bad
taste to use ‘eval’. By default, Guile automatically compiles any files
it encounters that have not been compiled yet (*note ‘--auto-compile’:
Invoking Guile.). The compiler can also be invoked explicitly from the
shell as ‘guild compile foo.scm’.
(Why are calls to ‘eval’ and ‘compile’ usually in bad taste? Because
they are limited, in that they can only really make sense for top-level
expressions. Also, most needs for “compile-time” computation are
fulfilled by macros and closures. Of course one good counterexample is
the REPL itself, or any code that reads expressions from a port.)
Automatic compilation generally works transparently, without any need
for user intervention. However Guile does not yet do proper dependency
tracking, so that if file ‘A.scm’ uses macros from ‘B.scm’, and B.SCM
changes, ‘A.scm’ would not be automatically recompiled. To forcibly
invalidate the auto-compilation cache, pass the ‘--fresh-auto-compile’
option to Guile, or set the ‘GUILE_AUTO_COMPILE’ environment variable to
‘fresh’ (instead of to ‘0’ or ‘1’).
For more information on the compiler itself, see *note Compiling to
the Virtual Machine::. For information on the virtual machine, see
*note A Virtual Machine for Guile::.
The command-line interface to Guile’s compiler is the ‘guild compile’
command:
-- Command: guild compile [option...] FILE...
Compile FILE, a source file, and store bytecode in the compilation
cache or in the file specified by the ‘-o’ option. The following
options are available:
‘-L DIR’
‘--load-path=DIR’
Add DIR to the front of the module load path.
‘-o OFILE’
‘--output=OFILE’
Write output bytecode to OFILE. By convention, bytecode file
names end in ‘.go’. When ‘-o’ is omitted, the output file
name is as for ‘compile-file’ (see below).
‘-x EXTENSION’
Recognize EXTENSION as a valid source file name extension.
For example, to compile R6RS code, you might want to pass ‘-x
.sls’ so that files ending in ‘.sls’ can be found.
‘-W WARNING’
‘--warn=WARNING’
Enable specific warning passes; use ‘-Whelp’ for a list of
available options. The default is ‘-W1’, which enables a
number of common warnings. Pass ‘-W0’ to disable all
warnings.
‘-O OPT’
‘--optimize=OPT’
Enable or disable specific compiler optimizations; use
‘-Ohelp’ for a list of available options. The default is
‘-O2’, which enables most optimizations. ‘-O0’ is recommended
if compilation speed is more important than the speed of the
compiled code. Pass ‘-Ono-OPT’ to disable a specific compiler
pass. Any number of ‘-O’ options can be passed to the
compiler, with later ones taking precedence.
‘--r6rs’
‘--r7rs’
Compile in an environment whose default bindings, reader
options, and load paths are adapted for specific Scheme
standards. *Note R6RS Support::, and *Note R7RS Support::.
‘-f LANG’
‘--from=LANG’
Use LANG as the source language of FILE. If this option is
omitted, ‘scheme’ is assumed.
‘-t LANG’
‘--to=LANG’
Use LANG as the target language of FILE. If this option is
omitted, ‘rtl’ is assumed.
‘-T TARGET’
‘--target=TARGET’
Produce code for TARGET instead of %HOST-TYPE (*note
%host-type: Build Config.). Target must be a valid GNU
triplet, such as ‘armv5tel-unknown-linux-gnueabi’ (*note
(autoconf)Specifying Target Triplets::).
Each FILE is assumed to be UTF-8-encoded, unless it contains a
coding declaration as recognized by ‘file-encoding’ (*note
Character Encoding of Source Files::).
The compiler can also be invoked directly by Scheme code. These
interfaces are in their own module:
(use-modules (system base compile))
-- Scheme Procedure: compile exp [#:env=#f] [#:from=(current-language)]
[#:to=value] [#:opts='()]
[#:optimization-level=(default-optimization-level)]
[#:warning-level=(default-warning-level)]
Compile the expression EXP in the environment ENV. If EXP is a
procedure, the result will be a compiled procedure; otherwise
‘compile’ is mostly equivalent to ‘eval’.
For a discussion of languages and compiler options, *Note Compiling
to the Virtual Machine::.
-- Scheme Procedure: compile-file file [#:output-file=#f]
[#:from=(current-language)] [#:to='rtl]
[#:env=(default-environment from)] [#:opts='()]
[#:optimization-level=(default-optimization-level)]
[#:warning-level=(default-warning-level)]
[#:canonicalization='relative]
Compile the file named FILE.
Output will be written to a OUTPUT-FILE. If you do not supply an
output file name, output is written to a file in the cache
directory, as computed by ‘(compiled-file-name FILE)’.
FROM and TO specify the source and target languages. *Note
Compiling to the Virtual Machine::, for more information on these
options, and on ENV and OPTS.
As with ‘guild compile’, FILE is assumed to be UTF-8-encoded unless
it contains a coding declaration.
-- Scheme Parameter: default-optimization-level
The default optimization level, as an integer from 0 to 9. The
default is 2.
-- Scheme Parameter: default-warning-level
The default warning level, as an integer from 0 to 9. The default
is 1.
*Note Parameters::, for more on how to set parameters.
-- Scheme Procedure: compiled-file-name file
Compute a cached location for a compiled version of a Scheme file
named FILE.
This file will usually be below the ‘$HOME/.cache/guile/ccache’
directory, depending on the value of the ‘XDG_CACHE_HOME’
environment variable. The intention is that ‘compiled-file-name’
provides a fallback location for caching auto-compiled files. If
you want to place a compile file in the ‘%load-compiled-path’, you
should pass the OUTPUT-FILE option to ‘compile-file’, explicitly.
-- Scheme Variable: %auto-compilation-options
This variable contains the options passed to the ‘compile-file’
procedure when auto-compiling source files. By default, it enables
useful compilation warnings. It can be customized from ‘~/.guile’.
6.16.7 Loading Scheme Code from File
------------------------------------
-- Scheme Procedure: load filename [reader]
Load FILENAME and evaluate its contents in the top-level
environment.
READER if provided should be either ‘#f’, or a procedure with the
signature ‘(lambda (port) ...)’ which reads the next expression
from PORT. If READER is ‘#f’ or absent, Guile’s built-in ‘read’
procedure is used (*note Scheme Read::).
The READER argument takes effect by setting the value of the
‘current-reader’ fluid (see below) before loading the file, and
restoring its previous value when loading is complete. The Scheme
code inside FILENAME can itself change the current reader procedure
on the fly by setting ‘current-reader’ fluid.
If the variable ‘%load-hook’ is defined, it should be bound to a
procedure that will be called before any code is loaded. See
documentation for ‘%load-hook’ later in this section.
-- Scheme Procedure: load-compiled filename
Load the compiled file named FILENAME.
Compiling a source file (*note Read/Load/Eval/Compile::) and then
calling ‘load-compiled’ on the resulting file is equivalent to
calling ‘load’ on the source file.
-- Scheme Procedure: primitive-load filename
-- C Function: scm_primitive_load (filename)
Load the file named FILENAME and evaluate its contents in the
top-level environment. FILENAME must either be a full pathname or
be a pathname relative to the current directory. If the variable
‘%load-hook’ is defined, it should be bound to a procedure that
will be called before any code is loaded. See the documentation
for ‘%load-hook’ later in this section.
-- C Function: SCM scm_c_primitive_load (const char *filename)
‘scm_primitive_load’, but taking a C string instead of an ‘SCM’.
-- Variable: current-reader
‘current-reader’ holds the read procedure that is currently being
used by the above loading procedures to read expressions (from the
file that they are loading). ‘current-reader’ is a fluid, so it
has an independent value in each dynamic root and should be read
and set using ‘fluid-ref’ and ‘fluid-set!’ (*note Fluids and
Dynamic States::).
Changing ‘current-reader’ is typically useful to introduce local
syntactic changes, such that code following the ‘fluid-set!’ call
is read using the newly installed reader. The ‘current-reader’
change should take place at evaluation time when the code is
evaluated, or at compilation time when the code is compiled:
(eval-when (compile eval)
(fluid-set! current-reader my-own-reader))
The ‘eval-when’ form above ensures that the ‘current-reader’ change
occurs at the right time.
-- Variable: %load-hook
A procedure to be called ‘(%load-hook FILENAME)’ whenever a file is
loaded, or ‘#f’ for no such call. ‘%load-hook’ is used by all of
the loading functions (‘load’ and ‘primitive-load’, and
‘load-from-path’ and ‘primitive-load-path’ documented in the next
section).
For example an application can set this to show what’s loaded,
(set! %load-hook (lambda (filename)
(format #t "Loading ~a ...\n" filename)))
(load-from-path "foo.scm")
⊣ Loading /usr/local/share/guile/site/foo.scm ...
-- Scheme Procedure: current-load-port
-- C Function: scm_current_load_port ()
Return the current-load-port. The load port is used internally by
‘primitive-load’.
6.16.8 Load Paths
-----------------
The procedure in the previous section look for Scheme code in the file
system at specific location. Guile also has some procedures to search
the load path for code.
-- Variable: %load-path
List of directories which should be searched for Scheme modules and
libraries. When Guile starts up, ‘%load-path’ is initialized to
the default load path ‘(list (%library-dir) (%site-dir)
(%global-site-dir) (%package-data-dir))’. The ‘GUILE_LOAD_PATH’
environment variable can be used to prepend or append additional
directories (*note Environment Variables::).
*Note Build Config::, for more on ‘%site-dir’ and related
procedures.
-- Scheme Procedure: load-from-path filename
Similar to ‘load’, but searches for FILENAME in the load paths.
Preferentially loads a compiled version of the file, if it is
available and up-to-date.
A user can extend the load path by calling ‘add-to-load-path’.
-- Scheme Syntax: add-to-load-path dir
Add DIR to the load path.
For example, a script might include this form to add the directory
that it is in to the load path:
(add-to-load-path (dirname (current-filename)))
It’s better to use ‘add-to-load-path’ than to modify ‘%load-path’
directly, because ‘add-to-load-path’ takes care of modifying the path
both at compile-time and at run-time.
-- Scheme Procedure: primitive-load-path filename
[exception-on-not-found]
-- C Function: scm_primitive_load_path (filename)
Search ‘%load-path’ for the file named FILENAME and load it into
the top-level environment. If FILENAME is a relative pathname and
is not found in the list of search paths, an error is signalled.
Preferentially loads a compiled version of the file, if it is
available and up-to-date.
If FILENAME is a relative pathname and is not found in the list of
search paths, one of three things may happen, depending on the
optional second argument, EXCEPTION-ON-NOT-FOUND. If it is ‘#f’,
‘#f’ will be returned. If it is a procedure, it will be called
with no arguments. (This allows a distinction to be made between
exceptions raised by loading a file, and exceptions related to the
loader itself.) Otherwise an error is signalled.
For compatibility with Guile 1.8 and earlier, the C function takes
only one argument, which can be either a string (the file name) or
an argument list.
-- Scheme Procedure: %search-load-path filename
-- C Function: scm_sys_search_load_path (filename)
Search ‘%load-path’ for the file named FILENAME, which must be
readable by the current user. If FILENAME is found in the list of
paths to search or is an absolute pathname, return its full
pathname. Otherwise, return ‘#f’. Filenames may have any of the
optional extensions in the ‘%load-extensions’ list;
‘%search-load-path’ will try each extension automatically.
-- Variable: %load-extensions
A list of default file extensions for files containing Scheme code.
‘%search-load-path’ tries each of these extensions when looking for
a file to load. By default, ‘%load-extensions’ is bound to the
list ‘("" ".scm")’.
As mentioned above, when Guile searches the ‘%load-path’ for a source
file, it will also search the ‘%load-compiled-path’ for a corresponding
compiled file. If the compiled file is as new or newer than the source
file, it will be loaded instead of the source file, using
‘load-compiled’.
-- Variable: %load-compiled-path
Like ‘%load-path’, but for compiled files. By default, this path
has two entries: one for compiled files from Guile itself, and one
for site packages. The ‘GUILE_LOAD_COMPILED_PATH’ environment
variable can be used to prepend or append additional directories
(*note Environment Variables::).
When ‘primitive-load-path’ searches the ‘%load-compiled-path’ for a
corresponding compiled file for a relative path it does so by appending
‘.go’ to the relative path. For example, searching for ‘ice-9/popen’
could find ‘/usr/lib/guile/3.0/ccache/ice-9/popen.go’, and use it
instead of ‘/usr/share/guile/3.0/ice-9/popen.scm’.
If ‘primitive-load-path’ does not find a corresponding ‘.go’ file in
the ‘%load-compiled-path’, or the ‘.go’ file is out of date, it will
search for a corresponding auto-compiled file in the fallback path,
possibly creating one if one does not exist.
*Note Installing Site Packages::, for more on how to correctly
install site packages. *Note Modules and the File System::, for more on
the relationship between load paths and modules. *Note Compilation::,
for more on the fallback path and auto-compilation.
Finally, there are a couple of helper procedures for general path
manipulation.
-- Scheme Procedure: parse-path path [tail]
-- C Function: scm_parse_path (path, tail)
Parse PATH, which is expected to be a colon-separated string, into
a list and return the resulting list with TAIL appended. If PATH
is ‘#f’, TAIL is returned.
-- Scheme Procedure: parse-path-with-ellipsis path base
-- C Function: scm_parse_path_with_ellipsis (path, base)
Parse PATH, which is expected to be a colon-separated string, into
a list and return the resulting list with BASE (a list) spliced in
place of the ‘...’ path component, if present, or else BASE is
added to the end. If PATH is ‘#f’, BASE is returned.
-- Scheme Procedure: search-path path filename [extensions
[require-exts?]]
-- C Function: scm_search_path (path, filename, rest)
Search PATH for a directory containing a file named FILENAME. The
file must be readable, and not a directory. If we find one, return
its full filename; otherwise, return ‘#f’. If FILENAME is
absolute, return it unchanged. If given, EXTENSIONS is a list of
strings; for each directory in PATH, we search for FILENAME
concatenated with each EXTENSION. If REQUIRE-EXTS? is true,
require that the returned file name have one of the given
extensions; if REQUIRE-EXTS? is not given, it defaults to ‘#f’.
For compatibility with Guile 1.8 and earlier, the C function takes
only three arguments.
6.16.9 Character Encoding of Source Files
-----------------------------------------
Scheme source code files are usually encoded in ASCII or UTF-8, but the
built-in reader can interpret other character encodings as well. When
Guile loads Scheme source code, it uses the ‘file-encoding’ procedure
(described below) to try to guess the encoding of the file. In the
absence of any hints, UTF-8 is assumed. One way to provide a hint about
the encoding of a source file is to place a coding declaration in the
top 500 characters of the file.
A coding declaration has the form ‘coding: XXXXXX’, where ‘XXXXXX’ is
the name of a character encoding in which the source code file has been
encoded. The coding declaration must appear in a scheme comment. It
can either be a semicolon-initiated comment, or the first block ‘#!’
comment in the file.
The name of the character encoding in the coding declaration is
typically lower case and containing only letters, numbers, and hyphens,
as recognized by ‘set-port-encoding!’ (*note ‘set-port-encoding!’:
Ports.). Common examples of character encoding names are ‘utf-8’ and
‘iso-8859-1’, as defined by IANA
(http://www.iana.org/assignments/character-sets). Thus, the coding
declaration is mostly compatible with Emacs.
However, there are some differences in encoding names recognized by
Emacs and encoding names defined by IANA, the latter being essentially a
subset of the former. For instance, ‘latin-1’ is a valid encoding name
for Emacs, but it’s not according to the IANA standard, which Guile
follows; instead, you should use ‘iso-8859-1’, which is both understood
by Emacs and dubbed by IANA (IANA writes it uppercase but Emacs wants it
lowercase and Guile is case insensitive.)
For source code, only a subset of all possible character encodings
can be interpreted by the built-in source code reader. Only those
character encodings in which ASCII text appears unmodified can be used.
This includes ‘UTF-8’ and ‘ISO-8859-1’ through ‘ISO-8859-15’. The
multi-byte character encodings ‘UTF-16’ and ‘UTF-32’ may not be used
because they are not compatible with ASCII.
There might be a scenario in which one would want to read non-ASCII
code from a port, such as with the function ‘read’, instead of with
‘load’. If the port’s character encoding is the same as the encoding of
the code to be read by the port, not other special handling is
necessary. The port will automatically do the character encoding
conversion. The functions ‘setlocale’ or by ‘set-port-encoding!’ are
used to set port encodings (*note Ports::).
If a port is used to read code of unknown character encoding, it can
accomplish this in three steps. First, the character encoding of the
port should be set to ISO-8859-1 using ‘set-port-encoding!’. Then, the
procedure ‘file-encoding’, described below, is used to scan for a coding
declaration when reading from the port. As a side effect, it rewinds
the port after its scan is complete. After that, the port’s character
encoding should be set to the encoding returned by ‘file-encoding’, if
any, again by using ‘set-port-encoding!’. Then the code can be read as
normal.
Alternatively, one can use the ‘#:guess-encoding’ keyword argument of
‘open-file’ and related procedures. *Note File Ports::.
-- Scheme Procedure: file-encoding port
-- C Function: scm_file_encoding (port)
Attempt to scan the first few hundred bytes from the PORT for hints
about its character encoding. Return a string containing the
encoding name or ‘#f’ if the encoding cannot be determined. The
port is rewound.
Currently, the only supported method is to look for an Emacs-like
character coding declaration (*note how Emacs recognizes file
encoding: (emacs)Recognize Coding.). The coding declaration is of
the form ‘coding: XXXXX’ and must appear in a Scheme comment.
Additional heuristics may be added in the future.
6.16.10 Delayed Evaluation
--------------------------
Promises are a convenient way to defer a calculation until its result is
actually needed, and to run such a calculation only once. Also *note
SRFI-45::.
-- syntax: delay expr
Return a promise object which holds the given EXPR expression,
ready to be evaluated by a later ‘force’.
-- Scheme Procedure: promise? obj
-- C Function: scm_promise_p (obj)
Return true if OBJ is a promise.
-- Scheme Procedure: force p
-- C Function: scm_force (p)
Return the value obtained from evaluating the EXPR in the given
promise P. If P has previously been forced then its EXPR is not
evaluated again, instead the value obtained at that time is simply
returned.
During a ‘force’, an EXPR can call ‘force’ again on its own
promise, resulting in a recursive evaluation of that EXPR. The
first evaluation to return gives the value for the promise. Higher
evaluations run to completion in the normal way, but their results
are ignored, ‘force’ always returns the first value.
6.16.11 Local Evaluation
------------------------
Guile includes a facility to capture a lexical environment, and later
evaluate a new expression within that environment. This code is
implemented in a module.
(use-modules (ice-9 local-eval))
-- syntax: the-environment
Captures and returns a lexical environment for use with
‘local-eval’ or ‘local-compile’.
-- Scheme Procedure: local-eval exp env
-- C Function: scm_local_eval (exp, env)
-- Scheme Procedure: local-compile exp env [opts=()]
Evaluate or compile the expression EXP in the lexical environment
ENV.
Here is a simple example, illustrating that it is the variable that
gets captured, not just its value at one point in time.
(define e (let ((x 100)) (the-environment)))
(define fetch-x (local-eval '(lambda () x) e))
(fetch-x)
⇒ 100
(local-eval '(set! x 42) e)
(fetch-x)
⇒ 42
While EXP is evaluated within the lexical environment of
‘(the-environment)’, it has the dynamic environment of the call to
‘local-eval’.
‘local-eval’ and ‘local-compile’ can only evaluate expressions, not
definitions.
(local-eval '(define foo 42)
(let ((x 100)) (the-environment)))
⇒ syntax error: definition in expression context
Note that the current implementation of ‘(the-environment)’ only
captures “normal” lexical bindings, and pattern variables bound by
‘syntax-case’. It does not currently capture local syntax transformers
bound by ‘let-syntax’, ‘letrec-syntax’ or non-top-level ‘define-syntax’
forms. Any attempt to reference such captured syntactic keywords via
‘local-eval’ or ‘local-compile’ produces an error.
6.16.12 Local Inclusion
-----------------------
This section has discussed various means of linking Scheme code
together: fundamentally, loading up files at run-time using ‘load’ and
‘load-compiled’. Guile provides another option to compose parts of
programs together at expansion-time instead of at run-time.
-- Scheme Syntax: include file-name
Open FILE-NAME, at expansion-time, and read the Scheme forms that
it contains, splicing them into the location of the ‘include’,
within a ‘begin’.
If FILE-NAME is a relative path, it is searched for relative to the
path that contains the file that the ‘include’ form appears in.
If you are a C programmer, if ‘load’ in Scheme is like ‘dlopen’ in C,
consider ‘include’ to be like the C preprocessor’s ‘#include’. When you
use ‘include’, it is as if the contents of the included file were typed
in instead of the ‘include’ form.
Because the code is included at compile-time, it is available to the
macroexpander. Syntax definitions in the included file are available to
later code in the form in which the ‘include’ appears, without the need
for ‘eval-when’. (*Note Eval When::.)
For the same reason, compiling a form that uses ‘include’ results in
one compilation unit, composed of multiple files. Loading the compiled
file is one ‘stat’ operation for the compilation unit, instead of ‘2*N’
in the case of ‘load’ (once for each loaded source file, and once each
corresponding compiled file, in the best case).
Unlike ‘load’, ‘include’ also works within nested lexical contexts.
It so happens that the optimizer works best within a lexical context,
because all of the uses of bindings in a lexical context are visible, so
composing files by including them within a ‘(let () ...)’ can sometimes
lead to important speed improvements.
On the other hand, ‘include’ does have all the disadvantages of early
binding: once the code with the ‘include’ is compiled, no change to the
included file is reflected in the future behavior of the including form.
Also, the particular form of ‘include’, which requires an absolute
path, or a path relative to the current directory at compile-time, is
not very amenable to compiling the source in one place, but then
installing the source to another place. For this reason, Guile provides
another form, ‘include-from-path’, which looks for the source file to
include within a load path.
-- Scheme Syntax: include-from-path file-name
Like ‘include’, but instead of expecting ‘file-name’ to be an
absolute file name, it is expected to be a relative path to search
in the ‘%load-path’.
‘include-from-path’ is more useful when you want to install all of
the source files for a package (as you should!). It makes it possible
to evaluate an installed file from source, instead of relying on the
‘.go’ file being up to date.
6.16.13 Sandboxed Evaluation
----------------------------
Sometimes you would like to evaluate code that comes from an untrusted
party. The safest way to do this is to buy a new computer, evaluate the
code on that computer, then throw the machine away. However if you are
unwilling to take this simple approach, Guile does include a limited
“sandbox” facility that can allow untrusted code to be evaluated with
some confidence.
To use the sandboxed evaluator, load its module:
(use-modules (ice-9 sandbox))
Guile’s sandboxing facility starts with the ability to restrict the
time and space used by a piece of code.
-- Scheme Procedure: call-with-time-limit limit thunk limit-reached
Call THUNK, but cancel it if LIMIT seconds of wall-clock time have
elapsed. If the computation is cancelled, call LIMIT-REACHED in
tail position. THUNK must not disable interrupts or prevent an
abort via a ‘dynamic-wind’ unwind handler.
-- Scheme Procedure: call-with-allocation-limit limit thunk
limit-reached
Call THUNK, but cancel it if LIMIT bytes have been allocated. If
the computation is cancelled, call LIMIT-REACHED in tail position.
THUNK must not disable interrupts or prevent an abort via a
‘dynamic-wind’ unwind handler.
This limit applies to both stack and heap allocation. The
computation will not be aborted before LIMIT bytes have been
allocated, but for the heap allocation limit, the check may be
postponed until the next garbage collection.
Note that as a current shortcoming, the heap size limit applies to
all threads; concurrent allocation by other unrelated threads
counts towards the allocation limit.
-- Scheme Procedure: call-with-time-and-allocation-limits time-limit
allocation-limit thunk
Invoke THUNK in a dynamic extent in which its execution is limited
to TIME-LIMIT seconds of wall-clock time, and its allocation to
ALLOCATION-LIMIT bytes. THUNK must not disable interrupts or
prevent an abort via a ‘dynamic-wind’ unwind handler.
If successful, return all values produced by invoking THUNK. Any
uncaught exception thrown by the thunk will propagate out. If the
time or allocation limit is exceeded, an exception will be thrown
to the ‘limit-exceeded’ key.
The time limit and stack limit are both very precise, but the heap
limit only gets checked asynchronously, after a garbage collection. In
particular, if the heap is already very large, the number of allocated
bytes between garbage collections will be large, and therefore the
precision of the check is reduced.
Additionally, due to the mechanism used by the allocation limit (the
‘after-gc-hook’), large single allocations like ‘(make-vector #e1e7)’
are only detected after the allocation completes, even if the allocation
itself causes garbage collection. It’s possible therefore for user code
to not only exceed the allocation limit set, but also to exhaust all
available memory, causing out-of-memory conditions at any allocation
site. Failure to allocate memory in Guile itself should be safe and
cause an exception to be thrown, but most systems are not designed to
handle ‘malloc’ failures. An allocation failure may therefore exercise
unexpected code paths in your system, so it is a weakness of the sandbox
(and therefore an interesting point of attack).
The main sandbox interface is ‘eval-in-sandbox’.
-- Scheme Procedure: eval-in-sandbox exp [#:time-limit 0.1]
[#:allocation-limit #e10e6] [#:bindings all-pure-bindings]
[#:module (make-sandbox-module bindings)] [#:sever-module? #t]
Evaluate the Scheme expression EXP within an isolated "sandbox".
Limit its execution to TIME-LIMIT seconds of wall-clock time, and
limit its allocation to ALLOCATION-LIMIT bytes.
The evaluation will occur in MODULE, which defaults to the result
of calling ‘make-sandbox-module’ on BINDINGS, which itself defaults
to ‘all-pure-bindings’. This is the core of the sandbox: creating
a scope for the expression that is “safe”.
A safe sandbox module has two characteristics. Firstly, it will
not allow the expression being evaluated to avoid being cancelled
due to time or allocation limits. This ensures that the expression
terminates in a timely fashion.
Secondly, a safe sandbox module will prevent the evaluation from
receiving information from previous evaluations, or from affecting
future evaluations. All combinations of binding sets exported by
‘(ice-9 sandbox)’ form safe sandbox modules.
The BINDINGS should be given as a list of import sets. One import
set is a list whose car names an interface, like ‘(ice-9 q)’, and
whose cdr is a list of imports. An import is either a bare symbol
or a pair of ‘(OUT . IN)’, where OUT and IN are both symbols and
denote the name under which a binding is exported from the module,
and the name under which to make the binding available,
respectively. Note that BINDINGS is only used as an input to the
default initializer for the MODULE argument; if you pass
‘#:module’, BINDINGS is unused. If SEVER-MODULE? is true (the
default), the module will be unlinked from the global module tree
after the evaluation returns, to allow MOD to be garbage-collected.
If successful, return all values produced by EXP. Any uncaught
exception thrown by the expression will propagate out. If the time
or allocation limit is exceeded, an exception will be thrown to the
‘limit-exceeded’ key.
Constructing a safe sandbox module is tricky in general. Guile
defines an easy way to construct safe modules from predefined sets of
bindings. Before getting to that interface, here are some general notes
on safety.
1. The time and allocation limits rely on the ability to interrupt and
cancel a computation. For this reason, no binding included in a
sandbox module should be able to indefinitely postpone interrupt
handling, nor should a binding be able to prevent an abort. In
practice this second consideration means that ‘dynamic-wind’ should
not be included in any binding set.
2. The time and allocation limits apply only to the ‘eval-in-sandbox’
call. If the call returns a procedure which is later called, no
limit is “automatically” in place. Users of ‘eval-in-sandbox’ have
to be very careful to reimpose limits when calling procedures that
escape from sandboxes.
3. Similarly, the dynamic environment of the ‘eval-in-sandbox’ call is
not necessarily in place when any procedure that escapes from the
sandbox is later called.
This detail prevents us from exposing ‘primitive-eval’ to the
sandbox, for two reasons. The first is that it’s possible for
legacy code to forge references to any binding, if the
‘allow-legacy-syntax-objects?’ parameter is true. The default for
this parameter is true; *note Syntax Transformer Helpers:: for the
details. The parameter is bound to ‘#f’ for the duration of the
‘eval-in-sandbox’ call itself, but that will not be in place during
calls to escaped procedures.
The second reason we don’t expose ‘primitive-eval’ is that
‘primitive-eval’ implicitly works in the current module, which for
an escaped procedure will probably be different than the module
that is current for the ‘eval-in-sandbox’ call itself.
The common denominator here is that if an interface exposed to the
sandbox relies on dynamic environments, it is easy to mistakenly
grant the sandboxed procedure additional capabilities in the form
of bindings that it should not have access to. For this reason,
the default sets of predefined bindings do not depend on any
dynamically scoped value.
4. Mutation may allow a sandboxed evaluation to break some invariant
in users of data supplied to it. A lot of code culturally doesn’t
expect mutation, but if you hand mutable data to a sandboxed
evaluation and you also grant mutating capabilities to that
evaluation, then the sandboxed code may indeed mutate that data.
The default set of bindings to the sandbox do not include any
mutating primitives.
Relatedly, ‘set!’ may allow a sandbox to mutate a primitive,
invalidating many system-wide invariants. Guile is currently quite
permissive when it comes to imported bindings and mutability.
Although ‘set!’ to a module-local or lexically bound variable would
be fine, we don’t currently have an easy way to disallow ‘set!’ to
an imported binding, so currently no binding set includes ‘set!’.
5. Mutation may allow a sandboxed evaluation to keep state, or make a
communication mechanism with other code. On the one hand this
sounds cool, but on the other hand maybe this is part of your
threat model. Again, the default set of bindings doesn’t include
mutating primitives, preventing sandboxed evaluations from keeping
state.
6. The sandbox should probably not be able to open a network
connection, or write to a file, or open a file from disk. The
default binding set includes no interaction with the operating
system.
If you, dear reader, find the above discussion interesting, you will
enjoy Jonathan Rees’ dissertation, “A Security Kernel Based on the
Lambda Calculus”.
-- Scheme Variable: all-pure-bindings
All “pure” bindings that together form a safe subset of those
bindings available by default to Guile user code.
-- Scheme Variable: all-pure-and-impure-bindings
Like ‘all-pure-bindings’, but additionally including mutating
primitives like ‘vector-set!’. This set is still safe in the sense
mentioned above, with the caveats about mutation.
The components of these composite sets are as follows:
-- Scheme Variable: alist-bindings
-- Scheme Variable: array-bindings
-- Scheme Variable: bit-bindings
-- Scheme Variable: bitvector-bindings
-- Scheme Variable: char-bindings
-- Scheme Variable: char-set-bindings
-- Scheme Variable: clock-bindings
-- Scheme Variable: core-bindings
-- Scheme Variable: error-bindings
-- Scheme Variable: fluid-bindings
-- Scheme Variable: hash-bindings
-- Scheme Variable: iteration-bindings
-- Scheme Variable: keyword-bindings
-- Scheme Variable: list-bindings
-- Scheme Variable: macro-bindings
-- Scheme Variable: nil-bindings
-- Scheme Variable: number-bindings
-- Scheme Variable: pair-bindings
-- Scheme Variable: predicate-bindings
-- Scheme Variable: procedure-bindings
-- Scheme Variable: promise-bindings
-- Scheme Variable: prompt-bindings
-- Scheme Variable: regexp-bindings
-- Scheme Variable: sort-bindings
-- Scheme Variable: srfi-4-bindings
-- Scheme Variable: string-bindings
-- Scheme Variable: symbol-bindings
-- Scheme Variable: unspecified-bindings
-- Scheme Variable: variable-bindings
-- Scheme Variable: vector-bindings
-- Scheme Variable: version-bindings
The components of ‘all-pure-bindings’.
-- Scheme Variable: mutating-alist-bindings
-- Scheme Variable: mutating-array-bindings
-- Scheme Variable: mutating-bitvector-bindings
-- Scheme Variable: mutating-fluid-bindings
-- Scheme Variable: mutating-hash-bindings
-- Scheme Variable: mutating-list-bindings
-- Scheme Variable: mutating-pair-bindings
-- Scheme Variable: mutating-sort-bindings
-- Scheme Variable: mutating-srfi-4-bindings
-- Scheme Variable: mutating-string-bindings
-- Scheme Variable: mutating-variable-bindings
-- Scheme Variable: mutating-vector-bindings
The additional components of ‘all-pure-and-impure-bindings’.
Finally, what do you do with a binding set? What is a binding set
anyway? ‘make-sandbox-module’ is here for you.
-- Scheme Procedure: make-sandbox-module bindings
Return a fresh module that only contains BINDINGS.
The BINDINGS should be given as a list of import sets. One import
set is a list whose car names an interface, like ‘(ice-9 q)’, and
whose cdr is a list of imports. An import is either a bare symbol
or a pair of ‘(OUT . IN)’, where OUT and IN are both symbols and
denote the name under which a binding is exported from the module,
and the name under which to make the binding available,
respectively.
So you see that binding sets are just lists, and
‘all-pure-and-impure-bindings’ is really just the result of appending
all of the component binding sets.
6.16.14 REPL Servers
--------------------
The procedures in this section are provided by
(use-modules (system repl server))
When an application is written in Guile, it is often convenient to
allow the user to be able to interact with it by evaluating Scheme
expressions in a REPL.
The procedures of this module allow you to spawn a “REPL server”,
which permits interaction over a local or TCP connection. Guile itself
uses them internally to implement the ‘--listen’ switch, *note
Command-line Options::.
-- Scheme Procedure: make-tcp-server-socket [#:host=#f] [#:addr]
[#:port=37146]
Return a stream socket bound to a given address ADDR and port
number PORT. If the HOST is given, and ADDR is not, then the HOST
string is converted to an address. If neither is given, we use the
loopback address.
-- Scheme Procedure: make-unix-domain-server-socket
[#:path="/tmp/guile-socket"]
Return a UNIX domain socket, bound to a given PATH.
-- Scheme Procedure: run-server [server-socket]
-- Scheme Procedure: spawn-server [server-socket]
Create and run a REPL, making it available over the given
SERVER-SOCKET. If SERVER-SOCKET is not provided, it defaults to
the socket created by calling ‘make-tcp-server-socket’ with no
arguments.
‘run-server’ runs the server in the current thread, whereas
‘spawn-server’ runs the server in a new thread.
-- Scheme Procedure: stop-server-and-clients!
Closes the connection on all running server sockets.
Please note that in the current implementation, the REPL threads
are cancelled without unwinding their stacks. If any of them are
holding mutexes or are within a critical section, the results are
unspecified.
6.16.15 Cooperative REPL Servers
--------------------------------
The procedures in this section are provided by
(use-modules (system repl coop-server))
Whereas ordinary REPL servers run in their own threads (*note REPL
Servers::), sometimes it is more convenient to provide REPLs that run at
specified times within an existing thread, for example in programs
utilizing an event loop or in single-threaded programs. This allows for
safe access and mutation of a program’s data structures from the REPL,
without concern for thread synchronization.
Although the REPLs are run in the thread that calls
‘spawn-coop-repl-server’ and ‘poll-coop-repl-server’, dedicated threads
are spawned so that the calling thread is not blocked. The spawned
threads read input for the REPLs and to listen for new connections.
Cooperative REPL servers must be polled periodically to evaluate any
pending expressions by calling ‘poll-coop-repl-server’ with the object
returned from ‘spawn-coop-repl-server’. The thread that calls
‘poll-coop-repl-server’ will be blocked for as long as the expression
takes to be evaluated or if the debugger is entered.
-- Scheme Procedure: spawn-coop-repl-server [server-socket]
Create and return a new cooperative REPL server object, and spawn a
new thread to listen for connections on SERVER-SOCKET. Proper
functioning of the REPL server requires that
‘poll-coop-repl-server’ be called periodically on the returned
server object.
-- Scheme Procedure: poll-coop-repl-server coop-server
Poll the cooperative REPL server COOP-SERVER and apply a pending
operation if there is one, such as evaluating an expression typed
at the REPL prompt. This procedure must be called from the same
thread that called ‘spawn-coop-repl-server’.
6.17 Memory Management and Garbage Collection
=============================================
Guile uses a _garbage collector_ to manage most of its objects. While
the garbage collector is designed to be mostly invisible, you sometimes
need to interact with it explicitly.
See *note Garbage Collection:: for a general discussion of how
garbage collection relates to using Guile from C.
6.17.1 Function related to Garbage Collection
---------------------------------------------
-- Scheme Procedure: gc
-- C Function: scm_gc ()
Finds all of the “live” ‘SCM’ objects and reclaims for further use
those that are no longer accessible. You normally don’t need to
call this function explicitly. Its functionality is invoked
automatically as needed.
-- C Function: SCM scm_gc_protect_object (SCM OBJ)
Protects OBJ from being freed by the garbage collector, when it
otherwise might be. When you are done with the object, call
‘scm_gc_unprotect_object’ on the object. Calls to
‘scm_gc_protect_object’/‘scm_gc_unprotect_object’ can be nested,
and the object remains protected until it has been unprotected as
many times as it was protected. It is an error to unprotect an
object more times than it has been protected. Returns the SCM
object it was passed.
Note that storing OBJ in a C global variable has the same
effect(1).
-- C Function: SCM scm_gc_unprotect_object (SCM OBJ)
Unprotects an object from the garbage collector which was protected
by ‘scm_gc_unprotect_object’. Returns the SCM object it was
passed.
-- C Function: SCM scm_permanent_object (SCM OBJ)
Similar to ‘scm_gc_protect_object’ in that it causes the collector
to always mark the object, except that it should not be nested
(only call ‘scm_permanent_object’ on an object once), and it has no
corresponding unpermanent function. Once an object is declared
permanent, it will never be freed. Returns the SCM object it was
passed.
-- C Macro: void scm_remember_upto_here_1 (SCM obj)
-- C Macro: void scm_remember_upto_here_2 (SCM obj1, SCM obj2)
Create a reference to the given object or objects, so they’re
certain to be present on the stack or in a register and hence will
not be freed by the garbage collector before this point.
Note that these functions can only be applied to ordinary C local
variables (ie. “automatics”). Objects held in global or static
variables or some malloced block or the like cannot be protected
with this mechanism.
-- Scheme Procedure: gc-stats
-- C Function: scm_gc_stats ()
Return an association list of statistics about Guile’s current use
of storage.
-- Scheme Procedure: gc-live-object-stats
-- C Function: scm_gc_live_object_stats ()
Return an alist of statistics of the current live objects.
-- Function: void scm_gc_mark (SCM X)
Mark the object X, and recurse on any objects X refers to. If X’s
mark bit is already set, return immediately. This function must
only be called during the mark-phase of garbage collection,
typically from a smob _mark_ function.
---------- Footnotes ----------
(1) In Guile up to version 1.8, C global variables were not visited
by the garbage collector in the mark phase; hence,
‘scm_gc_protect_object’ was the only way in C to prevent a Scheme object
from being freed.
6.17.2 Memory Blocks
--------------------
In C programs, dynamic management of memory blocks is normally done with
the functions malloc, realloc, and free. Guile has additional functions
for dynamic memory allocation that are integrated into the garbage
collector and the error reporting system.
Memory blocks that are associated with Scheme objects (for example a
foreign object) should be allocated with ‘scm_gc_malloc’ or
‘scm_gc_malloc_pointerless’. These two functions will either return a
valid pointer or signal an error. Memory blocks allocated this way may
be released explicitly; however, this is not strictly needed, and we
recommend _not_ calling ‘scm_gc_free’. All memory allocated with
‘scm_gc_malloc’ or ‘scm_gc_malloc_pointerless’ is automatically
reclaimed when the garbage collector no longer sees any live reference
to it(1).
When garbage collection occurs, Guile will visit the words in memory
allocated with ‘scm_gc_malloc’, looking for live pointers. This means
that if ‘scm_gc_malloc’-allocated memory contains a pointer to some
other part of the memory, the garbage collector notices it and prevents
it from being reclaimed(2). Conversely, memory allocated with
‘scm_gc_malloc_pointerless’ is assumed to be “pointer-less” and is not
scanned for pointers.
For memory that is not associated with a Scheme object, you can use
‘scm_malloc’ instead of ‘malloc’. Like ‘scm_gc_malloc’, it will either
return a valid pointer or signal an error. However, it will not assume
that the new memory block can be freed by a garbage collection. The
memory must be explicitly freed with ‘free’.
There is also ‘scm_gc_realloc’ and ‘scm_realloc’, to be used in place
of ‘realloc’ when appropriate, and ‘scm_gc_calloc’ and ‘scm_calloc’, to
be used in place of ‘calloc’ when appropriate.
The function ‘scm_dynwind_free’ can be useful when memory should be
freed with libc’s ‘free’ when leaving a dynwind context, *Note Dynamic
Wind::.
-- C Function: void * scm_malloc (size_t SIZE)
-- C Function: void * scm_calloc (size_t SIZE)
Allocate SIZE bytes of memory and return a pointer to it. When
SIZE is 0, return ‘NULL’. When not enough memory is available,
signal an error. This function runs the GC to free up some memory
when it deems it appropriate.
The memory is allocated by the libc ‘malloc’ function and can be
freed with ‘free’. There is no ‘scm_free’ function to go with
‘scm_malloc’ to make it easier to pass memory back and forth
between different modules.
The function ‘scm_calloc’ is similar to ‘scm_malloc’, but
initializes the block of memory to zero as well.
These functions will (indirectly) call
‘scm_gc_register_allocation’.
-- C Function: void * scm_realloc (void *MEM, size_t NEW_SIZE)
Change the size of the memory block at MEM to NEW_SIZE and return
its new location. When NEW_SIZE is 0, this is the same as calling
‘free’ on MEM and ‘NULL’ is returned. When MEM is ‘NULL’, this
function behaves like ‘scm_malloc’ and allocates a new block of
size NEW_SIZE.
When not enough memory is available, signal an error. This
function runs the GC to free up some memory when it deems it
appropriate.
This function will call ‘scm_gc_register_allocation’.
-- C Function: void * scm_gc_malloc (size_t SIZE, const char *WHAT)
-- C Function: void * scm_gc_malloc_pointerless (size_t SIZE, const
char *WHAT)
-- C Function: void * scm_gc_realloc (void *MEM, size_t OLD_SIZE,
size_t NEW_SIZE, const char *WHAT);
-- C Function: void * scm_gc_calloc (size_t SIZE, const char *WHAT)
Allocate SIZE bytes of automatically-managed memory. The memory is
automatically freed when no longer referenced from any live memory
block.
When garbage collection occurs, Guile will visit the words in
memory allocated with ‘scm_gc_malloc’ or ‘scm_gc_calloc’, looking
for pointers to other memory allocations that are managed by the
GC. In contrast, memory allocated by ‘scm_gc_malloc_pointerless’ is
not scanned for pointers.
The ‘scm_gc_realloc’ call preserves the “pointerlessness” of the
memory area pointed to by MEM. Note that you need to pass the old
size of a reallocated memory block as well. See below for a
motivation.
-- C Function: void scm_gc_free (void *MEM, size_t SIZE, const char
*WHAT)
Explicitly free the memory block pointed to by MEM, which was
previously allocated by one of the above ‘scm_gc’ functions. This
function is almost always unnecessary, except for codebases that
still need to compile on Guile 1.8.
Note that you need to explicitly pass the SIZE parameter. This is
done since it should normally be easy to provide this parameter
(for memory that is associated with GC controlled objects) and help
keep the memory management overhead very low. However, in Guile
2.x, SIZE is always ignored.
-- C Function: void scm_gc_register_allocation (size_t SIZE)
Informs the garbage collector that SIZE bytes have been allocated,
which the collector would otherwise not have known about.
In general, Scheme will decide to collect garbage only after some
amount of memory has been allocated. Calling this function will
make the Scheme garbage collector know about more allocation, and
thus run more often (as appropriate).
It is especially important to call this function when large
unmanaged allocations, like images, may be freed by small Scheme
allocations, like foreign objects.
-- C Function: void scm_dynwind_free (void *mem)
Equivalent to ‘scm_dynwind_unwind_handler (free, MEM,
SCM_F_WIND_EXPLICITLY)’. That is, the memory block at MEM will be
freed (using ‘free’ from the C library) when the current dynwind is
left.
-- Scheme Procedure: malloc-stats
Return an alist ((WHAT . N) ...) describing number of malloced
objects. WHAT is the second argument to ‘scm_gc_malloc’, N is the
number of objects of that type currently allocated.
This function is only available if the ‘GUILE_DEBUG_MALLOC’
preprocessor macro was defined when Guile was compiled.
---------- Footnotes ----------
(1) In Guile up to version 1.8, memory allocated with ‘scm_gc_malloc’
_had_ to be freed with ‘scm_gc_free’.
(2) In Guile up to 1.8, memory allocated with ‘scm_gc_malloc’ was
_not_ visited by the collector in the mark phase. Consequently, the GC
had to be told explicitly about pointers to live objects contained in
the memory block, e.g., via SMOB mark functions (*note
‘scm_set_smob_mark’: Smobs.)
6.17.3 Weak References
----------------------
[FIXME: This chapter is based on Mikael Djurfeldt’s answer to a question
by Michael Livshin. Any mistakes are not theirs, of course. ]
Weak references let you attach bookkeeping information to data so
that the additional information automatically disappears when the
original data is no longer in use and gets garbage collected. In a weak
key hash, the hash entry for that key disappears as soon as the key is
no longer referenced from anywhere else. For weak value hashes, the
same happens as soon as the value is no longer in use. Entries in a
doubly weak hash disappear when either the key or the value are not used
anywhere else anymore.
Object properties offer the same kind of functionality as weak key
hashes in many situations. (*note Object Properties::)
Here’s an example (a little bit strained perhaps, but one of the
examples is actually used in Guile):
Assume that you’re implementing a debugging system where you want to
associate information about filename and position of source code
expressions with the expressions themselves.
Hashtables can be used for that, but if you use ordinary hash tables
it will be impossible for the scheme interpreter to "forget" old source
when, for example, a file is reloaded.
To implement the mapping from source code expressions to positional
information it is necessary to use weak-key tables since we don’t want
the expressions to be remembered just because they are in our table.
To implement a mapping from source file line numbers to source code
expressions you would use a weak-value table.
To implement a mapping from source code expressions to the procedures
they constitute a doubly-weak table has to be used.
6.17.3.1 Weak hash tables
.........................
-- Scheme Procedure: make-weak-key-hash-table [size]
-- Scheme Procedure: make-weak-value-hash-table [size]
-- Scheme Procedure: make-doubly-weak-hash-table [size]
-- C Function: scm_make_weak_key_hash_table (size)
-- C Function: scm_make_weak_value_hash_table (size)
-- C Function: scm_make_doubly_weak_hash_table (size)
Return a weak hash table with SIZE buckets. As with any hash
table, choosing a good size for the table requires some caution.
You can modify weak hash tables in exactly the same way you would
modify regular hash tables, with the exception of the routines that
act on handles. Weak tables have a different implementation behind
the scenes that doesn’t have handles. *note Hash Tables::, for
more on ‘hashq-ref’ et al.
Note that in a weak-key hash table, the reference to the value is
strong. This means that if the value references the key, even
indirectly, the key will never be collected, which can lead to a memory
leak. The reverse is true for weak value tables.
-- Scheme Procedure: weak-key-hash-table? obj
-- Scheme Procedure: weak-value-hash-table? obj
-- Scheme Procedure: doubly-weak-hash-table? obj
-- C Function: scm_weak_key_hash_table_p (obj)
-- C Function: scm_weak_value_hash_table_p (obj)
-- C Function: scm_doubly_weak_hash_table_p (obj)
Return ‘#t’ if OBJ is the specified weak hash table. Note that a
doubly weak hash table is neither a weak key nor a weak value hash
table.
6.17.3.2 Weak vectors
.....................
-- Scheme Procedure: make-weak-vector size [fill]
-- C Function: scm_make_weak_vector (size, fill)
Return a weak vector with SIZE elements. If the optional argument
FILL is given, all entries in the vector will be set to FILL. The
default value for FILL is the empty list.
-- Scheme Procedure: weak-vector elem ...
-- Scheme Procedure: list->weak-vector l
-- C Function: scm_weak_vector (l)
Construct a weak vector from a list: ‘weak-vector’ uses the list of
its arguments while ‘list->weak-vector’ uses its only argument L (a
list) to construct a weak vector the same way ‘list->vector’ would.
-- Scheme Procedure: weak-vector? obj
-- C Function: scm_weak_vector_p (obj)
Return ‘#t’ if OBJ is a weak vector.
-- Scheme Procedure: weak-vector-ref wvect k
-- C Function: scm_weak_vector_ref (wvect, k)
Return the Kth element of the weak vector WVECT, or ‘#f’ if that
element has been collected.
-- Scheme Procedure: weak-vector-set! wvect k elt
-- C Function: scm_weak_vector_set_x (wvect, k, elt)
Set the Kth element of the weak vector WVECT to ELT.
6.17.4 Guardians
----------------
Guardians provide a way to be notified about objects that would
otherwise be collected as garbage. Guarding them prevents the objects
from being collected and cleanup actions can be performed on them, for
example.
See R. Kent Dybvig, Carl Bruggeman, and David Eby (1993) "Guardians
in a Generation-Based Garbage Collector". ACM SIGPLAN Conference on
Programming Language Design and Implementation, June 1993.
-- Scheme Procedure: make-guardian
-- C Function: scm_make_guardian ()
Create a new guardian. A guardian protects a set of objects from
garbage collection, allowing a program to apply cleanup or other
actions.
‘make-guardian’ returns a procedure representing the guardian.
Calling the guardian procedure with an argument adds the argument
to the guardian’s set of protected objects. Calling the guardian
procedure without an argument returns one of the protected objects
which are ready for garbage collection, or ‘#f’ if no such object
is available. Objects which are returned in this way are removed
from the guardian.
You can put a single object into a guardian more than once and you
can put a single object into more than one guardian. The object
will then be returned multiple times by the guardian procedures.
An object is eligible to be returned from a guardian when it is no
longer referenced from outside any guardian.
There is no guarantee about the order in which objects are returned
from a guardian. If you want to impose an order on finalization
actions, for example, you can do that by keeping objects alive in
some global data structure until they are no longer needed for
finalizing other objects.
Being an element in a weak vector, a key in a hash table with weak
keys, or a value in a hash table with weak values does not prevent
an object from being returned by a guardian. But as long as an
object can be returned from a guardian it will not be removed from
such a weak vector or hash table. In other words, a weak link does
not prevent an object from being considered collectable, but being
inside a guardian prevents a weak link from being broken.
A key in a weak key hash table can be thought of as having a strong
reference to its associated value as long as the key is accessible.
Consequently, when the key is only accessible from within a
guardian, the reference from the key to the value is also
considered to be coming from within a guardian. Thus, if there is
no other reference to the value, it is eligible to be returned from
a guardian.
6.18 Modules
============
When programs become large, naming conflicts can occur when a function
or global variable defined in one file has the same name as a function
or global variable in another file. Even just a _similarity_ between
function names can cause hard-to-find bugs, since a programmer might
type the wrong function name.
The approach used to tackle this problem is called _information
encapsulation_, which consists of packaging functional units into a
given name space that is clearly separated from other name spaces.
The language features that allow this are usually called _the module
system_ because programs are broken up into modules that are compiled
separately (or loaded separately in an interpreter).
Older languages, like C, have limited support for name space
manipulation and protection. In C a variable or function is public by
default, and can be made local to a module with the ‘static’ keyword.
But you cannot reference public variables and functions from another
module with different names.
More advanced module systems have become a common feature in recently
designed languages: ML, Python, Perl, and Modula 3 all allow the
_renaming_ of objects from a foreign module, so they will not clutter
the global name space.
In addition, Guile offers variables as first-class objects. They can
be used for interacting with the module system.
6.18.1 General Information about Modules
----------------------------------------
A Guile module can be thought of as a collection of named procedures,
variables and macros. More precisely, it is a set of “bindings” of
symbols (names) to Scheme objects.
Within a module, all bindings are visible. Certain bindings can be
declared “public”, in which case they are added to the module’s
so-called “export list”; this set of public bindings is called the
module’s “public interface” (*note Creating Guile Modules::).
A client module “uses” a providing module’s bindings by either
accessing the providing module’s public interface, or by building a
custom interface (and then accessing that). In a custom interface, the
client module can “select” which bindings to access and can also
algorithmically “rename” bindings. In contrast, when using the
providing module’s public interface, the entire export list is available
without renaming (*note Using Guile Modules::).
All Guile modules have a unique “module name”, for example ‘(ice-9
popen)’ or ‘(srfi srfi-11)’. Module names are lists of one or more
symbols.
When Guile goes to use an interface from a module, for example
‘(ice-9 popen)’, Guile first looks to see if it has loaded ‘(ice-9
popen)’ for any reason. If the module has not been loaded yet, Guile
searches a “load path” for a file that might define it, and loads that
file.
The following subsections go into more detail on using, creating,
installing, and otherwise manipulating modules and the module system.
6.18.2 Using Guile Modules
--------------------------
To use a Guile module is to access either its public interface or a
custom interface (*note General Information about Modules::). Both
types of access are handled by the syntactic form ‘use-modules’, which
accepts one or more interface specifications and, upon evaluation,
arranges for those interfaces to be available to the current module.
This process may include locating and loading code for a given module if
that code has not yet been loaded, following ‘%load-path’ (*note Modules
and the File System::).
An “interface specification” has one of two forms. The first
variation is simply to name the module, in which case its public
interface is the one accessed. For example:
(use-modules (ice-9 popen))
Here, the interface specification is ‘(ice-9 popen)’, and the result
is that the current module now has access to ‘open-pipe’, ‘close-pipe’,
‘open-input-pipe’, and so on (*note Pipes::).
Note in the previous example that if the current module had already
defined ‘open-pipe’, that definition would be overwritten by the
definition in ‘(ice-9 popen)’. For this reason (and others), there is a
second variation of interface specification that not only names a module
to be accessed, but also selects bindings from it and renames them to
suit the current module’s needs. For example:
(use-modules ((ice-9 popen)
#:select ((open-pipe . pipe-open) close-pipe)
#:renamer (symbol-prefix-proc 'unixy:)))
or more simply:
(use-modules ((ice-9 popen)
#:select ((open-pipe . pipe-open) close-pipe)
#:prefix unixy:))
Here, the interface specification is more complex than before, and
the result is that a custom interface with only two bindings is created
and subsequently accessed by the current module. The mapping of old to
new names is as follows:
(ice-9 popen) sees: current module sees:
open-pipe unixy:pipe-open
close-pipe unixy:close-pipe
This example also shows how to use the convenience procedure
‘symbol-prefix-proc’.
You can also directly refer to bindings in a module by using the ‘@’
syntax. For example, instead of using the ‘use-modules’ statement from
above and writing ‘unixy:pipe-open’ to refer to the ‘pipe-open’ from the
‘(ice-9 popen)’, you could also write ‘(@ (ice-9 popen) open-pipe)’.
Thus an alternative to the complete ‘use-modules’ statement would be
(define unixy:pipe-open (@ (ice-9 popen) open-pipe))
(define unixy:close-pipe (@ (ice-9 popen) close-pipe))
There is also ‘@@’, which can be used like ‘@’, but does not check
whether the variable that is being accessed is actually exported. Thus,
‘@@’ can be thought of as the impolite version of ‘@’ and should only be
used as a last resort or for debugging, for example.
Note that just as with a ‘use-modules’ statement, any module that has
not yet been loaded will be loaded when referenced by a ‘@’ or ‘@@’
form.
You can also use the ‘@’ and ‘@@’ syntaxes as the target of a ‘set!’
when the binding refers to a variable.
-- Scheme Procedure: symbol-prefix-proc prefix-sym
Return a procedure that prefixes its arg (a symbol) with
PREFIX-SYM.
-- syntax: use-modules spec ...
Resolve each interface specification SPEC into an interface and
arrange for these to be accessible by the current module. The
return value is unspecified.
SPEC can be a list of symbols, in which case it names a module
whose public interface is found and used.
SPEC can also be of the form:
(MODULE-NAME [#:select SELECTION]
[#:prefix PREFIX]
[#:renamer RENAMER])
in which case a custom interface is newly created and used.
MODULE-NAME is a list of symbols, as above; SELECTION is a list of
selection-specs; PREFIX is a symbol that is prepended to imported
names; and RENAMER is a procedure that takes a symbol and returns
its new name. A selection-spec is either a symbol or a pair of
symbols ‘(ORIG . SEEN)’, where ORIG is the name in the used module
and SEEN is the name in the using module. Note that SEEN is also
modified by PREFIX and RENAMER.
The ‘#:select’, ‘#:prefix’, and ‘#:renamer’ clauses are optional.
If all are omitted, the returned interface has no bindings. If the
‘#:select’ clause is omitted, PREFIX and RENAMER operate on the
used module’s public interface.
In addition to the above, SPEC can also include a ‘#:version’
clause, of the form:
#:version VERSION-SPEC
where VERSION-SPEC is an R6RS-compatible version reference. An
error will be signaled in the case in which a module with the same
name has already been loaded, if that module specifies a version
and that version is not compatible with VERSION-SPEC. *Note R6RS
Version References::, for more on version references.
If the module name is not resolvable, ‘use-modules’ will signal an
error.
-- syntax: @ module-name binding-name
Refer to the binding named BINDING-NAME in module MODULE-NAME. The
binding must have been exported by the module.
-- syntax: @@ module-name binding-name
Refer to the binding named BINDING-NAME in module MODULE-NAME. The
binding must not have been exported by the module. This syntax is
only intended for debugging purposes or as a last resort. *Note
Declarative Modules::, for some limitations on the use of ‘@@’.
6.18.3 Creating Guile Modules
-----------------------------
When you want to create your own modules, you have to take the following
steps:
• Create a Scheme source file and add all variables and procedures
you wish to export, or which are required by the exported
procedures.
• Add a ‘define-module’ form at the beginning.
• Export all bindings which should be in the public interface, either
by using ‘define-public’ or ‘export’ (both documented below).
-- syntax: define-module module-name option ...
MODULE-NAME is a list of one or more symbols.
(define-module (ice-9 popen))
‘define-module’ makes this module available to Guile programs under
the given MODULE-NAME.
OPTION ... are keyword/value pairs which specify more about the
defined module. The recognized options and their meaning are shown
in the following table.
‘#:use-module INTERFACE-SPECIFICATION’
Equivalent to a ‘(use-modules INTERFACE-SPECIFICATION)’ (*note
Using Guile Modules::).
‘#:autoload MODULE SYMBOL-LIST’
Load MODULE when any of SYMBOL-LIST are accessed. For
example,
(define-module (my mod)
#:autoload (srfi srfi-1) (partition delete-duplicates))
...
(when something
(set! foo (delete-duplicates ...)))
When a module is autoloaded, only the bindings in SYMBOL-LIST
become available(1).
An autoload is a good way to put off loading a big module
until it’s really needed, for instance for faster startup or
if it will only be needed in certain circumstances.
‘#:export LIST’
Export all identifiers in LIST which must be a list of symbols
or pairs of symbols. This is equivalent to ‘(export LIST)’ in
the module body.
‘#:re-export LIST’
Re-export all identifiers in LIST which must be a list of
symbols or pairs of symbols. The symbols in LIST must be
imported by the current module from other modules. This is
equivalent to ‘re-export’ below.
‘#:replace LIST’
Export all identifiers in LIST (a list of symbols or pairs of
symbols) and mark them as “replacing bindings”. In the module
user’s name space, this will have the effect of replacing any
binding with the same name that is not also “replacing”.
Normally a replacement results in an “override” warning
message, ‘#:replace’ avoids that.
In general, a module that exports a binding for which the
‘(guile)’ module already has a definition should use
‘#:replace’ instead of ‘#:export’. ‘#:replace’, in a sense,
lets Guile know that the module _purposefully_ replaces a core
binding. It is important to note, however, that this binding
replacement is confined to the name space of the module user.
In other words, the value of the core binding in question
remains unchanged for other modules.
Note that although it is often a good idea for the replaced
binding to remain compatible with a binding in ‘(guile)’, to
avoid surprising the user, sometimes the bindings will be
incompatible. For example, SRFI-19 exports its own version of
‘current-time’ (*note SRFI-19 Time::) which is not compatible
with the core ‘current-time’ function (*note Time::). Guile
assumes that a user importing a module knows what she is
doing, and uses ‘#:replace’ for this binding rather than
‘#:export’.
A ‘#:replace’ clause is equivalent to ‘(export! LIST)’ in the
module body.
The ‘#:duplicates’ (see below) provides fine-grain control
about duplicate binding handling on the module-user side.
‘#:re-export-and-replace LIST’
Like ‘#:re-export’, but also marking the bindings as
replacements in the sense of ‘#:replace’.
‘#:version LIST’
Specify a version for the module in the form of LIST, a list
of zero or more exact, nonnegative integers. The
corresponding ‘#:version’ option in the ‘use-modules’ form
allows callers to restrict the value of this option in various
ways.
‘#:duplicates LIST’
Tell Guile to handle duplicate bindings for the bindings
imported by the current module according to the policy defined
by LIST, a list of symbols. LIST must contain symbols
representing a duplicate binding handling policy chosen among
the following:
‘check’
Raises an error when a binding is imported from more than
one place.
‘warn’
Issue a warning when a binding is imported from more than
one place and leave the responsibility of actually
handling the duplication to the next duplicate binding
handler.
‘replace’
When a new binding is imported that has the same name as
a previously imported binding, then do the following:
1. If the old binding was said to be “replacing” (via
the ‘#:replace’ option above) and the new binding is
not replacing, the keep the old binding.
2. If the old binding was not said to be replacing and
the new binding is replacing, then replace the old
binding with the new one.
3. If neither the old nor the new binding is replacing,
then keep the old one.
‘warn-override-core’
Issue a warning when a core binding is being overwritten
and actually override the core binding with the new one.
‘first’
In case of duplicate bindings, the firstly imported
binding is always the one which is kept.
‘last’
In case of duplicate bindings, the lastly imported
binding is always the one which is kept.
‘noop’
In case of duplicate bindings, leave the responsibility
to the next duplicate handler.
If LIST contains more than one symbol, then the duplicate
binding handlers which appear first will be used first when
resolving a duplicate binding situation. As mentioned above,
some resolution policies may explicitly leave the
responsibility of handling the duplication to the next handler
in LIST.
If GOOPS has been loaded before the ‘#:duplicates’ clause is
processed, there are additional strategies available for
dealing with generic functions. *Note Merging Generics::, for
more information.
The default duplicate binding resolution policy is given by
the ‘default-duplicate-binding-handler’ procedure, and is
(replace warn-override-core warn last)
‘#:pure’
Create a “pure” module, that is a module which does not
contain any of the standard procedure bindings except for the
syntax forms. This is useful if you want to create “safe”
modules, that is modules which do not know anything about
dangerous procedures.
-- syntax: export variable ...
Add all VARIABLEs (which must be symbols or pairs of symbols) to
the list of exported bindings of the current module. If VARIABLE
is a pair, its ‘car’ gives the name of the variable as seen by the
current module and its ‘cdr’ specifies a name for the binding in
the current module’s public interface.
-- syntax: define-public ...
Equivalent to ‘(begin (define foo ...) (export foo))’.
-- syntax: re-export variable ...
Add all VARIABLEs (which must be symbols or pairs of symbols) to
the list of re-exported bindings of the current module. Pairs of
symbols are handled as in ‘export’. Re-exported bindings must be
imported by the current module from some other module.
-- syntax: export! variable ...
Like ‘export’, but marking the exported variables as replacing.
Using a module with replacing bindings will cause any existing
bindings to be replaced without issuing any warnings. See the
discussion of ‘#:replace’ above.
---------- Footnotes ----------
(1) In Guile 2.2 and earlier, _all_ the module bindings would become
available; SYMBOL-LIST was just the list of bindings that will first
trigger the load.
6.18.4 Modules and the File System
----------------------------------
Typical programs only use a small subset of modules installed on a Guile
system. In order to keep startup time down, Guile only loads modules
when a program uses them, on demand.
When a program evaluates ‘(use-modules (ice-9 popen))’, and the
module is not loaded, Guile searches for a conventionally-named file
from in the “load path”.
In this case, loading ‘(ice-9 popen)’ will eventually cause Guile to
run ‘(primitive-load-path "ice-9/popen")’. ‘primitive-load-path’ will
search for a file ‘ice-9/popen’ in the ‘%load-path’ (*note Load
Paths::). For each directory in ‘%load-path’, Guile will try to find
the file name, concatenated with the extensions from ‘%load-extensions’.
By default, this will cause Guile to ‘stat’ ‘ice-9/popen.scm’, and then
‘ice-9/popen’. *Note Load Paths::, for more on ‘primitive-load-path’.
If a corresponding compiled ‘.go’ file is found in the
‘%load-compiled-path’ or in the fallback path, and is as fresh as the
source file, it will be loaded instead of the source file. If no
compiled file is found, Guile may try to compile the source file and
cache away the resulting ‘.go’ file. *Note Compilation::, for more on
compilation.
Once Guile finds a suitable source or compiled file is found, the
file will be loaded. If, after loading the file, the module under
consideration is still not defined, Guile will signal an error.
For more information on where and how to install Scheme modules,
*Note Installing Site Packages::.
6.18.5 R6RS Version References
------------------------------
Guile’s module system includes support for locating modules based on a
declared version specifier of the same form as the one described in R6RS
(*note R6RS Library Form: (r6rs)Library form.). By using the
‘#:version’ keyword in a ‘define-module’ form, a module may specify a
version as a list of zero or more exact, nonnegative integers.
This version can then be used to locate the module during the module
search process. Client modules and callers of the ‘use-modules’
function may specify constraints on the versions of target modules by
providing a “version reference”, which has one of the following forms:
(SUB-VERSION-REFERENCE ...)
(and VERSION-REFERENCE ...)
(or VERSION-REFERENCE ...)
(not VERSION-REFERENCE)
in which SUB-VERSION-REFERENCE is in turn one of:
(SUB-VERSION)
(>= SUB-VERSION)
(<= SUB-VERSION)
(and SUB-VERSION-REFERENCE ...)
(or SUB-VERSION-REFERENCE ...)
(not SUB-VERSION-REFERENCE)
in which SUB-VERSION is an exact, nonnegative integer as above. A
version reference matches a declared module version if each element of
the version reference matches a corresponding element of the module
version, according to the following rules:
• The ‘and’ sub-form matches a version or version element if every
element in the tail of the sub-form matches the specified version
or version element.
• The ‘or’ sub-form matches a version or version element if any
element in the tail of the sub-form matches the specified version
or version element.
• The ‘not’ sub-form matches a version or version element if the tail
of the sub-form does not match the version or version element.
• The ‘>=’ sub-form matches a version element if the element is
greater than or equal to the SUB-VERSION in the tail of the
sub-form.
• The ‘<=’ sub-form matches a version element if the version is less
than or equal to the SUB-VERSION in the tail of the sub-form.
• A SUB-VERSION matches a version element if one is EQV? to the
other.
For example, a module declared as:
(define-module (mylib mymodule) #:version (1 2 0))
would be successfully loaded by any of the following ‘use-modules’
expressions:
(use-modules ((mylib mymodule) #:version (1 2 (>= 0))))
(use-modules ((mylib mymodule) #:version (or (1 2 0) (1 2 1))))
(use-modules ((mylib mymodule) #:version ((and (>= 1) (not 2)) 2 0)))
6.18.6 R6RS Libraries
---------------------
In addition to the API described in the previous sections, you also have
the option to create modules using the portable ‘library’ form described
in R6RS (*note R6RS Library Form: (r6rs)Library form.), and to import
libraries created in this format by other programmers. Guile’s R6RS
library implementation takes advantage of the flexibility built into the
module system by expanding the R6RS library form into a corresponding
Guile ‘define-module’ form that specifies equivalent import and export
requirements and includes the same body expressions. The library
expression:
(library (mylib (1 2))
(export mybinding)
(import (otherlib (3))))
is equivalent to the module definition:
(define-module (mylib)
#:version (1 2)
#:use-module ((otherlib) #:version (3))
#:export (mybinding))
Central to the mechanics of R6RS libraries is the concept of import
and export “levels”, which control the visibility of bindings at various
phases of a library’s lifecycle — macros necessary to expand forms in
the library’s body need to be available at expand time; variables used
in the body of a procedure exported by the library must be available at
runtime. R6RS specifies the optional ‘for’ sub-form of an _import set_
specification (see below) as a mechanism by which a library author can
indicate that a particular library import should take place at a
particular phase with respect to the lifecycle of the importing library.
Guile’s library implementation uses a technique called “implicit
phasing” (first described by Abdulaziz Ghuloum and R. Kent Dybvig),
which allows the expander and compiler to automatically determine the
necessary visibility of a binding imported from another library. As
such, the ‘for’ sub-form described below is ignored by Guile (but may be
required by Schemes in which phasing is explicit).
-- Scheme Syntax: library name (export export-spec ...) (import
import-spec ...) body ...
Defines a new library with the specified name, exports, and
imports, and evaluates the specified body expressions in this
library’s environment.
The library NAME is a non-empty list of identifiers, optionally
ending with a version specification of the form described above
(*note Creating Guile Modules::).
Each EXPORT-SPEC is the name of a variable defined or imported by
the library, or must take the form ‘(rename (internal-name
external-name) ...)’, where the identifier INTERNAL-NAME names a
variable defined or imported by the library and EXTERNAL-NAME is
the name by which the variable is seen by importing libraries.
Each IMPORT-SPEC must be either an “import set” (see below) or must
be of the form ‘(for import-set import-level ...)’, where each
IMPORT-LEVEL is one of:
run
expand
(meta LEVEL)
where LEVEL is an integer. Note that since Guile does not require
explicit phase specification, any IMPORT-SETs found inside of ‘for’
sub-forms will be “unwrapped” during expansion and processed as if
they had been specified directly.
Import sets in turn take one of the following forms:
LIBRARY-REFERENCE
(library LIBRARY-REFERENCE)
(only IMPORT-SET IDENTIFIER ...)
(except IMPORT-SET IDENTIFIER ...)
(prefix IMPORT-SET IDENTIFIER)
(rename IMPORT-SET (INTERNAL-IDENTIFIER EXTERNAL-IDENTIFIER) ...)
where LIBRARY-REFERENCE is a non-empty list of identifiers ending
with an optional version reference (*note R6RS Version
References::), and the other sub-forms have the following
semantics, defined recursively on nested IMPORT-SETs:
• The ‘library’ sub-form is used to specify libraries for import
whose names begin with the identifier “library.”
• The ‘only’ sub-form imports only the specified IDENTIFIERs
from the given IMPORT-SET.
• The ‘except’ sub-form imports all of the bindings exported by
IMPORT-SET except for those that appear in the specified list
of IDENTIFIERs.
• The ‘prefix’ sub-form imports all of the bindings exported by
IMPORT-SET, first prefixing them with the specified
IDENTIFIER.
• The ‘rename’ sub-form imports all of the identifiers exported
by IMPORT-SET. The binding for each INTERNAL-IDENTIFIER among
these identifiers is made visible to the importing library as
the corresponding EXTERNAL-IDENTIFIER; all other bindings are
imported using the names provided by IMPORT-SET.
Note that because Guile translates R6RS libraries into module
definitions, an import specification may be used to declare a
dependency on a native Guile module — although doing so may make
your libraries less portable to other Schemes.
-- Scheme Syntax: import import-spec ...
Import into the current environment the libraries specified by the
given import specifications, where each IMPORT-SPEC takes the same
form as in the ‘library’ form described above.
6.18.7 Variables
----------------
Each module has its own hash table, sometimes known as an “obarray”,
that maps the names defined in that module to their corresponding
variable objects.
A variable is a box-like object that can hold any Scheme value. It
is said to be “undefined” if its box holds a special Scheme value that
denotes undefined-ness (which is different from all other Scheme values,
including for example ‘#f’); otherwise the variable is “defined”.
On its own, a variable object is anonymous. A variable is said to be
“bound” when it is associated with a name in some way, usually a symbol
in a module obarray. When this happens, the name is said to be bound to
the variable, in that module.
(That’s the theory, anyway. In practice, defined-ness and bound-ness
sometimes get confused, because Lisp and Scheme implementations have
often conflated — or deliberately drawn no distinction between — a name
that is unbound and a name that is bound to a variable whose value is
undefined. We will try to be clear about the difference and explain any
confusion where it is unavoidable.)
Variables do not have a read syntax. Most commonly they are created
and bound implicitly by ‘define’ expressions: a top-level ‘define’
expression of the form
(define NAME VALUE)
creates a variable with initial value VALUE and binds it to the name
NAME in the current module. But they can also be created dynamically by
calling one of the constructor procedures ‘make-variable’ and
‘make-undefined-variable’.
-- Scheme Procedure: make-undefined-variable
-- C Function: scm_make_undefined_variable ()
Return a variable that is initially unbound.
-- Scheme Procedure: make-variable init
-- C Function: scm_make_variable (init)
Return a variable initialized to value INIT.
-- Scheme Procedure: variable-bound? var
-- C Function: scm_variable_bound_p (var)
Return ‘#t’ if VAR is bound to a value, or ‘#f’ otherwise. Throws
an error if VAR is not a variable object.
-- Scheme Procedure: variable-ref var
-- C Function: scm_variable_ref (var)
Dereference VAR and return its value. VAR must be a variable
object; see ‘make-variable’ and ‘make-undefined-variable’.
-- Scheme Procedure: variable-set! var val
-- C Function: scm_variable_set_x (var, val)
Set the value of the variable VAR to VAL. VAR must be a variable
object, VAL can be any value. Return an unspecified value.
-- Scheme Procedure: variable-unset! var
-- C Function: scm_variable_unset_x (var)
Unset the value of the variable VAR, leaving VAR unbound.
-- Scheme Procedure: variable? obj
-- C Function: scm_variable_p (obj)
Return ‘#t’ if OBJ is a variable object, else return ‘#f’.
6.18.8 Module System Reflection
-------------------------------
The previous sections have described a declarative view of the module
system. You can also work with it programmatically by accessing and
modifying various parts of the Scheme objects that Guile uses to
implement the module system.
At any time, there is a “current module”. This module is the one
where a top-level ‘define’ and similar syntax will add new bindings.
You can find other module objects with ‘resolve-module’, for example.
These module objects can be used as the second argument to ‘eval’.
-- Scheme Procedure: current-module
-- C Function: scm_current_module ()
Return the current module object.
-- Scheme Procedure: set-current-module module
-- C Function: scm_set_current_module (module)
Set the current module to MODULE and return the previous current
module.
-- Scheme Procedure: save-module-excursion thunk
Call THUNK within a ‘dynamic-wind’ such that the module that is
current at invocation time is restored when THUNK’s dynamic extent
is left (*note Dynamic Wind::).
More precisely, if THUNK escapes non-locally, the current module
(at the time of escape) is saved, and the original current module
(at the time THUNK’s dynamic extent was last entered) is restored.
If THUNK’s dynamic extent is re-entered, then the current module is
saved, and the previously saved inner module is set current again.
-- Scheme Procedure: resolve-module name [autoload=#t] [version=#f]
[#:ensure=#t]
-- C Function: scm_resolve_module (name)
Find the module named NAME and return it. When it has not already
been defined and AUTOLOAD is true, try to auto-load it. When it
can’t be found that way either, create an empty module if ENSURE is
true, otherwise return ‘#f’. If VERSION is true, ensure that the
resulting module is compatible with the given version reference
(*note R6RS Version References::). The name is a list of symbols.
-- Scheme Procedure: resolve-interface name [#:select=#f] [#:hide='()]
[#:prefix=#f] [#:renamer=#f] [#:version=#f]
Find the module named NAME as with ‘resolve-module’ and return its
interface. The interface of a module is also a module object, but
it contains only the exported bindings.
-- Scheme Procedure: module-uses module
Return a list of the interfaces used by MODULE.
-- Scheme Procedure: module-use! module interface
Add INTERFACE to the front of the use-list of MODULE. Both
arguments should be module objects, and INTERFACE should very
likely be a module returned by ‘resolve-interface’.
-- Scheme Procedure: reload-module module
Revisit the source file that corresponds to MODULE. Raises an
error if no source file is associated with the given module.
As mentioned in the previous section, modules contain a mapping
between identifiers (as symbols) and storage locations (as variables).
Guile defines a number of procedures to allow access to this mapping.
If you are programming in C, *note Accessing Modules from C::.
-- Scheme Procedure: module-variable module name
Return the variable bound to NAME (a symbol) in MODULE, or ‘#f’ if
NAME is unbound.
-- Scheme Procedure: module-add! module name var
Define a new binding between NAME (a symbol) and VAR (a variable)
in MODULE.
-- Scheme Procedure: module-ref module name
Look up the value bound to NAME in MODULE. Like ‘module-variable’,
but also does a ‘variable-ref’ on the resulting variable, raising
an error if NAME is unbound.
-- Scheme Procedure: module-define! module name value
Locally bind NAME to VALUE in MODULE. If NAME was already locally
bound in MODULE, i.e., defined locally and not by an imported
module, the value stored in the existing variable will be updated.
Otherwise, a new variable will be added to the module, via
‘module-add!’.
-- Scheme Procedure: module-set! module name value
Update the binding of NAME in MODULE to VALUE, raising an error if
NAME is not already bound in MODULE.
There are many other reflective procedures available in the default
environment. If you find yourself using one of them, please contact the
Guile developers so that we can commit to stability for that interface.
6.18.9 Declarative Modules
--------------------------
The first-class access to modules and module variables described in the
previous subsection is very powerful and allows Guile users to build
many tools to dynamically learn things about their Guile systems.
However, as Scheme godparent Mathias Felleisen wrote in “On the
Expressive Power of Programming Languages”, a more expressive language
is necessarily harder to reason about. There are transformations that
Guile’s compiler would like to make which can’t be done if every
top-level definition is subject to mutation at any time.
Consider this module:
(define-module (boxes)
#:export (make-box box-ref box-set! box-swap!))
(define (make-box x) (list x))
(define (box-ref box) (car box))
(define (box-set! box x) (set-car! box x))
(define (box-swap! box x)
(let ((y (box-ref box)))
(box-set! box x)
y))
Ideally you’d like for the ‘box-ref’ in ‘box-swap!’ to be inlined to
‘car’. Guile’s compiler can do this, but only if it knows that
‘box-ref’’s definition is what it appears to be in the text. However,
in the general case it could be that a programmer could reach into the
‘(boxes)’ module at any time and change the value of ‘box-ref’.
To allow Guile to reason about the values of top-levels from a
module, a module can be marked as “declarative”. This flag applies only
to the subset of top-level definitions that are themselves declarative:
those that are defined within the compilation unit, and not assigned
(‘set!’) or redefined within the compilation unit.
To explicitly mark a module as being declarative, pass the
‘#:declarative?’ keyword argument when declaring a module:
(define-module (boxes)
#:export (make-box box-ref box-set! box-swap!)
#:declarative? #t)
By default, modules are compiled declaratively if the
‘user-modules-declarative?’ parameter is true when the module is
compiled.
-- Scheme Parameter: user-modules-declarative?
A boolean indicating whether definitions in modules created by
‘define-module’ or implicitly as part of a compilation unit without
an explicit module can be treated as declarative.
Because it’s usually what you want, the default value of
‘user-modules-declarative?’ is ‘#t’.
Should I Mark My Module As Declarative?
.......................................
In the vast majority of use cases, declarative modules are what you
want. However, there are exceptions.
Consider the ‘(boxes)’ module above. Let’s say you want to be able
to go in and change the definition of ‘box-set!’ at run-time:
scheme@(guile-user)> (use-modules (boxes))
scheme@(guile-user)> ,module boxes
scheme@(boxes)> (define (box-set! x y) (set-car! x (pk y)))
However, considering that ‘(boxes)’ is a declarative module, it could
be that ‘box-swap!’ inlined the call to ‘box-set!’ – so it may be that
you are surprised if you call ‘(box-swap! x y)’ and you don’t see the
new definition being used. (Note, however, that Guile has no guarantees
about what definitions its compiler will or will not inline.)
If you want to allow the definition of ‘box-set!’ to be changed and
to have all of its uses updated, then probably the best option is to
edit the module and reload the whole thing:
scheme@(guile-user)> ,reload (boxes)
The advantage of the reloading approach is that you maintain the
optimizations that declarative modules enable, while also being able to
live-update the code. If the module keeps precious program state, those
definitions can be marked as ‘define-once’ to prevent reloads from
overwriting them. *Note Top Level::, for more on ‘define-once’.
Incidentally, ‘define-once’ also prevents declarative-definition
optimizations, so if there’s a limited subset of redefinable bindings,
‘define-once’ could be an interesting tool to mark those definitions as
works-in-progress for interactive program development.
To users, whether a module is declarative or not is mostly
immaterial: besides normal use via ‘use-modules’, users can reference
and redefine public or private bindings programmatically or
interactively. The only difference is that changing a declarative
definition may not change all of its uses. If this use-case is
important to you, and if reloading whole modules is insufficient, then
you can mark all definitions in a module as non-declarative by adding
‘#:declarative? #f’ to the module definition.
The default of whether modules are declarative or not can be
controlled via the ‘(user-modules-declarative?)’ parameter mentioned
above, but care should be taken to set this parameter when the modules
are compiled, e.g. via ‘(eval-when (expand) (user-modules-declarative?
#f))’. *Note Eval When::.
Alternately you can prevent declarative-definition optimizations by
compiling at the ‘-O1’ optimization level instead of the default ‘-O2’,
or via explicitly passing ‘-Ono-letrectify’ to the ‘guild compile’
invocation. *Note Compilation::, for more on compiler options.
One final note. Currently, definitions from declarative modules can
only be inlined within the module they are defined in, and within a
compilation unit. This may change in the future to allow Guile to
inline imported declarative definitions as well (cross-module inlining).
To Guile, whether a definition is inlinable or not is a property of the
definition, not its use. We hope to improve compiler tooling in the
future to allow the user to identify definitions that are out of date
when a declarative binding is redefined.
6.18.10 Accessing Modules from C
--------------------------------
The last sections have described how modules are used in Scheme code,
which is the recommended way of creating and accessing modules. You can
also work with modules from C, but it is more cumbersome.
The following procedures are available.
-- C Function: SCM scm_c_call_with_current_module (SCM MODULE, SCM
(*FUNC)(void *), void *DATA)
Call FUNC and make MODULE the current module during the call. The
argument DATA is passed to FUNC. The return value of
‘scm_c_call_with_current_module’ is the return value of FUNC.
-- C Function: SCM scm_public_variable (SCM MODULE_NAME, SCM NAME)
-- C Function: SCM scm_c_public_variable (const char *MODULE_NAME,
const char *NAME)
Find a the variable bound to the symbol NAME in the public
interface of the module named MODULE_NAME.
MODULE_NAME should be a list of symbols, when represented as a
Scheme object, or a space-separated string, in the ‘const char *’
case. See ‘scm_c_define_module’ below, for more examples.
Signals an error if no module was found with the given name. If
NAME is not bound in the module, just returns ‘#f’.
-- C Function: SCM scm_private_variable (SCM MODULE_NAME, SCM NAME)
-- C Function: SCM scm_c_private_variable (const char *MODULE_NAME,
const char *NAME)
Like ‘scm_public_variable’, but looks in the internals of the
module named MODULE_NAME instead of the public interface.
Logically, these procedures should only be called on modules you
write.
-- C Function: SCM scm_public_lookup (SCM MODULE_NAME, SCM NAME)
-- C Function: SCM scm_c_public_lookup (const char *MODULE_NAME, const
char *NAME)
-- C Function: SCM scm_private_lookup (SCM MODULE_NAME, SCM NAME)
-- C Function: SCM scm_c_private_lookup (const char *MODULE_NAME, const
char *NAME)
Like ‘scm_public_variable’ or ‘scm_private_variable’, but if the
NAME is not bound in the module, signals an error. Returns a
variable, always.
static SCM eval_string_var;
/* NOTE: It is important that the call to 'my_init'
happens-before all calls to 'my_eval_string'. */
void my_init (void)
{
eval_string_var = scm_c_public_lookup ("ice-9 eval-string",
"eval-string");
}
SCM my_eval_string (SCM str)
{
return scm_call_1 (scm_variable_ref (eval_string_var), str);
}
-- C Function: SCM scm_public_ref (SCM MODULE_NAME, SCM NAME)
-- C Function: SCM scm_c_public_ref (const char *MODULE_NAME, const
char *NAME)
-- C Function: SCM scm_private_ref (SCM MODULE_NAME, SCM NAME)
-- C Function: SCM scm_c_private_ref (const char *MODULE_NAME, const
char *NAME)
Like ‘scm_public_lookup’ or ‘scm_private_lookup’, but additionally
dereferences the variable. If the variable object is unbound,
signals an error. Returns the value bound to NAME in MODULE_NAME.
In addition, there are a number of other lookup-related procedures.
We suggest that you use the ‘scm_public_’ and ‘scm_private_’ family of
procedures instead, if possible.
-- C Function: SCM scm_c_lookup (const char *NAME)
Return the variable bound to the symbol indicated by NAME in the
current module. If there is no such binding or the symbol is not
bound to a variable, signal an error.
-- C Function: SCM scm_lookup (SCM NAME)
Like ‘scm_c_lookup’, but the symbol is specified directly.
-- C Function: SCM scm_c_module_lookup (SCM MODULE, const char *NAME)
-- C Function: SCM scm_module_lookup (SCM MODULE, SCM NAME)
Like ‘scm_c_lookup’ and ‘scm_lookup’, but the specified module is
used instead of the current one.
-- C Function: SCM scm_module_variable (SCM MODULE, SCM NAME)
Like ‘scm_module_lookup’, but if the binding does not exist, just
returns ‘#f’ instead of raising an error.
To define a value, use ‘scm_define’:
-- C Function: SCM scm_c_define (const char *NAME, SCM VAL)
Bind the symbol indicated by NAME to a variable in the current
module and set that variable to VAL. When NAME is already bound to
a variable, use that. Else create a new variable.
-- C Function: SCM scm_define (SCM NAME, SCM VAL)
Like ‘scm_c_define’, but the symbol is specified directly.
-- C Function: SCM scm_c_module_define (SCM MODULE, const char *NAME,
SCM VAL)
-- C Function: SCM scm_module_define (SCM MODULE, SCM NAME, SCM VAL)
Like ‘scm_c_define’ and ‘scm_define’, but the specified module is
used instead of the current one.
In some rare cases, you may need to access the variable that
‘scm_module_define’ would have accessed, without changing the binding of
the existing variable, if one is present. In that case, use
‘scm_module_ensure_local_variable’:
-- C Function: SCM scm_module_ensure_local_variable (SCM MODULE, SCM
SYM)
Like ‘scm_module_define’, but if the SYM is already locally bound
in that module, the variable’s existing binding is not reset.
Returns a variable.
-- C Function: SCM scm_module_reverse_lookup (SCM MODULE, SCM VARIABLE)
Find the symbol that is bound to VARIABLE in MODULE. When no such
binding is found, return ‘#f’.
-- C Function: SCM scm_c_define_module (const char *NAME, void
(*INIT)(void *), void *DATA)
Define a new module named NAME and make it current while INIT is
called, passing it DATA. Return the module.
The parameter NAME is a string with the symbols that make up the
module name, separated by spaces. For example, ‘"foo bar"’ names
the module ‘(foo bar)’.
When there already exists a module named NAME, it is used
unchanged, otherwise, an empty module is created.
-- C Function: SCM scm_c_resolve_module (const char *NAME)
Find the module name NAME and return it. When it has not already
been defined, try to auto-load it. When it can’t be found that way
either, create an empty module. The name is interpreted as for
‘scm_c_define_module’.
-- C Function: SCM scm_c_use_module (const char *NAME)
Add the module named NAME to the uses list of the current module,
as with ‘(use-modules NAME)’. The name is interpreted as for
‘scm_c_define_module’.
-- C Function: void scm_c_export (const char *NAME, ...)
Add the bindings designated by NAME, ... to the public interface
of the current module. The list of names is terminated by ‘NULL’.
6.18.11 provide and require
---------------------------
Aubrey Jaffer, mostly to support his portable Scheme library SLIB,
implemented a provide/require mechanism for many Scheme implementations.
Library files in SLIB _provide_ a feature, and when user programs
_require_ that feature, the library file is loaded in.
For example, the file ‘random.scm’ in the SLIB package contains the
line
(provide 'random)
so to use its procedures, a user would type
(require 'random)
and they would magically become available, _but still have the same
names!_ So this method is nice, but not as good as a full-featured
module system.
When SLIB is used with Guile, provide and require can be used to
access its facilities.
6.18.12 Environments
--------------------
Scheme, as defined in R5RS, does _not_ have a full module system.
However it does define the concept of a top-level “environment”. Such
an environment maps identifiers (symbols) to Scheme objects such as
procedures and lists: *note About Closure::. In other words, it
implements a set of “bindings”.
Environments in R5RS can be passed as the second argument to ‘eval’
(*note Fly Evaluation::). Three procedures are defined to return
environments: ‘scheme-report-environment’, ‘null-environment’ and
‘interaction-environment’ (*note Fly Evaluation::).
In addition, in Guile any module can be used as an R5RS environment,
i.e., passed as the second argument to ‘eval’.
Note: the following two procedures are available only when the
‘(ice-9 r5rs)’ module is loaded:
(use-modules (ice-9 r5rs))
-- Scheme Procedure: scheme-report-environment version
-- Scheme Procedure: null-environment version
VERSION must be the exact integer ‘5’, corresponding to revision 5
of the Scheme report (the Revised^5 Report on Scheme).
‘scheme-report-environment’ returns a specifier for an environment
that is empty except for all bindings defined in the report that
are either required or both optional and supported by the
implementation. ‘null-environment’ returns a specifier for an
environment that is empty except for the (syntactic) bindings for
all syntactic keywords defined in the report that are either
required or both optional and supported by the implementation.
Currently Guile does not support values of VERSION for other
revisions of the report.
The effect of assigning (through the use of ‘eval’) a variable
bound in a ‘scheme-report-environment’ (for example ‘car’) is
unspecified. Currently the environments specified by
‘scheme-report-environment’ are not immutable in Guile.
6.19 Foreign Function Interface
===============================
Sometimes you need to use libraries written in C or Rust or some other
non-Scheme language. More rarely, you might need to write some C to
extend Guile. This section describes how to load these “foreign
libraries”, look up data and functions inside them, and so on.
6.19.1 Foreign Libraries
------------------------
Just as Guile can load up Scheme libraries at run-time, Guile can also
load some system libraries written in C or other low-level languages.
We refer to these as dynamically-loadable modules as “foreign
libraries”, to distinguish them from native libraries written in Scheme
or other languages implemented by Guile.
Foreign libraries usually come in two forms. Some foreign libraries
are part of the operating system, such as the compression library
‘libz’. These shared libraries are built in such a way that many
programs can use their functionality without duplicating their code.
When a program written in C is built, it can declare that it uses a
specific set of shared libraries. When the program is run, the
operating system takes care of locating and loading the shared
libraries.
The operating system components that can dynamically load and link
shared libraries when a program is run are also available
programmatically during a program’s execution. This is the interface
that’s most useful for Guile, and this is what we mean in Guile when we
refer to “dynamic linking”. Dynamic linking at run-time is sometimes
called “dlopening”, to distinguish it from the dynamic linking that
happens at program start-up.
The other kind of foreign library is sometimes known as a module,
plug-in, bundle, or an extension. These foreign libraries aren’t meant
to be linked to by C programs, but rather only to be dynamically loaded
at run-time – they extend some main program with functionality, but
don’t stand on their own. Sometimes a Guile library will implement some
of its functionality in a loadable module.
In either case, the interface on the Guile side is the same. You
load the interface using ‘load-foreign-library’. The resulting foreign
library object implements a simple lookup interface whereby the user can
get addresses of data or code exported by the library. There is no
facility to inspect foreign libraries; you have to know what’s in there
already before you look.
Routines for loading foreign libraries and accessing their contents
are implemented in the ‘(system foreign-library)’ module.
(use-modules (system foreign-library))
-- Scheme Procedure: load-foreign-library [library]
[#:extensions=system-library-extensions]
[#:search-ltdl-library-path?=#t] [#:search-path=search-path]
[#:search-system-paths?=#t] [#:lazy?=#t] [#:global=#f]
[#:rename-on-cygwin?=#t] Find the shared library denoted by LIBRARY
(a string or ‘#f’) and link it into the running Guile application.
When everything works out, return a Scheme object suitable for
representing the linked object file. Otherwise an error is thrown.
If LIBRARY argument is omitted, it defaults to ‘#f’. If ‘library’
is false, the resulting foreign library gives access to all symbols
available for dynamic linking in the main binary.
It is not necessary to include any extension such as ‘.so’ in
LIBRARY. For each system, Guile has a default set of extensions
that it will try. On GNU systems, the default extension set is
just ‘.so’; on Windows, just ‘.dll’; and on Darwin (Mac OS), it is
‘.bundle’, ‘.so’, and ‘.dylib’. Pass ‘#:extensions EXTENSIONS’ to
override the default extensions list. If LIBRARY contains one of
the extensions, no extensions are tried, so it is possible to
specify the extension if you know exactly what file to load.
Unless LIBRARY denotes an absolute file name or otherwise contains
a directory separator (‘/’, and also ‘\’ on Windows), Guile will
search for the library in the directories listed in SEARCH-PATHS.
The default search path has three components, which can all be
overriden by colon-delimited (semicolon on Windows) environment
variables:
‘GUILE_EXTENSIONS_PATH’
This is the main environment variable for users to add
directories containing Guile extensions. The default value
has no entries. This environment variable was added in Guile
3.0.6.
‘LTDL_LIBRARY_PATH’
Before Guile 3.0.6, Guile loaded foreign libraries using
‘libltdl’, the dynamic library loader provided by libtool.
This loader used ‘LTDL_LIBRARY_PATH’, and for backwards
compatibility we still support that path.
However, ‘libltdl’ would not only open ‘.so’ (or ‘.dll’ and so
on) files, but also the ‘.la’ files created by libtool. In
installed libraries – libraries that are in the target
directories of ‘make install’ – ‘.la’ files are never needed,
to the extent that most GNU/Linux distributions remove them
entirely. It is sufficient to just load the ‘.so’ (or ‘.dll’
and so on) files, which are always located in the same
directory as the ‘.la’ files.
But for uninstalled dynamic libraries, like those in a build
tree, the situation is a bit of a mess. If you have a project
that uses libtool to build libraries – which is the case for
Guile, and for most projects using autotools – and you build
‘foo.so’ in directory ‘D’, libtool will put ‘foo.la’ in ‘D’,
but ‘foo.so’ gets put into ‘D/.libs’.
Users were mostly oblivious to this situation, as ‘libltdl’
had special logic to be able to read the ‘.la’ file to know
where to find the ‘.so’, even from an uninstalled build tree,
preventing the existence of ‘.libs’ from leaking out to the
user.
We don’t use libltdl now, essentially for flexibility and
error-reporting reasons. But, to keep this old use-case
working, if SEARCH-LTDL-LIBRARY-PATH? is true, we add each
entry of ‘LTDL_LIBRARY_PATH’ to the default extensions load
path, additionally adding the ‘.libs’ subdirextories for each
entry, in case there are ‘.so’ files there instead of
alongside the ‘.la’ files.
‘GUILE_SYSTEM_EXTENSIONS_PATH’
The last path in Guile’s search path belongs to Guile itself,
and defaults to the libdir and the extensiondir, in that
order. For example, if you install to ‘/opt/guile’, these
would probably be ‘/opt/guile/lib’ and
‘/opt/guile/lib/guile/3.0/extensions’, respectively. *Note
Parallel Installations::, for more details on ‘extensionsdir’.
Finally, if no library is found in the search path, and if LIBRARY
is not absolute and does not include directory separators, and if
SEARCH-SYSTEM-PATHS? is true, the operating system may have its own
logic for where to locate LIBRARY. For example, on GNU, there will
be a default set of paths (often ‘/usr/lib’ and ‘/lib’, though it
depends on the system), and the ‘LD_LIBRARY_PATH’ environment
variable can add additional paths. Other operating systems have
other conventions.
Falling back to the operating system for search is usually not a
great thing; it is a recipe for making programs that work on one
machine but not on others. Still, when wrapping system libraries,
it can be the only way to get things working at all.
If LAZY? is true (the default), Guile will request the operating
system to resolve symbols used by the loaded library as they are
first used. If GLOBAL? is true, symbols defined by the loaded
library will be available when other modules need to resolve
symbols; the default is ‘#f’, which keeps symbols local.
If RENAME-ON-CYGWIN? is true (the default) – on Cygwin hosts only –
the search behavior is modified such that a filename that starts
with “lib” will be searched for under the name “cyg”, as is
customary for Cygwin.
The environment variables mentioned above are parsed when the
foreign-library module is first loaded and bound to parameters. Null
path components, for example the three components of
‘GUILE_SYSTEM_EXTENSIONS_PATH="::"’, are ignored.
-- Scheme Parameter: guile-extensions-path
-- Scheme Parameter: ltdl-library-path
-- Scheme Parameter: guile-system-extensions-path
Parameters whose initial values are taken from
‘GUILE_EXTENSIONS_PATH’, ‘LTDL_LIBRARY_PATH’, and
‘GUILE_SYSTEM_EXTENSIONS_PATH’, respectively. *Note Parameters::.
The current values of these parameters are used when building the
search path when ‘load-foreign-library’ is called, unless the
caller explicitly passes a ‘#:search-path’ argument.
-- Scheme Procedure: foreign-library? obj
Return ‘#t’ if OBJ is a foreign library, or ‘#f’ otherwise.
6.19.2 Foreign Extensions
-------------------------
One way to use shared libraries is to extend Guile. Such loadable
modules generally define one distinguished initialization function that,
when called, will use the ‘libguile’ API to define procedures in the
current module.
Concretely, you might extend Guile with an implementation of the
Bessel function, ‘j0’:
#include
#include
SCM
j0_wrapper (SCM x)
{
return scm_from_double (j0 (scm_to_double (x, "j0")));
}
void
init_math_bessel (void)
{
scm_c_define_gsubr ("j0", 1, 0, 0, j0_wrapper);
}
The C source file would then need to be compiled into a shared
library. On GNU/Linux, the compiler invocation might look like this:
gcc -shared -o bessel.so -fPIC bessel.c
A good default place to put shared libraries that extend Guile is
into the extensions dir. From the command line or a build script,
invoke ‘pkg-config --variable=extensionsdir guile-3.0’ to print the
extensions dir. *Note Parallel Installations::, for more details.
Guile can load up ‘bessel.so’ via ‘load-extension’.
-- Scheme Procedure: load-extension lib init
-- C Function: scm_load_extension (lib, init)
Load and initialize the extension designated by LIB and INIT.
The normal way for a extension to be used is to write a small Scheme
file that defines a module, and to load the extension into this module.
When the module is auto-loaded, the extension is loaded as well. For
example:
(define-module (math bessel)
#:export (j0))
(load-extension "bessel" "init_math_bessel")
This ‘load-extension’ invocation loads the ‘bessel’ library via
‘(load-foreign-library "bessel")’, then looks up the ‘init_math_bessel’
symbol in the library, treating it as a function of no arguments, and
calls that function.
If you decide to put your extension outside the default search path
for ‘load-foreign-library’, probably you should adapt the Scheme module
to specify its absolute path. For example, if you use ‘automake’ to
build your extension and place it in ‘$(pkglibdir)’, you might define a
build-parameters module that gets created by the build system:
(define-module (math config)
#:export (extensiondir))
(define extensiondir "PKGLIBDIR")
This file would be ‘config.scm.in’. You would define a ‘make’ rule
to substitute in the absolute installed file name:
config.scm: config.scm.in
sed 's|PKGLIBDIR|$(pkglibdir)|' <$< >$
Then your ‘(math bessel)’ would import ‘(math config)’, then
‘(load-extension (in-vicinity extensiondir "bessel")
"init_math_bessel")’.
An alternate approach would be to rebind the ‘guile-extensions-path’
parameter, or its corresponding environment variable, but note that
changing those parameters applies to other users of
‘load-foreign-library’ as well.
Note that the new primitives that the extension adds to Guile with
‘scm_c_define_gsubr’ (*note Primitive Procedures::) or with any of the
other mechanisms are placed into the module that is current when the
‘scm_c_define_gsubr’ is executed, so to be clear about what goes vwhere
it’s best to include the ‘load-extension’ in a module, as above.
Alternately, the C code can use ‘scm_c_define_module’ to specify which
module is being created:
static void
do_init (void *unused)
{
scm_c_define_gsubr ("j0", 1, 0, 0, j0_wrapper);
scm_c_export ("j0", NULL);
}
void
init_math_bessel ()
{
scm_c_define_module ("math bessel", do_init, NULL);
}
And yet... if what we want is just the ‘j0’ function, it seems like
a lot of ceremony to have to compile a Guile-specific wrapper library
complete with an initialization function and wraper module to allow
Guile users to call it. There is another way, but to get there, we have
to talk about function pointers and function types first. *Note Foreign
Functions::, to skip to the good parts.
6.19.3 Foreign Pointers
-----------------------
Foreign libraries are essentially key-value mappings, where the keys are
names of definitions and the values are the addresses of those
definitions. To look up the address of a definition, use
‘foreign-library-pointer’ from the ‘(system foreign-library)’ module.
-- Scheme Procedure: foreign-library-pointer lib name
Return a “wrapped pointer” for the symbol NAME in the shared object
referred to by LIB. The returned pointer points to a C object.
As a convenience, if LIB is not a foreign library, it will be
passed to ‘load-foreign-library’.
If we continue with the ‘bessel.so’ example from before, we can get
the address of the ‘init_math_bessel’ function via:
(use-modules (system foreign-library))
(define init (foreign-library-pointer "bessel" "init_math_bessel"))
init
⇒ #
A value returned by ‘foreign-library-pointer’ is a Scheme wrapper for
a C pointer. Pointers are a data type in Guile that is disjoint from
all other types. The next section discusses ways to dereference
pointers, but before then we describe the usual type predicates and so
on.
Note that the rest of the interfaces in this section are part of the
‘(system foreign)’ library:
(use-modules (system foreign))
-- Scheme Procedure: pointer-address pointer
-- C Function: scm_pointer_address (pointer)
Return the numerical value of POINTER.
(pointer-address init)
⇒ 139984413364296 ; YMMV
-- Scheme Procedure: make-pointer address [finalizer]
Return a foreign pointer object pointing to ADDRESS. If FINALIZER
is passed, it should be a pointer to a one-argument C function that
will be called when the pointer object becomes unreachable.
-- Scheme Procedure: pointer? obj
Return ‘#t’ if OBJ is a pointer object, or ‘#f’ otherwise.
-- Scheme Variable: %null-pointer
A foreign pointer whose value is 0.
-- Scheme Procedure: null-pointer? pointer
Return ‘#t’ if POINTER is the null pointer, ‘#f’ otherwise.
For the purpose of passing SCM values directly to foreign functions,
and allowing them to return SCM values, Guile also supports some unsafe
casting operators.
-- Scheme Procedure: scm->pointer scm
Return a foreign pointer object with the ‘object-address’ of SCM.
-- Scheme Procedure: pointer->scm pointer
Unsafely cast POINTER to a Scheme object. Cross your fingers!
Sometimes you want to give C extensions access to the dynamic FFI. At
that point, the names get confusing, because “pointer” can refer to a
‘SCM’ object that wraps a pointer, or to a ‘void*’ value. We will try
to use “pointer object” to refer to Scheme objects, and “pointer value”
to refer to ‘void *’ values.
-- C Function: SCM scm_from_pointer (void *ptr, void (*finalizer)
(void*))
Create a pointer object from a pointer value.
If FINALIZER is non-null, Guile arranges to call it on the pointer
value at some point after the pointer object becomes collectable.
-- C Function: void* scm_to_pointer (SCM obj)
Unpack the pointer value from a pointer object.
6.19.4 Foreign Types
--------------------
From Scheme’s perspective, foreign pointers are shards of chaos. The
user can create a foreign pointer for any address, and do with it what
they will. The only thing that lends a sense of order to the whole is a
shared hallucination that certain storage locations have certain types.
When making Scheme wrappers for foreign interfaces, we hide the madness
by explicitly representing the the data types of parameters and fields.
These “foreign type values” may be constructed using the constants
and procedures from the ‘(system foreign)’ module, which may be loaded
like this:
(use-modules (system foreign))
‘(system foreign)’ exports a number of values expressing the basic C
types.
-- Scheme Variable: int8
-- Scheme Variable: uint8
-- Scheme Variable: uint16
-- Scheme Variable: int16
-- Scheme Variable: uint32
-- Scheme Variable: int32
-- Scheme Variable: uint64
-- Scheme Variable: int64
-- Scheme Variable: float
-- Scheme Variable: double
-- Scheme Variable: complex-double
-- Scheme Variable: complex-float
These values represent the C numeric types of the specified sizes
and signednesses. ‘complex-float’ and ‘complex-double’ stand for
C99 ‘float _Complex’ and ‘double _Complex’ respecively.
In addition there are some convenience bindings for indicating types
of platform-dependent size.
-- Scheme Variable: int
-- Scheme Variable: unsigned-int
-- Scheme Variable: long
-- Scheme Variable: unsigned-long
-- Scheme Variable: short
-- Scheme Variable: unsigned-short
-- Scheme Variable: size_t
-- Scheme Variable: ssize_t
-- Scheme Variable: ptrdiff_t
-- Scheme Variable: intptr_t
-- Scheme Variable: uintptr_t
Values exported by the ‘(system foreign)’ module, representing C
numeric types. For example, ‘long’ may be ‘equal?’ to ‘int64’ on a
64-bit platform.
-- Scheme Variable: void
The ‘void’ type. It can be used as the first argument to
‘pointer->procedure’ to wrap a C function that returns nothing.
In addition, the symbol ‘*’ is used by convention to denote pointer
types. Procedures detailed in the following sections, such as
‘pointer->procedure’, accept it as a type descriptor.
6.19.5 Foreign Functions
------------------------
The most natural thing to do with a dynamic library is to grovel around
in it for a function pointer: a “foreign function”. Load the ‘(system
foreign)’ module to use these Scheme interfaces.
(use-modules (system foreign))
-- Scheme Procedure: pointer->procedure return_type func_ptr arg_types
[#:return-errno?=#f]
-- C Function: scm_pointer_to_procedure (return_type, func_ptr,
arg_types)
-- C Function: scm_pointer_to_procedure_with_errno (return_type,
func_ptr, arg_types)
Make a foreign function.
Given the foreign void pointer FUNC_PTR, its argument and return
types ARG_TYPES and RETURN_TYPE, return a procedure that will pass
arguments to the foreign function and return appropriate values.
ARG_TYPES should be a list of foreign types. ‘return_type’ should
be a foreign type. *Note Foreign Types::, for more information on
foreign types.
If RETURN-ERRNO? is true, or when calling
‘scm_pointer_to_procedure_with_errno’, the returned procedure will
return two values, with ‘errno’ as the second value.
Finally, in ‘(system foreign-library)’ there is a convenient wrapper
function, joining together ‘foreign-libary-pointer’ and
‘procedure->pointer’:
-- Scheme Procedure: foreign-library-function lib name
[#:return-type=void] [#:arg-types='()] [#:return-errno?=#f]
Load the address of NAME from LIB, and treat it as a function
taking arguments ARG-TYPES and returning RETURN-TYPE, optionally
also with errno.
An invocation of ‘foreign-library-function’ is entirely equivalent
to:
(pointer->procedure RETURN-TYPE
(foreign-library-pointer LIB NAME)
ARG-TYPES
#:return-errno? RETURN-ERRNO?).
Pulling all this together, here is a better definition of ‘(math
bessel)’:
(define-module (math bessel)
#:use-module (system foreign)
#:use-module (system foreign-library)
#:export (j0))
(define j0
(foreign-library-function "libm" "j0"
#:return-type double
#:arg-types (list double)))
That’s it! No C at all.
Before going on to more detailed examples, the next two sections
discuss how to deal with data that is more complex than, say, ‘int8’.
*Note More Foreign Functions::, to continue with foreign function
examples.
6.19.6 Void Pointers and Byte Access
------------------------------------
Wrapped pointers are untyped, so they are essentially equivalent to C
‘void’ pointers. As in C, the memory region pointed to by a pointer can
be accessed at the byte level. This is achieved using _bytevectors_
(*note Bytevectors::). The ‘(rnrs bytevectors)’ module contains
procedures that can be used to convert byte sequences to Scheme objects
such as strings, floating point numbers, or integers.
Load the ‘(system foreign)’ module to use these Scheme interfaces.
(use-modules (system foreign))
-- Scheme Procedure: pointer->bytevector pointer len [offset
[uvec_type]]
-- C Function: scm_pointer_to_bytevector (pointer, len, offset,
uvec_type)
Return a bytevector aliasing the LEN bytes pointed to by POINTER.
The user may specify an alternate default interpretation for the
memory by passing the UVEC_TYPE argument, to indicate that the
memory is an array of elements of that type. UVEC_TYPE should be
something that ‘array-type’ would return, like ‘f32’ or ‘s16’.
When OFFSET is passed, it specifies the offset in bytes relative to
POINTER of the memory region aliased by the returned bytevector.
Mutating the returned bytevector mutates the memory pointed to by
POINTER, so buckle your seatbelts.
-- Scheme Procedure: bytevector->pointer bv [offset]
-- C Function: scm_bytevector_to_pointer (bv, offset)
Return a pointer aliasing the memory pointed to by BV or OFFSET
bytes after BV when OFFSET is passed.
In addition to these primitives, convenience procedures are
available:
-- Scheme Procedure: dereference-pointer pointer
Assuming POINTER points to a memory region that holds a pointer,
return this pointer.
-- Scheme Procedure: string->pointer string [encoding]
Return a foreign pointer to a nul-terminated copy of STRING in the
given ENCODING, defaulting to the current locale encoding. The C
string is freed when the returned foreign pointer becomes
unreachable.
This is the Scheme equivalent of ‘scm_to_stringn’.
-- Scheme Procedure: pointer->string pointer [length] [encoding]
Return the string representing the C string pointed to by POINTER.
If LENGTH is omitted or ‘-1’, the string is assumed to be
nul-terminated. Otherwise LENGTH is the number of bytes in memory
pointed to by POINTER. The C string is assumed to be in the given
ENCODING, defaulting to the current locale encoding.
This is the Scheme equivalent of ‘scm_from_stringn’.
Most object-oriented C libraries use pointers to specific data
structures to identify objects. It is useful in such cases to reify the
different pointer types as disjoint Scheme types. The
‘define-wrapped-pointer-type’ macro simplifies this.
-- Scheme Syntax: define-wrapped-pointer-type type-name pred wrap
unwrap print
Define helper procedures to wrap pointer objects into Scheme
objects with a disjoint type. Specifically, this macro defines:
• PRED, a predicate for the new Scheme type;
• WRAP, a procedure that takes a pointer object and returns an
object that satisfies PRED;
• UNWRAP, which does the reverse.
WRAP preserves pointer identity, for two pointer objects P1 and P2
that are ‘equal?’, ‘(eq? (WRAP P1) (WRAP P2)) ⇒ #t’.
Finally, PRINT should name a user-defined procedure to print such
objects. The procedure is passed the wrapped object and a port to
write to.
For example, assume we are wrapping a C library that defines a
type, ‘bottle_t’, and functions that can be passed ‘bottle_t *’
pointers to manipulate them. We could write:
(define-wrapped-pointer-type bottle
bottle?
wrap-bottle unwrap-bottle
(lambda (b p)
(format p "#"
(bottle-contents b)
(pointer-address (unwrap-bottle b)))))
(define grab-bottle
;; Wrapper for `bottle_t *grab (void)'.
(let ((grab (foreign-library-function libbottle "grab_bottle"
#:return-type '*)))
(lambda ()
"Return a new bottle."
(wrap-bottle (grab)))))
(define bottle-contents
;; Wrapper for `const char *bottle_contents (bottle_t *)'.
(let ((contents (foreign-library-function libbottle "bottle_contents"
#:return-type '*
#:arg-types '(*))))
(lambda (b)
"Return the contents of B."
(pointer->string (contents (unwrap-bottle b))))))
(write (grab-bottle))
⇒ #
In this example, ‘grab-bottle’ is guaranteed to return a genuine
‘bottle’ object satisfying ‘bottle?’. Likewise, ‘bottle-contents’
errors out when its argument is not a genuine ‘bottle’ object.
As another example, currently Guile has a variable, ‘scm_numptob’, as
part of its API. It is declared as a C ‘long’. So, to read its value,
we can do:
(use-modules (system foreign))
(use-modules (rnrs bytevectors))
(define numptob
(foreign-library-pointer #f "scm_numptob"))
numptob
(bytevector-uint-ref (pointer->bytevector numptob (sizeof long))
0 (native-endianness)
(sizeof long))
⇒ 8
If we wanted to corrupt Guile’s internal state, we could set
‘scm_numptob’ to another value; but we shouldn’t, because that variable
is not meant to be set. Indeed this point applies more widely: the C
API is a dangerous place to be. Not only might setting a value crash
your program, simply accessing the data pointed to by a dangling pointer
or similar can prove equally disastrous.
6.19.7 Foreign Structs
----------------------
Finally, one last note on foreign values before moving on to actually
calling foreign functions. Sometimes you need to deal with C structs,
which requires interpreting each element of the struct according to the
its type, offset, and alignment. The ‘(system foreign)’ module has some
primitives to support this.
(use-modules (system foreign))
-- Scheme Procedure: sizeof type
-- C Function: scm_sizeof (type)
Return the size of TYPE, in bytes.
TYPE should be a valid C type, like ‘int’. Alternately TYPE may be
the symbol ‘*’, in which case the size of a pointer is returned.
TYPE may also be a list of types, in which case the size of a
‘struct’ with ABI-conventional packing is returned.
-- Scheme Procedure: alignof type
-- C Function: scm_alignof (type)
Return the alignment of TYPE, in bytes.
TYPE should be a valid C type, like ‘int’. Alternately TYPE may be
the symbol ‘*’, in which case the alignment of a pointer is
returned. TYPE may also be a list of types, in which case the
alignment of a ‘struct’ with ABI-conventional packing is returned.
Guile also provides some convenience methods to pack and unpack
foreign pointers wrapping C structs.
-- Scheme Procedure: make-c-struct types vals
Create a foreign pointer to a C struct containing VALS with types
‘types’.
VALS and ‘types’ should be lists of the same length.
-- Scheme Procedure: parse-c-struct foreign types
Parse a foreign pointer to a C struct, returning a list of values.
‘types’ should be a list of C types.
For example, to create and parse the equivalent of a ‘struct {
int64_t a; uint8_t b; }’:
(parse-c-struct (make-c-struct (list int64 uint8)
(list 300 43))
(list int64 uint8))
⇒ (300 43)
As yet, Guile only has convenience routines to support
conventionally-packed structs. But given the ‘bytevector->pointer’ and
‘pointer->bytevector’ routines, one can create and parse tightly packed
structs and unions by hand. See the code for ‘(system foreign)’ for
details.
6.19.8 More Foreign Functions
-----------------------------
It is possible to pass pointers to foreign functions, and to return them
as well. In that case the type of the argument or return value should
be the symbol ‘*’, indicating a pointer. For example, the following
code makes ‘memcpy’ available to Scheme:
(use-modules (system foreign))
(define memcpy
(foreign-library-function #f "memcpy"
#:return-type '*
#:arg-types (list '* '* size_t)))
To invoke ‘memcpy’, one must pass it foreign pointers:
(use-modules (rnrs bytevectors))
(define src-bits
(u8-list->bytevector '(0 1 2 3 4 5 6 7)))
(define src
(bytevector->pointer src-bits))
(define dest
(bytevector->pointer (make-bytevector 16 0)))
(memcpy dest src (bytevector-length src-bits))
(bytevector->u8-list (pointer->bytevector dest 16))
⇒ (0 1 2 3 4 5 6 7 0 0 0 0 0 0 0 0)
One may also pass structs as values, passing structs as foreign
pointers. *Note Foreign Structs::, for more information on how to
express struct types and struct values.
“Out” arguments are passed as foreign pointers. The memory pointed
to by the foreign pointer is mutated in place.
;; struct timeval {
;; time_t tv_sec; /* seconds */
;; suseconds_t tv_usec; /* microseconds */
;; };
;; assuming fields are of type "long"
(define gettimeofday
(let ((f (foreign-library-function #f "gettimeofday"
#:return-type int
#:arg-types (list '* '*)))
(tv-type (list long long)))
(lambda ()
(let* ((timeval (make-c-struct tv-type (list 0 0)))
(ret (f timeval %null-pointer)))
(if (zero? ret)
(apply values (parse-c-struct timeval tv-type))
(error "gettimeofday returned an error" ret))))))
(gettimeofday)
⇒ 1270587589
⇒ 499553
As you can see, this interface to foreign functions is at a very low,
somewhat dangerous level(1).
The FFI can also work in the opposite direction: making Scheme
procedures callable from C. This makes it possible to use Scheme
procedures as “callbacks” expected by C function.
-- Scheme Procedure: procedure->pointer return-type proc arg-types
-- C Function: scm_procedure_to_pointer (return_type, proc, arg_types)
Return a pointer to a C function of type RETURN-TYPE taking
arguments of types ARG-TYPES (a list) and behaving as a proxy to
procedure PROC. Thus PROC’s arity, supported argument types, and
return type should match RETURN-TYPE and ARG-TYPES.
As an example, here’s how the C library’s ‘qsort’ array sorting
function can be made accessible to Scheme (*note ‘qsort’: (libc)Array
Sort Function.):
(define qsort!
(let ((qsort (foreign-library-function
#f "qsort" #:arg-types (list '* size_t size_t '*))))
(lambda (bv compare)
;; Sort bytevector BV in-place according to comparison
;; procedure COMPARE.
(let ((ptr (procedure->pointer int
(lambda (x y)
;; X and Y are pointers so,
;; for convenience, dereference
;; them before calling COMPARE.
(compare (dereference-uint8* x)
(dereference-uint8* y)))
(list '* '*))))
(qsort (bytevector->pointer bv)
(bytevector-length bv) 1 ;; we're sorting bytes
ptr)))))
(define (dereference-uint8* ptr)
;; Helper function: dereference the byte pointed to by PTR.
(let ((b (pointer->bytevector ptr 1)))
(bytevector-u8-ref b 0)))
(define bv
;; An unsorted array of bytes.
(u8-list->bytevector '(7 1 127 3 5 4 77 2 9 0)))
;; Sort BV.
(qsort! bv (lambda (x y) (- x y)))
;; Let's see what the sorted array looks like:
(bytevector->u8-list bv)
⇒ (0 1 2 3 4 5 7 9 77 127)
And voilà!
Note that ‘procedure->pointer’ is not supported (and not defined) on
a few exotic architectures. Thus, user code may need to check
‘(defined? 'procedure->pointer)’. Nevertheless, it is available on many
architectures, including (as of libffi 3.0.9) x86, ia64, SPARC, PowerPC,
ARM, and MIPS, to name a few.
---------- Footnotes ----------
(1) A contribution to Guile in the form of a high-level FFI would be
most welcome.
6.20 Foreign Objects
====================
This chapter contains reference information related to defining and
working with foreign objects. *Note Defining New Foreign Object
Types::, for a tutorial-like introduction to foreign objects.
-- C Type: scm_t_struct_finalize
This function type returns ‘void’ and takes one ‘SCM’ argument.
-- C Function: SCM scm_make_foreign_object_type (SCM name, SCM slots,
scm_t_struct_finalize finalizer)
Create a fresh foreign object type. NAME is a symbol naming the
type. SLOTS is a list of symbols, each one naming a field in the
foreign object type. FINALIZER indicates the finalizer, and may be
‘NULL’.
We recommend that finalizers be avoided if possible. *Note Foreign
Object Memory Management::. Finalizers must be async-safe and
thread-safe. Again, *note Foreign Object Memory Management::. If you
are embedding Guile in an application that is not thread-safe, and you
define foreign object types that need finalization, you might want to
disable automatic finalization, and arrange to call
‘scm_manually_run_finalizers ()’ yourself.
-- C Function: int scm_set_automatic_finalization_enabled (int
enabled_p)
Enable or disable automatic finalization. By default, Guile
arranges to invoke object finalizers automatically, in a separate
thread if possible. Passing a zero value for ENABLED_P will
disable automatic finalization for Guile as a whole. If you
disable automatic finalization, you will have to call
‘scm_run_finalizers ()’ periodically.
Unlike most other Guile functions, you can call
‘scm_set_automatic_finalization_enabled’ before Guile has been
initialized.
Return the previous status of automatic finalization.
-- C Function: int scm_run_finalizers (void)
Invoke any pending finalizers. Returns the number of finalizers
that were invoked. This function should be called when automatic
finalization is disabled, though it may be called if it is enabled
as well.
-- C Function: void scm_assert_foreign_object_type (SCM type, SCM val)
When VAL is a foreign object of the given TYPE, do nothing.
Otherwise, signal an error.
-- C Function: SCM scm_make_foreign_object_0 (SCM type)
-- C Function: SCM scm_make_foreign_object_1 (SCM type, void *val0)
-- C Function: SCM scm_make_foreign_object_2 (SCM type, void *val0,
void *val1)
-- C Function: SCM scm_make_foreign_object_3 (SCM type, void *val0,
void *val1, void *val2)
-- C Function: SCM scm_make_foreign_object_n (SCM type, size_t n, void
*vals[])
Make a new foreign object of the type with type TYPE and initialize
the first N fields to the given values, as appropriate.
The number of fields for objects of a given type is fixed when the
type is created. It is an error to give more initializers than
there are fields in the value. It is perfectly fine to give fewer
initializers than needed; this is convenient when some fields are
of non-pointer types, and would be easier to initialize with the
setters described below.
-- C Function: void* scm_foreign_object_ref (SCM obj, size_t n);
-- C Function: scm_t_bits scm_foreign_object_unsigned_ref (SCM obj,
size_t n);
-- C Function: scm_t_signed_bits scm_foreign_object_signed_ref (SCM
obj, size_t n);
Return the value of the Nth field of the foreign object OBJ. The
backing store for the fields is as wide as a ‘scm_t_bits’ value,
which is at least as wide as a pointer. The different variants
handle casting in a portable way.
-- C Function: void scm_foreign_object_set_x (SCM obj, size_t n, void
*val);
-- C Function: void scm_foreign_object_unsigned_set_x (SCM obj, size_t
n, scm_t_bits val);
-- C Function: void scm_foreign_object_signed_set_x (SCM obj, size_t n,
scm_t_signed_bits val);
Set the value of the Nth field of the foreign object OBJ to VAL,
after portably converting to a ‘scm_t_bits’ value, if needed.
One can also access foreign objects from Scheme. *Note Foreign
Objects and Scheme::, for some examples.
(use-modules (system foreign-object))
-- Scheme Procedure: make-foreign-object-type name slots
[#:finalizer=#f]
Make a new foreign object type. See the above documentation for
‘scm_make_foreign_object_type’; these functions are exactly
equivalent, except for the way in which the finalizer gets attached
to instances (an internal detail).
The resulting value is a GOOPS class. *Note GOOPS::, for more on
classes in Guile.
-- Scheme Syntax: define-foreign-object-type name constructor (slot
...) [#:finalizer=#f]
A convenience macro to define a type, using
‘make-foreign-object-type’, and bind it to NAME. A constructor
will be bound to CONSTRUCTOR, and getters will be bound to each of
SLOT....
6.21 Smobs
==========
A “smob” is a “small object”. Before foreign objects were introduced in
Guile 2.0.12 (*note Foreign Objects::), smobs were the preferred way to
for C code to define new kinds of Scheme objects. With the exception of
the so-called “applicable SMOBs” discussed below, smobs are now a legacy
interface and are headed for eventual deprecation. *Note Deprecation::.
New code should use the foreign object interface.
This section contains reference information related to defining and
working with smobs. For a tutorial-like introduction to smobs, see
“Defining New Types (Smobs)” in previous versions of this manual.
-- Function: scm_t_bits scm_make_smob_type (const char *name, size_t
size)
This function adds a new smob type, named NAME, with instance size
SIZE, to the system. The return value is a tag that is used in
creating instances of the type.
If SIZE is 0, the default _free_ function will do nothing.
If SIZE is not 0, the default _free_ function will deallocate the
memory block pointed to by ‘SCM_SMOB_DATA’ with ‘scm_gc_free’. The
WHAT parameter in the call to ‘scm_gc_free’ will be NAME.
Default values are provided for the _mark_, _free_, _print_, and
_equalp_ functions. If you want to customize any of these
functions, the call to ‘scm_make_smob_type’ should be immediately
followed by calls to one or several of ‘scm_set_smob_mark’,
‘scm_set_smob_free’, ‘scm_set_smob_print’, and/or
‘scm_set_smob_equalp’.
-- C Function: void scm_set_smob_free (scm_t_bits tc, size_t (*free)
(SCM obj))
This function sets the smob freeing procedure (sometimes referred
to as a “finalizer”) for the smob type specified by the tag TC. TC
is the tag returned by ‘scm_make_smob_type’.
The FREE procedure must deallocate all resources that are directly
associated with the smob instance OBJ. It must assume that all
‘SCM’ values that it references have already been freed and are
thus invalid.
It must also not call any libguile function or macro except
‘scm_gc_free’, ‘SCM_SMOB_FLAGS’, ‘SCM_SMOB_DATA’,
‘SCM_SMOB_DATA_2’, and ‘SCM_SMOB_DATA_3’.
The FREE procedure must return 0.
Note that defining a freeing procedure is not necessary if the
resources associated with OBJ consists only of memory allocated
with ‘scm_gc_malloc’ or ‘scm_gc_malloc_pointerless’ because this
memory is automatically reclaimed by the garbage collector when it
is no longer needed (*note ‘scm_gc_malloc’: Memory Blocks.).
Smob free functions must be thread-safe. *Note Foreign Object Memory
Management::, for a discussion on finalizers and concurrency. If you
are embedding Guile in an application that is not thread-safe, and you
define smob types that need finalization, you might want to disable
automatic finalization, and arrange to call ‘scm_manually_run_finalizers
()’ yourself. *Note Foreign Objects::.
-- C Function: void scm_set_smob_mark (scm_t_bits tc, SCM (*mark) (SCM
obj))
This function sets the smob marking procedure for the smob type
specified by the tag TC. TC is the tag returned by
‘scm_make_smob_type’.
Defining a marking procedure is almost always the wrong thing to
do. It is much, much preferable to allocate smob data with the
‘scm_gc_malloc’ and ‘scm_gc_malloc_pointerless’ functions, and
allow the GC to trace pointers automatically.
Any mark procedures you see currently almost surely date from the
time of Guile 1.8, before the switch to the Boehm-Demers-Weiser
collector. Such smob implementations should be changed to just use
‘scm_gc_malloc’ and friends, and to lose their mark function.
If you decide to keep the mark function, note that it may be called
on objects that are on the free list. Please read and digest the
comments from the BDW GC’s ‘gc/gc_mark.h’ header.
The MARK procedure must cause ‘scm_gc_mark’ to be called for every
‘SCM’ value that is directly referenced by the smob instance OBJ.
One of these ‘SCM’ values can be returned from the procedure and
Guile will call ‘scm_gc_mark’ for it. This can be used to avoid
deep recursions for smob instances that form a list.
It must not call any libguile function or macro except
‘scm_gc_mark’, ‘SCM_SMOB_FLAGS’, ‘SCM_SMOB_DATA’,
‘SCM_SMOB_DATA_2’, and ‘SCM_SMOB_DATA_3’.
-- C Function: void scm_set_smob_print (scm_t_bits tc, int (*print)
(SCM obj, SCM port, scm_print_state* pstate))
This function sets the smob printing procedure for the smob type
specified by the tag TC. TC is the tag returned by
‘scm_make_smob_type’.
The PRINT procedure should output a textual representation of the
smob instance OBJ to PORT, using information in PSTATE.
The textual representation should be of the form ‘#’.
This ensures that ‘read’ will not interpret it as some other Scheme
value.
It is often best to ignore PSTATE and just print to PORT with
‘scm_display’, ‘scm_write’, ‘scm_simple_format’, and ‘scm_puts’.
-- C Function: void scm_set_smob_equalp (scm_t_bits tc, SCM (*equalp)
(SCM obj1, SCM obj2))
This function sets the smob equality-testing predicate for the smob
type specified by the tag TC. TC is the tag returned by
‘scm_make_smob_type’.
The EQUALP procedure should return ‘SCM_BOOL_T’ when OBJ1 is
‘equal?’ to OBJ2. Else it should return ‘SCM_BOOL_F’. Both OBJ1
and OBJ2 are instances of the smob type TC.
-- C Function: void scm_assert_smob_type (scm_t_bits tag, SCM val)
When VAL is a smob of the type indicated by TAG, do nothing. Else,
signal an error.
-- C Macro: int SCM_SMOB_PREDICATE (scm_t_bits tag, SCM exp)
Return true if EXP is a smob instance of the type indicated by TAG,
or false otherwise. The expression EXP can be evaluated more than
once, so it shouldn’t contain any side effects.
-- C Function: SCM scm_new_smob (scm_t_bits tag, void *data)
-- C Function: SCM scm_new_double_smob (scm_t_bits tag, void *data,
void *data2, void *data3)
Make a new smob of the type with tag TAG and smob data DATA, DATA2,
and DATA3, as appropriate.
The TAG is what has been returned by ‘scm_make_smob_type’. The
initial values DATA, DATA2, and DATA3 are of type ‘scm_t_bits’;
when you want to use them for ‘SCM’ values, these values need to be
converted to a ‘scm_t_bits’ first by using ‘SCM_UNPACK’.
The flags of the smob instance start out as zero.
-- C Macro: scm_t_bits SCM_SMOB_FLAGS (SCM obj)
Return the 16 extra bits of the smob OBJ. No meaning is predefined
for these bits, you can use them freely.
-- C Macro: scm_t_bits SCM_SET_SMOB_FLAGS (SCM obj, scm_t_bits flags)
Set the 16 extra bits of the smob OBJ to FLAGS. No meaning is
predefined for these bits, you can use them freely.
-- C Macro: scm_t_bits SCM_SMOB_DATA (SCM obj)
-- C Macro: scm_t_bits SCM_SMOB_DATA_2 (SCM obj)
-- C Macro: scm_t_bits SCM_SMOB_DATA_3 (SCM obj)
Return the first (second, third) immediate word of the smob OBJ as
a ‘scm_t_bits’ value. When the word contains a ‘SCM’ value, use
‘SCM_SMOB_OBJECT’ (etc.) instead.
-- C Macro: void SCM_SET_SMOB_DATA (SCM obj, scm_t_bits val)
-- C Macro: void SCM_SET_SMOB_DATA_2 (SCM obj, scm_t_bits val)
-- C Macro: void SCM_SET_SMOB_DATA_3 (SCM obj, scm_t_bits val)
Set the first (second, third) immediate word of the smob OBJ to
VAL. When the word should be set to a ‘SCM’ value, use
‘SCM_SMOB_SET_OBJECT’ (etc.) instead.
-- C Macro: SCM SCM_SMOB_OBJECT (SCM obj)
-- C Macro: SCM SCM_SMOB_OBJECT_2 (SCM obj)
-- C Macro: SCM SCM_SMOB_OBJECT_3 (SCM obj)
Return the first (second, third) immediate word of the smob OBJ as
a ‘SCM’ value. When the word contains a ‘scm_t_bits’ value, use
‘SCM_SMOB_DATA’ (etc.) instead.
-- C Macro: void SCM_SET_SMOB_OBJECT (SCM obj, SCM val)
-- C Macro: void SCM_SET_SMOB_OBJECT_2 (SCM obj, SCM val)
-- C Macro: void SCM_SET_SMOB_OBJECT_3 (SCM obj, SCM val)
Set the first (second, third) immediate word of the smob OBJ to
VAL. When the word should be set to a ‘scm_t_bits’ value, use
‘SCM_SMOB_SET_DATA’ (etc.) instead.
-- C Macro: SCM * SCM_SMOB_OBJECT_LOC (SCM obj)
-- C Macro: SCM * SCM_SMOB_OBJECT_2_LOC (SCM obj)
-- C Macro: SCM * SCM_SMOB_OBJECT_3_LOC (SCM obj)
Return a pointer to the first (second, third) immediate word of the
smob OBJ. Note that this is a pointer to ‘SCM’. If you need to
work with ‘scm_t_bits’ values, use ‘SCM_PACK’ and ‘SCM_UNPACK’, as
appropriate.
-- Function: SCM scm_markcdr (SCM X)
Mark the references in the smob X, assuming that X’s first data
word contains an ordinary Scheme object, and X refers to no other
objects. This function simply returns X’s first data word.
6.22 Threads, Mutexes, Asyncs and Dynamic Roots
===============================================
6.22.1 Threads
--------------
Guile supports POSIX threads, unless it was configured with
‘--without-threads’ or the host lacks POSIX thread support. When thread
support is available, the ‘threads’ feature is provided (*note
‘provided?’: Feature Manipulation.).
The procedures below manipulate Guile threads, which are wrappers
around the system’s POSIX threads. For application-level parallelism,
using higher-level constructs, such as futures, is recommended (*note
Futures::).
To use these facilities, load the ‘(ice-9 threads)’ module.
(use-modules (ice-9 threads))
-- Scheme Procedure: all-threads
-- C Function: scm_all_threads ()
Return a list of all threads.
-- Scheme Procedure: current-thread
-- C Function: scm_current_thread ()
Return the thread that called this function.
-- Scheme Procedure: call-with-new-thread thunk [handler]
Call ‘thunk’ in a new thread and with a new dynamic state,
returning the new thread. The procedure THUNK is called via
‘with-continuation-barrier’.
When HANDLER is specified, then THUNK is called from within a
‘catch’ with tag ‘#t’ that has HANDLER as its handler. This catch
is established inside the continuation barrier.
Once THUNK or HANDLER returns, the return value is made the _exit
value_ of the thread and the thread is terminated.
-- C Function: SCM scm_spawn_thread (scm_t_catch_body body, void
*body_data, scm_t_catch_handler handler, void *handler_data)
Call BODY in a new thread, passing it BODY_DATA, returning the new
thread. The function BODY is called via
‘scm_c_with_continuation_barrier’.
When HANDLER is non-‘NULL’, BODY is called via ‘scm_internal_catch’
with tag ‘SCM_BOOL_T’ that has HANDLER and HANDLER_DATA as the
handler and its data. This catch is established inside the
continuation barrier.
Once BODY or HANDLER returns, the return value is made the _exit
value_ of the thread and the thread is terminated.
-- Scheme Procedure: thread? obj
-- C Function: scm_thread_p (obj)
Return ‘#t’ ff OBJ is a thread; otherwise, return ‘#f’.
-- Scheme Procedure: join-thread thread [timeout [timeoutval]]
-- C Function: scm_join_thread (thread)
-- C Function: scm_join_thread_timed (thread, timeout, timeoutval)
Wait for THREAD to terminate and return its exit value. Only
threads that were created with ‘call-with-new-thread’ or
‘scm_spawn_thread’ can be joinable; attempting to join a foreign
thread will raise an error.
When TIMEOUT is given, it specifies a point in time where the
waiting should be aborted. It can be either an integer as returned
by ‘current-time’ or a pair as returned by ‘gettimeofday’. When
the waiting is aborted, TIMEOUTVAL is returned (if it is specified;
‘#f’ is returned otherwise).
-- Scheme Procedure: thread-exited? thread
-- C Function: scm_thread_exited_p (thread)
Return ‘#t’ if THREAD has exited, or ‘#f’ otherwise.
-- Scheme Procedure: yield
-- C Function: scm_yield (thread)
If one or more threads are waiting to execute, calling yield forces
an immediate context switch to one of them. Otherwise, yield has
no effect.
-- Scheme Procedure: cancel-thread thread . values
-- C Function: scm_cancel_thread (thread)
Asynchronously interrupt THREAD and ask it to terminate.
‘dynamic-wind’ post thunks will run, but throw handlers will not.
If THREAD has already terminated or been signaled to terminate,
this function is a no-op. Calling ‘join-thread’ on the thread will
return the given VALUES, if the cancel succeeded.
Under the hood, thread cancellation uses ‘system-async-mark’ and
‘abort-to-prompt’. *Note Asyncs:: for more on asynchronous
interrupts.
-- macro: make-thread proc arg ...
Apply PROC to ARG ... in a new thread formed by
‘call-with-new-thread’ using a default error handler that displays
the error to the current error port. The ARG ... expressions are
evaluated in the new thread.
-- macro: begin-thread expr1 expr2 ...
Evaluate forms EXPR1 EXPR2 ... in a new thread formed by
‘call-with-new-thread’ using a default error handler that displays
the error to the current error port.
One often wants to limit the number of threads running to be
proportional to the number of available processors. These interfaces
are therefore exported by (ice-9 threads) as well.
-- Scheme Procedure: total-processor-count
-- C Function: scm_total_processor_count ()
Return the total number of processors of the machine, which is
guaranteed to be at least 1. A “processor” here is a thread
execution unit, which can be either:
• an execution core in a (possibly multi-core) chip, in a
(possibly multi- chip) module, in a single computer, or
• a thread execution unit inside a core in the case of
“hyper-threaded” CPUs.
Which of the two definitions is used, is unspecified.
-- Scheme Procedure: current-processor-count
-- C Function: scm_current_processor_count ()
Like ‘total-processor-count’, but return the number of processors
available to the current process. See ‘setaffinity’ and
‘getaffinity’ for more information.
6.22.2 Thread-Local Variables
-----------------------------
Sometimes you want to establish a variable binding that is only valid
for a given thread: a “thread-local variable”.
You would think that fluids or parameters would be Guile’s answer for
thread-local variables, since establishing a new fluid binding doesn’t
affect bindings in other threads. *Note Fluids and Dynamic States::, or
*Note Parameters::. However, new threads inherit the fluid bindings
that were in place in their creator threads. In this way, a binding
established using a fluid (or a parameter) in a thread can escape to
other threads, which might not be what you want. Or, it might escape
via explicit reification via ‘current-dynamic-state’.
Of course, this dynamic scoping might be exactly what you want;
that’s why fluids and parameters work this way, and is what you want for
for many common parameters such as the current input and output ports,
the current locale conversion parameters, and the like. Perhaps this is
the case for most parameters, even. If your use case for thread-local
bindings comes from a desire to isolate a binding from its setting in
unrelated threads, then fluids and parameters apply nicely.
On the other hand, if your use case is to prevent concurrent access
to a value from multiple threads, then using vanilla fluids or
parameters is not appropriate. For this purpose, Guile has
“thread-local fluids”. A fluid created with ‘make-thread-local-fluid’
won’t be captured by ‘current-dynamic-state’ and won’t be propagated to
new threads.
-- Scheme Procedure: make-thread-local-fluid [dflt]
-- C Function: scm_make_thread_local_fluid (dflt)
Return a newly created fluid, whose initial value is DFLT, or ‘#f’
if DFLT is not given. Unlike fluids made with ‘make-fluid’, thread
local fluids are not captured by ‘make-dynamic-state’. Similarly,
a newly spawned child thread does not inherit thread-local fluid
values from the parent thread.
-- Scheme Procedure: fluid-thread-local? fluid
-- C Function: scm_fluid_thread_local_p (fluid)
Return ‘#t’ if the fluid FLUID is is thread-local, or ‘#f’
otherwise.
For example:
(define %thread-local (make-thread-local-fluid))
(with-fluids ((%thread-local (compute-data)))
... (fluid-ref %thread-local) ...)
You can also make a thread-local parameter out of a thread-local
fluid using the normal ‘fluid->parameter’:
(define param (fluid->parameter (make-thread-local-fluid)))
(parameterize ((param (compute-data)))
... (param) ...)
6.22.3 Asynchronous Interrupts
------------------------------
Every Guile thread can be interrupted. Threads running Guile code will
periodically check if there are pending interrupts and run them if
necessary. To interrupt a thread, call ‘system-async-mark’ on that
thread.
-- Scheme Procedure: system-async-mark proc [thread]
-- C Function: scm_system_async_mark (proc)
-- C Function: scm_system_async_mark_for_thread (proc, thread)
Enqueue PROC (a procedure with zero arguments) for future execution
in THREAD. When PROC has already been enqueued for THREAD but has
not been executed yet, this call has no effect. When THREAD is
omitted, the thread that called ‘system-async-mark’ is used.
Note that ‘scm_system_async_mark_for_thread’ is not
“async-signal-safe” and so cannot be called from a C signal handler.
(Indeed in general, ‘libguile’ functions are not safe to call from C
signal handlers.)
Though an interrupt procedure can have any side effect permitted to
Guile code, asynchronous interrupts are generally used either for
profiling or for prematurely cancelling a computation. The former case
is mostly transparent to the program being run, by design, but the
latter case can introduce bugs. Like finalizers (*note Foreign Object
Memory Management::), asynchronous interrupts introduce concurrency in a
program. An asyncronous interrupt can run in the middle of some
mutex-protected operation, for example, and potentially corrupt the
program’s state.
If some bit of Guile code needs to temporarily inhibit interrupts, it
can use ‘call-with-blocked-asyncs’. This function works by temporarily
increasing the _async blocking level_ of the current thread while a
given procedure is running. The blocking level starts out at zero, and
whenever a safe point is reached, a blocking level greater than zero
will prevent the execution of queued asyncs.
Analogously, the procedure ‘call-with-unblocked-asyncs’ will
temporarily decrease the blocking level of the current thread. You can
use it when you want to disable asyncs by default and only allow them
temporarily.
In addition to the C versions of ‘call-with-blocked-asyncs’ and
‘call-with-unblocked-asyncs’, C code can use ‘scm_dynwind_block_asyncs’
and ‘scm_dynwind_unblock_asyncs’ inside a “dynamic context” (*note
Dynamic Wind::) to block or unblock asyncs temporarily.
-- Scheme Procedure: call-with-blocked-asyncs proc
-- C Function: scm_call_with_blocked_asyncs (proc)
Call PROC and block the execution of asyncs by one level for the
current thread while it is running. Return the value returned by
PROC. For the first two variants, call PROC with no arguments; for
the third, call it with DATA.
-- C Function: void * scm_c_call_with_blocked_asyncs (void * (*proc)
(void *data), void *data)
The same but with a C function PROC instead of a Scheme thunk.
-- Scheme Procedure: call-with-unblocked-asyncs proc
-- C Function: scm_call_with_unblocked_asyncs (proc)
Call PROC and unblock the execution of asyncs by one level for the
current thread while it is running. Return the value returned by
PROC. For the first two variants, call PROC with no arguments; for
the third, call it with DATA.
-- C Function: void * scm_c_call_with_unblocked_asyncs (void *(*proc)
(void *data), void *data)
The same but with a C function PROC instead of a Scheme thunk.
-- C Function: void scm_dynwind_block_asyncs ()
During the current dynwind context, increase the blocking of asyncs
by one level. This function must be used inside a pair of calls to
‘scm_dynwind_begin’ and ‘scm_dynwind_end’ (*note Dynamic Wind::).
-- C Function: void scm_dynwind_unblock_asyncs ()
During the current dynwind context, decrease the blocking of asyncs
by one level. This function must be used inside a pair of calls to
‘scm_dynwind_begin’ and ‘scm_dynwind_end’ (*note Dynamic Wind::).
Sometimes you want to interrupt a thread that might be waiting for
something to happen, for example on a file descriptor or a condition
variable. In that case you can inform Guile of how to interrupt that
wait using the following procedures:
-- C Function: int scm_c_prepare_to_wait_on_fd (int fd)
Inform Guile that the current thread is about to sleep, and that if
an asynchronous interrupt is signalled on this thread, Guile should
wake up the thread by writing a zero byte to FD. Returns zero if
the prepare succeeded, or nonzero if the thread already has a
pending async and that it should avoid waiting.
-- C Function: int scm_c_prepare_to_wait_on_cond (scm_i_pthread_mutex_t
*mutex, scm_i_pthread_cond_t *cond)
Inform Guile that the current thread is about to sleep, and that if
an asynchronous interrupt is signalled on this thread, Guile should
wake up the thread by acquiring MUTEX and signalling COND. The
caller must already hold MUTEX and only drop it as part of the
‘pthread_cond_wait’ call. Returns zero if the prepare succeeded,
or nonzero if the thread already has a pending async and that it
should avoid waiting.
-- C Function: void scm_c_wait_finished (void)
Inform Guile that the current thread has finished waiting, and that
asynchronous interrupts no longer need any special wakeup action;
the current thread will periodically poll its internal queue
instead.
Guile’s own interface to ‘sleep’, ‘wait-condition-variable’,
‘select’, and so on all call the above routines as appropriate.
Finally, note that threads can also be interrupted via POSIX signals.
*Note Signals::. As an implementation detail, signal handlers will
effectively call ‘system-async-mark’ in a signal-safe way, eventually
running the signal handler using the same async mechanism. In this way
you can temporarily inhibit signal handlers from running using the above
interfaces.
6.22.4 Atomics
--------------
When accessing data in parallel from multiple threads, updates made by
one thread are not generally guaranteed to be visible by another thread.
It could be that your hardware requires special instructions to be
emitted to propagate a change from one CPU core to another. Or, it
could be that your hardware updates values with a sequence of
instructions, and a parallel thread could see a value that is in the
process of being updated but not fully updated.
Atomic references solve this problem. Atomics are a standard,
primitive facility to allow for concurrent access and update of mutable
variables from multiple threads with guaranteed forward-progress and
well-defined intermediate states.
Atomic references serve not only as a hardware memory barrier but
also as a compiler barrier. Normally a compiler might choose to reorder
or elide certain memory accesses due to optimizations like common
subexpression elimination. Atomic accesses however will not be
reordered relative to each other, and normal memory accesses will not be
reordered across atomic accesses.
As an implementation detail, currently all atomic accesses and
updates use the sequential consistency memory model from C11. We may
relax this in the future to the acquire/release semantics, which still
issues a memory barrier so that non-atomic updates are not reordered
across atomic accesses or updates.
To use Guile’s atomic operations, load the ‘(ice-9 atomic)’ module:
(use-modules (ice-9 atomic))
-- Scheme Procedure: make-atomic-box init
Return an atomic box initialized to value INIT.
-- Scheme Procedure: atomic-box? obj
Return ‘#t’ if OBJ is an atomic-box object, else return ‘#f’.
-- Scheme Procedure: atomic-box-ref box
Fetch the value stored in the atomic box BOX and return it.
-- Scheme Procedure: atomic-box-set! box val
Store VAL into the atomic box BOX.
-- Scheme Procedure: atomic-box-swap! box val
Store VAL into the atomic box BOX, and return the value that was
previously stored in the box.
-- Scheme Procedure: atomic-box-compare-and-swap! box expected desired
If the value of the atomic box BOX is the same as, EXPECTED (in the
sense of ‘eq?’), replace the contents of the box with DESIRED.
Otherwise does not update the box. Returns the previous value of
the box in either case, so you can know if the swap worked by
checking if the return value is ‘eq?’ to EXPECTED.
6.22.5 Mutexes and Condition Variables
--------------------------------------
Mutexes are low-level primitives used to coordinate concurrent access to
mutable data. Short for “mutual exclusion”, the name “mutex” indicates
that only one thread at a time can acquire access to data that is
protected by a mutex – threads are excluded from accessing data at the
same time. If one thread has locked a mutex, then another thread
attempting to lock that same mutex will wait until the first thread is
done.
Mutexes can be used to build robust multi-threaded programs that take
advantage of multiple cores. However, they provide very low-level
functionality and are somewhat dangerous; usually you end up wanting to
acquire multiple mutexes at the same time to perform a multi-object
access, but this can easily lead to deadlocks if the program is not
carefully written. For example, if objects A and B are protected by
associated mutexes M and N, respectively, then to access both of them
then you need to acquire both mutexes. But what if one thread acquires
M first and then N, at the same time that another thread acquires N them
M? You can easily end up in a situation where one is waiting for the
other.
There’s no easy way around this problem on the language level. A
function A that uses mutexes does not necessarily compose nicely with a
function B that uses mutexes. For this reason we suggest using atomic
variables when you can (*note Atomics::), as they do not have this
problem.
Still, if you as a programmer are responsible for a whole system,
then you can use mutexes as a primitive to provide safe concurrent
abstractions to your users. (For example, given all locks in a system,
if you establish an order such that M is consistently acquired before N,
you can avoid the “deadly-embrace” deadlock described above. The
problem is enumerating all mutexes and establishing this order from a
system perspective.) Guile gives you the low-level facilities to build
such systems.
In Guile there are additional considerations beyond the usual ones in
other programming languages: non-local control flow and asynchronous
interrupts. What happens if you hold a mutex, but somehow you cause an
exception to be thrown? There is no one right answer. You might want
to keep the mutex locked to prevent any other code from ever entering
that critical section again. Or, your critical section might be fine if
you unlock the mutex “on the way out”, via an exception handler or
‘dynamic-wind’. *Note Exceptions::, and *Note Dynamic Wind::.
But if you arrange to unlock the mutex when leaving a dynamic extent
via ‘dynamic-wind’, what to do if control re-enters that dynamic extent
via a continuation invocation? Surely re-entering the dynamic extent
without the lock is a bad idea, so there are two options on the table:
either prevent re-entry via ‘with-continuation-barrier’ or similar, or
reacquire the lock in the entry thunk of a ‘dynamic-wind’.
You might think that because you don’t use continuations, that you
don’t have to think about this, and you might be right. If you control
the whole system, you can reason about continuation use globally. Or,
if you know all code that can be called in a dynamic extent, and none of
that code can call continuations, then you don’t have to worry about
re-entry, and you might not have to worry about early exit either.
However, do consider the possibility of asynchronous interrupts
(*note Asyncs::). If the user interrupts your code interactively, that
can cause an exception; or your thread might be cancelled, which does
the same; or the user could be running your code under some pre-emptive
system that periodically causes lightweight task switching. (Guile does
not currently include such a system, but it’s possible to implement as a
library.) Probably you also want to defer asynchronous interrupt
processing while you hold the mutex, and probably that also means that
you should not hold the mutex for very long.
All of these additional Guile-specific considerations mean that from
a system perspective, you would do well to avoid these hazards if you
can by not requiring mutexes. Instead, work with immutable data that
can be shared between threads without hazards, or use persistent data
structures with atomic updates based on the atomic variable library
(*note Atomics::).
There are three types of mutexes in Guile: “standard”, “recursive”,
and “unowned”.
Calling ‘make-mutex’ with no arguments makes a standard mutex. A
standard mutex can only be locked once. If you try to lock it again
from the thread that locked it to begin with (the "owner" thread), it
throws an error. It can only be unlocked from the thread that locked it
in the first place.
Calling ‘make-mutex’ with the symbol ‘recursive’ as the argument, or
calling ‘make-recursive-mutex’, will give you a recursive mutex. A
recursive mutex can be locked multiple times by its owner. It then has
to be unlocked the corresponding number of times, and like standard
mutexes can only be unlocked by the owner thread.
Finally, calling ‘make-mutex’ with the symbol ‘allow-external-unlock’
creates an unowned mutex. An unowned mutex is like a standard mutex,
except that it can be unlocked by any thread. A corollary of this
behavior is that a thread’s attempt to lock a mutex that it already owns
will block instead of signalling an error, as it could be that some
other thread unlocks the mutex, allowing the owner thread to proceed.
This kind of mutex is a bit strange and is here for use by SRFI-18.
The mutex procedures in Guile can operate on all three kinds of
mutexes.
To use these facilities, load the ‘(ice-9 threads)’ module.
(use-modules (ice-9 threads))
-- Scheme Procedure: make-mutex [kind]
-- C Function: scm_make_mutex ()
-- C Function: scm_make_mutex_with_kind (SCM kind)
Return a new mutex. It will be a standard non-recursive mutex,
unless the ‘recursive’ symbol is passed as the optional KIND
argument, in which case it will be recursive. It’s also possible
to pass ‘unowned’ for semantics tailored to SRFI-18’s use case; see
above for details.
-- Scheme Procedure: mutex? obj
-- C Function: scm_mutex_p (obj)
Return ‘#t’ if OBJ is a mutex; otherwise, return ‘#f’.
-- Scheme Procedure: make-recursive-mutex
-- C Function: scm_make_recursive_mutex ()
Create a new recursive mutex. It is initially unlocked. Calling
this function is equivalent to calling ‘make-mutex’ with the
‘recursive’ kind.
-- Scheme Procedure: lock-mutex mutex [timeout]
-- C Function: scm_lock_mutex (mutex)
-- C Function: scm_timed_lock_mutex (mutex, timeout)
Lock MUTEX and return ‘#t’. If the mutex is already locked, then
block and return only when MUTEX has been acquired.
When TIMEOUT is given, it specifies a point in time where the
waiting should be aborted. It can be either an integer as returned
by ‘current-time’ or a pair as returned by ‘gettimeofday’. When
the waiting is aborted, ‘#f’ is returned.
For standard mutexes (‘make-mutex’), an error is signalled if the
thread has itself already locked MUTEX.
For a recursive mutex (‘make-recursive-mutex’), if the thread has
itself already locked MUTEX, then a further ‘lock-mutex’ call
increments the lock count. An additional ‘unlock-mutex’ will be
required to finally release.
When an asynchronous interrupt (*note Asyncs::) is scheduled for a
thread blocked in ‘lock-mutex’, Guile will interrupt the wait, run
the interrupts, and then resume the wait.
-- C Function: void scm_dynwind_lock_mutex (SCM mutex)
Arrange for MUTEX to be locked whenever the current dynwind context
is entered and to be unlocked when it is exited.
-- Scheme Procedure: try-mutex mx
-- C Function: scm_try_mutex (mx)
Try to lock MUTEX and return ‘#t’ if successful, or ‘#f’ otherwise.
This is like calling ‘lock-mutex’ with an expired timeout.
-- Scheme Procedure: unlock-mutex mutex
-- C Function: scm_unlock_mutex (mutex)
Unlock MUTEX. An error is signalled if MUTEX is not locked.
“Standard” and “recursive” mutexes can only be unlocked by the
thread that locked them; Guile detects this situation and signals
an error. “Unowned” mutexes can be unlocked by any thread.
-- Scheme Procedure: mutex-owner mutex
-- C Function: scm_mutex_owner (mutex)
Return the current owner of MUTEX, in the form of a thread or ‘#f’
(indicating no owner). Note that a mutex may be unowned but still
locked.
-- Scheme Procedure: mutex-level mutex
-- C Function: scm_mutex_level (mutex)
Return the current lock level of MUTEX. If MUTEX is currently
unlocked, this value will be 0; otherwise, it will be the number of
times MUTEX has been recursively locked by its current owner.
-- Scheme Procedure: mutex-locked? mutex
-- C Function: scm_mutex_locked_p (mutex)
Return ‘#t’ if MUTEX is locked, regardless of ownership; otherwise,
return ‘#f’.
-- Scheme Procedure: make-condition-variable
-- C Function: scm_make_condition_variable ()
Return a new condition variable.
-- Scheme Procedure: condition-variable? obj
-- C Function: scm_condition_variable_p (obj)
Return ‘#t’ if OBJ is a condition variable; otherwise, return ‘#f’.
-- Scheme Procedure: wait-condition-variable condvar mutex [time]
-- C Function: scm_wait_condition_variable (condvar, mutex, time)
Wait until CONDVAR has been signalled. While waiting, MUTEX is
atomically unlocked (as with ‘unlock-mutex’) and is locked again
when this function returns. When TIME is given, it specifies a
point in time where the waiting should be aborted. It can be
either a integer as returned by ‘current-time’ or a pair as
returned by ‘gettimeofday’. When the waiting is aborted, ‘#f’ is
returned. When the condition variable has in fact been signalled,
‘#t’ is returned. The mutex is re-locked in any case before
‘wait-condition-variable’ returns.
When an async is activated for a thread that is blocked in a call
to ‘wait-condition-variable’, the waiting is interrupted, the mutex
is locked, and the async is executed. When the async returns, the
mutex is unlocked again and the waiting is resumed. When the
thread block while re-acquiring the mutex, execution of asyncs is
blocked.
-- Scheme Procedure: signal-condition-variable condvar
-- C Function: scm_signal_condition_variable (condvar)
Wake up one thread that is waiting for CONDVAR.
-- Scheme Procedure: broadcast-condition-variable condvar
-- C Function: scm_broadcast_condition_variable (condvar)
Wake up all threads that are waiting for CONDVAR.
Guile also includes some higher-level abstractions for working with
mutexes.
-- macro: with-mutex mutex body1 body2 ...
Lock MUTEX, evaluate the body BODY1 BODY2 ..., then unlock MUTEX.
The return value is that returned by the last body form.
The lock, body and unlock form the branches of a ‘dynamic-wind’
(*note Dynamic Wind::), so MUTEX is automatically unlocked if an
error or new continuation exits the body, and is re-locked if the
body is re-entered by a captured continuation.
-- macro: monitor body1 body2 ...
Evaluate the body form BODY1 BODY2 ... with a mutex locked so only
one thread can execute that code at any one time. The return value
is the return from the last body form.
Each ‘monitor’ form has its own private mutex and the locking and
evaluation is as per ‘with-mutex’ above. A standard mutex
(‘make-mutex’) is used, which means the body must not recursively
re-enter the ‘monitor’ form.
The term “monitor” comes from operating system theory, where it
means a particular bit of code managing access to some resource and
which only ever executes on behalf of one process at any one time.
6.22.6 Blocking in Guile Mode
-----------------------------
Up to Guile version 1.8, a thread blocked in guile mode would prevent
the garbage collector from running. Thus threads had to explicitly
leave guile mode with ‘scm_without_guile ()’ before making a potentially
blocking call such as a mutex lock, a ‘select ()’ system call, etc. The
following functions could be used to temporarily leave guile mode or to
perform some common blocking operations in a supported way.
Starting from Guile 2.0, blocked threads no longer hinder garbage
collection. Thus, the functions below are not needed anymore. They can
still be used to inform the GC that a thread is about to block, giving
it a (small) optimization opportunity for “stop the world” garbage
collections, should they occur while the thread is blocked.
-- C Function: void * scm_without_guile (void *(*func) (void *), void
*data)
Leave guile mode, call FUNC on DATA, enter guile mode and return
the result of calling FUNC.
While a thread has left guile mode, it must not call any libguile
functions except ‘scm_with_guile’ or ‘scm_without_guile’ and must
not use any libguile macros. Also, local variables of type ‘SCM’
that are allocated while not in guile mode are not protected from
the garbage collector.
When used from non-guile mode, calling ‘scm_without_guile’ is still
allowed: it simply calls FUNC. In that way, you can leave guile
mode without having to know whether the current thread is in guile
mode or not.
-- C Function: int scm_pthread_mutex_lock (pthread_mutex_t *mutex)
Like ‘pthread_mutex_lock’, but leaves guile mode while waiting for
the mutex.
-- C Function: int scm_pthread_cond_wait (pthread_cond_t *cond,
pthread_mutex_t *mutex)
-- C Function: int scm_pthread_cond_timedwait (pthread_cond_t *cond,
pthread_mutex_t *mutex, struct timespec *abstime)
Like ‘pthread_cond_wait’ and ‘pthread_cond_timedwait’, but leaves
guile mode while waiting for the condition variable.
-- C Function: int scm_std_select (int nfds, fd_set *readfds, fd_set
*writefds, fd_set *exceptfds, struct timeval *timeout)
Like ‘select’ but leaves guile mode while waiting. Also, the
delivery of an async causes this function to be interrupted with
error code ‘EINTR’.
-- C Function: unsigned int scm_std_sleep (unsigned int seconds)
Like ‘sleep’, but leaves guile mode while sleeping. Also, the
delivery of an async causes this function to be interrupted.
-- C Function: unsigned long scm_std_usleep (unsigned long usecs)
Like ‘usleep’, but leaves guile mode while sleeping. Also, the
delivery of an async causes this function to be interrupted.
6.22.7 Futures
--------------
The ‘(ice-9 futures)’ module provides “futures”, a construct for
fine-grain parallelism. A future is a wrapper around an expression
whose computation may occur in parallel with the code of the calling
thread, and possibly in parallel with other futures. Like promises,
futures are essentially proxies that can be queried to obtain the value
of the enclosed expression:
(touch (future (+ 2 3)))
⇒ 5
However, unlike promises, the expression associated with a future may
be evaluated on another CPU core, should one be available. This
supports “fine-grain parallelism”, because even relatively small
computations can be embedded in futures. Consider this sequential code:
(define (find-prime lst1 lst2)
(or (find prime? lst1)
(find prime? lst2)))
The two arms of ‘or’ are potentially computation-intensive. They are
independent of one another, yet, they are evaluated sequentially when
the first one returns ‘#f’. Using futures, one could rewrite it like
this:
(define (find-prime lst1 lst2)
(let ((f (future (find prime? lst2))))
(or (find prime? lst1)
(touch f))))
This preserves the semantics of ‘find-prime’. On a multi-core
machine, though, the computation of ‘(find prime? lst2)’ may be done in
parallel with that of the other ‘find’ call, which can reduce the
execution time of ‘find-prime’.
Futures may be nested: a future can itself spawn and then ‘touch’
other futures, leading to a directed acyclic graph of futures. Using
this facility, a parallel ‘map’ procedure can be defined along these
lines:
(use-modules (ice-9 futures) (ice-9 match))
(define (par-map proc lst)
(match lst
(()
'())
((head tail ...)
(let ((tail (future (par-map proc tail)))
(head (proc head)))
(cons head (touch tail))))))
Note that futures are intended for the evaluation of purely
functional expressions. Expressions that have side-effects or rely on
I/O may require additional care, such as explicit synchronization (*note
Mutexes and Condition Variables::).
Guile’s futures are implemented on top of POSIX threads (*note
Threads::). Internally, a fixed-size pool of threads is used to
evaluate futures, such that offloading the evaluation of an expression
to another thread doesn’t incur thread creation costs. By default, the
pool contains one thread per available CPU core, minus one, to account
for the main thread. The number of available CPU cores is determined
using ‘current-processor-count’ (*note Processes::).
When a thread touches a future that has not completed yet, it
processes any pending future while waiting for it to complete, or just
waits if there are no pending futures. When ‘touch’ is called from
within a future, the execution of the calling future is suspended,
allowing its host thread to process other futures, and resumed when the
touched future has completed. This suspend/resume is achieved by
capturing the calling future’s continuation, and later reinstating it
(*note delimited continuations: Prompts.).
-- Scheme Syntax: future exp
Return a future for expression EXP. This is equivalent to:
(make-future (lambda () exp))
-- Scheme Procedure: make-future thunk
Return a future for THUNK, a zero-argument procedure.
This procedure returns immediately. Execution of THUNK may begin
in parallel with the calling thread’s computations, if idle CPU
cores are available, or it may start when ‘touch’ is invoked on the
returned future.
If the execution of THUNK throws an exception, that exception will
be re-thrown when ‘touch’ is invoked on the returned future.
-- Scheme Procedure: future? obj
Return ‘#t’ if OBJ is a future.
-- Scheme Procedure: touch f
Return the result of the expression embedded in future F.
If the result was already computed in parallel, ‘touch’ returns
instantaneously. Otherwise, it waits for the computation to
complete, if it already started, or initiates it. In the former
case, the calling thread may process other futures in the meantime.
6.22.8 Parallel forms
---------------------
The functions described in this section are available from
(use-modules (ice-9 threads))
They provide high-level parallel constructs. The following functions
are implemented in terms of futures (*note Futures::). Thus they are
relatively cheap as they re-use existing threads, and portable, since
they automatically use one thread per available CPU core.
-- syntax: parallel expr ...
Evaluate each EXPR expression in parallel, each in its own thread.
Return the results of N expressions as a set of N multiple values
(*note Multiple Values::).
-- syntax: letpar ((var expr) ...) body1 body2 ...
Evaluate each EXPR in parallel, each in its own thread, then bind
the results to the corresponding VAR variables, and then evaluate
BODY1 BODY2 ...
‘letpar’ is like ‘let’ (*note Local Bindings::), but all the
expressions for the bindings are evaluated in parallel.
-- Scheme Procedure: par-map proc lst1 lst2 ...
-- Scheme Procedure: par-for-each proc lst1 lst2 ...
Call PROC on the elements of the given lists. ‘par-map’ returns a
list comprising the return values from PROC. ‘par-for-each’
returns an unspecified value, but waits for all calls to complete.
The PROC calls are ‘(PROC ELEM1 ELEM2 ...)’, where each ELEM is
from the corresponding LST . Each LST must be the same length.
The calls are potentially made in parallel, depending on the number
of CPU cores available.
These functions are like ‘map’ and ‘for-each’ (*note List
Mapping::), but make their PROC calls in parallel.
Unlike those above, the functions described below take a number of
threads as an argument. This makes them inherently non-portable since
the specified number of threads may differ from the number of available
CPU cores as returned by ‘current-processor-count’ (*note Processes::).
In addition, these functions create the specified number of threads when
they are called and terminate them upon completion, which makes them
quite expensive.
Therefore, they should be avoided.
-- Scheme Procedure: n-par-map n proc lst1 lst2 ...
-- Scheme Procedure: n-par-for-each n proc lst1 lst2 ...
Call PROC on the elements of the given lists, in the same way as
‘par-map’ and ‘par-for-each’ above, but use no more than N threads
at any one time. The order in which calls are initiated within
that threads limit is unspecified.
These functions are good for controlling resource consumption if
PROC calls might be costly, or if there are many to be made. On a
dual-CPU system for instance N=4 might be enough to keep the CPUs
utilized, and not consume too much memory.
-- Scheme Procedure: n-for-each-par-map n sproc pproc lst1 lst2 ...
Apply PPROC to the elements of the given lists, and apply SPROC to
each result returned by PPROC. The final return value is
unspecified, but all calls will have been completed before
returning.
The calls made are ‘(SPROC (PPROC ELEM1 ... ELEMN))’, where each
ELEM is from the corresponding LST. Each LST must have the same
number of elements.
The PPROC calls are made in parallel, in separate threads. No more
than N threads are used at any one time. The order in which PPROC
calls are initiated within that limit is unspecified.
The SPROC calls are made serially, in list element order, one at a
time. PPROC calls on later elements may execute in parallel with
the SPROC calls. Exactly which thread makes each SPROC call is
unspecified.
This function is designed for individual calculations that can be
done in parallel, but with results needing to be handled serially,
for instance to write them to a file. The N limit on threads
controls system resource usage when there are many calculations or
when they might be costly.
It will be seen that ‘n-for-each-par-map’ is like a combination of
‘n-par-map’ and ‘for-each’,
(for-each sproc (n-par-map n pproc lst1 ... lstN))
But the actual implementation is more efficient since each SPROC
call, in turn, can be initiated once the relevant PPROC call has
completed, it doesn’t need to wait for all to finish.
6.23 Configuration, Features and Runtime Options
================================================
Why is my Guile different from your Guile? There are three kinds of
possible variation:
• build differences — different versions of the Guile source code,
installation directories, configuration flags that control pieces
of functionality being included or left out, etc.
• differences in dynamically loaded code — behaviour and features
provided by modules that can be dynamically loaded into a running
Guile
• different runtime options — some of the options that are provided
for controlling Guile’s behaviour may be set differently.
Guile provides “introspective” variables and procedures to query all
of these possible variations at runtime. For runtime options, it also
provides procedures to change the settings of options and to obtain
documentation on what the options mean.
6.23.1 Configuration, Build and Installation
--------------------------------------------
The following procedures and variables provide information about how
Guile was configured, built and installed on your system.
-- Scheme Procedure: version
-- Scheme Procedure: effective-version
-- Scheme Procedure: major-version
-- Scheme Procedure: minor-version
-- Scheme Procedure: micro-version
-- C Function: scm_version ()
-- C Function: scm_effective_version ()
-- C Function: scm_major_version ()
-- C Function: scm_minor_version ()
-- C Function: scm_micro_version ()
Return a string describing Guile’s full version number, effective
version number, major, minor or micro version number, respectively.
The ‘effective-version’ function returns the version name that
should remain unchanged during a stable series. Currently that
means that it omits the micro version. The effective version
should be used for items like the versioned share directory name
i.e. ‘/usr/share/guile/3.0/’
(version) ⇒ "3.0.0"
(effective-version) ⇒ "3.0"
(major-version) ⇒ "3"
(minor-version) ⇒ "0"
(micro-version) ⇒ "0"
-- Scheme Procedure: %package-data-dir
-- C Function: scm_sys_package_data_dir ()
Return the name of the directory under which Guile Scheme files in
general are stored. On Unix-like systems, this is usually
‘/usr/local/share/guile’ or ‘/usr/share/guile’.
-- Scheme Procedure: %library-dir
-- C Function: scm_sys_library_dir ()
Return the name of the directory where the Guile Scheme files that
belong to the core Guile installation (as opposed to files from a
3rd party package) are installed. On Unix-like systems this is
usually ‘/usr/local/share/guile/GUILE_EFFECTIVE_VERSION’ or
‘/usr/share/guile/GUILE_EFFECTIVE_VERSION’;
for example ‘/usr/local/share/guile/3.0’.
-- Scheme Procedure: %site-dir
-- C Function: scm_sys_site_dir ()
Return the name of the directory where Guile Scheme files specific
to your site should be installed. On Unix-like systems, this is
usually ‘/usr/local/share/guile/site’ or ‘/usr/share/guile/site’.
-- Scheme Procedure: %site-ccache-dir
-- C Function: scm_sys_site_ccache_dir ()
Return the directory where users should install compiled ‘.go’
files for use with this version of Guile. Might look something
like ‘/usr/lib/guile/3.0/site-ccache’.
-- Variable: %guile-build-info
Alist of information collected during the building of a particular
Guile. Entries can be grouped into one of several categories:
directories, env vars, and versioning info.
Briefly, here are the keys in ‘%guile-build-info’, by group:
directories
srcdir, top_srcdir, prefix, exec_prefix, bindir, sbindir,
libexecdir, datadir, sysconfdir, sharedstatedir,
localstatedir, libdir, infodir, mandir, includedir,
pkgdatadir, pkglibdir, pkgincludedir
env vars
LIBS
versioning info
guileversion, libguileinterface, buildstamp
Values are all strings. The value for ‘LIBS’ is typically found
also as a part of ‘pkg-config --libs guile-3.0’ output. The value
for ‘guileversion’ has form X.Y.Z, and should be the same as
returned by ‘(version)’. The value for ‘libguileinterface’ is
libtool compatible and has form CURRENT:REVISION:AGE (*note Library
interface versions: (libtool)Versioning.). The value for
‘buildstamp’ is the output of the command ‘date -u +'%Y-%m-%d %T'’
(UTC).
In the source, ‘%guile-build-info’ is initialized from
libguile/libpath.h, which is completely generated, so deleting this
file before a build guarantees up-to-date values for that build.
-- Variable: %host-type
The canonical host type (GNU triplet) of the host Guile was
configured for, e.g., ‘"x86_64-unknown-linux-gnu"’ (*note
(autoconf)Canonicalizing::).
6.23.2 Feature Tracking
-----------------------
Guile has a Scheme level variable ‘*features*’ that keeps track to some
extent of the features that are available in a running Guile.
‘*features*’ is a list of symbols, for example ‘threads’, each of which
describes a feature of the running Guile process.
-- Variable: *features*
A list of symbols describing available features of the Guile
process.
You shouldn’t modify the ‘*features*’ variable directly using ‘set!’.
Instead, see the procedures that are provided for this purpose in the
following subsection.
6.23.2.1 Feature Manipulation
.............................
To check whether a particular feature is available, use the ‘provided?’
procedure:
-- Scheme Procedure: provided? feature
-- Deprecated Scheme Procedure: feature? feature
Return ‘#t’ if the specified FEATURE is available, otherwise ‘#f’.
To advertise a feature from your own Scheme code, you can use the
‘provide’ procedure:
-- Scheme Procedure: provide feature
Add FEATURE to the list of available features in this Guile
process.
For C code, the equivalent function takes its feature name as a ‘char
*’ argument for convenience:
-- C Function: void scm_add_feature (const char *str)
Add a symbol with name STR to the list of available features in
this Guile process.
6.23.2.2 Common Feature Symbols
...............................
In general, a particular feature may be available for one of two
reasons. Either because the Guile library was configured and compiled
with that feature enabled — i.e. the feature is built into the library
on your system. Or because some C or Scheme code that was dynamically
loaded by Guile has added that feature to the list.
In the first category, here are the features that the current version
of Guile may define (depending on how it is built), and what they mean.
‘array’
Indicates support for arrays (*note Arrays::).
‘array-for-each’
Indicates availability of ‘array-for-each’ and other array mapping
procedures (*note Arrays::).
‘char-ready?’
Indicates that the ‘char-ready?’ function is available (*note
Venerable Port Interfaces::).
‘complex’
Indicates support for complex numbers.
‘current-time’
Indicates availability of time-related functions: ‘times’,
‘get-internal-run-time’ and so on (*note Time::).
‘debug-extensions’
Indicates that the debugging evaluator is available, together with
the options for controlling it.
‘delay’
Indicates support for promises (*note Delayed Evaluation::).
‘EIDs’
Indicates that the ‘geteuid’ and ‘getegid’ really return effective
user and group IDs (*note Processes::).
‘inexact’
Indicates support for inexact numbers.
‘i/o-extensions’
Indicates availability of the following extended I/O procedures:
‘ftell’, ‘redirect-port’, ‘dup->fdes’, ‘dup2’, ‘fileno’, ‘isatty?’,
‘fdopen’, ‘primitive-move->fdes’ and ‘fdes->ports’ (*note Ports and
File Descriptors::).
‘net-db’
Indicates availability of network database functions:
‘scm_gethost’, ‘scm_getnet’, ‘scm_getproto’, ‘scm_getserv’,
‘scm_sethost’, ‘scm_setnet’, ‘scm_setproto’, ‘scm_setserv’, and
their ‘byXXX’ variants (*note Network Databases::).
‘posix’
Indicates support for POSIX functions: ‘pipe’, ‘getgroups’, ‘kill’,
‘execl’ and so on (*note POSIX::).
‘fork’
Indicates support for the POSIX ‘fork’ function (*note
‘primitive-fork’: Processes.).
‘popen’
Indicates support for ‘open-pipe’ in the ‘(ice-9 popen)’ module
(*note Pipes::).
‘random’
Indicates availability of random number generation functions:
‘random’, ‘copy-random-state’, ‘random-uniform’ and so on (*note
Random::).
‘reckless’
Indicates that Guile was built with important checks omitted — you
should never see this!
‘regex’
Indicates support for POSIX regular expressions using
‘make-regexp’, ‘regexp-exec’ and friends (*note Regexp
Functions::).
‘socket’
Indicates availability of socket-related functions: ‘socket’,
‘bind’, ‘connect’ and so on (*note Network Sockets and
Communication::).
‘sort’
Indicates availability of sorting and merging functions (*note
Sorting::).
‘system’
Indicates that the ‘system’ function is available (*note
Processes::).
‘threads’
Indicates support for multithreading (*note Threads::).
‘values’
Indicates support for multiple return values using ‘values’ and
‘call-with-values’ (*note Multiple Values::).
Available features in the second category depend, by definition, on
what additional code your Guile process has loaded in. The following
table lists features that you might encounter for this reason.
‘defmacro’
Indicates that the ‘defmacro’ macro is available (*note Macros::).
‘describe’
Indicates that the ‘(oop goops describe)’ module has been loaded,
which provides a procedure for describing the contents of GOOPS
instances.
‘readline’
Indicates that Guile has loaded in Readline support, for command
line editing (*note Readline Support::).
‘record’
Indicates support for record definition using ‘make-record-type’
and friends (*note Records::).
Although these tables may seem exhaustive, it is probably unwise in
practice to rely on them, as the correspondences between feature symbols
and available procedures/behaviour are not strictly defined. If you are
writing code that needs to check for the existence of some procedure, it
is probably safer to do so directly using the ‘defined?’ procedure than
to test for the corresponding feature using ‘provided?’.
6.23.3 Runtime Options
----------------------
There are a number of runtime options available for paramaterizing
built-in procedures, like ‘read’, and built-in behavior, like what
happens on an uncaught error.
For more information on reader options, *Note Scheme Read::.
For more information on print options, *Note Scheme Write::.
Finally, for more information on debugger options, *Note Debug
Options::.
6.23.3.1 Examples of option use
...............................
Here is an example of a session in which some read and debug option
handling procedures are used. In this example, the user
1. Notices that the symbols ‘abc’ and ‘aBc’ are not the same
2. Examines the ‘read-options’, and sees that ‘case-insensitive’ is
set to “no”.
3. Enables ‘case-insensitive’
4. Quits the recursive prompt
5. Verifies that now ‘aBc’ and ‘abc’ are the same
scheme@(guile-user)> (define abc "hello")
scheme@(guile-user)> abc
$1 = "hello"
scheme@(guile-user)> aBc
: warning: possibly unbound variable `aBc'
ERROR: In procedure module-lookup:
ERROR: Unbound variable: aBc
Entering a new prompt. Type `,bt' for a backtrace or `,q' to continue.
scheme@(guile-user) [1]> (read-options 'help)
copy no Copy source code expressions.
positions yes Record positions of source code expressions.
case-insensitive no Convert symbols to lower case.
keywords #f Style of keyword recognition: #f, 'prefix or 'postfix.
r6rs-hex-escapes no Use R6RS variable-length character and string hex escapes.
square-brackets yes Treat `[' and `]' as parentheses, for R6RS compatibility.
hungry-eol-escapes no In strings, consume leading whitespace after an
escaped end-of-line.
curly-infix no Support SRFI-105 curly infix expressions.
scheme@(guile-user) [1]> (read-enable 'case-insensitive)
$2 = (square-brackets keywords #f case-insensitive positions)
scheme@(guile-user) [1]> ,q
scheme@(guile-user)> aBc
$3 = "hello"
6.24 Support for Other Languages
================================
In addition to Scheme, a user may write a Guile program in an increasing
number of other languages. Currently supported languages include Emacs
Lisp and ECMAScript.
Guile is still fundamentally a Scheme, but it tries to support a wide
variety of language building-blocks, so that other languages can be
implemented on top of Guile. This allows users to write or extend
applications in languages other than Scheme, too. This section
describes the languages that have been implemented.
(For details on how to implement a language, *Note Compiling to the
Virtual Machine::.)
6.24.1 Using Other Languages
----------------------------
There are currently only two ways to access other languages from within
Guile: at the REPL, and programmatically, via ‘compile’,
‘read-and-compile’, and ‘compile-file’.
The REPL is Guile’s command prompt (*note Using Guile
Interactively::). The REPL has a concept of the “current language”,
which defaults to Scheme. The user may change that language, via the
meta-command ‘,language’.
For example, the following meta-command enables Emacs Lisp input:
scheme@(guile-user)> ,language elisp
Happy hacking with Emacs Lisp! To switch back, type `,L scheme'.
elisp@(guile-user)> (eq 1 2)
$1 = #nil
Each language has its short name: for example, ‘elisp’, for Elisp.
The same short name may be used to compile source code programmatically,
via ‘compile’:
elisp@(guile-user)> ,L scheme
Happy hacking with Guile Scheme! To switch back, type `,L elisp'.
scheme@(guile-user)> (compile '(eq 1 2) #:from 'elisp)
$2 = #nil
Granted, as the input to ‘compile’ is a datum, this works best for
Lispy languages, which have a straightforward datum representation.
Other languages that need more parsing are better dealt with as strings.
The easiest way to deal with syntax-heavy language is with files, via
‘compile-file’ and friends. However it is possible to invoke a
language’s reader on a port, and then compile the resulting expression
(which is a datum at that point). For more information, *Note
Compilation::.
For more details on introspecting aspects of different languages,
*Note Compiler Tower::.
6.24.2 Emacs Lisp
-----------------
Emacs Lisp (Elisp) is a dynamically-scoped Lisp dialect used in the
Emacs editor. *Note Overview: (elisp)top, for more information on Emacs
Lisp.
We hope that eventually Guile’s implementation of Elisp will be good
enough to replace Emacs’ own implementation of Elisp. For that reason,
we have thought long and hard about how to support the various features
of Elisp in a performant and compatible manner.
Readers familiar with Emacs Lisp might be curious about how exactly
these various Elisp features are supported in Guile. The rest of this
section focuses on addressing these concerns of the Elisp elect.
6.24.2.1 Nil
............
‘nil’ in ELisp is an amalgam of Scheme’s ‘#f’ and ‘'()’. It is false,
and it is the end-of-list; thus it is a boolean, and a list as well.
Guile has chosen to support ‘nil’ as a separate value, distinct from
‘#f’ and ‘'()’. This allows existing Scheme and Elisp code to maintain
their current semantics. ‘nil’, which in Elisp would just be written
and read as ‘nil’, in Scheme has the external representation ‘#nil’.
In Elisp code, ‘#nil’, ‘#f’, and ‘'()’ behave like ‘nil’, in the
sense that they are all interpreted as ‘nil’ by Elisp ‘if’, ‘cond’,
‘when’, ‘not’, ‘null’, etc. To test whether Elisp would interpret an
object as ‘nil’ from within Scheme code, use ‘nil?’:
-- Scheme Procedure: nil? obj
Return ‘#t’ if OBJ would be interpreted as ‘nil’ by Emacs Lisp
code, else return ‘#f’.
(nil? #nil) ⇒ #t
(nil? #f) ⇒ #t
(nil? '()) ⇒ #t
(nil? 3) ⇒ #f
This decision to have ‘nil’ as a low-level distinct value facilitates
interoperability between the two languages. Guile has chosen to have
Scheme deal with ‘nil’ as follows:
(boolean? #nil) ⇒ #t
(not #nil) ⇒ #t
(null? #nil) ⇒ #t
And in C, one has:
scm_is_bool (SCM_ELISP_NIL) ⇒ 1
scm_is_false (SCM_ELISP_NIL) ⇒ 1
scm_is_null (SCM_ELISP_NIL) ⇒ 1
In this way, a version of ‘fold’ written in Scheme can correctly fold
a function written in Elisp (or in fact any other language) over a
nil-terminated list, as Elisp makes. The converse holds as well; a
version of ‘fold’ written in Elisp can fold over a ‘'()’-terminated
list, as made by Scheme.
On a low level, the bit representations for ‘#f’, ‘#t’, ‘nil’, and
‘'()’ are made in such a way that they differ by only one bit, and so a
test for, for example, ‘#f’-or-‘nil’ may be made very efficiently. See
‘libguile/boolean.h’, for more information.
Equality
........
Since Scheme’s ‘equal?’ must be transitive, and ‘'()’ is not ‘equal?’ to
‘#f’, to Scheme ‘nil’ is not ‘equal?’ to ‘#f’ or ‘'()’.
(eq? #f '()) ⇒ #f
(eq? #nil '()) ⇒ #f
(eq? #nil #f) ⇒ #f
(eqv? #f '()) ⇒ #f
(eqv? #nil '()) ⇒ #f
(eqv? #nil #f) ⇒ #f
(equal? #f '()) ⇒ #f
(equal? #nil '()) ⇒ #f
(equal? #nil #f) ⇒ #f
However, in Elisp, ‘'()’, ‘#f’, and ‘nil’ are all ‘equal’ (though not
‘eq’).
(defvar f (make-scheme-false))
(defvar eol (make-scheme-null))
(eq f eol) ⇒ nil
(eq nil eol) ⇒ nil
(eq nil f) ⇒ nil
(equal f eol) ⇒ t
(equal nil eol) ⇒ t
(equal nil f) ⇒ t
These choices facilitate interoperability between Elisp and Scheme
code, but they are not perfect. Some code that is correct standard
Scheme is not correct in the presence of a second false and null value.
For example:
(define (truthiness x)
(if (eq? x #f)
#f
#t))
This code seems to be meant to test a value for truth, but now that
there are two false values, ‘#f’ and ‘nil’, it is no longer correct.
Similarly, there is the loop:
(define (my-length l)
(let lp ((l l) (len 0))
(if (eq? l '())
len
(lp (cdr l) (1+ len)))))
Here, ‘my-length’ will raise an error if L is a ‘nil’-terminated
list.
Both of these examples are correct standard Scheme, but, depending on
what they really want to do, they are not correct Guile Scheme.
Correctly written, they would test the _properties_ of falsehood or
nullity, not the individual members of that set. That is to say, they
should use ‘not’ or ‘null?’ to test for falsehood or nullity, not ‘eq?’
or ‘memv’ or the like.
Fortunately, using ‘not’ and ‘null?’ is in good style, so all
well-written standard Scheme programs are correct, in Guile Scheme.
Here are correct versions of the above examples:
(define (truthiness* x)
(if (not x)
#f
#t))
;; or: (define (t* x) (not (not x)))
;; or: (define (t** x) x)
(define (my-length* l)
(let lp ((l l) (len 0))
(if (null? l)
len
(lp (cdr l) (1+ len)))))
This problem has a mirror-image case in Elisp:
(defun my-falsep (x)
(if (eq x nil)
t
nil))
Guile can warn when compiling code that has equality comparisons with
‘#f’, ‘'()’, or ‘nil’. *Note Compilation::, for details.
6.24.2.2 Dynamic Binding
........................
In contrast to Scheme, which uses “lexical scoping”, Emacs Lisp scopes
its variables dynamically. Guile supports dynamic scoping with its
“fluids” facility. *Note Fluids and Dynamic States::, for more
information.
6.24.2.3 Other Elisp Features
.............................
Buffer-local and mode-local variables should be mentioned here, along
with buckybits on characters, Emacs primitive data types, the
Lisp-2-ness of Elisp, and other things. Contributions to the
documentation are most welcome!
6.24.3 ECMAScript
-----------------
ECMAScript
(http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-262.pdf)
was not the first non-Schemey language implemented by Guile, but it was
the first implemented for Guile’s bytecode compiler. The goal was to
support ECMAScript version 3.1, a relatively small language, but the
implementor was completely irresponsible and got distracted by other
things before finishing the standard library, and even some bits of the
syntax. So, ECMAScript does deserve a mention in the manual, but it
doesn’t deserve an endorsement until its implementation is completed,
perhaps by some more responsible hacker.
In the meantime, the charitable user might investigate such
invocations as ‘,L ecmascript’ and ‘cat
test-suite/tests/ecmascript.test’.
6.25 Support for Internationalization
=====================================
Guile provides internationalization(1) support for Scheme programs in
two ways. First, procedures to manipulate text and data in a way that
conforms to particular cultural conventions (i.e., in a
“locale-dependent” way) are provided in the ‘(ice-9 i18n)’. Second,
Guile allows the use of GNU ‘gettext’ to translate program message
strings.
---------- Footnotes ----------
(1) For concision and style, programmers often like to refer to
internationalization as “i18n”.
6.25.1 Internationalization with Guile
--------------------------------------
In order to make use of the functions described thereafter, the ‘(ice-9
i18n)’ module must be imported in the usual way:
(use-modules (ice-9 i18n))
The ‘(ice-9 i18n)’ module provides procedures to manipulate text and
other data in a way that conforms to the cultural conventions chosen by
the user. Each region of the world or language has its own customs to,
for instance, represent real numbers, classify characters, collate text,
etc. All these aspects comprise the so-called “cultural conventions” of
that region or language.
Computer systems typically refer to a set of cultural conventions as
a “locale”. For each particular aspect that comprise those cultural
conventions, a “locale category” is defined. For instance, the way
characters are classified is defined by the ‘LC_CTYPE’ category, while
the language in which program messages are issued to the user is defined
by the ‘LC_MESSAGES’ category (*note General Locale Information:
Locales. for details).
The procedures provided by this module allow the development of
programs that adapt automatically to any locale setting. As we will see
later, many of these procedures can optionally take a “locale object”
argument. This additional argument defines the locale settings that
must be followed by the invoked procedure. When it is omitted, then the
current locale settings of the process are followed (*note ‘setlocale’:
Locales.).
The following procedures allow the manipulation of such locale
objects.
-- Scheme Procedure: make-locale category-list locale-name
[base-locale]
-- C Function: scm_make_locale (category_list, locale_name,
base_locale)
Return a reference to a data structure representing a set of locale
datasets. LOCALE-NAME should be a string denoting a particular
locale (e.g., ‘"aa_DJ"’) and CATEGORY-LIST should be either a list
of locale categories or a single category as used with ‘setlocale’
(*note ‘setlocale’: Locales.). Optionally, if ‘base-locale’ is
passed, it should be a locale object denoting settings for
categories not listed in CATEGORY-LIST.
The following invocation creates a locale object that combines the
use of Swedish for messages and character classification with the
default settings for the other categories (i.e., the settings of
the default ‘C’ locale which usually represents conventions in use
in the USA):
(make-locale (list LC_MESSAGES LC_CTYPE) "sv_SE")
The following example combines the use of Esperanto messages and
conventions with monetary conventions from Croatia:
(make-locale LC_MONETARY "hr_HR"
(make-locale LC_ALL "eo_EO"))
A ‘system-error’ exception (*note Handling Errors::) is raised by
‘make-locale’ when LOCALE-NAME does not match any of the locales
compiled on the system. Note that on non-GNU systems, this error
may be raised later, when the locale object is actually used.
-- Scheme Procedure: locale? obj
-- C Function: scm_locale_p (obj)
Return true if OBJ is a locale object.
-- Scheme Variable: %global-locale
-- C Variable: scm_global_locale
This variable is bound to a locale object denoting the current
process locale as installed using ‘setlocale ()’ (*note Locales::).
It may be used like any other locale object, including as a third
argument to ‘make-locale’, for instance.
6.25.2 Text Collation
---------------------
The following procedures provide support for text collation, i.e.,
locale-dependent string and character sorting.
-- Scheme Procedure: string-locale? s1 s2 [locale]
-- C Function: scm_string_locale_gt (s1, s2, locale)
-- Scheme Procedure: string-locale-ci? s1 s2 [locale]
-- C Function: scm_string_locale_ci_gt (s1, s2, locale)
Compare strings S1 and S2 in a locale-dependent way. If LOCALE is
provided, it should be locale object (as returned by ‘make-locale’)
and will be used to perform the comparison; otherwise, the current
system locale is used. For the ‘-ci’ variants, the comparison is
made in a case-insensitive way.
-- Scheme Procedure: string-locale-ci=? s1 s2 [locale]
-- C Function: scm_string_locale_ci_eq (s1, s2, locale)
Compare strings S1 and S2 in a case-insensitive, and
locale-dependent way. If LOCALE is provided, it should be a locale
object (as returned by ‘make-locale’) and will be used to perform
the comparison; otherwise, the current system locale is used.
-- Scheme Procedure: char-locale? c1 c2 [locale]
-- C Function: scm_char_locale_gt (c1, c2, locale)
-- Scheme Procedure: char-locale-ci? c1 c2 [locale]
-- C Function: scm_char_locale_ci_gt (c1, c2, locale)
Compare characters C1 and C2 according to either LOCALE (a locale
object as returned by ‘make-locale’) or the current locale. For
the ‘-ci’ variants, the comparison is made in a case-insensitive
way.
-- Scheme Procedure: char-locale-ci=? c1 c2 [locale]
-- C Function: scm_char_locale_ci_eq (c1, c2, locale)
Return true if character C1 is equal to C2, in a case insensitive
way according to LOCALE or to the current locale.
6.25.3 Character Case Mapping
-----------------------------
The procedures below provide support for “character case mapping”, i.e.,
to convert characters or strings to their upper-case or lower-case
equivalent. Note that SRFI-13 provides procedures that look similar
(*note Alphabetic Case Mapping::). However, the SRFI-13 procedures are
locale-independent. Therefore, they do not take into account
specificities of the customs in use in a particular language or region
of the world. For instance, while most languages using the Latin
alphabet map lower-case letter “i” to upper-case letter “I”, Turkish
maps lower-case “i” to “Latin capital letter I with dot above”. The
following procedures allow programmers to provide idiomatic character
mapping.
-- Scheme Procedure: char-locale-downcase chr [locale]
-- C Function: scm_char_locale_upcase (chr, locale)
Return the lowercase character that corresponds to CHR according to
either LOCALE or the current locale.
-- Scheme Procedure: char-locale-upcase chr [locale]
-- C Function: scm_char_locale_downcase (chr, locale)
Return the uppercase character that corresponds to CHR according to
either LOCALE or the current locale.
-- Scheme Procedure: char-locale-titlecase chr [locale]
-- C Function: scm_char_locale_titlecase (chr, locale)
Return the titlecase character that corresponds to CHR according to
either LOCALE or the current locale.
-- Scheme Procedure: string-locale-upcase str [locale]
-- C Function: scm_string_locale_upcase (str, locale)
Return a new string that is the uppercase version of STR according
to either LOCALE or the current locale.
-- Scheme Procedure: string-locale-downcase str [locale]
-- C Function: scm_string_locale_downcase (str, locale)
Return a new string that is the down-case version of STR according
to either LOCALE or the current locale.
-- Scheme Procedure: string-locale-titlecase str [locale]
-- C Function: scm_string_locale_titlecase (str, locale)
Return a new string that is the titlecase version of STR according
to either LOCALE or the current locale.
6.25.4 Number Input and Output
------------------------------
The following procedures allow programs to read and write numbers
written according to a particular locale. As an example, in English,
“ten thousand and a half” is usually written ‘10,000.5’ while in French
it is written ‘10 000,5’. These procedures allow such differences to be
taken into account.
-- Scheme Procedure: locale-string->integer str [base [locale]]
-- C Function: scm_locale_string_to_integer (str, base, locale)
Convert string STR into an integer according to either LOCALE (a
locale object as returned by ‘make-locale’) or the current process
locale. If BASE is specified, then it determines the base of the
integer being read (e.g., ‘16’ for an hexadecimal number, ‘10’ for
a decimal number); by default, decimal numbers are read. Return
two values (*note Multiple Values::): an integer (on success) or
‘#f’, and the number of characters read from STR (‘0’ on failure).
This function is based on the C library’s ‘strtol’ function (*note
‘strtol’: (libc)Parsing of Integers.).
-- Scheme Procedure: locale-string->inexact str [locale]
-- C Function: scm_locale_string_to_inexact (str, locale)
Convert string STR into an inexact number according to either
LOCALE (a locale object as returned by ‘make-locale’) or the
current process locale. Return two values (*note Multiple
Values::): an inexact number (on success) or ‘#f’, and the number
of characters read from STR (‘0’ on failure).
This function is based on the C library’s ‘strtod’ function (*note
‘strtod’: (libc)Parsing of Floats.).
-- Scheme Procedure: number->locale-string number [fraction-digits
[locale]]
Convert NUMBER (an inexact) into a string according to the cultural
conventions of either LOCALE (a locale object) or the current
locale. By default, print as many fractional digits as necessary,
up to an upper bound. Optionally, FRACTION-DIGITS may be bound to
an integer specifying the number of fractional digits to be
displayed.
-- Scheme Procedure: monetary-amount->locale-string amount intl?
[locale]
Convert AMOUNT (an inexact denoting a monetary amount) into a
string according to the cultural conventions of either LOCALE (a
locale object) or the current locale. If INTL? is true, then the
international monetary format for the given locale is used (*note
international and locale monetary formats: (libc)Currency Symbol.).
6.25.5 Accessing Locale Information
-----------------------------------
It is sometimes useful to obtain very specific information about a
locale such as the word it uses for days or months, its format for
representing floating-point figures, etc. The ‘(ice-9 i18n)’ module
provides support for this in a way that is similar to the libc functions
‘nl_langinfo ()’ and ‘localeconv ()’ (*note accessing locale information
from C: (libc)Locale Information.). The available functions are listed
below.
-- Scheme Procedure: locale-encoding [locale]
Return the name of the encoding (a string whose interpretation is
system-dependent) of either LOCALE or the current locale.
The following functions deal with dates and times.
-- Scheme Procedure: locale-day day [locale]
-- Scheme Procedure: locale-day-short day [locale]
-- Scheme Procedure: locale-month month [locale]
-- Scheme Procedure: locale-month-short month [locale]
Return the word (a string) used in either LOCALE or the current
locale to name the day (or month) denoted by DAY (or MONTH), an
integer between 1 and 7 (or 1 and 12). The ‘-short’ variants
provide an abbreviation instead of a full name.
-- Scheme Procedure: locale-am-string [locale]
-- Scheme Procedure: locale-pm-string [locale]
Return a (potentially empty) string that is used to denote ante
meridiem (or post meridiem) hours in 12-hour format.
-- Scheme Procedure: locale-date+time-format [locale]
-- Scheme Procedure: locale-date-format [locale]
-- Scheme Procedure: locale-time-format [locale]
-- Scheme Procedure: locale-time+am/pm-format [locale]
-- Scheme Procedure: locale-era-date-format [locale]
-- Scheme Procedure: locale-era-date+time-format [locale]
-- Scheme Procedure: locale-era-time-format [locale]
These procedures return format strings suitable to ‘strftime’
(*note Time::) that may be used to display (part of) a date/time
according to certain constraints and to the conventions of either
LOCALE or the current locale (*note the ‘nl_langinfo ()’ items:
(libc)The Elegant and Fast Way.).
-- Scheme Procedure: locale-era [locale]
-- Scheme Procedure: locale-era-year [locale]
These functions return, respectively, the era and the year of the
relevant era used in LOCALE or the current locale. Most locales do
not define this value. In this case, the empty string is returned.
An example of a locale that does define this value is the Japanese
one.
The following procedures give information about number
representation.
-- Scheme Procedure: locale-decimal-point [locale]
-- Scheme Procedure: locale-thousands-separator [locale]
These functions return a string denoting the representation of the
decimal point or that of the thousand separator (respectively) for
either LOCALE or the current locale.
-- Scheme Procedure: locale-digit-grouping [locale]
Return a (potentially circular) list of integers denoting how
digits of the integer part of a number are to be grouped, starting
at the decimal point and going to the left. The list contains
integers indicating the size of the successive groups, from right
to left. If the list is non-circular, then no grouping occurs for
digits beyond the last group.
For instance, if the returned list is a circular list that contains
only ‘3’ and the thousand separator is ‘","’ (as is the case with
English locales), then the number ‘12345678’ should be printed
‘12,345,678’.
The following procedures deal with the representation of monetary
amounts. Some of them take an additional INTL? argument (a boolean)
that tells whether the international or local monetary conventions for
the given locale are to be used.
-- Scheme Procedure: locale-monetary-decimal-point [locale]
-- Scheme Procedure: locale-monetary-thousands-separator [locale]
-- Scheme Procedure: locale-monetary-grouping [locale]
These are the monetary counterparts of the above procedures. These
procedures apply to monetary amounts.
-- Scheme Procedure: locale-currency-symbol intl? [locale]
Return the currency symbol (a string) of either LOCALE or the
current locale.
The following example illustrates the difference between the local
and international monetary formats:
(define us (make-locale LC_MONETARY "en_US"))
(locale-currency-symbol #f us)
⇒ "-$"
(locale-currency-symbol #t us)
⇒ "USD "
-- Scheme Procedure: locale-monetary-fractional-digits intl? [locale]
Return the number of fractional digits to be used when printing
monetary amounts according to either LOCALE or the current locale.
If the locale does not specify it, then ‘#f’ is returned.
-- Scheme Procedure: locale-currency-symbol-precedes-positive? intl?
[locale]
-- Scheme Procedure: locale-currency-symbol-precedes-negative? intl?
[locale]
-- Scheme Procedure: locale-positive-separated-by-space? intl? [locale]
-- Scheme Procedure: locale-negative-separated-by-space? intl? [locale]
These procedures return a boolean indicating whether the currency
symbol should precede a positive/negative number, and whether a
whitespace should be inserted between the currency symbol and a
positive/negative amount.
-- Scheme Procedure: locale-monetary-positive-sign [locale]
-- Scheme Procedure: locale-monetary-negative-sign [locale]
Return a string denoting the positive (respectively negative) sign
that should be used when printing a monetary amount.
-- Scheme Procedure: locale-positive-sign-position
-- Scheme Procedure: locale-negative-sign-position
These functions return a symbol telling where a sign of a
positive/negative monetary amount is to appear when printing it.
The possible values are:
‘parenthesize’
The currency symbol and quantity should be surrounded by
parentheses.
‘sign-before’
Print the sign string before the quantity and currency symbol.
‘sign-after’
Print the sign string after the quantity and currency symbol.
‘sign-before-currency-symbol’
Print the sign string right before the currency symbol.
‘sign-after-currency-symbol’
Print the sign string right after the currency symbol.
‘unspecified’
Unspecified. We recommend you print the sign after the
currency symbol.
Finally, the two following procedures may be helpful when programming
user interfaces:
-- Scheme Procedure: locale-yes-regexp [locale]
-- Scheme Procedure: locale-no-regexp [locale]
Return a string that can be used as a regular expression to
recognize a positive (respectively, negative) response to a yes/no
question. For the C locale, the default values are typically
‘"^[yY]"’ and ‘"^[nN]"’, respectively.
Here is an example:
(use-modules (ice-9 rdelim))
(format #t "Does Guile rock?~%")
(let lp ((answer (read-line)))
(cond ((string-match (locale-yes-regexp) answer)
(format #t "High fives!~%"))
((string-match (locale-no-regexp) answer)
(format #t "How about now? Does it rock yet?~%")
(lp (read-line)))
(else
(format #t "What do you mean?~%")
(lp (read-line)))))
For an internationalized yes/no string output, ‘gettext’ should be
used (*note Gettext Support::).
Example uses of some of these functions are the implementation of the
‘number->locale-string’ and ‘monetary-amount->locale-string’ procedures
(*note Number Input and Output::), as well as that the SRFI-19 date and
time conversion to/from strings (*note SRFI-19::).
6.25.6 Gettext Support
----------------------
Guile provides an interface to GNU ‘gettext’ for translating message
strings (*note (gettext)Introduction::).
Messages are collected in domains, so different libraries and
programs maintain different message catalogues. The DOMAIN parameter in
the functions below is a string (it becomes part of the message catalog
filename).
When ‘gettext’ is not available, or if Guile was configured
‘--without-nls’, dummy functions doing no translation are provided.
When ‘gettext’ support is available in Guile, the ‘i18n’ feature is
provided (*note Feature Tracking::).
-- Scheme Procedure: gettext msg [domain [category]]
-- C Function: scm_gettext (msg, domain, category)
Return the translation of MSG in DOMAIN. DOMAIN is optional and
defaults to the domain set through ‘textdomain’ below. CATEGORY is
optional and defaults to ‘LC_MESSAGES’ (*note Locales::).
Normal usage is for MSG to be a literal string. ‘xgettext’ can
extract those from the source to form a message catalogue ready for
translators (*note Invoking the ‘xgettext’ Program:
(gettext)xgettext Invocation.).
(display (gettext "You are in a maze of twisty passages."))
It is conventional to use ‘G_’ as a shorthand for ‘gettext’.(1)
Libraries can define ‘G_’ in such a way to look up translations
using its specific DOMAIN, allowing different parts of a program to
have different translation sources.
(define (G_ msg) (gettext msg "mylibrary"))
(display (G_ "File not found."))
‘G_’ is also a good place to perhaps strip disambiguating extra
text from the message string, as for instance in *note How to use
‘gettext’ in GUI programs: (gettext)GUI program problems.
-- Scheme Procedure: ngettext msg msgplural n [domain [category]]
-- C Function: scm_ngettext (msg, msgplural, n, domain, category)
Return the translation of MSG/MSGPLURAL in DOMAIN, with a plural
form chosen appropriately for the number N. DOMAIN is optional and
defaults to the domain set through ‘textdomain’ below. CATEGORY is
optional and defaults to ‘LC_MESSAGES’ (*note Locales::).
MSG is the singular form, and MSGPLURAL the plural. When no
translation is available, MSG is used if N = 1, or MSGPLURAL
otherwise. When translated, the message catalogue can have a
different rule, and can have more than two possible forms.
As per ‘gettext’ above, normal usage is for MSG and MSGPLURAL to be
literal strings, since ‘xgettext’ can extract them from the source
to build a message catalogue. For example,
(define (done n)
(format #t (ngettext "~a file processed\n"
"~a files processed\n" n)
n))
(done 1) ⊣ 1 file processed
(done 3) ⊣ 3 files processed
It’s important to use ‘ngettext’ rather than plain ‘gettext’ for
plurals, since the rules for singular and plural forms in English
are not the same in other languages. Only ‘ngettext’ will allow
translators to give correct forms (*note Additional functions for
plural forms: (gettext)Plural forms.).
-- Scheme Procedure: textdomain [domain]
-- C Function: scm_textdomain (domain)
Get or set the default gettext domain. When called with no
parameter the current domain is returned. When called with a
parameter, DOMAIN is set as the current domain, and that new value
returned. For example,
(textdomain "myprog")
⇒ "myprog"
-- Scheme Procedure: bindtextdomain domain [directory]
-- C Function: scm_bindtextdomain (domain, directory)
Get or set the directory under which to find message files for
DOMAIN. When called without a DIRECTORY the current setting is
returned. When called with a DIRECTORY, DIRECTORY is set for
DOMAIN and that new setting returned. For example,
(bindtextdomain "myprog" "/my/tree/share/locale")
⇒ "/my/tree/share/locale"
When using Autoconf/Automake, an application should arrange for the
configured ‘localedir’ to get into the program (by substituting, or
by generating a config file) and set that for its domain. This
ensures the catalogue can be found even when installed in a
non-standard location.
-- Scheme Procedure: bind-textdomain-codeset domain [encoding]
-- C Function: scm_bind_textdomain_codeset (domain, encoding)
Get or set the text encoding to be used by ‘gettext’ for messages
from DOMAIN. ENCODING is a string, the name of a coding system,
for instance "8859_1". (On a Unix/POSIX system the ‘iconv’ program
can list all available encodings.)
When called without an ENCODING the current setting is returned, or
‘#f’ if none yet set. When called with an ENCODING, it is set for
DOMAIN and that new setting returned. For example,
(bind-textdomain-codeset "myprog")
⇒ #f
(bind-textdomain-codeset "myprog" "latin-9")
⇒ "latin-9"
The encoding requested can be different from the translated data
file, messages will be recoded as necessary. But note that when
there is no translation, ‘gettext’ returns its MSG unchanged, ie.
without any recoding. For that reason source message strings are
best as plain ASCII.
Currently Guile has no understanding of multi-byte characters, and
string functions won’t recognise character boundaries in multi-byte
strings. An application will at least be able to pass such strings
through to some output though. Perhaps this will change in the
future.
---------- Footnotes ----------
(1) Users of ‘gettext’ might be a bit surprised that ‘G_’ is the
conventional abbreviation for ‘gettext’. In most other languages, the
conventional shorthand is ‘_’. Guile uses ‘G_’ because ‘_’ is already
taken, as it is bound to a syntactic keyword used by ‘syntax-rules’,
‘match’, and other macros.
6.26 Debugging Infrastructure
=============================
In order to understand Guile’s debugging facilities, you first need to
understand a little about how Guile represents the Scheme control stack.
With that in place we explain the low level trap calls that the virtual
machine can be configured to make, and the trap and breakpoint
infrastructure that builds on top of those calls.
6.26.1 Evaluation and the Scheme Stack
--------------------------------------
The idea of the Scheme stack is central to a lot of debugging. The
Scheme stack is a reified representation of the pending function returns
in an expression’s continuation. As Guile implements function calls
using a stack, this reification takes the form of a number of nested
stack frames, each of which corresponds to the application of a
procedure to a set of arguments.
A Scheme stack always exists implicitly, and can be summoned into
concrete existence as a first-class Scheme value by the ‘make-stack’
call, so that an introspective Scheme program – such as a debugger – can
present it in some way and allow the user to query its details. The
first thing to understand, therefore, is how Guile’s function call
convention creates the stack.
Broadly speaking, Guile represents all control flow on a stack.
Calling a function involves pushing an empty frame on the stack, then
evaluating the procedure and its arguments, then fixing up the new frame
so that it points to the old one. Frames on the stack are thus linked
together. A tail call is the same, except it reuses the existing frame
instead of pushing on a new one.
In this way, the only frames that are on the stack are “active”
frames, frames which need to do some work before the computation is
complete. On the other hand, a function that has tail-called another
function will not be on the stack, as it has no work left to do.
Therefore, when an error occurs in a running program, or the program
hits a breakpoint, or in fact at any point that the programmer chooses,
its state at that point can be represented by a “stack” of all the
procedure applications that are logically in progress at that time, each
of which is known as a “frame”. The programmer can learn more about the
program’s state at that point by inspecting the stack and its frames.
6.26.1.1 Stack Capture
......................
A Scheme program can use the ‘make-stack’ primitive anywhere in its
code, with first arg ‘#t’, to construct a Scheme value that describes
the Scheme stack at that point.
(make-stack #t)
⇒
#
Use ‘start-stack’ to limit the stack extent captured by future
‘make-stack’ calls.
-- Scheme Procedure: make-stack obj arg ...
-- C Function: scm_make_stack (obj, args)
Create a new stack. If OBJ is ‘#t’, the current evaluation stack
is used for creating the stack frames, otherwise the frames are
taken from OBJ (which must be a continuation or a frame object).
ARG ... can be any combination of integer, procedure, address
range, and prompt tag values.
These values specify various ways of cutting away uninteresting
stack frames from the top and bottom of the stack that ‘make-stack’
returns. They come in pairs like this: ‘(INNER_CUT_1 OUTER_CUT_1
INNER_CUT_2 OUTER_CUT_2 ...)’.
Each INNER_CUT_I can be an integer, a procedure, an address range,
or a prompt tag. An integer means to cut away exactly that number
of frames. A procedure means to cut away all frames up to but
excluding the frame whose procedure matches the specified one. An
address range is a pair of integers indicating the low and high
addresses of a procedure’s code, and is the same as cutting away to
a procedure (though with less work). Anything else is interpreted
as a prompt tag which cuts away all frames that are inside a prompt
with the given tag.
Each OUTER_CUT_I can likewise be an integer, a procedure, an
address range, or a prompt tag. An integer means to cut away that
number of frames. A procedure means to cut away frames down to but
excluding the frame whose procedure matches the specified one. An
address range is the same, but with the procedure’s code specified
as an address range. Anything else is taken to be a prompt tag,
which cuts away all frames that are outside a prompt with the given
tag.
If the OUTER_CUT_I of the last pair is missing, it is taken as 0.
-- Scheme Syntax: start-stack id exp
Evaluate EXP on a new calling stack with identity ID. If EXP is
interrupted during evaluation, backtraces will not display frames
farther back than EXP’s top-level form. This macro is a way of
artificially limiting backtraces and stack procedures, largely as a
convenience to the user.
6.26.1.2 Stacks
...............
-- Scheme Procedure: stack? obj
-- C Function: scm_stack_p (obj)
Return ‘#t’ if OBJ is a calling stack.
-- Scheme Procedure: stack-id stack
-- C Function: scm_stack_id (stack)
Return the identifier given to STACK by ‘start-stack’.
-- Scheme Procedure: stack-length stack
-- C Function: scm_stack_length (stack)
Return the length of STACK.
-- Scheme Procedure: stack-ref stack index
-- C Function: scm_stack_ref (stack, index)
Return the INDEX’th frame from STACK.
-- Scheme Procedure: display-backtrace stack port [first [depth
[highlights]]]
-- C Function: scm_display_backtrace_with_highlights (stack, port,
first, depth, highlights)
-- C Function: scm_display_backtrace (stack, port, first, depth)
Display a backtrace to the output port PORT. STACK is the stack to
take the backtrace from, FIRST specifies where in the stack to
start and DEPTH how many frames to display. FIRST and DEPTH can be
‘#f’, which means that default values will be used. If HIGHLIGHTS
is given it should be a list; the elements of this list will be
highlighted wherever they appear in the backtrace.
6.26.1.3 Frames
...............
-- Scheme Procedure: frame? obj
-- C Function: scm_frame_p (obj)
Return ‘#t’ if OBJ is a stack frame.
-- Scheme Procedure: frame-previous frame
-- C Function: scm_frame_previous (frame)
Return the previous frame of FRAME, or ‘#f’ if FRAME is the first
frame in its stack.
-- Scheme Procedure: frame-procedure-name frame
-- C Function: scm_frame_procedure_name (frame)
Return the name of the procedure being applied in FRAME, as a
symbol, or ‘#f’ if the procedure has no name.
-- Scheme Procedure: frame-arguments frame
-- C Function: scm_frame_arguments (frame)
Return the arguments of FRAME.
-- Scheme Procedure: frame-address frame
-- Scheme Procedure: frame-instruction-pointer frame
-- Scheme Procedure: frame-stack-pointer frame
Accessors for the three VM registers associated with this frame:
the frame pointer (fp), instruction pointer (ip), and stack pointer
(sp), respectively. *Note VM Concepts::, for more information.
-- Scheme Procedure: frame-dynamic-link frame
-- Scheme Procedure: frame-return-address frame
-- Scheme Procedure: frame-mv-return-address frame
Accessors for the three saved VM registers in a frame: the previous
frame pointer, the single-value return address, and the
multiple-value return address. *Note Stack Layout::, for more
information.
-- Scheme Procedure: frame-bindings frame
Return a list of binding records indicating the local variables
that are live in a frame.
-- Scheme Procedure: frame-lookup-binding frame var
Fetch the bindings in FRAME, and return the first one whose name is
VAR, or ‘#f’ otherwise.
-- Scheme Procedure: binding-index binding
-- Scheme Procedure: binding-name binding
-- Scheme Procedure: binding-slot binding
-- Scheme Procedure: binding-representation binding
Accessors for the various fields in a binding. The implicit
“callee” argument is index 0, the first argument is index 1, and so
on to the end of the arguments. After that are temporary
variables. Note that if a variable is dead, it might not be
available.
-- Scheme Procedure: binding-ref binding
-- Scheme Procedure: binding-set! binding val
Accessors for the values of local variables in a frame.
-- Scheme Procedure: display-application frame [port [indent]]
-- C Function: scm_display_application (frame, port, indent)
Display a procedure application FRAME to the output port PORT.
INDENT specifies the indentation of the output.
Additionally, the ‘(system vm frame)’ module defines a number of
higher-level introspective procedures, for example to retrieve the names
of local variables, and the source location to correspond to a frame.
See its source code for more details.
6.26.2 Source Properties
------------------------
How best to associate source locations with datums parsed from a port?
The right way to do this is to annotate all components of each parsed
datum. *Note Annotated Scheme Read::, for more on ‘read-syntax’.
Guile only switched to use ‘read-syntax’ in 2021, however. For the
previous thirty years, it used a mechanism known as “source properties”.
As Guile reads in Scheme code from file or from standard input, it
can record the file name, line number and column number where each
expression begins in a side table.
The way that this side table associates datums with source properties
has a limitation, however: Guile can only associate source properties
with freshly allocated objects. This notably excludes individual
symbols, keywords, characters, booleans, or small integers. This
limitation finally motivated the switch to ‘read-syntax’.
-- Scheme Procedure: supports-source-properties? obj
-- C Function: scm_supports_source_properties_p (obj)
Return #t if source properties can be associated with OBJ,
otherwise return #f.
The recording of source properties is controlled by the read option
named “positions” (*note Scheme Read::). This option is switched _on_
by default. Now that ‘read-syntax’ is available, however, Guile may
change the default for this flag to off in the future.
The following procedures can be used to access and set the source
properties of read expressions.
-- Scheme Procedure: set-source-properties! obj alist
-- C Function: scm_set_source_properties_x (obj, alist)
Install the association list ALIST as the source property list for
OBJ.
-- Scheme Procedure: set-source-property! obj key datum
-- C Function: scm_set_source_property_x (obj, key, datum)
Set the source property of object OBJ, which is specified by KEY to
DATUM. Normally, the key will be a symbol.
-- Scheme Procedure: source-properties obj
-- C Function: scm_source_properties (obj)
Return the source property association list of OBJ.
-- Scheme Procedure: source-property obj key
-- C Function: scm_source_property (obj, key)
Return the property specified by KEY from OBJ’s source properties.
If the ‘positions’ reader option is enabled, supported expressions
will have values set for the ‘filename’, ‘line’ and ‘column’ properties.
Source properties are also associated with syntax objects.
Procedural macros can get at the source location of their input using
the ‘syntax-source’ accessor. *Note Syntax Transformer Helpers::, for
more.
Guile also defines a couple of convenience macros built on
‘syntax-source’:
-- Scheme Syntax: current-source-location
Expands to the source properties corresponding to the location of
the ‘(current-source-location)’ form.
-- Scheme Syntax: current-filename
Expands to the current filename: the filename that the
‘(current-filename)’ form appears in. Expands to ‘#f’ if this
information is unavailable.
If you’re stuck with defmacros (*note Defmacros::), and want to
preserve source information, the following helper function might be
useful to you:
-- Scheme Procedure: cons-source xorig x y
-- C Function: scm_cons_source (xorig, x, y)
Create and return a new pair whose car and cdr are X and Y. Any
source properties associated with XORIG are also associated with
the new pair.
6.26.3 Programmatic Error Handling
----------------------------------
For better or for worse, all programs have bugs, and dealing with bugs
is part of programming. This section deals with that class of bugs that
causes an exception to be raised – from your own code, from within a
library, or from Guile itself.
6.26.3.1 Catching Exceptions
............................
A common requirement is to be able to show as much useful context as
possible when a Scheme program hits an error. The most immediate
information about an error is the kind of error that it is – such as
“division by zero” – and any parameters that the code which signalled
the error chose explicitly to provide. This information originates with
the ‘error’ or ‘raise-exception’ call (or their C code equivalents, if
the error is detected by C code) that signals the error, and is passed
automatically to the handler procedure of the innermost applicable
exception handler.
Therefore, to catch errors that occur within a chunk of Scheme code,
and to intercept basic information about those errors, you need to
execute that code inside the dynamic context of a
‘with-exception-handler’, or the equivalent in C.
For example, to print out a message and return #f when an error
occurs, you might use:
(define (catch-all thunk)
(with-exception-handler
(lambda (exn)
(format (current-error-port)
"Uncaught exception: ~s\n" exn)
#f)
thunk
#:unwind? #t))
(catch-all
(lambda () (error "Not a vegetable: tomato")))
⊣ Uncaught exception: #<&exception-with-kind-and-args ...>
⇒ #f
*Note Exceptions::, for full details.
6.26.3.2 Pre-Unwind Debugging
.............................
Sometimes when something goes wrong, what you want is not just a
representation of the exceptional situation, but the context that
brought about that situation. The example in the previous section
passed ‘#:unwind #t’ to ‘with-exception-handler’, indicating that
‘raise-exception’ should unwind the stack before invoking the exception
handler. However if you don’t take this approach and instead let the
exception handler be invoked in the context of the ‘raise-exception’,
you can print a backtrace, launch a recursive debugger, or take other
“pre-unwind” actions.
The most basic idea would be to simply print a backtrace:
(define (call-with-backtrace thunk)
(with-exception-handler
(lambda (exn)
(backtrace)
(raise-exception exn))
thunk))
Here we use the built-in ‘backtrace’ procedure to print the
backtrace.
-- Scheme Procedure: backtrace [highlights]
-- C Function: scm_backtrace_with_highlights (highlights)
-- C Function: scm_backtrace ()
Display a backtrace of the current stack to the current output
port. If HIGHLIGHTS is given it should be a list; the elements of
this list will be highlighted wherever they appear in the
backtrace.
By re-raising the exception, ‘call-with-backtrace’ doesn’t actually
handle the error. We could define a version that instead aborts the
computation:
(use-modules (ice-9 control))
(define (call-with-backtrace thunk)
(let/ec cancel
(with-exception-handler
(lambda (exn)
(backtrace)
(cancel #f))
thunk)))
In this second example, we use an escape continuation to abort the
computation after printing the backtrace, returning ‘#f’ instead.
It could be that you want to only print a limited backtrace. In that
case, use ‘start-stack’:
(use-modules (ice-9 control))
(define (call-with-backtrace thunk)
(let/ec cancel
(start-stack 'stack-with-backtrace
(with-exception-handler
(lambda (exn)
(backtrace)
(cancel #f))
thunk))))
There are also more powerful, programmatic ways to walk the stack
using ‘make-stack’ and friends; see the API described in *note Stacks::
and *note Frames::.
6.26.3.3 call-with-error-handling
.................................
The Guile REPL code (in ‘system/repl/repl.scm’ and related files) uses a
‘catch’ with a pre-unwind handler to capture the stack when an error
occurs in an expression that was typed into the REPL, and debug that
stack interactively in the context of the error.
These procedures are available for use by user programs, in the
‘(system repl error-handling)’ module.
(use-modules (system repl error-handling))
-- Scheme Procedure: call-with-error-handling thunk [#:on-error
on-error='debug] [#:post-error post-error='catch] [#:pass-keys
pass-keys='(quit)] [#:report-keys
report-keys='(stack-overflow)] [#:trap-handler
trap-handler='debug]
Call a thunk in a context in which errors are handled.
Note that this function was written when ‘throw’/‘catch’ were the
fundamental exception handling primitives in Guile, and so exposes
some aspects of that interface (notably in the form of the
procedural handlers). Guile will probably replace this function
with a ‘call-with-standard-exception-handling’ in the future.
There are five keyword arguments:
ON-ERROR
Specifies what to do before the stack is unwound.
Valid options are ‘debug’ (the default), which will enter a
debugger; ‘pass’, in which case nothing is done, and the
exception is rethrown; or a procedure, which will be the
pre-unwind handler.
POST-ERROR
Specifies what to do after the stack is unwound.
Valid options are ‘catch’ (the default), which will silently
catch errors, returning the unspecified value; ‘report’, which
prints out a description of the error (via ‘display-error’),
and then returns the unspecified value; or a procedure, which
will be the catch handler.
TRAP-HANDLER
Specifies a trap handler: what to do when a breakpoint is hit.
Valid options are ‘debug’, which will enter the debugger;
‘pass’, which does nothing; or ‘disabled’, which disables
traps entirely. *Note Traps::, for more information.
PASS-KEYS
A set of keys to ignore, as a list.
REPORT-KEYS
A set of keys to always report even if the post-error handler
is ‘catch’, as a list.
6.26.3.4 Stack Overflow
.......................
Every time a Scheme program makes a call that is not in tail position,
it pushes a new frame onto the stack. Returning a value from a function
pops the top frame off the stack. Stack frames take up memory, and as
nobody has an infinite amount of memory, deep recursion could cause
Guile to run out of memory. Running out of stack memory is called
“stack overflow”.
Stack Limits
............
Most languages have a terrible stack overflow story. For example, in C,
if you use too much stack, your program will exhibit “undefined
behavior”, which if you are lucky means that it will crash. It’s
especially bad in C, as you neither know ahead of time how much stack
your functions use, nor the stack limit imposed by the user’s system,
and the stack limit is often quite small relative to the total memory
size.
Managed languages like Python have a better error story, as they are
defined to raise an exception on stack overflow – but like C, Python and
most dynamic languages still have a fixed stack size limit that is
usually much smaller than the heap.
Arbitrary stack limits would have an unfortunate effect on Guile
programs. For example, the following implementation of the inner loop
of ‘map’ is clean and elegant:
(define (map f l)
(if (pair? l)
(cons (f (car l))
(map f (cdr l)))
'()))
However, if there were a stack limit, that would limit the size of
lists that can be processed with this ‘map’. Eventually, you would have
to rewrite it to use iteration with an accumulator:
(define (map f l)
(let lp ((l l) (out '()))
(if (pair? l)
(lp (cdr l) (cons (f (car l)) out))
(reverse out))))
This second version is sadly not as clear, and it also allocates more
heap memory (once to build the list in reverse, and then again to
reverse the list). You would be tempted to use the destructive
‘reverse!’ to save memory and time, but then your code would not be
continuation-safe – if F returned again after the map had finished, it
would see an OUT list that had already been reversed. The recursive
‘map’ has none of these problems.
Guile has no stack limit for Scheme code. When a thread makes its
first Guile call, a small stack is allocated – just one page of memory.
Whenever that memory limit would be reached, Guile arranges to grow the
stack by a factor of two. When garbage collection happens, Guile
arranges to return the unused part of the stack to the operating system,
but without causing the stack to shrink. In this way, the stack can
grow to consume up to all memory available to the Guile process, and
when the recursive computation eventually finishes, that stack memory is
returned to the system.
Exceptional Situations
......................
Of course, it’s still possible to run out of stack memory. The most
common cause of this is program bugs that cause unbounded recursion, as
in:
(define (faulty-map f l)
(if (pair? l)
(cons (f (car l)) (faulty-map f l))
'()))
Did you spot the bug? The recursive call to ‘faulty-map’ recursed on
L, not ‘(cdr L)’. Running this program would cause Guile to use up all
memory in your system, and eventually Guile would fail to grow the
stack. At that point you have a problem: Guile needs to raise an
exception to unwind the stack and return memory to the system, but the
user might have exception handlers in place (*note Raising and Handling
Exceptions::) that want to run before the stack is unwound, and we don’t
have any stack in which to run them.
Therefore in this case, Guile raises an unwind-only exception that
does not run pre-unwind handlers. Because this is such an odd case,
Guile prints out a message on the console, in case the user was
expecting to be able to get a backtrace from any pre-unwind handler.
Runaway Recursion
.................
Still, this failure mode is not so nice. If you are running an
environment in which you are interactively building a program while it
is running, such as at a REPL, you might want to impose an artificial
stack limit on the part of your program that you are building to detect
accidental runaway recursion. For that purpose, there is
‘call-with-stack-overflow-handler’, from ‘(system vm vm)’.
(use-module (system vm vm))
-- Scheme Procedure: call-with-stack-overflow-handler limit thunk
handler
Call THUNK in an environment in which the stack limit has been
reduced to LIMIT additional words. If the limit is reached,
HANDLER (a thunk) will be invoked in the dynamic environment of the
error. For the extent of the call to HANDLER, the stack limit and
handler are restored to the values that were in place when
‘call-with-stack-overflow-handler’ was called.
Usually, HANDLER should raise an exception or abort to an outer
prompt. However if HANDLER does return, it should return a number
of additional words of stack space to allow to the inner
environment.
A stack overflow handler may only ever “credit” the inner thunk with
stack space that was available when the handler was instated. When
Guile first starts, there is no stack limit in place, so the outer
handler may allow the inner thunk an arbitrary amount of space, but any
nested stack overflow handler will not be able to consume more than its
limit.
Unlike the unwind-only exception that is thrown if Guile is unable to
grow its stack, any exception thrown by a stack overflow handler might
invoke pre-unwind handlers. Indeed, the stack overflow handler is
itself a pre-unwind handler of sorts. If the code imposing the stack
limit wants to protect itself against malicious pre-unwind handlers from
the inner thunk, it should abort to a prompt of its own making instead
of throwing an exception that might be caught by the inner thunk.
C Stack Usage
.............
It is also possible for Guile to run out of space on the C stack. If
you call a primitive procedure which then calls a Scheme procedure in a
loop, you will consume C stack space. Guile tries to detect excessive
consumption of C stack space, throwing an error when you have hit 80% of
the process’ available stack (as allocated by the operating system), or
160 kilowords in the absence of a strict limit.
For example, looping through ‘call-with-vm’, a primitive that calls a
thunk, gives us the following:
scheme@(guile-user)> (use-modules (system vm vm))
scheme@(guile-user)> (let lp () (call-with-vm lp))
ERROR: Stack overflow
Unfortunately, that’s all the information we get. Overrunning the C
stack will throw an unwind-only exception, because it’s not safe to do
very much when you are close to the C stack limit.
If you get an error like this, you can either try rewriting your code
to use less stack space, or increase the maximum stack size. To
increase the maximum stack size, use ‘debug-set!’, for example:
(debug-set! stack 200000)
The next section describes ‘debug-set!’ more thoroughly. Of course
the best thing is to have your code operate without so much resource
consumption by avoiding loops through C trampolines.
6.26.3.5 Debug options
......................
The behavior of the ‘backtrace’ procedure and of the default error
handler can be parameterized via the debug options.
-- Scheme Procedure: debug-options [setting]
Display the current settings of the debug options. If SETTING is
omitted, only a short form of the current read options is printed.
Otherwise if SETTING is the symbol ‘help’, a complete options
description is displayed.
The set of available options, and their default values, may be had by
invoking ‘debug-options’ at the prompt.
scheme@(guile-user)>
backwards no Display backtrace in anti-chronological order.
width 79 Maximal width of backtrace.
depth 20 Maximal length of printed backtrace.
backtrace yes Show backtrace on error.
stack 1048576 Stack size limit (measured in words;
0 = no check).
show-file-name #t Show file names and line numbers in backtraces
when not `#f'. A value of `base' displays only
base names, while `#t' displays full names.
warn-deprecated no Warn when deprecated features are used.
The boolean options may be toggled with ‘debug-enable’ and
‘debug-disable’. The non-boolean options must be set using
‘debug-set!’.
-- Scheme Procedure: debug-enable option-name
-- Scheme Procedure: debug-disable option-name
-- Scheme Syntax: debug-set! option-name value
Modify the debug options. ‘debug-enable’ should be used with
boolean options and switches them on, ‘debug-disable’ switches them
off.
‘debug-set!’ can be used to set an option to a specific value. Due
to historical oddities, it is a macro that expects an unquoted
option name.
6.26.4 Traps
------------
Guile’s virtual machine can be configured to call out at key points to
arbitrary user-specified procedures.
In principle, these “hooks” allow Scheme code to implement any model
it chooses for examining the evaluation stack as program execution
proceeds, and for suspending execution to be resumed later.
VM hooks are very low-level, though, and so Guile also has a library
of higher-level “traps” on top of the VM hooks. A trap is an execution
condition that, when fulfilled, will fire a handler. For example, Guile
defines a trap that fires when control reaches a certain source
location.
Finally, Guile also defines a third level of abstractions: per-thread
“trap states”. A trap state exists to give names to traps, and to hold
on to the set of traps so that they can be enabled, disabled, or
removed. The trap state infrastructure defines the most useful
abstractions for most cases. For example, Guile’s REPL uses trap state
functions to set breakpoints and tracepoints.
The following subsections describe all this in detail, for both the
user wanting to use traps, and the developer interested in understanding
how the interface hangs together.
6.26.4.1 VM Hooks
.................
Everything that runs in Guile runs on its virtual machine, a C program
that defines a number of operations that Scheme programs can perform.
Note that there are multiple VM “engines” for Guile. Only some of
them have support for hooks compiled in. Normally the deal is that you
get hooks if you are running interactively, and otherwise they are
disabled, as they do have some overhead (about 10 or 20 percent).
To ensure that you are running with hooks, pass ‘--debug’ to Guile
when running your program, or otherwise use the ‘call-with-vm’ and
‘set-vm-engine!’ procedures to ensure that you are running in a VM with
the ‘debug’ engine.
To digress, Guile’s VM has 4 different hooks that can be fired at
different times. For implementation reasons, these hooks are not
actually implemented with first-class Scheme hooks (*note Hooks::); they
are managed using an ad-hoc interface.
VM hooks are called with one argument: the current frame. *Note
Frames::. Since these hooks may be fired very frequently, Guile does a
terrible thing: it allocates the frames on the C stack instead of the
garbage-collected heap.
The upshot here is that the frames are only valid within the dynamic
extent of the call to the hook. If a hook procedure keeps a reference
to the frame outside the extent of the hook, bad things will happen.
The interface to hooks is provided by the ‘(system vm vm)’ module:
(use-modules (system vm vm))
All of these functions implicitly act on the VM for the current thread
only.
-- Scheme Procedure: vm-add-next-hook! f
Arrange to call F when before an instruction is retired (and
executed).
-- Scheme Procedure: vm-add-apply-hook! f
Arrange to call F whenever a procedure is applied. The frame
locals will be the callee, followed by the arguments to the call.
Note that procedure application is somewhat orthogonal to
continuation pushes and pops. To know whether a call is a tail
call or not, with respect to the frame previously in place, check
the value of the frame pointer compared the previous frame pointer.
-- Scheme Procedure: vm-add-return-hook! f
Arrange to call F before returning from a frame. The values in the
frame will be the frame’s return values.
Note that it’s possible to return from an “inner” frame: one that
was not immediately proceeded by a call with that frame pointer.
In that case, it corresponds to a non-local control flow jump,
either because of applying a composable continuation or because of
restoring a saved undelimited continuation.
-- Scheme Procedure: vm-add-abort-hook!
Arrange to call F after aborting to a prompt. *Note Prompts::.
Unfortunately, the values passed to the prompt handler are not
easily available to F.
-- Scheme Procedure: vm-remove-next-hook! f
-- Scheme Procedure: vm-remove-apply-hook! f
-- Scheme Procedure: vm-remove-return-hook! f
-- Scheme Procedure: vm-remove-abort-hook! f
Remove F from the corresponding VM hook for the current thread.
These hooks do impose a performance penalty, if they are on.
Obviously, the ‘vm-next-hook’ has quite an impact, performance-wise.
Therefore Guile exposes a single, heavy-handed knob to turn hooks on or
off, the “VM trace level”. If the trace level is positive, hooks run;
otherwise they don’t.
For convenience, when the VM fires a hook, it does so with the trap
level temporarily set to 0. That way the hooks don’t fire while you’re
handling a hook. The trace level is restored to whatever it was once
the hook procedure finishes.
-- Scheme Procedure: vm-trace-level
Retrieve the “trace level” of the VM. If positive, the trace hooks
associated with VM will be run. The initial trace level is 0.
-- Scheme Procedure: set-vm-trace-level! level
Set the “trace level” of the VM.
*Note A Virtual Machine for Guile::, for more information on Guile’s
virtual machine.
6.26.4.2 Trap Interface
.......................
The capabilities provided by hooks are great, but hooks alone rarely
correspond to what users want to do.
For example, if a user wants to break when and if control reaches a
certain source location, how do you do it? If you install a “next”
hook, you get unacceptable overhead for the execution of the entire
program. It would be possible to install an “apply” hook, then if the
procedure encompasses those source locations, install a “next” hook, but
already you’re talking about one concept that might be implemented by a
varying number of lower-level concepts.
It’s best to be clear about things and define one abstraction for all
such conditions: the “trap”.
Considering the myriad capabilities offered by the hooks though,
there is only a minimum of functionality shared by all traps. Guile’s
current take is to reduce this to the absolute minimum, and have the
only standard interface of a trap be “turn yourself on” or “turn
yourself off”.
This interface sounds a bit strange, but it is useful to procedurally
compose higher-level traps from lower-level building blocks. For
example, Guile defines a trap that calls one handler when control enters
a procedure, and another when control leaves the procedure. Given that
trap, one can define a trap that adds to the next-hook only when within
a given procedure. Building further, one can define a trap that fires
when control reaches particular instructions within a procedure.
Or of course you can stop at any of these intermediate levels. For
example, one might only be interested in calls to a given procedure.
But the point is that a simple enable/disable interface is all the
commonality that exists between the various kinds of traps, and
furthermore that such an interface serves to allow “higher-level” traps
to be composed from more primitive ones.
Specifically, a trap, in Guile, is a procedure. When a trap is
created, by convention the trap is enabled; therefore, the procedure
that is the trap will, when called, disable the trap, and return a
procedure that will enable the trap, and so on.
Trap procedures take one optional argument: the current frame. (A
trap may want to add to different sets of hooks depending on the frame
that is current at enable-time.)
If this all sounds very complicated, it’s because it is. Some of it
is essential, but probably most of it is not. The advantage of using
this minimal interface is that composability is more lexically apparent
than when, for example, using a stateful interface based on GOOPS. But
perhaps this reflects the cognitive limitations of the programmer who
made the current interface more than anything else.
6.26.4.3 Low-Level Traps
........................
To summarize the last sections, traps are enabled or disabled, and when
they are enabled, they add to various VM hooks.
Note, however, that _traps do not increase the VM trace level_. So
if you create a trap, it will be enabled, but unless something else
increases the VM’s trace level (*note VM Hooks::), the trap will not
fire. It turns out that getting the VM trace level right is tricky
without a global view of what traps are enabled. *Note Trap States::,
for Guile’s answer to this problem.
Traps are created by calling procedures. Most of these procedures
share a set of common keyword arguments, so rather than document them
separately, we discuss them all together here:
‘#:vm’
The VM to instrument. Defaults to the current thread’s VM.
‘#:current-frame’
For traps that enable more hooks depending on their dynamic
context, this argument gives the current frame that the trap is
running in. Defaults to ‘#f’.
To have access to these procedures, you’ll need to have imported the
‘(system vm traps)’ module:
(use-modules (system vm traps))
-- Scheme Procedure: trap-at-procedure-call proc handler [#:vm]
A trap that calls HANDLER when PROC is applied.
-- Scheme Procedure: trap-in-procedure proc enter-handler exit-handler
[#:current-frame] [#:vm]
A trap that calls ENTER-HANDLER when control enters PROC, and
EXIT-HANDLER when control leaves PROC.
Control can enter a procedure via:
• A procedure call.
• A return to a procedure’s frame on the stack.
• A continuation returning directly to an application of this
procedure.
Control can leave a procedure via:
• A normal return from the procedure.
• An application of another procedure.
• An invocation of a continuation.
• An abort.
-- Scheme Procedure: trap-instructions-in-procedure proc next-handler
exit-handler [#:current-frame] [#:vm]
A trap that calls NEXT-HANDLER for every instruction executed in
PROC, and EXIT-HANDLER when execution leaves PROC.
-- Scheme Procedure: trap-at-procedure-ip-in-range proc range handler
[#:current-frame] [#:vm]
A trap that calls HANDLER when execution enters a range of
instructions in PROC. RANGE is a simple of pairs, ‘((START . END)
...)’. The START addresses are inclusive, and END addresses are
exclusive.
-- Scheme Procedure: trap-at-source-location file user-line handler
[#:current-frame] [#:vm]
A trap that fires when control reaches a given source location.
The USER-LINE parameter is one-indexed, as a user counts lines,
instead of zero-indexed, as Guile counts lines.
-- Scheme Procedure: trap-frame-finish frame return-handler
abort-handler [#:vm]
A trap that fires when control leaves the given frame. FRAME
should be a live frame in the current continuation. RETURN-HANDLER
will be called on a normal return, and ABORT-HANDLER on a nonlocal
exit.
-- Scheme Procedure: trap-in-dynamic-extent proc enter-handler
return-handler abort-handler [#:vm]
A more traditional dynamic-wind trap, which fires ENTER-HANDLER
when control enters PROC, RETURN-HANDLER on a normal return, and
ABORT-HANDLER on a nonlocal exit.
Note that rewinds are not handled, so there is no rewind handler.
-- Scheme Procedure: trap-calls-in-dynamic-extent proc apply-handler
return-handler [#:current-frame] [#:vm]
A trap that calls APPLY-HANDLER every time a procedure is applied,
and RETURN-HANDLER for returns, but only during the dynamic extent
of an application of PROC.
-- Scheme Procedure: trap-instructions-in-dynamic-extent proc
next-handler [#:current-frame] [#:vm]
A trap that calls NEXT-HANDLER for all retired instructions within
the dynamic extent of a call to PROC.
-- Scheme Procedure: trap-calls-to-procedure proc apply-handler
return-handler [#:vm]
A trap that calls APPLY-HANDLER whenever PROC is applied, and
RETURN-HANDLER when it returns, but with an additional argument,
the call depth.
That is to say, the handlers will get two arguments: the frame in
question, and the call depth (a non-negative integer).
-- Scheme Procedure: trap-matching-instructions frame-pred handler
[#:vm]
A trap that calls FRAME-PRED at every instruction, and if
FRAME-PRED returns a true value, calls HANDLER on the frame.
6.26.4.4 Tracing Traps
......................
The ‘(system vm trace)’ module defines a number of traps for tracing of
procedure applications. When a procedure is “traced”, it means that
every call to that procedure is reported to the user during a program
run. The idea is that you can mark a collection of procedures for
tracing, and Guile will subsequently print out a line of the form
| | (PROCEDURE ARGS ...)
whenever a marked procedure is about to be applied to its arguments.
This can help a programmer determine whether a function is being called
at the wrong time or with the wrong set of arguments.
In addition, the indentation of the output is useful for
demonstrating how the traced applications are or are not tail recursive
with respect to each other. Thus, a trace of a non-tail recursive
factorial implementation looks like this:
scheme@(guile-user)> (define (fact1 n)
(if (zero? n) 1
(* n (fact1 (1- n)))))
scheme@(guile-user)> ,trace (fact1 4)
trace: (fact1 4)
trace: | (fact1 3)
trace: | | (fact1 2)
trace: | | | (fact1 1)
trace: | | | | (fact1 0)
trace: | | | | 1
trace: | | | 1
trace: | | 2
trace: | 6
trace: 24
While a typical tail recursive implementation would look more like
this:
scheme@(guile-user)> (define (facti acc n)
(if (zero? n) acc
(facti (* n acc) (1- n))))
scheme@(guile-user)> (define (fact2 n) (facti 1 n))
scheme@(guile-user)> ,trace (fact2 4)
trace: (fact2 4)
trace: (facti 1 4)
trace: (facti 4 3)
trace: (facti 12 2)
trace: (facti 24 1)
trace: (facti 24 0)
trace: 24
The low-level traps below (*note Low-Level Traps::) share some common
options:
‘#:width’
The maximum width of trace output. Trace printouts will try not to
exceed this column, but for highly nested procedure calls, it may
be unavoidable. Defaults to 80.
‘#:vm’
The VM on which to add the traps. Defaults to the current thread’s
VM.
‘#:prefix’
A string to print out before each trace line. As seen above in the
examples, defaults to ‘"trace: "’.
To have access to these procedures, you’ll need to have imported the
‘(system vm trace)’ module:
(use-modules (system vm trace))
-- Scheme Procedure: trace-calls-to-procedure proc [#:width] [#:vm]
[#:prefix]
Print a trace at applications of and returns from PROC.
-- Scheme Procedure: trace-calls-in-procedure proc [#:width] [#:vm]
[#:prefix]
Print a trace at all applications and returns within the dynamic
extent of calls to PROC.
-- Scheme Procedure: trace-instructions-in-procedure proc [#:width]
[#:vm]
Print a trace at all instructions executed in the dynamic extent of
calls to PROC.
In addition, Guile defines a procedure to call a thunk, tracing all
procedure calls and returns within the thunk.
-- Scheme Procedure: call-with-trace thunk [#:calls?=#t]
[#:instructions?=#f] [#:width=80]
Call THUNK, tracing all execution within its dynamic extent.
If CALLS? is true, Guile will print a brief report at each
procedure call and return, as given above.
If INSTRUCTIONS? is true, Guile will also print a message each time
an instruction is executed. This is a lot of output, but it is
sometimes useful when doing low-level optimization.
Note that because this procedure manipulates the VM trace level
directly, it doesn’t compose well with traps at the REPL.
*Note Profile Commands::, for more information on tracing at the
REPL.
6.26.4.5 Trap States
....................
When multiple traps are present in a system, we begin to have a
bookkeeping problem. How are they named? How does one disable, enable,
or delete them?
Guile’s answer to this is to keep an implicit per-thread “trap
state”. The trap state object is not exposed to the user; rather, API
that works on trap states fetches the current trap state from the
dynamic environment.
Traps are identified by integers. A trap can be enabled, disabled,
or removed, and can have an associated user-visible name.
These procedures have their own module:
(use-modules (system vm trap-state))
-- Scheme Procedure: add-trap! trap name
Add a trap to the current trap state, associating the given NAME
with it. Returns a fresh trap identifier (an integer).
Note that usually the more specific functions detailed in *note
High-Level Traps:: are used in preference to this one.
-- Scheme Procedure: list-traps
List the current set of traps, both enabled and disabled. Returns
a list of integers.
-- Scheme Procedure: trap-name idx
Returns the name associated with trap IDX, or ‘#f’ if there is no
such trap.
-- Scheme Procedure: trap-enabled? idx
Returns ‘#t’ if trap IDX is present and enabled, or ‘#f’ otherwise.
-- Scheme Procedure: enable-trap! idx
Enables trap IDX.
-- Scheme Procedure: disable-trap! idx
Disables trap IDX.
-- Scheme Procedure: delete-trap! idx
Removes trap IDX, disabling it first, if necessary.
6.26.4.6 High-Level Traps
.........................
The low-level trap API allows one to make traps that call procedures,
and the trap state API allows one to keep track of what traps are there.
But neither of these APIs directly helps you when you want to set a
breakpoint, because it’s unclear what to do when the trap fires. Do you
enter a debugger, or mail a summary of the situation to your great-aunt,
or what?
So for the common case in which you just want to install breakpoints,
and then have them all result in calls to one parameterizable procedure,
we have the high-level trap interface.
Perhaps we should have started this section with this interface, as
it’s clearly the one most people should use. But as its capabilities
and limitations proceed from the lower layers, we felt that the
character-building exercise of building a mental model might be helpful.
These procedures share a module with trap states:
(use-modules (system vm trap-state))
-- Scheme Procedure: with-default-trap-handler handler thunk
Call THUNK in a dynamic context in which HANDLER is the current
trap handler.
Additionally, during the execution of THUNK, the VM trace level
(*note VM Hooks::) is set to the number of enabled traps. This
ensures that traps will in fact fire.
HANDLER may be ‘#f’, in which case VM hooks are not enabled as they
otherwise would be, as there is nothing to handle the traps.
The trace-level-setting behavior of ‘with-default-trap-handler’ is
one of its more useful aspects, but if you are willing to forgo that,
and just want to install a global trap handler, there’s a function for
that too:
-- Scheme Procedure: install-trap-handler! handler
Set the current thread’s trap handler to HANDLER.
Trap handlers are called when traps installed by procedures from this
module fire. The current “consumer” of this API is Guile’s REPL, but
one might easily imagine other trap handlers being used to integrate
with other debugging tools.
-- Scheme Procedure: add-trap-at-procedure-call! proc
Install a trap that will fire when PROC is called.
This is a breakpoint.
-- Scheme Procedure: add-trace-at-procedure-call! proc
Install a trap that will print a tracing message when PROC is
called. *Note Tracing Traps::, for more information.
This is a tracepoint.
-- Scheme Procedure: add-trap-at-source-location! file user-line
Install a trap that will fire when control reaches the given source
location. USER-LINE is one-indexed, as users count lines, instead
of zero-indexed, as Guile counts lines.
This is a source breakpoint.
-- Scheme Procedure: add-ephemeral-trap-at-frame-finish! frame handler
Install a trap that will call HANDLER when FRAME finishes
executing. The trap will be removed from the trap state after
firing, or on nonlocal exit.
This is a finish trap, used to implement the “finish” REPL command.
-- Scheme Procedure: add-ephemeral-stepping-trap! frame handler
[#:into?] [#:instruction?]
Install a trap that will call HANDLER after stepping to a different
source line or instruction. The trap will be removed from the trap
state after firing, or on nonlocal exit.
If INSTRUCTION? is false (the default), the trap will fire when
control reaches a new source line. Otherwise it will fire when
control reaches a new instruction.
Additionally, if INTO? is false (not the default), the trap will
only fire for frames at or prior to the given frame. If INTO? is
true (the default), the trap may step into nested procedure
invocations.
This is a stepping trap, used to implement the “step”, “next”,
“step-instruction”, and “next-instruction” REPL commands.
6.26.5 GDB Support
------------------
Sometimes, you may find it necessary to debug Guile applications at the
C level. Doing so can be tedious, in particular because the debugger is
oblivious to Guile’s ‘SCM’ type, and thus unable to display ‘SCM’ values
in any meaningful way:
(gdb) frame
#0 scm_display (obj=0xf04310, port=0x6f9f30) at print.c:1437
To address that, Guile comes with an extension of the GNU Debugger
(GDB) that contains a “pretty-printer” for ‘SCM’ values. With this GDB
extension, the C frame in the example above shows up like this:
(gdb) frame
#0 scm_display (obj=("hello" GDB!), port=#) at print.c:1437
Here GDB was able to decode the list pointed to by OBJ, and to print it
using Scheme’s read syntax.
That extension is a ‘.scm’ file installed alongside the ‘libguile’
shared library. When GDB 7.8 or later is installed and compiled with
support for extensions written in Guile, the extension is automatically
loaded when debugging a program linked against ‘libguile’ (*note
(gdb)Auto-loading::). Note that the directory where ‘libguile’ is
installed must be among GDB’s auto-loading “safe directories” (*note
(gdb)Auto-loading safe path::).
6.27 Code Coverage Reports
==========================
When writing a test suite for a program or library, it is desirable to
know what part of the code is “covered” by the test suite. The ‘(system
vm coverage)’ module provides tools to gather code coverage data and to
present them, as detailed below.
-- Scheme Procedure: with-code-coverage thunk
Run THUNK, a zero-argument procedure, while instrumenting Guile’s
virtual machine to collect code coverage data. Return code
coverage data and the values returned by THUNK.
-- Scheme Procedure: coverage-data? obj
Return ‘#t’ if OBJ is a “coverage data” object as returned by
‘with-code-coverage’.
-- Scheme Procedure: coverage-data->lcov data port #:key modules
Traverse code coverage information DATA, as obtained with
‘with-code-coverage’, and write coverage information to port in the
‘.info’ format used by LCOV
(http://ltp.sourceforge.net/coverage/lcov.php). The report will
include all of MODULES (or, by default, all the currently loaded
modules) even if their code was not executed.
The generated data can be fed to LCOV’s ‘genhtml’ command to
produce an HTML report, which aids coverage data visualization.
Here’s an example use:
(use-modules (system vm coverage)
(system vm vm))
(call-with-values (lambda ()
(with-code-coverage
(lambda ()
(do-something-tricky))))
(lambda (data result)
(let ((port (open-output-file "lcov.info")))
(coverage-data->lcov data port)
(close port))))
In addition, the module provides low-level procedures that would make
it possible to write other user interfaces to the coverage data.
-- Scheme Procedures: instrumented-source-files data
Return the list of “instrumented” source files, i.e., source files
whose code was loaded at the time DATA was collected.
-- Scheme Procedures: line-execution-counts data file
Return a list of line number/execution count pairs for FILE, or
‘#f’ if FILE is not among the files covered by DATA. This includes
lines with zero count.
-- Scheme Procedures: instrumented/executed-lines data file
Return the number of instrumented and the number of executed source
lines in FILE according to DATA.
-- Scheme Procedures: procedure-execution-count data proc
Return the number of times PROC’s code was executed, according to
DATA, or ‘#f’ if PROC was not executed. When PROC is a closure,
the number of times its code was executed is returned, not the
number of times this code associated with this particular closure
was executed.
7 Guile Modules
***************
7.1 SLIB
========
SLIB is a portable library of Scheme packages which can be used with
Guile and other Scheme implementations. SLIB is not included in the
Guile distribution, but can be installed separately (*note SLIB
installation::). It is available from
.
After SLIB is installed, the following Scheme expression must be
executed before the SLIB facilities can be used:
(use-modules (ice-9 slib))
‘require’ can then be used in the usual way (*note (slib)Require::).
For example,
(use-modules (ice-9 slib))
(require 'primes)
(prime? 13)
⇒ #t
A few Guile core functions are overridden by the SLIB setups; for
example the SLIB version of ‘delete-file’ returns a boolean indicating
success or failure, whereas the Guile core version throws an error for
failure. In general (and as might be expected) when SLIB is loaded it’s
the SLIB specifications that are followed.
7.1.1 SLIB installation
-----------------------
The following procedure works, e.g., with SLIB version 3a3 (*note SLIB
installation: (slib)Installation.):
1. Unpack SLIB and install it using ‘make install’ from its directory.
By default, this will install SLIB in ‘/usr/local/lib/slib/’.
Running ‘make install-info’ installs its documentation, by default
under ‘/usr/local/info/’.
2. Define the ‘SCHEME_LIBRARY_PATH’ environment variable:
$ SCHEME_LIBRARY_PATH=/usr/local/lib/slib/
$ export SCHEME_LIBRARY_PATH
Alternatively, you can create a symlink in the Guile directory to
SLIB, e.g.:
ln -s /usr/local/lib/slib /usr/local/share/guile/3.0/slib
3. Use Guile to create the catalog file, e.g.,:
# guile
guile> (use-modules (ice-9 slib))
guile> (require 'new-catalog)
guile> (quit)
The catalog data should now be in
‘/usr/local/share/guile/3.0/slibcat’.
If instead you get an error such as:
Unbound variable: scheme-implementation-type
then a solution is to get a newer version of Guile, or to modify
‘ice-9/slib.scm’ to use ‘define-public’ for the offending
variables.
7.1.2 JACAL
-----------
Jacal is a symbolic math package written in Scheme by Aubrey Jaffer. It
is usually installed as an extra package in SLIB.
You can use Guile’s interface to SLIB to invoke Jacal:
(use-modules (ice-9 slib))
(slib:load "math")
(math)
For complete documentation on Jacal, please read the Jacal manual. If
it has been installed on line, you can look at *note Jacal: (jacal)Top.
Otherwise you can find it on the web at
7.2 POSIX System Calls and Networking
=====================================
7.2.1 POSIX Interface Conventions
---------------------------------
These interfaces provide access to operating system facilities. They
provide a simple wrapping around the underlying C interfaces to make
usage from Scheme more convenient. They are also used to implement the
Guile port of scsh (*note The Scheme shell (scsh)::).
Generally there is a single procedure for each corresponding Unix
facility. There are some exceptions, such as procedures implemented for
speed and convenience in Scheme with no primitive Unix equivalent, e.g.
‘copy-file’.
The interfaces are intended as far as possible to be portable across
different versions of Unix. In some cases procedures which can’t be
implemented on particular systems may become no-ops, or perform limited
actions. In other cases they may throw errors.
General naming conventions are as follows:
• The Scheme name is often identical to the name of the underlying
Unix facility.
• Underscores in Unix procedure names are converted to hyphens.
• Procedures which destructively modify Scheme data have exclamation
marks appended, e.g., ‘recv!’.
• Predicates (returning only ‘#t’ or ‘#f’) have question marks
appended, e.g., ‘access?’.
• Some names are changed to avoid conflict with dissimilar interfaces
defined by scsh, e.g., ‘primitive-fork’.
• Unix preprocessor names such as ‘EPERM’ or ‘R_OK’ are converted to
Scheme variables of the same name (underscores are not replaced
with hyphens).
Unexpected conditions are generally handled by raising exceptions.
There are a few procedures which return a special value if they don’t
succeed, e.g., ‘getenv’ returns ‘#f’ if it the requested string is not
found in the environment. These cases are noted in the documentation.
For ways to deal with exceptions, see *note Exceptions::.
Errors which the C library would report by returning a null pointer
or through some other means are reported by raising a ‘system-error’
exception with ‘scm-error’ (*note Error Reporting::). The DATA
parameter is a list containing the Unix ‘errno’ value (an integer). For
example,
(define (my-handler key func fmt fmtargs data)
(display key) (newline)
(display func) (newline)
(apply format #t fmt fmtargs) (newline)
(display data) (newline))
(catch 'system-error
(lambda () (dup2 -123 -456))
my-handler)
⊣
system-error
dup2
Bad file descriptor
(9)
-- Function: system-error-errno arglist
Return the ‘errno’ value from a list which is the arguments to an
exception handler. If the exception is not a ‘system-error’, then
the return is ‘#f’. For example,
(catch
'system-error
(lambda ()
(mkdir "/this-ought-to-fail-if-I'm-not-root"))
(lambda stuff
(let ((errno (system-error-errno stuff)))
(cond
((= errno EACCES)
(display "You're not allowed to do that."))
((= errno EEXIST)
(display "Already exists."))
(#t
(display (strerror errno))))
(newline))))
7.2.2 Ports and File Descriptors
--------------------------------
Conventions generally follow those of scsh, *note The Scheme shell
(scsh)::.
Each open file port has an associated operating system file
descriptor. File descriptors are generally not useful in Scheme
programs; however they may be needed when interfacing with foreign code
and the Unix environment.
A file descriptor can be extracted from a port and a new port can be
created from a file descriptor. However a file descriptor is just an
integer and the garbage collector doesn’t recognize it as a reference to
the port. If all other references to the port were dropped, then it’s
likely that the garbage collector would free the port, with the
side-effect of closing the file descriptor prematurely.
To assist the programmer in avoiding this problem, each port has an
associated “revealed count” which can be used to keep track of how many
times the underlying file descriptor has been stored in other places.
If a port’s revealed count is greater than zero, the file descriptor
will not be closed when the port is garbage collected. A programmer can
therefore ensure that the revealed count will be greater than zero if
the file descriptor is needed elsewhere.
For the simple case where a file descriptor is “imported” once to
become a port, it does not matter if the file descriptor is closed when
the port is garbage collected. There is no need to maintain a revealed
count. Likewise when “exporting” a file descriptor to the external
environment, setting the revealed count is not required provided the
port is kept open (i.e., is pointed to by a live Scheme binding) while
the file descriptor is in use.
To correspond with traditional Unix behaviour, three file descriptors
(0, 1, and 2) are automatically imported when a program starts up and
assigned to the initial values of the current/standard input, output,
and error ports, respectively. The revealed count for each is initially
set to one, so that dropping references to one of these ports will not
result in its garbage collection: it could be retrieved with ‘fdopen’ or
‘fdes->ports’.
Guile’s ports can be buffered. This means that writing a byte to a
file port goes to the internal buffer first, and only when the buffer is
full (or the user invokes ‘force-output’ on the port) is the data
actually written to the file descriptor. Likewise on input, bytes are
read in from the file descriptor in blocks and placed in a buffer.
Reading a character via ‘read-char’ first goes to the buffer, filling it
as needed. Usually read buffering is more or less transparent, but
write buffering can sometimes cause writes to be delayed unexpectedly,
if you forget to call ‘force-output’. *Note Buffering::, for more on
how to control port buffers.
Note however that some procedures (e.g., ‘recv!’) will accept ports
as arguments, but will actually operate directly on the file descriptor
underlying the port. Any port buffering is ignored, including the
buffer which implements ‘peek-char’ and ‘unread-char’.
-- Scheme Procedure: port-revealed port
-- C Function: scm_port_revealed (port)
Return the revealed count for PORT.
-- Scheme Procedure: set-port-revealed! port rcount
-- C Function: scm_set_port_revealed_x (port, rcount)
Sets the revealed count for a PORT to RCOUNT. The return value is
unspecified.
-- Scheme Procedure: fileno port
-- C Function: scm_fileno (port)
Return the integer file descriptor underlying PORT. Does not
change its revealed count.
-- Scheme Procedure: port->fdes port
Returns the integer file descriptor underlying PORT. As a side
effect the revealed count of PORT is incremented.
-- Scheme Procedure: fdopen fdes modes
-- C Function: scm_fdopen (fdes, modes)
Return a new port based on the file descriptor FDES. Modes are
given by the string MODES. The revealed count of the port is
initialized to zero. The MODES string is the same as that accepted
by ‘open-file’ (*note open-file: File Ports.).
-- Scheme Procedure: fdes->ports fdes
-- C Function: scm_fdes_to_ports (fdes)
Return a list of existing ports which have FDES as an underlying
file descriptor, without changing their revealed counts.
-- Scheme Procedure: fdes->inport fdes
Returns an existing input port which has FDES as its underlying
file descriptor, if one exists, and increments its revealed count.
Otherwise, returns a new input port with a revealed count of 1.
-- Scheme Procedure: fdes->outport fdes
Returns an existing output port which has FDES as its underlying
file descriptor, if one exists, and increments its revealed count.
Otherwise, returns a new output port with a revealed count of 1.
-- Scheme Procedure: primitive-move->fdes port fdes
-- C Function: scm_primitive_move_to_fdes (port, fdes)
Moves the underlying file descriptor for PORT to the integer value
FDES without changing the revealed count of PORT. Any other ports
already using this descriptor will be automatically shifted to new
descriptors and their revealed counts reset to zero. The return
value is ‘#f’ if the file descriptor already had the required value
or ‘#t’ if it was moved.
-- Scheme Procedure: move->fdes port fdes
Moves the underlying file descriptor for PORT to the integer value
FDES and sets its revealed count to one. Any other ports already
using this descriptor will be automatically shifted to new
descriptors and their revealed counts reset to zero. The return
value is unspecified.
-- Scheme Procedure: release-port-handle port
Decrements the revealed count for a port.
-- Scheme Procedure: fsync port_or_fd
-- C Function: scm_fsync (port_or_fd)
Copies any unwritten data for the specified output file descriptor
to disk. If PORT_OR_FD is a port, its buffer is flushed before the
underlying file descriptor is fsync’d. The return value is
unspecified.
-- Scheme Procedure: open path flags [mode]
-- C Function: scm_open (path, flags, mode)
Open the file named by PATH for reading and/or writing. FLAGS is
an integer specifying how the file should be opened. MODE is an
integer specifying the permission bits of the file, if it needs to
be created, before the umask (*note Processes::) is applied. The
default is 666 (Unix itself has no default).
FLAGS can be constructed by combining variables using ‘logior’.
Basic flags are:
-- Variable: O_RDONLY
Open the file read-only.
-- Variable: O_WRONLY
Open the file write-only.
-- Variable: O_RDWR
Open the file read/write.
-- Variable: O_APPEND
Append to the file instead of truncating.
-- Variable: O_CREAT
Create the file if it does not already exist.
*Note (libc)File Status Flags::, for additional flags.
-- Scheme Procedure: openat dir path flags [mode]
-- C Function: scm_openat (dir, path, flags, mode)
Similar to ‘open’, but resolve the file name PATH relative to the
directory referred to by the file port DIR instead.
-- Scheme Procedure: open-fdes path flags [mode]
-- C Function: scm_open_fdes (path, flags, mode)
Similar to ‘open’ but return a file descriptor instead of a port.
-- Scheme Procedure: open-fdes-at dir path flags [mode]
-- C Function: scm_open_fdes_at (dir, path, flags, mode)
Similar to ‘openat’, but return a file descriptor instead of a
port.
-- Scheme Procedure: close fd_or_port
-- C Function: scm_close (fd_or_port)
Similar to ‘close-port’ (*note close-port: Ports.), but also works
on file descriptors. A side effect of closing a file descriptor is
that any ports using that file descriptor are moved to a different
file descriptor and have their revealed counts set to zero.
-- Scheme Procedure: close-fdes fd
-- C Function: scm_close_fdes (fd)
A simple wrapper for the ‘close’ system call. Close file
descriptor FD, which must be an integer. Unlike ‘close’, the file
descriptor will be closed even if a port is using it. The return
value is unspecified.
-- Scheme Procedure: pipe [flags]
-- C Function: scm_pipe ()
Return a newly created pipe: a pair of ports which are linked
together on the local machine. The CAR is the input port and the
CDR is the output port. Data written (and flushed) to the output
port can be read from the input port. Pipes are commonly used for
communication with a newly forked child process. The need to flush
the output port can be avoided by making it unbuffered using
‘setvbuf’ (*note Buffering::).
Optionally, on systems that support it such as GNU/Linux and
GNU/Hurd, FLAGS can specify a bitwise-or of the following
constants:
‘O_CLOEXEC’
Mark the returned file descriptors as close-on-exec;
‘O_DIRECT’
Create a pipe that performs input/output in “packet" mode—see
‘man 2 pipe’ for details;
‘O_NONBLOCK’
Set the ‘O_NONBLOCK’ status flag (non-blocking input and
output) on the file descriptors.
On systems that do _not_ support it, passing a non-zero FLAGS value
triggers a ‘system-error’ exception.
-- Variable: PIPE_BUF
A write of up to ‘PIPE_BUF’ many bytes to a pipe is atomic,
meaning when done it goes into the pipe instantaneously and as
a contiguous block (*note Atomicity of Pipe I/O: (libc)Pipe
Atomicity.).
Note that the output port is likely to block if too much data has
been written but not yet read from the input port. Typically the
capacity is ‘PIPE_BUF’ bytes.
The next group of procedures perform a ‘dup2’ system call, if NEWFD
(an integer) is supplied, otherwise a ‘dup’. The file descriptor to be
duplicated can be supplied as an integer or contained in a port. The
type of value returned varies depending on which procedure is used.
All procedures also have the side effect when performing ‘dup2’ that
any ports using NEWFD are moved to a different file descriptor and have
their revealed counts set to zero.
-- Scheme Procedure: dup->fdes fd_or_port [fd]
-- C Function: scm_dup_to_fdes (fd_or_port, fd)
Return a new integer file descriptor referring to the open file
designated by FD_OR_PORT, which must be either an open file port or
a file descriptor.
-- Scheme Procedure: dup->inport port/fd [newfd]
Returns a new input port using the new file descriptor.
-- Scheme Procedure: dup->outport port/fd [newfd]
Returns a new output port using the new file descriptor.
-- Scheme Procedure: dup port/fd [newfd]
Returns a new port if PORT/FD is a port, with the same mode as the
supplied port, otherwise returns an integer file descriptor.
-- Scheme Procedure: dup->port port/fd mode [newfd]
Returns a new port using the new file descriptor. MODE supplies a
mode string for the port (*note open-file: File Ports.).
-- Scheme Procedure: duplicate-port port modes
Returns a new port which is opened on a duplicate of the file
descriptor underlying PORT, with mode string MODES as for *note
open-file: File Ports. The two ports will share a file position
and file status flags.
Unexpected behaviour can result if both ports are subsequently used
and the original and/or duplicate ports are buffered. The mode
string can include ‘0’ to obtain an unbuffered duplicate port.
This procedure is equivalent to ‘(dup->port PORT MODES)’.
-- Scheme Procedure: redirect-port old_port new_port
-- C Function: scm_redirect_port (old_port, new_port)
This procedure takes two ports and duplicates the underlying file
descriptor from OLD_PORT into NEW_PORT. The current file
descriptor in NEW_PORT will be closed. After the redirection the
two ports will share a file position and file status flags.
The return value is unspecified.
Unexpected behaviour can result if both ports are subsequently used
and the original and/or duplicate ports are buffered.
This procedure does not have any side effects on other ports or
revealed counts.
-- Scheme Procedure: dup2 oldfd newfd
-- C Function: scm_dup2 (oldfd, newfd)
A simple wrapper for the ‘dup2’ system call. Copies the file
descriptor OLDFD to descriptor number NEWFD, replacing the previous
meaning of NEWFD. Both OLDFD and NEWFD must be integers. Unlike
for ‘dup->fdes’ or ‘primitive-move->fdes’, no attempt is made to
move away ports which are using NEWFD. The return value is
unspecified.
-- Scheme Procedure: port-for-each proc
-- C Function: scm_port_for_each (SCM proc)
-- C Function: scm_c_port_for_each (void (*proc)(void *, SCM), void
*data)
Apply PROC to each port in the Guile port table (FIXME: what is the
Guile port table?) in turn. The return value is unspecified.
More specifically, PROC is applied exactly once to every port that
exists in the system at the time ‘port-for-each’ is invoked.
Changes to the port table while ‘port-for-each’ is running have no
effect as far as ‘port-for-each’ is concerned.
The C function ‘scm_port_for_each’ takes a Scheme procedure encoded
as a ‘SCM’ value, while ‘scm_c_port_for_each’ takes a pointer to a
C function and passes along a arbitrary DATA cookie.
-- Scheme Procedure: fcntl port/fd cmd [value]
-- C Function: scm_fcntl (object, cmd, value)
Apply CMD on PORT/FD, either a port or file descriptor. The VALUE
argument is used by the ‘SET’ commands described below, it’s an
integer value.
Values for CMD are:
-- Variable: F_DUPFD
Duplicate the file descriptor, the same as ‘dup->fdes’ above
does.
-- Variable: F_GETFD
-- Variable: F_SETFD
Get or set flags associated with the file descriptor. The
only flag is the following,
-- Variable: FD_CLOEXEC
“Close on exec”, meaning the file descriptor will be
closed on an ‘exec’ call (a successful such call). For
example to set that flag,
(fcntl port F_SETFD FD_CLOEXEC)
Or better, set it but leave any other possible future
flags unchanged,
(fcntl port F_SETFD (logior FD_CLOEXEC
(fcntl port F_GETFD)))
-- Variable: F_GETFL
-- Variable: F_SETFL
Get or set flags associated with the open file. These flags
are ‘O_RDONLY’ etc described under ‘open’ above.
A common use is to set ‘O_NONBLOCK’ on a network socket. The
following sets that flag, and leaves other flags unchanged.
(fcntl sock F_SETFL (logior O_NONBLOCK
(fcntl sock F_GETFL)))
-- Variable: F_GETOWN
-- Variable: F_SETOWN
Get or set the process ID of a socket’s owner, for ‘SIGIO’
signals.
-- Scheme Procedure: flock file operation
-- C Function: scm_flock (file, operation)
Apply or remove an advisory lock on an open file. OPERATION
specifies the action to be done:
-- Variable: LOCK_SH
Shared lock. More than one process may hold a shared lock for
a given file at a given time.
-- Variable: LOCK_EX
Exclusive lock. Only one process may hold an exclusive lock
for a given file at a given time.
-- Variable: LOCK_UN
Unlock the file.
-- Variable: LOCK_NB
Don’t block when locking. This is combined with one of the
other operations using ‘logior’ (*note Bitwise Operations::).
If ‘flock’ would block an ‘EWOULDBLOCK’ error is thrown (*note
Conventions::).
The return value is not specified. FILE may be an open file
descriptor or an open file descriptor port.
Note that ‘flock’ does not lock files across NFS.
-- Scheme Procedure: select reads writes excepts [secs [usecs]]
-- C Function: scm_select (reads, writes, excepts, secs, usecs)
This procedure has a variety of uses: waiting for the ability to
provide input, accept output, or the existence of exceptional
conditions on a collection of ports or file descriptors, or waiting
for a timeout to occur.
When an error occurs, this procedure throws a ‘system-error’
exception (*note ‘system-error’: Conventions.). Note that ‘select’
may return early for other reasons, for example due to pending
interrupts. *Note Asyncs::, for more on interrupts.
READS, WRITES and EXCEPTS can be lists or vectors, with each member
a port or a file descriptor. The value returned is a list of three
corresponding lists or vectors containing only the members which
meet the specified requirement. The ability of port buffers to
provide input or accept output is taken into account. Ordering of
the input lists or vectors is not preserved.
The optional arguments SECS and USECS specify the timeout. Either
SECS can be specified alone, as either an integer or a real number,
or both SECS and USECS can be specified as integers, in which case
USECS is an additional timeout expressed in microseconds. If SECS
is omitted or is ‘#f’ then select will wait for as long as it takes
for one of the other conditions to be satisfied.
The scsh version of ‘select’ differs as follows: Only vectors are
accepted for the first three arguments. The USECS argument is not
supported. Multiple values are returned instead of a list.
Duplicates in the input vectors appear only once in output. An
additional ‘select!’ interface is provided.
While it is sometimes necessary to operate at the level of file
descriptors, this is an operation whose correctness can only be
considered as part of a whole program. So for example while the effects
of ‘(string-set! x 34 #\y)’ are limited to the bits of code that can
access X, ‘(close-fdes 34)’ mutates the state of the entire process. In
particular if another thread is using file descriptor 34 then their
state might be corrupted; and another thread which opens a file might
cause file descriptor 34 to be re-used, so that corruption could
manifest itself in a strange way.
However when working with file descriptors, it’s common to want to
associate information with the file descriptor, perhaps in a side table.
To support this use case and to allow user code to remove an association
when a file descriptor is closed, Guile offers “fdes finalizers”.
As the name indicates, fdes finalizers are finalizers – they can run
in response to garbage collection, and they can also run in response to
explicit calls to ‘close-port’, ‘close-fdes’, or the like. As such they
inherit many of the pitfalls of finalizers: they may be invoked from
concurrent threads, or not at all. *Note Foreign Object Memory
Management::, for more on finalizers.
To use fdes finalizers, import their module;
(use-modules (ice-9 fdes-finalizers))
-- Scheme Procedure: add-fdes-finalizer! fdes finalizer
-- Scheme Procedure: remove-fdes-finalizer! fdes finalizer
Add or remove a finalizer for FDES. A finalizer is a procedure
that is called by Guile when a file descriptor is closed. The file
descriptor being closed is passed as the one argument to the
finalizer. If a finalizer has been added multiple times to a file
descriptor, to remove it would require that number of calls to
‘remove-fdes-finalizer!’.
The finalizers added to a file descriptor are called by Guile in an
unspecified order, and their return values are ignored.
7.2.3 File System
-----------------
These procedures allow querying and setting file system attributes (such
as owner, permissions, sizes and types of files); deleting, copying,
renaming and linking files; creating and removing directories and
querying their contents; syncing the file system and creating special
files.
-- Scheme Procedure: access? path how
-- C Function: scm_access (path, how)
Test accessibility of a file under the real UID and GID of the
calling process. The return is ‘#t’ if PATH exists and the
permissions requested by HOW are all allowed, or ‘#f’ if not.
HOW is an integer which is one of the following values, or a
bitwise-OR (‘logior’) of multiple values.
-- Variable: R_OK
Test for read permission.
-- Variable: W_OK
Test for write permission.
-- Variable: X_OK
Test for execute permission.
-- Variable: F_OK
Test for existence of the file. This is implied by each of
the other tests, so there’s no need to combine it with them.
It’s important to note that ‘access?’ does not simply indicate what
will happen on attempting to read or write a file. In normal
circumstances it does, but in a set-UID or set-GID program it
doesn’t because ‘access?’ tests the real ID, whereas an open or
execute attempt uses the effective ID.
A program which will never run set-UID/GID can ignore the
difference between real and effective IDs, but for maximum
generality, especially in library functions, it’s best not to use
‘access?’ to predict the result of an open or execute, instead
simply attempt that and catch any exception.
The main use for ‘access?’ is to let a set-UID/GID program
determine what the invoking user would have been allowed to do,
without the greater (or perhaps lesser) privileges afforded by the
effective ID. For more on this, see *note (libc)Testing File
Access::.
-- Scheme Procedure: stat object [exception-on-error?]
-- C Function: scm_stat (object, exception_on_error)
Return an object containing various information about the file
determined by OBJECT. OBJECT can be a string containing a file
name or a port or integer file descriptor which is open on a file
(in which case ‘fstat’ is used as the underlying system call).
If the optional EXCEPTION_ON_ERROR argument is true, which is the
default, an exception will be raised if the underlying system call
returns an error, for example if the file is not found or is not
readable. Otherwise, an error will cause ‘stat’ to return ‘#f’.
The object returned by ‘stat’ can be passed as a single parameter
to the following procedures, all of which return integers:
-- Scheme Procedure: stat:dev st
The device number containing the file.
-- Scheme Procedure: stat:ino st
The file serial number, which distinguishes this file from all
other files on the same device.
-- Scheme Procedure: stat:mode st
The mode of the file. This is an integer which incorporates
file type information and file permission bits. See also
‘stat:type’ and ‘stat:perms’ below.
-- Scheme Procedure: stat:nlink st
The number of hard links to the file.
-- Scheme Procedure: stat:uid st
The user ID of the file’s owner.
-- Scheme Procedure: stat:gid st
The group ID of the file.
-- Scheme Procedure: stat:rdev st
Device ID; this entry is defined only for character or block
special files. On some systems this field is not available at
all, in which case ‘stat:rdev’ returns ‘#f’.
-- Scheme Procedure: stat:size st
The size of a regular file in bytes.
-- Scheme Procedure: stat:atime st
The last access time for the file, in seconds.
-- Scheme Procedure: stat:mtime st
The last modification time for the file, in seconds.
-- Scheme Procedure: stat:ctime st
The last modification time for the attributes of the file, in
seconds.
-- Scheme Procedure: stat:atimensec st
-- Scheme Procedure: stat:mtimensec st
-- Scheme Procedure: stat:ctimensec st
The fractional part of a file’s access, modification, or
attribute modification time, in nanoseconds. Nanosecond
timestamps are only available on some operating systems and
file systems. If Guile cannot retrieve nanosecond-level
timestamps for a file, these fields will be set to 0.
-- Scheme Procedure: stat:blksize st
The optimal block size for reading or writing the file, in
bytes. On some systems this field is not available, in which
case ‘stat:blksize’ returns a sensible suggested block size.
-- Scheme Procedure: stat:blocks st
The amount of disk space that the file occupies measured in
units of 512 byte blocks. On some systems this field is not
available, in which case ‘stat:blocks’ returns ‘#f’.
In addition, the following procedures return the information from
‘stat:mode’ in a more convenient form:
-- Scheme Procedure: stat:type st
A symbol representing the type of file. Possible values are
‘regular’, ‘directory’, ‘symlink’, ‘block-special’,
‘char-special’, ‘fifo’, ‘socket’, and ‘unknown’.
-- Scheme Procedure: stat:perms st
An integer representing the access permission bits.
-- Scheme Procedure: lstat path
-- C Function: scm_lstat (path)
Similar to ‘stat’, but does not follow symbolic links, i.e., it
will return information about a symbolic link itself, not the file
it points to. PATH must be a string.
-- Scheme Procedure: statat dir filename [flags]
-- C Function: scm_statat dir filename flags
Like ‘stat’, but resolve FILENAME relative to the directory
referred to by the file port DIR instead. The optional argument
FLAGS argument can be ‘AT_SYMLINK_NOFOLLOW’, in which case FILENAME
will not be dereferenced even if it is a symbolic link.
-- Scheme Procedure: readlink path
-- C Function: scm_readlink (path)
Return the value of the symbolic link named by PATH (a string, or a
port if supported by the system), i.e., the file that the link
points to.
To read a symbolic link represented by a port, the symbolic link
must have been opened with the ‘O_NOFOLLOW’ and ‘O_PATH’ flags.
‘(provided? 'readlink-port)’ reports whether ports are supported.
-- Scheme Procedure: chown object owner group
-- C Function: scm_chown (object, owner, group)
Change the ownership and group of the file referred to by OBJECT to
the integer values OWNER and GROUP. OBJECT can be a string
containing a file name or, if the platform supports ‘fchown’ (*note
(libc)File Owner::), a port or integer file descriptor which is
open on the file. The return value is unspecified.
If OBJECT is a symbolic link, either the ownership of the link or
the ownership of the referenced file will be changed depending on
the operating system (lchown is unsupported at present). If OWNER
or GROUP is specified as ‘-1’, then that ID is not changed.
-- Scheme Procedure: chownat dir name owner group [flags]
-- C Function: scm_chownat (dir, name, owner, group, flags)
Like ‘chown’, but modify the owner and/or group of the file named
NAME in the directory referred to by the file port DIR instead.
The optional argument FLAGS is a bitmask. If ‘AT_SYMLINK_NOFOLLOW’
is present, then NAME will not be dereferenced if it is a symbolic
link.
-- Scheme Procedure: chmod object mode
-- C Function: scm_chmod (object, mode)
Changes the permissions of the file referred to by OBJECT. OBJECT
can be a string containing a file name or a port or integer file
descriptor which is open on a file (in which case ‘fchmod’ is used
as the underlying system call). MODE specifies the new permissions
as a decimal number, e.g., ‘(chmod "foo" #o755)’. The return value
is unspecified.
-- Scheme Procedure: utime object [actime [modtime [actimens [modtimens
[flags]]]]]
-- C Function: scm_utime (object, actime, modtime, actimens, modtimens,
flags)
‘utime’ sets the access and modification times for the file named
by OBJECT. If ACTIME or MODTIME is not supplied, then the current
time is used. ACTIME and MODTIME must be integer time values as
returned by the ‘current-time’ procedure.
OBJECT must be a file name or a port (if supported by the system).
The optional ACTIMENS and MODTIMENS are nanoseconds to add ACTIME
and MODTIME. Nanosecond precision is only supported on some
combinations of file systems and operating systems.
(utime "foo" (- (current-time) 3600))
will set the access time to one hour in the past and the
modification time to the current time.
Last, FLAGS may be either ‘0’ or the ‘AT_SYMLINK_NOFOLLOW’
constant, to set the time of OBJECT even if it is a symbolic link.
On GNU/Linux systems, at least when using the Linux kernel 5.10.46,
if OBJECT is a port, it may not be a symbolic link, even if
‘AT_SYMLINK_NOFOLLOW’ is set. This is either a bug in Linux or Guile’s
wrappers. The exact cause is unclear.
-- Scheme Procedure: delete-file str
-- C Function: scm_delete_file (str)
Deletes (or “unlinks”) the file whose path is specified by STR.
-- Scheme Procedure: delete-file-at dir str [flags]
-- C Function: scm_delete_file_at (dir, str, flags)
Like ‘unlink’, but resolve STR relative to the directory referred
to by the file port DIR instead.
The optional FLAGS argument can be ‘AT_REMOVEDIR’, in which case
‘delete-file-at’ will act like ‘rmdir’ instead of ‘delete-file’.
Why doesn’t POSIX have a ‘rmdirat’ function for this instead? No
idea!
-- Scheme Procedure: copy-file oldfile newfile
-- C Function: scm_copy_file (oldfile, newfile)
Copy the file specified by OLDFILE to NEWFILE. The return value is
unspecified.
-- Scheme Procedure: sendfile out in count [offset]
-- C Function: scm_sendfile (out, in, count, offset)
Send COUNT bytes from IN to OUT, both of which must be either open
file ports or file descriptors. When OFFSET is omitted, start
reading from IN’s current position; otherwise, start reading at
OFFSET. Return the number of bytes actually sent.
When IN is a port, it is often preferable to specify OFFSET,
because IN’s offset as a port may be different from the offset of
its underlying file descriptor.
On systems that support it, such as GNU/Linux, this procedure uses
the ‘sendfile’ libc function, which usually corresponds to a system
call. This is faster than doing a series of ‘read’ and ‘write’
system calls. A typical application is to send a file over a
socket.
In some cases, the ‘sendfile’ libc function may return ‘EINVAL’ or
‘ENOSYS’. In that case, Guile’s ‘sendfile’ procedure automatically
falls back to doing a series of ‘read’ and ‘write’ calls.
In other cases, the libc function may send fewer bytes than
COUNT—for instance because OUT is a slow or limited device, such as
a pipe. When that happens, Guile’s ‘sendfile’ automatically
retries until exactly COUNT bytes were sent or an error occurs.
-- Scheme Procedure: rename-file oldname newname
-- C Function: scm_rename (oldname, newname)
Renames the file specified by OLDNAME to NEWNAME. The return value
is unspecified.
-- Scheme Procedure: rename-file-at olddir oldname newdir newname
-- C Function: scm_renameat (olddir, oldname, newdir, newname)
Like ‘rename-file’, but when OLDDIR or NEWDIR is true, resolve
OLDNAME or NEWNAME relative to the directory specified by the file
port OLDDIR or NEWDIR instead of the current working directory.
-- Scheme Procedure: link oldpath newpath
-- C Function: scm_link (oldpath, newpath)
Creates a new name NEWPATH in the file system for the file named by
OLDPATH. If OLDPATH is a symbolic link, the link may or may not be
followed depending on the system.
-- Scheme Procedure: symlink oldpath newpath
-- C Function: scm_symlink (oldpath, newpath)
Create a symbolic link named NEWPATH with the value (i.e., pointing
to) OLDPATH. The return value is unspecified.
-- Scheme Procedure: symlinkat dir oldpath newpath
-- C Function: scm_symlinkat (dir, oldpath, newpath)
Like ‘symlink’, but resolve NEWPATH relative to the directory
referred to by the file port DIR.
-- Scheme Procedure: mkdir path [mode]
-- C Function: scm_mkdir (path, mode)
Create a new directory named by PATH. If MODE is omitted then the
permissions of the directory are set to ‘#o777’ masked with the
current umask (*note ‘umask’: Processes.). Otherwise they are set
to the value specified with MODE masked with the current umask.
The return value is unspecified.
-- Scheme Procedure: mkdirat dir path [mode]
-- C Function: scm_mkdirat (dir, path, mode)
Like ‘mkdir’, but resolve PATH relative to the directory referred
to by the file port DIR instead.
-- Scheme Procedure: rmdir path
-- C Function: scm_rmdir (path)
Remove the existing directory named by PATH. The directory must be
empty for this to succeed. The return value is unspecified.
-- Scheme Procedure: opendir dirname
-- C Function: scm_opendir (dirname)
Open the directory specified by DIRNAME and return a directory
stream.
Before using this and the procedures below, make sure to see the
higher-level procedures for directory traversal that are available
(*note File Tree Walk::).
-- Scheme Procedure: directory-stream? object
-- C Function: scm_directory_stream_p (object)
Return a boolean indicating whether OBJECT is a directory stream as
returned by ‘opendir’.
-- Scheme Procedure: readdir stream
-- C Function: scm_readdir (stream)
Return (as a string) the next directory entry from the directory
stream STREAM. If there is no remaining entry to be read then the
end of file object is returned.
-- Scheme Procedure: rewinddir stream
-- C Function: scm_rewinddir (stream)
Reset the directory port STREAM so that the next call to ‘readdir’
will return the first directory entry.
-- Scheme Procedure: closedir stream
-- C Function: scm_closedir (stream)
Close the directory stream STREAM. The return value is
unspecified.
Here is an example showing how to display all the entries in a
directory:
(define dir (opendir "/usr/lib"))
(do ((entry (readdir dir) (readdir dir)))
((eof-object? entry))
(display entry)(newline))
(closedir dir)
-- Scheme Procedure: sync
-- C Function: scm_sync ()
Flush the operating system disk buffers. The return value is
unspecified.
-- Scheme Procedure: mknod path type perms dev
-- C Function: scm_mknod (path, type, perms, dev)
Creates a new special file, such as a file corresponding to a
device. PATH specifies the name of the file. TYPE should be one
of the following symbols: ‘regular’, ‘directory’, ‘symlink’,
‘block-special’, ‘char-special’, ‘fifo’, or ‘socket’. PERMS (an
integer) specifies the file permissions. DEV (an integer)
specifies which device the special file refers to. Its exact
interpretation depends on the kind of special file being created.
E.g.,
(mknod "/dev/fd0" 'block-special #o660 (+ (* 2 256) 2))
The return value is unspecified.
-- Scheme Procedure: tmpnam
-- C Function: scm_tmpnam ()
Return an auto-generated name of a temporary file, a file which
doesn’t already exist. The name includes a path, it’s usually in
‘/tmp’ but that’s system dependent.
Care must be taken when using ‘tmpnam’. In between choosing the
name and creating the file another program might use that name, or
an attacker might even make it a symlink pointing at something
important and causing you to overwrite that.
The safe way is to create the file using ‘open’ with ‘O_EXCL’ to
avoid any overwriting. A loop can try again with another name if
the file exists (error ‘EEXIST’). ‘mkstemp’ below does that.
-- Scheme Procedure: mkstemp tmpl [mode]
Create a new unique file in the file system and return a new
buffered port open for reading and writing to the file.
TMPL is a string specifying where the file should be created: it
must end with ‘XXXXXX’. The name of the newly created file will be
the same as TMPL, but with those ‘X’s changed, and can be
determined by calling ‘port-filename’ on the returned port.
Note that the newly created file is not deleted automatically by
Guile; probably the caller should arrange to call ‘delete-file’
when the file is no longer needed.
POSIX doesn’t specify the permissions mode of the file. On GNU and
most systems it’s ‘#o600’; an application can use ‘chmod’ to relax
that if desired. For example ‘#o666’ less ‘umask’, which is usual
for ordinary file creation,
(let ((port (mkstemp "/tmp/myfile-XXXXXX")))
(chmod port (logand #o666 (lognot (umask))))
...)
The optional MODE argument specifies a mode with which to open the
new file, as a string in the same format that ‘open-file’ takes.
It defaults to ‘"w+"’.
-- Scheme Procedure: tmpfile
-- C Function: scm_tmpfile ()
Return an input/output port to a unique temporary file named using
the path prefix ‘P_tmpdir’ defined in ‘stdio.h’. The file is
automatically deleted when the port is closed or the program
terminates.
-- Scheme Procedure: mkdtemp tmpl
-- C Function: scm_mkdtemp (tmpl)
Create a new directory named in accordance with the template string
TMPL.
TMPL is a string specifying the directory’s name. The last six
characters of TMPL must be ‘XXXXXX’. Upon successful execution,
the name of the new directory is returned which has the same form
as TMPL but with the ‘XXXXXX’ characters modified to ensure the
directory name is unique.
The permissions of the directory created are OS dependent, but, are
usually ‘#o700’.
An error may be thrown if the template has the wrong format or if
the directory cannot be created.
-- Scheme Procedure: dirname filename
-- C Function: scm_dirname (filename)
Return the directory name component of the file name FILENAME. If
FILENAME does not contain a directory component, ‘.’ is returned.
-- Scheme Procedure: basename filename [suffix]
-- C Function: scm_basename (filename, suffix)
Return the base name of the file name FILENAME. The base name is
the file name without any directory components. If SUFFIX is
provided, and is equal to the end of BASENAME, it is removed also.
(basename "/tmp/test.xml" ".xml")
⇒ "test"
-- Scheme Procedure: canonicalize-path path
-- C Function: scm_canonicalize_path (path)
Return the canonical (absolute) path of PATH. A canonical path has
no ‘.’ or ‘..’ components, nor any repeated path separators (‘/’)
nor symlinks.
Raises an error if any component of PATH does not exist.
(canonicalize-path "test.xml")
⇒ "/tmp/test.xml"
-- Scheme Procedure: file-exists? filename
Return ‘#t’ if the file named FILENAME exists, ‘#f’ if not.
Many operating systems, such as GNU, use ‘/’ (forward slash) to
separate the components of a file name; any file name starting with ‘/’
is considered an “absolute file name”. These conventions are specified
by the POSIX Base Definitions, which refer to conforming file names as
“pathnames”. Some operating systems use a different convention; in
particular, Windows uses ‘\’ (backslash) as the file name separator, and
also has the notion of “volume names” like ‘C:\’ for absolute file
names. The following procedures and variables provide support for
portable file name manipulations.
-- Scheme Procedure: system-file-name-convention
Return either ‘posix’ or ‘windows’, depending on what kind of
system this Guile is running on.
-- Scheme Procedure: file-name-separator? c
Return true if character C is a file name separator on the host
platform.
-- Scheme Procedure: absolute-file-name? file-name
Return true if FILE-NAME denotes an absolute file name on the host
platform.
-- Scheme Variable: file-name-separator-string
The preferred file name separator.
Note that on MinGW builds for Windows, both ‘/’ and ‘\’ are valid
separators. Thus, programs should not assume that
‘file-name-separator-string’ is the _only_ file name
separator—e.g., when extracting the components of a file name.
7.2.4 User Information
----------------------
The facilities in this section provide an interface to the user and
group database. They should be used with care since they are not
reentrant.
The following functions accept an object representing user
information and return a selected component:
-- Scheme Procedure: passwd:name pw
The name of the userid.
-- Scheme Procedure: passwd:passwd pw
The encrypted passwd.
-- Scheme Procedure: passwd:uid pw
The user id number.
-- Scheme Procedure: passwd:gid pw
The group id number.
-- Scheme Procedure: passwd:gecos pw
The full name.
-- Scheme Procedure: passwd:dir pw
The home directory.
-- Scheme Procedure: passwd:shell pw
The login shell.
-- Scheme Procedure: getpwuid uid
Look up an integer userid in the user database.
-- Scheme Procedure: getpwnam name
Look up a user name string in the user database.
-- Scheme Procedure: setpwent
Initializes a stream used by ‘getpwent’ to read from the user
database. The next use of ‘getpwent’ will return the first entry.
The return value is unspecified.
-- Scheme Procedure: getpwent
Read the next entry in the user database stream. The return is a
passwd user object as above, or ‘#f’ when no more entries.
-- Scheme Procedure: endpwent
Closes the stream used by ‘getpwent’. The return value is
unspecified.
-- Scheme Procedure: setpw [arg]
-- C Function: scm_setpwent (arg)
If called with a true argument, initialize or reset the password
data stream. Otherwise, close the stream. The ‘setpwent’ and
‘endpwent’ procedures are implemented on top of this.
-- Scheme Procedure: getpw [user]
-- C Function: scm_getpwuid (user)
Look up an entry in the user database. USER can be an integer, a
string, or omitted, giving the behaviour of getpwuid, getpwnam or
getpwent respectively.
The following functions accept an object representing group
information and return a selected component:
-- Scheme Procedure: group:name gr
The group name.
-- Scheme Procedure: group:passwd gr
The encrypted group password.
-- Scheme Procedure: group:gid gr
The group id number.
-- Scheme Procedure: group:mem gr
A list of userids which have this group as a supplementary group.
-- Scheme Procedure: getgrgid gid
Look up an integer group id in the group database.
-- Scheme Procedure: getgrnam name
Look up a group name in the group database.
-- Scheme Procedure: setgrent
Initializes a stream used by ‘getgrent’ to read from the group
database. The next use of ‘getgrent’ will return the first entry.
The return value is unspecified.
-- Scheme Procedure: getgrent
Return the next entry in the group database, using the stream set
by ‘setgrent’.
-- Scheme Procedure: endgrent
Closes the stream used by ‘getgrent’. The return value is
unspecified.
-- Scheme Procedure: setgr [arg]
-- C Function: scm_setgrent (arg)
If called with a true argument, initialize or reset the group data
stream. Otherwise, close the stream. The ‘setgrent’ and
‘endgrent’ procedures are implemented on top of this.
-- Scheme Procedure: getgr [group]
-- C Function: scm_getgrgid (group)
Look up an entry in the group database. GROUP can be an integer, a
string, or omitted, giving the behaviour of getgrgid, getgrnam or
getgrent respectively.
In addition to the accessor procedures for the user database, the
following shortcut procedure is also available.
-- Scheme Procedure: getlogin
-- C Function: scm_getlogin ()
Return a string containing the name of the user logged in on the
controlling terminal of the process, or ‘#f’ if this information
cannot be obtained.
7.2.5 Time
----------
-- Scheme Procedure: current-time
-- C Function: scm_current_time ()
Return the number of seconds since 1970-01-01 00:00:00 UTC,
excluding leap seconds.
-- Scheme Procedure: gettimeofday
-- C Function: scm_gettimeofday ()
Return a pair containing the number of seconds and microseconds
since 1970-01-01 00:00:00 UTC, excluding leap seconds. Note:
whether true microsecond resolution is available depends on the
operating system.
The following procedures either accept an object representing a
broken down time and return a selected component, or accept an object
representing a broken down time and a value and set the component to the
value. The numbers in parentheses give the usual range.
-- Scheme Procedure: tm:sec tm
-- Scheme Procedure: set-tm:sec tm val
Seconds (0-59).
-- Scheme Procedure: tm:min tm
-- Scheme Procedure: set-tm:min tm val
Minutes (0-59).
-- Scheme Procedure: tm:hour tm
-- Scheme Procedure: set-tm:hour tm val
Hours (0-23).
-- Scheme Procedure: tm:mday tm
-- Scheme Procedure: set-tm:mday tm val
Day of the month (1-31).
-- Scheme Procedure: tm:mon tm
-- Scheme Procedure: set-tm:mon tm val
Month (0-11).
-- Scheme Procedure: tm:year tm
-- Scheme Procedure: set-tm:year tm val
Year (70-), the year minus 1900.
-- Scheme Procedure: tm:wday tm
-- Scheme Procedure: set-tm:wday tm val
Day of the week (0-6) with Sunday represented as 0.
-- Scheme Procedure: tm:yday tm
-- Scheme Procedure: set-tm:yday tm val
Day of the year (0-364, 365 in leap years).
-- Scheme Procedure: tm:isdst tm
-- Scheme Procedure: set-tm:isdst tm val
Daylight saving indicator (0 for “no”, greater than 0 for “yes”,
less than 0 for “unknown”).
-- Scheme Procedure: tm:gmtoff tm
-- Scheme Procedure: set-tm:gmtoff tm val
Time zone offset in seconds west of UTC (-46800 to 43200). For
example on East coast USA (zone ‘EST+5’) this would be 18000 (ie.
5*60*60) in winter, or 14400 (ie. 4*60*60) during daylight savings.
Note ‘tm:gmtoff’ is not the same as ‘tm_gmtoff’ in the C ‘tm’
structure. ‘tm_gmtoff’ is seconds east and hence the negative of
the value here.
-- Scheme Procedure: tm:zone tm
-- Scheme Procedure: set-tm:zone tm val
Time zone label (a string), not necessarily unique.
-- Scheme Procedure: localtime time [zone]
-- C Function: scm_localtime (time, zone)
Return an object representing the broken down components of TIME,
an integer like the one returned by ‘current-time’. The time zone
for the calculation is optionally specified by ZONE (a string),
otherwise the ‘TZ’ environment variable or the system default is
used.
-- Scheme Procedure: gmtime time
-- C Function: scm_gmtime (time)
Return an object representing the broken down components of TIME,
an integer like the one returned by ‘current-time’. The values are
calculated for UTC.
-- Scheme Procedure: mktime sbd-time [zone]
-- C Function: scm_mktime (sbd_time, zone)
For a broken down time object SBD-TIME, return a pair the ‘car’ of
which is an integer time like ‘current-time’, and the ‘cdr’ of
which is a new broken down time with normalized fields.
ZONE is a timezone string, or the default is the ‘TZ’ environment
variable or the system default (*note Specifying the Time Zone with
‘TZ’: (libc)TZ Variable.). SBD-TIME is taken to be in that ZONE.
The following fields of SBD-TIME are used: ‘tm:year’, ‘tm:mon’,
‘tm:mday’, ‘tm:hour’, ‘tm:min’, ‘tm:sec’, ‘tm:isdst’. The values
can be outside their usual ranges. For example ‘tm:hour’ normally
goes up to 23, but a value say 33 would mean 9 the following day.
‘tm:isdst’ in SBD-TIME says whether the time given is with daylight
savings or not. This is ignored if ZONE doesn’t have any daylight
savings adjustment amount.
The broken down time in the return normalizes the values of
SBD-TIME by bringing them into their usual ranges, and using the
actual daylight savings rule for that time in ZONE (which may
differ from what SBD-TIME had). The easiest way to think of this
is that SBD-TIME plus ZONE converts to the integer UTC time, then a
‘localtime’ is applied to get the normal presentation of that time,
in ZONE.
-- Scheme Procedure: tzset
-- C Function: scm_tzset ()
Initialize the timezone from the ‘TZ’ environment variable or the
system default. It’s not usually necessary to call this procedure
since it’s done automatically by other procedures that depend on
the timezone.
-- Scheme Procedure: strftime format tm
-- C Function: scm_strftime (format, tm)
Return a string which is broken-down time structure TM formatted
according to the given FORMAT string.
FORMAT contains field specifications introduced by a ‘%’ character.
See *note (libc)Formatting Calendar Time::, or ‘man 3 strftime’,
for the available formatting.
(strftime "%c" (localtime (current-time)))
⇒ "Mon Mar 11 20:17:43 2002"
If ‘setlocale’ has been called (*note Locales::), month and day
names are from the current locale and in the locale character set.
-- Scheme Procedure: strptime format string
-- C Function: scm_strptime (format, string)
Performs the reverse action to ‘strftime’, parsing STRING according
to the specification supplied in FORMAT. The interpretation of
month and day names is dependent on the current locale. The value
returned is a pair. The CAR has an object with time components in
the form returned by ‘localtime’ or ‘gmtime’, but the time zone
components are not usefully set. The CDR reports the number of
characters from STRING which were used for the conversion.
-- Variable: internal-time-units-per-second
The value of this variable is the number of time units per second
reported by the following procedures.
-- Scheme Procedure: times
-- C Function: scm_times ()
Return an object with information about real and processor time.
The following procedures accept such an object as an argument and
return a selected component:
-- Scheme Procedure: tms:clock tms
The current real time, expressed as time units relative to an
arbitrary base.
-- Scheme Procedure: tms:utime tms
The CPU time units used by the calling process.
-- Scheme Procedure: tms:stime tms
The CPU time units used by the system on behalf of the calling
process.
-- Scheme Procedure: tms:cutime tms
The CPU time units used by terminated child processes of the
calling process, whose status has been collected (e.g., using
‘waitpid’).
-- Scheme Procedure: tms:cstime tms
Similarly, the CPU times units used by the system on behalf of
terminated child processes.
-- Scheme Procedure: get-internal-real-time
-- C Function: scm_get_internal_real_time ()
Return the number of time units since the interpreter was started.
-- Scheme Procedure: get-internal-run-time
-- C Function: scm_get_internal_run_time ()
Return the number of time units of processor time used by the
interpreter. Both _system_ and _user_ time are included but
subprocesses are not.
7.2.6 Runtime Environment
-------------------------
-- Scheme Procedure: program-arguments
-- Scheme Procedure: command-line
-- Scheme Procedure: set-program-arguments
-- C Function: scm_program_arguments ()
-- C Function: scm_set_program_arguments_scm (lst)
Get the command line arguments passed to Guile, or set new
arguments.
The arguments are a list of strings, the first of which is the
invoked program name. This is just "guile" (or the executable
path) when run interactively, or it’s the script name when running
a script with ‘-s’ (*note Invoking Guile::).
guile -L /my/extra/dir -s foo.scm abc def
(program-arguments) ⇒ ("foo.scm" "abc" "def")
‘set-program-arguments’ allows a library module or similar to
modify the arguments, for example to strip options it recognises,
leaving the rest for the mainline.
The argument list is held in a fluid, which means it’s separate for
each thread. Neither the list nor the strings within it are copied
at any point and normally should not be mutated.
The two names ‘program-arguments’ and ‘command-line’ are an
historical accident, they both do exactly the same thing. The name
‘scm_set_program_arguments_scm’ has an extra ‘_scm’ on the end to
avoid clashing with the C function below.
-- C Function: void scm_set_program_arguments (int argc, char **argv,
char *first)
Set the list of command line arguments for ‘program-arguments’ and
‘command-line’ above.
ARGV is an array of null-terminated strings, as in a C ‘main’
function. ARGC is the number of strings in ARGV, or if it’s
negative then a ‘NULL’ in ARGV marks its end.
FIRST is an extra string put at the start of the arguments, or
‘NULL’ for no such extra. This is a convenient way to pass the
program name after advancing ARGV to strip option arguments. Eg.
{
char *progname = argv[0];
for (argv++; argv[0] != NULL && argv[0][0] == '-'; argv++)
{
/* munch option ... */
}
/* remaining args for scheme level use */
scm_set_program_arguments (-1, argv, progname);
}
This sort of thing is often done at startup under ‘scm_boot_guile’
with options handled at the C level removed. The given strings are
all copied, so the C data is not accessed again once
‘scm_set_program_arguments’ returns.
-- Scheme Procedure: getenv name
-- C Function: scm_getenv (name)
Looks up the string NAME in the current environment. The return
value is ‘#f’ unless a string of the form ‘NAME=VALUE’ is found, in
which case the string ‘VALUE’ is returned.
-- Scheme Procedure: setenv name value
Modifies the environment of the current process, which is also the
default environment inherited by child processes.
If VALUE is ‘#f’, then NAME is removed from the environment.
Otherwise, the string NAME=VALUE is added to the environment,
replacing any existing string with name matching NAME.
The return value is unspecified.
-- Scheme Procedure: unsetenv name
Remove variable NAME from the environment. The name can not
contain a ‘=’ character.
-- Scheme Procedure: environ [env]
-- C Function: scm_environ (env)
If ENV is omitted, return the current environment (in the Unix
sense) as a list of strings. Otherwise set the current
environment, which is also the default environment for child
processes, to the supplied list of strings. Each member of ENV
should be of the form NAME=VALUE and values of NAME should not be
duplicated. If ENV is supplied then the return value is
unspecified.
-- Scheme Procedure: putenv str
-- C Function: scm_putenv (str)
Modifies the environment of the current process, which is also the
default environment inherited by child processes.
If STR is of the form ‘NAME=VALUE’ then it will be written directly
into the environment, replacing any existing environment string
with name matching ‘NAME’. If STR does not contain an equal sign,
then any existing string with name matching STR will be removed.
The return value is unspecified.
7.2.7 Processes
---------------
-- Scheme Procedure: chdir str
-- C Function: scm_chdir (str)
Change the current working directory to STR. STR can be a string
containing a file name, or a port if supported by the system.
‘(provided? 'chdir-port)’ reports whether ports are supported. The
return value is unspecified.
-- Scheme Procedure: getcwd
-- C Function: scm_getcwd ()
Return the name of the current working directory.
-- Scheme Procedure: umask [mode]
-- C Function: scm_umask (mode)
If MODE is omitted, returns a decimal number representing the
current file creation mask. Otherwise the file creation mask is
set to MODE and the previous value is returned. *Note Assigning
File Permissions: (libc)Setting Permissions, for more on how to use
umasks.
E.g., ‘(umask #o022)’ sets the mask to octal 22/decimal 18.
-- Scheme Procedure: chroot path
-- C Function: scm_chroot (path)
Change the root directory to that specified in PATH. This
directory will be used for path names beginning with ‘/’. The root
directory is inherited by all children of the current process.
Only the superuser may change the root directory.
-- Scheme Procedure: getpid
-- C Function: scm_getpid ()
Return an integer representing the current process ID.
-- Scheme Procedure: getgroups
-- C Function: scm_getgroups ()
Return a vector of integers representing the current supplementary
group IDs.
-- Scheme Procedure: getppid
-- C Function: scm_getppid ()
Return an integer representing the process ID of the parent
process.
-- Scheme Procedure: getuid
-- C Function: scm_getuid ()
Return an integer representing the current real user ID.
-- Scheme Procedure: getgid
-- C Function: scm_getgid ()
Return an integer representing the current real group ID.
-- Scheme Procedure: geteuid
-- C Function: scm_geteuid ()
Return an integer representing the current effective user ID. If
the system does not support effective IDs, then the real ID is
returned. ‘(provided? 'EIDs)’ reports whether the system supports
effective IDs.
-- Scheme Procedure: getegid
-- C Function: scm_getegid ()
Return an integer representing the current effective group ID. If
the system does not support effective IDs, then the real ID is
returned. ‘(provided? 'EIDs)’ reports whether the system supports
effective IDs.
-- Scheme Procedure: setgroups vec
-- C Function: scm_setgroups (vec)
Set the current set of supplementary group IDs to the integers in
the given vector VEC. The return value is unspecified.
Generally only the superuser can set the process group IDs (*note
Setting the Group IDs: (libc)Setting Groups.).
-- Scheme Procedure: setuid id
-- C Function: scm_setuid (id)
Sets both the real and effective user IDs to the integer ID,
provided the process has appropriate privileges. The return value
is unspecified.
-- Scheme Procedure: setgid id
-- C Function: scm_setgid (id)
Sets both the real and effective group IDs to the integer ID,
provided the process has appropriate privileges. The return value
is unspecified.
-- Scheme Procedure: seteuid id
-- C Function: scm_seteuid (id)
Sets the effective user ID to the integer ID, provided the process
has appropriate privileges. If effective IDs are not supported,
the real ID is set instead—‘(provided? 'EIDs)’ reports whether the
system supports effective IDs. The return value is unspecified.
-- Scheme Procedure: setegid id
-- C Function: scm_setegid (id)
Sets the effective group ID to the integer ID, provided the process
has appropriate privileges. If effective IDs are not supported,
the real ID is set instead—‘(provided? 'EIDs)’ reports whether the
system supports effective IDs. The return value is unspecified.
-- Scheme Procedure: getpgrp
-- C Function: scm_getpgrp ()
Return an integer representing the current process group ID. This
is the POSIX definition, not BSD.
-- Scheme Procedure: setpgid pid pgid
-- C Function: scm_setpgid (pid, pgid)
Move the process PID into the process group PGID. PID or PGID must
be integers: they can be zero to indicate the ID of the current
process. Fails on systems that do not support job control. The
return value is unspecified.
-- Scheme Procedure: setsid
-- C Function: scm_setsid ()
Creates a new session. The current process becomes the session
leader and is put in a new process group. The process will be
detached from its controlling terminal if it has one. The return
value is an integer representing the new process group ID.
-- Scheme Procedure: getsid pid
-- C Function: scm_getsid (pid)
Returns the session ID of process PID. (The session ID of a
process is the process group ID of its session leader.)
-- Scheme Procedure: waitpid pid [options]
-- C Function: scm_waitpid (pid, options)
This procedure collects status information from a child process
which has terminated or (optionally) stopped. Normally it will
suspend the calling process until this can be done. If more than
one child process is eligible then one will be chosen by the
operating system.
The value of PID determines the behaviour:
PID greater than 0
Request status information from the specified child process.
PID equal to -1 or ‘WAIT_ANY’
Request status information for any child process.
PID equal to 0 or ‘WAIT_MYPGRP’
Request status information for any child process in the
current process group.
PID less than -1
Request status information for any child process whose process
group ID is −PID.
The OPTIONS argument, if supplied, should be the bitwise OR of the
values of zero or more of the following variables:
-- Variable: WNOHANG
Return immediately even if there are no child processes to be
collected.
-- Variable: WUNTRACED
Report status information for stopped processes as well as
terminated processes.
The return value is a pair containing:
1. The process ID of the child process, or 0 if ‘WNOHANG’ was
specified and no process was collected.
2. The integer status value (*note (libc)Process Completion
Status::).
The following three functions can be used to decode the integer
status value returned by ‘waitpid’.
-- Scheme Procedure: status:exit-val status
-- C Function: scm_status_exit_val (status)
Return the exit status value, as would be set if a process ended
normally through a call to ‘exit’ or ‘_exit’, if any, otherwise
‘#f’.
-- Scheme Procedure: status:term-sig status
-- C Function: scm_status_term_sig (status)
Return the signal number which terminated the process, if any,
otherwise ‘#f’.
-- Scheme Procedure: status:stop-sig status
-- C Function: scm_status_stop_sig (status)
Return the signal number which stopped the process, if any,
otherwise ‘#f’.
-- Scheme Procedure: system [cmd]
-- C Function: scm_system (cmd)
Execute CMD using the operating system’s “command processor”.
Under Unix this is usually the default shell ‘sh’. The value
returned is CMD’s exit status as returned by ‘waitpid’, which can
be interpreted using the functions above.
If ‘system’ is called without arguments, return a boolean
indicating whether the command processor is available.
-- Scheme Procedure: system* arg1 arg2 ...
-- C Function: scm_system_star (args)
Execute the command indicated by ARG1 ARG2 .... The first element
must be a string indicating the command to be executed, and the
remaining items must be strings representing each of the arguments
to that command.
This function returns the exit status of the command as provided by
‘waitpid’. This value can be handled with ‘status:exit-val’ and
the related functions.
‘system*’ is similar to ‘system’, but accepts only one string
per-argument, and performs no shell interpretation. The command is
executed using fork and execlp. Accordingly this function may be
safer than ‘system’ in situations where shell interpretation is not
required.
Example: (system* "echo" "foo" "bar")
-- Scheme Procedure: quit [status]
-- Scheme Procedure: exit [status]
Terminate the current process with proper unwinding of the Scheme
stack. The exit status zero if STATUS is not supplied. If STATUS
is supplied, and it is an integer, that integer is used as the exit
status. If STATUS is ‘#t’ or ‘#f’, the exit status is EXIT_SUCCESS
or EXIT_FAILURE, respectively.
The procedure ‘exit’ is an alias of ‘quit’. They have the same
functionality.
-- Scheme Variable: EXIT_SUCCESS
-- Scheme Variable: EXIT_FAILURE
These constants represent the standard exit codes for success
(zero) or failure (one.)
-- Scheme Procedure: primitive-exit [status]
-- Scheme Procedure: primitive-_exit [status]
-- C Function: scm_primitive_exit (status)
-- C Function: scm_primitive__exit (status)
Terminate the current process without unwinding the Scheme stack.
The exit status is STATUS if supplied, otherwise zero.
‘primitive-exit’ uses the C ‘exit’ function and hence runs usual C
level cleanups (flush output streams, call ‘atexit’ functions, etc,
see *note (libc)Normal Termination::)).
‘primitive-_exit’ is the ‘_exit’ system call (*note
(libc)Termination Internals::). This terminates the program
immediately, with neither Scheme-level nor C-level cleanups.
The typical use for ‘primitive-_exit’ is from a child process
created with ‘primitive-fork’. For example in a Gdk program the
child process inherits the X server connection and a C-level
‘atexit’ cleanup which will close that connection. But closing in
the child would upset the protocol in the parent, so
‘primitive-_exit’ should be used to exit without that.
-- Scheme Procedure: execl filename arg ...
-- C Function: scm_execl (filename, args)
Executes the file named by FILENAME as a new process image. The
remaining arguments are supplied to the process; from a C program
they are accessible as the ‘argv’ argument to ‘main’.
Conventionally the first ARG is the same as FILENAME. All
arguments must be strings.
If ARG is missing, FILENAME is executed with a null argument list,
which may have system-dependent side-effects.
This procedure is currently implemented using the ‘execv’ system
call, but we call it ‘execl’ because of its Scheme calling
interface.
-- Scheme Procedure: execlp filename arg ...
-- C Function: scm_execlp (filename, args)
Similar to ‘execl’, however if FILENAME does not contain a slash
then the file to execute will be located by searching the
directories listed in the ‘PATH’ environment variable.
This procedure is currently implemented using the ‘execvp’ system
call, but we call it ‘execlp’ because of its Scheme calling
interface.
-- Scheme Procedure: execle filename env arg ...
-- C Function: scm_execle (filename, env, args)
Similar to ‘execl’, but the environment of the new process is
specified by ENV, which must be a list of strings as returned by
the ‘environ’ procedure.
This procedure is currently implemented using the ‘execve’ system
call, but we call it ‘execle’ because of its Scheme calling
interface.
-- Scheme Procedure: primitive-fork
-- C Function: scm_fork ()
Creates a new “child” process by duplicating the current “parent”
process. In the child the return value is 0. In the parent the
return value is the integer process ID of the child.
Note that it is unsafe to fork a process that has multiple threads
running, as only the thread that calls ‘primitive-fork’ will
persist in the child. Any resources that other threads held, such
as locked mutexes or open file descriptors, are lost. Indeed,
POSIX specifies that only async-signal-safe procedures are safe to
call after a multithreaded fork, which is a very limited set.
Guile issues a warning if it detects a fork from a multi-threaded
program.
Note: If you are looking to spawn a process with some pipes
set up, using the ‘spawn’ procedure described below will be
more robust (in particular in multi-threaded contexts), more
portable, and usually more efficient than the combination of
‘primitive-fork’ and ‘execl’.
This procedure has been renamed from ‘fork’ to avoid a naming
conflict with the scsh fork.
-- Scheme Procedure: spawn PROGRAM ARGUMENTS [#:environment=(environ)]
[#:input=(current-input-port)]
[#:output=(current-output-port)]
[#:error=(current-error-port)] [#:search-path?=#t]
Spawn a new child process executing PROGRAM with the given
ARGUMENTS, a list of one or more strings (by convention, the first
argument is typically PROGRAM), and return its PID. Raise a
‘system-error’ exception if PROGRAM could not be found or could not
be executed.
If the keyword argument ‘#:search-path?’ is true, it selects
whether the ‘PATH’ environment variable should be inspected to find
PROGRAM. It is true by default.
The ‘#:environment’ keyword parameter specifies the list of
environment variables of the child process. It defaults to
‘(environ)’.
The keyword arguments ‘#:input’, ‘#:output’, and ‘#:error’ specify
the port or file descriptor for the child process to use as
standard input, standard output, and standard error. No other file
descriptors are inherited from the parent process.
The example below shows how to spawn the ‘uname’ program with the
‘-o’ option (*note (coreutils)uname invocation::), redirect its standard
output to a pipe, and read from it:
(use-modules (rnrs io ports))
(let* ((input+output (pipe))
(pid (spawn "uname" '("uname" "-o")
#:output (cdr input+output))))
(close-port (cdr input+output))
(format #t "read ~s~%" (get-string-all (car input+output)))
(close-port (car input+output))
(waitpid pid))
⊣ read "GNU/Linux\n"
⇒ (1234 . 0)
-- Scheme Procedure: nice incr
-- C Function: scm_nice (incr)
Increment the priority of the current process by INCR. A higher
priority value means that the process runs less often. The return
value is unspecified.
-- Scheme Procedure: setpriority which who prio
-- C Function: scm_setpriority (which, who, prio)
Set the scheduling priority of the process, process group or user,
as indicated by WHICH and WHO. WHICH is one of the variables
‘PRIO_PROCESS’, ‘PRIO_PGRP’ or ‘PRIO_USER’, and WHO is interpreted
relative to WHICH (a process identifier for ‘PRIO_PROCESS’, process
group identifier for ‘PRIO_PGRP’, and a user identifier for
‘PRIO_USER’. A zero value of WHO denotes the current process,
process group, or user. PRIO is a value in the range [−20,20].
The default priority is 0; lower priorities (in numerical terms)
cause more favorable scheduling. Sets the priority of all of the
specified processes. Only the super-user may lower priorities.
The return value is not specified.
-- Scheme Procedure: getpriority which who
-- C Function: scm_getpriority (which, who)
Return the scheduling priority of the process, process group or
user, as indicated by WHICH and WHO. WHICH is one of the variables
‘PRIO_PROCESS’, ‘PRIO_PGRP’ or ‘PRIO_USER’, and WHO should be
interpreted depending on WHICH (a process identifier for
‘PRIO_PROCESS’, process group identifier for ‘PRIO_PGRP’, and a
user identifier for ‘PRIO_USER’). A zero value of WHO denotes the
current process, process group, or user. Return the highest
priority (lowest numerical value) of any of the specified
processes.
-- Scheme Procedure: getaffinity pid
-- C Function: scm_getaffinity (pid)
Return a bitvector representing the CPU affinity mask for process
PID. Each CPU the process has affinity with has its corresponding
bit set in the returned bitvector. The number of bits set is a
good estimate of how many CPUs Guile can use without stepping on
other processes’ toes.
Currently this procedure is only defined on GNU variants (*note
‘sched_getaffinity’: (libc)CPU Affinity.).
-- Scheme Procedure: setaffinity pid mask
-- C Function: scm_setaffinity (pid, mask)
Install the CPU affinity mask MASK, a bitvector, for the process or
thread with ID PID. The return value is unspecified.
Currently this procedure is only defined on GNU variants (*note
‘sched_setaffinity’: (libc)CPU Affinity.).
*Note Threads::, for information on how get the number of processors
available on a system.
7.2.8 Signals
-------------
The following procedures raise, handle and wait for signals.
Scheme code signal handlers are run via an async (*note Asyncs::), so
they’re called in the handler’s thread at the next safe opportunity.
Generally this is after any currently executing primitive procedure
finishes (which could be a long time for primitives that wait for an
external event).
-- Scheme Procedure: kill pid sig
-- C Function: scm_kill (pid, sig)
Sends a signal to the specified process or group of processes.
PID specifies the processes to which the signal is sent:
PID greater than 0
The process whose identifier is PID.
PID equal to 0
All processes in the current process group.
PID less than -1
The process group whose identifier is -PID
PID equal to -1
If the process is privileged, all processes except for some
special system processes. Otherwise, all processes with the
current effective user ID.
SIG should be specified using a variable corresponding to the Unix
symbolic name, e.g.,
-- Variable: SIGHUP
Hang-up signal.
-- Variable: SIGINT
Interrupt signal.
A full list of signals on the GNU system may be found in *note
(libc)Standard Signals::.
-- Scheme Procedure: raise sig
-- C Function: scm_raise (sig)
Sends a specified signal SIG to the current process, where SIG is
as described for the ‘kill’ procedure.
-- Scheme Procedure: sigaction signum [handler [flags [thread]]]
-- C Function: scm_sigaction (signum, handler, flags)
-- C Function: scm_sigaction_for_thread (signum, handler, flags,
thread)
Install or report the signal handler for a specified signal.
SIGNUM is the signal number, which can be specified using the value
of variables such as ‘SIGINT’.
If HANDLER is omitted, ‘sigaction’ returns a pair: the CAR is the
current signal hander, which will be either an integer with the
value ‘SIG_DFL’ (default action) or ‘SIG_IGN’ (ignore), or the
Scheme procedure which handles the signal, or ‘#f’ if a non-Scheme
procedure handles the signal. The CDR contains the current
‘sigaction’ flags for the handler.
If HANDLER is provided, it is installed as the new handler for
SIGNUM. HANDLER can be a Scheme procedure taking one argument, or
the value of ‘SIG_DFL’ (default action) or ‘SIG_IGN’ (ignore), or
‘#f’ to restore whatever signal handler was installed before
‘sigaction’ was first used. When a scheme procedure has been
specified, that procedure will run in the given THREAD. When no
thread has been given, the thread that made this call to
‘sigaction’ is used.
FLAGS is a ‘logior’ (*note Bitwise Operations::) of the following
(where provided by the system), or ‘0’ for none.
-- Variable: SA_NOCLDSTOP
By default, ‘SIGCHLD’ is signalled when a child process stops
(ie. receives ‘SIGSTOP’), and when a child process terminates.
With the ‘SA_NOCLDSTOP’ flag, ‘SIGCHLD’ is only signalled for
termination, not stopping.
‘SA_NOCLDSTOP’ has no effect on signals other than ‘SIGCHLD’.
-- Variable: SA_RESTART
If a signal occurs while in a system call, deliver the signal
then restart the system call (as opposed to returning an
‘EINTR’ error from that call).
Guile handles signals asynchronously. When it receives a signal,
the synchronous signal handler just records the fact that a signal
was received and sets a flag to tell the relevant Guile thread that
it has a pending signal. When the Guile thread checks the
pending-interrupt flag, it will arrange to run the asynchronous
part of the signal handler, which is the handler attached by
‘sigaction’.
This strategy has some perhaps-unexpected interactions with the
‘SA_RESTART’ flag, though: because the synchronous handler doesn’t
do very much, and notably it doesn’t run the Guile handler, it’s
impossible to interrupt a thread stuck in a long-running system
call via a signal handler that is installed with ‘SA_RESTART’: the
synchronous handler just records the pending interrupt, but then
the system call resumes and Guile doesn’t have a chance to actually
check the flag and run the asynchronous handler. That’s just how
it is.
The return value is a pair with information about the old handler
as described above.
This interface does not provide access to the “signal blocking”
facility. Maybe this is not needed, since the thread support may
provide solutions to the problem of consistent access to data
structures.
-- Scheme Procedure: restore-signals
-- C Function: scm_restore_signals ()
Return all signal handlers to the values they had before any call
to ‘sigaction’ was made. The return value is unspecified.
-- Scheme Procedure: alarm i
-- C Function: scm_alarm (i)
Set a timer to raise a ‘SIGALRM’ signal after the specified number
of seconds (an integer). It’s advisable to install a signal
handler for ‘SIGALRM’ beforehand, since the default action is to
terminate the process.
The return value indicates the time remaining for the previous
alarm, if any. The new value replaces the previous alarm. If
there was no previous alarm, the return value is zero.
-- Scheme Procedure: pause
-- C Function: scm_pause ()
Pause the current process (thread?) until a signal arrives whose
action is to either terminate the current process or invoke a
handler procedure. The return value is unspecified.
-- Scheme Procedure: sleep secs
-- Scheme Procedure: usleep usecs
-- C Function: scm_sleep (secs)
-- C Function: scm_usleep (usecs)
Wait the given period SECS seconds or USECS microseconds (both
integers). If a signal arrives the wait stops and the return value
is the time remaining, in seconds or microseconds respectively. If
the period elapses with no signal the return is zero.
On most systems the process scheduler is not microsecond accurate
and the actual period slept by ‘usleep’ might be rounded to a
system clock tick boundary, which might be 10 milliseconds for
instance.
See ‘scm_std_sleep’ and ‘scm_std_usleep’ for equivalents at the C
level (*note Blocking::).
-- Scheme Procedure: getitimer which_timer
-- Scheme Procedure: setitimer which_timer interval_seconds
interval_microseconds value_seconds value_microseconds
-- C Function: scm_getitimer (which_timer)
-- C Function: scm_setitimer (which_timer, interval_seconds,
interval_microseconds, value_seconds, value_microseconds)
Get or set the periods programmed in certain system timers.
These timers have two settings. The first setting, the interval,
is the value at which the timer will be reset when the current
timer expires. The second is the current value of the timer,
indicating when the next expiry will be signalled.
WHICH_TIMER is one of the following values:
-- Variable: ITIMER_REAL
A real-time timer, counting down elapsed real time. At zero
it raises ‘SIGALRM’. This is like ‘alarm’ above, but with a
higher resolution period.
-- Variable: ITIMER_VIRTUAL
A virtual-time timer, counting down while the current process
is actually using CPU. At zero it raises ‘SIGVTALRM’.
-- Variable: ITIMER_PROF
A profiling timer, counting down while the process is running
(like ‘ITIMER_VIRTUAL’) and also while system calls are
running on the process’s behalf. At zero it raises a
‘SIGPROF’.
This timer is intended for profiling where a program is
spending its time (by looking where it is when the timer goes
off).
‘getitimer’ returns the restart timer value and its current value,
as a list containing two pairs. Each pair is a time in seconds and
microseconds: ‘((INTERVAL_SECS . INTERVAL_USECS) (VALUE_SECS .
VALUE_USECS))’.
‘setitimer’ sets the timer values similarly, in seconds and
microseconds (which must be integers). The interval value can be
zero to have the timer run down just once. The return value is the
timer’s previous setting, in the same form as ‘getitimer’ returns.
(setitimer ITIMER_REAL
5 500000 ;; Raise SIGALRM every 5.5 seconds
2 0) ;; with the first SIGALRM in 2 seconds
Although the timers are programmed in microseconds, the actual
accuracy might not be that high.
Note that ‘ITIMER_PROF’ and ‘ITIMER_VIRTUAL’ are not functional on
all platforms and may always error when called. ‘(provided?
'ITIMER_PROF)’ and ‘(provided? 'ITIMER_VIRTUAL)’ can be used to
test if the those itimers are supported on the given host.
‘ITIMER_REAL’ is supported on all platforms that support
‘setitimer’.
7.2.9 Terminals and Ptys
------------------------
-- Scheme Procedure: isatty? port
-- C Function: scm_isatty_p (port)
Return ‘#t’ if PORT is using a serial non–file device, otherwise
‘#f’.
-- Scheme Procedure: ttyname port
-- C Function: scm_ttyname (port)
Return a string with the name of the serial terminal device
underlying PORT.
-- Scheme Procedure: ctermid
-- C Function: scm_ctermid ()
Return a string containing the file name of the controlling
terminal for the current process.
-- Scheme Procedure: tcgetpgrp port
-- C Function: scm_tcgetpgrp (port)
Return the process group ID of the foreground process group
associated with the terminal open on the file descriptor underlying
PORT.
If there is no foreground process group, the return value is a
number greater than 1 that does not match the process group ID of
any existing process group. This can happen if all of the
processes in the job that was formerly the foreground job have
terminated, and no other job has yet been moved into the
foreground.
-- Scheme Procedure: tcsetpgrp port pgid
-- C Function: scm_tcsetpgrp (port, pgid)
Set the foreground process group ID for the terminal used by the
file descriptor underlying PORT to the integer PGID. The calling
process must be a member of the same session as PGID and must have
the same controlling terminal. The return value is unspecified.
7.2.10 Pipes
------------
The following procedures are similar to the ‘popen’ and ‘pclose’ system
routines. The code is in a separate “popen” module(1):
(use-modules (ice-9 popen))
-- Scheme Procedure: open-pipe command mode
-- Scheme Procedure: open-pipe* mode prog [args...]
Execute a command in a subprocess, with a pipe to it or from it, or
with pipes in both directions.
‘open-pipe’ runs the shell COMMAND using ‘/bin/sh -c’.
‘open-pipe*’ executes PROG directly, with the optional ARGS
arguments (all strings).
MODE should be one of the following values. ‘OPEN_READ’ is an
input pipe, ie. to read from the subprocess. ‘OPEN_WRITE’ is an
output pipe, ie. to write to it.
-- Variable: OPEN_READ
-- Variable: OPEN_WRITE
-- Variable: OPEN_BOTH
For an input pipe, the child’s standard output is the pipe and
standard input is inherited from ‘current-input-port’. For an
output pipe, the child’s standard input is the pipe and standard
output is inherited from ‘current-output-port’. In all cases the
child’s standard error is inherited from ‘current-error-port’
(*note Default Ports::).
If those ‘current-X-ports’ are not files of some kind, and hence
don’t have file descriptors for the child, then ‘/dev/null’ is used
instead.
Care should be taken with ‘OPEN_BOTH’, a deadlock will occur if
both parent and child are writing, and waiting until the write
completes before doing any reading. Each direction has ‘PIPE_BUF’
bytes of buffering (*note Buffering::), which will be enough for
small writes, but not for say putting a big file through a filter.
-- Scheme Procedure: open-input-pipe command
Equivalent to ‘open-pipe’ with mode ‘OPEN_READ’.
(let* ((port (open-input-pipe "date --utc"))
(str (read-line port))) ; from (ice-9 rdelim)
(close-pipe port)
str)
⇒ "Mon Mar 11 20:10:44 UTC 2002"
-- Scheme Procedure: open-output-pipe command
Equivalent to ‘open-pipe’ with mode ‘OPEN_WRITE’.
(let ((port (open-output-pipe "lpr")))
(display "Something for the line printer.\n" port)
(if (not (eqv? 0 (status:exit-val (close-pipe port))))
(error "Cannot print")))
-- Scheme Procedure: open-input-output-pipe command
Equivalent to ‘open-pipe’ with mode ‘OPEN_BOTH’.
-- Scheme Procedure: close-pipe port
Close a pipe created by ‘open-pipe’, wait for the process to
terminate, and return the wait status code. The status is as per
‘waitpid’ and can be decoded with ‘status:exit-val’ etc (*note
Processes::)
‘waitpid WAIT_ANY’ should not be used when pipes are open, since it
can reap a pipe’s child process, causing an error from a subsequent
‘close-pipe’.
‘close-port’ (*note Ports::) can close a pipe, but it doesn’t reap
the child process.
The garbage collector will close a pipe no longer in use, and reap
the child process with ‘waitpid’. If the child hasn’t yet terminated
the garbage collector doesn’t block, but instead checks again in the
next GC.
Many systems have per-user and system-wide limits on the number of
processes, and a system-wide limit on the number of pipes, so pipes
should be closed explicitly when no longer needed, rather than letting
the garbage collector pick them up at some later time.
-- Scheme Procedure: pipeline COMMANDS
Execute a pipeline of COMMANDS, where each command is a list of a
program and its arguments as strings, returning an input port to
the end of the pipeline, an output port to the beginning of the
pipeline and a list of PIDs of the processes executing the
COMMANDS.
(let ((commands '(("git" "ls-files")
("tar" "-cf-" "-T-")
("sha1sum" "-")))
(success? (lambda (pid)
(zero?
(status:exit-val (cdr (waitpid pid)))))))
(receive (from to pids) (pipeline commands)
(let* ((sha1 (read-delimited " " from))
(index (list-index (negate success?) (reverse pids))))
(close to)
(close from)
(if (not index)
sha1
(string-append "pipeline failed in command: "
(string-join (list-ref commands index)))))))
⇒ "52f99d234503fca8c84ef94b1005a3a28d8b3bc1"
---------- Footnotes ----------
(1) This module is only available on systems where the ‘popen’
feature is provided (*note Common Feature Symbols::).
7.2.11 Networking
-----------------
7.2.11.1 Network Address Conversion
...................................
This section describes procedures which convert internet addresses
between numeric and string formats.
IPv4 Address Conversion
.......................
An IPv4 Internet address is a 4-byte value, represented in Guile as an
integer in host byte order, so that say “0.0.0.1” is 1, or “1.0.0.0” is
16777216.
Some underlying C functions use network byte order for addresses,
Guile converts as necessary so that at the Scheme level its host byte
order everywhere.
-- Variable: INADDR_ANY
For a server, this can be used with ‘bind’ (*note Network Sockets
and Communication::) to allow connections from any interface on the
machine.
-- Variable: INADDR_BROADCAST
The broadcast address on the local network.
-- Variable: INADDR_LOOPBACK
The address of the local host using the loopback device, ie.
‘127.0.0.1’.
-- Scheme Procedure: inet-netof address
-- C Function: scm_inet_netof (address)
Return the network number part of the given IPv4 Internet address.
E.g.,
(inet-netof 2130706433) ⇒ 127
-- Scheme Procedure: inet-lnaof address
-- C Function: scm_lnaof (address)
Return the local-address-with-network part of the given IPv4
Internet address, using the obsolete class A/B/C system. E.g.,
(inet-lnaof 2130706433) ⇒ 1
-- Scheme Procedure: inet-makeaddr net lna
-- C Function: scm_inet_makeaddr (net, lna)
Make an IPv4 Internet address by combining the network number NET
with the local-address-within-network number LNA. E.g.,
(inet-makeaddr 127 1) ⇒ 2130706433
IPv6 Address Conversion
.......................
An IPv6 Internet address is a 16-byte value, represented in Guile as an
integer in host byte order, so that say “::1” is 1. The following
constants are defined for convenience.
-- Variable: IN6ADDR_ANY
For a server, this can be used with ‘bind’ (*note Network Sockets
and Communication::) to allow connections from any IPv6 interface
on the machine.
-- Variable: IN6ADDR_LOOPBACK
The address of the local host using the loopback device, ie. ‘::1’.
The procedures below convert an IPv6 _or_ an IPv4 address to and from
its textual representation.
-- Scheme Procedure: inet-ntop family address
-- C Function: scm_inet_ntop (family, address)
Convert a network address from an integer to a printable string.
FAMILY can be ‘AF_INET’ or ‘AF_INET6’. E.g.,
(inet-ntop AF_INET 2130706433) ⇒ "127.0.0.1"
(inet-ntop AF_INET6 (- (expt 2 128) 1))
⇒ "ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff"
-- Scheme Procedure: inet-pton family address
-- C Function: scm_inet_pton (family, address)
Convert a string containing a printable network address to an
integer address. FAMILY can be ‘AF_INET’ or ‘AF_INET6’. E.g.,
(inet-pton AF_INET "127.0.0.1") ⇒ 2130706433
(inet-pton AF_INET6 "::1") ⇒ 1
7.2.11.2 Network Databases
..........................
This section describes procedures which query various network databases.
Care should be taken when using the database routines since they are not
reentrant.
‘getaddrinfo’
.............
The ‘getaddrinfo’ procedure maps host and service names to socket
addresses and associated information in a protocol-independent way.
-- Scheme Procedure: getaddrinfo name service [hint_flags [hint_family
[hint_socktype [hint_protocol]]]]
-- C Function: scm_getaddrinfo (name, service, hint_flags, hint_family,
hint_socktype, hint_protocol)
Return a list of ‘addrinfo’ structures containing a socket address
and associated information for host NAME and/or SERVICE to be used
in creating a socket with which to address the specified service.
(let* ((ai (car (getaddrinfo "www.gnu.org" "http")))
(s (socket (addrinfo:fam ai) (addrinfo:socktype ai)
(addrinfo:protocol ai))))
(connect s (addrinfo:addr ai))
s)
When SERVICE is omitted or is ‘#f’, return network-level addresses
for NAME. When NAME is ‘#f’ SERVICE must be provided and service
locations local to the caller are returned.
Additional hints can be provided. When specified, HINT_FLAGS
should be a bitwise-or of zero or more constants among the
following:
‘AI_PASSIVE’
Socket address is intended for ‘bind’.
‘AI_CANONNAME’
Request for canonical host name, available via
‘addrinfo:canonname’. This makes sense mainly when DNS
lookups are involved.
‘AI_NUMERICHOST’
Specifies that NAME is a numeric host address string (e.g.,
‘"127.0.0.1"’), meaning that name resolution will not be used.
‘AI_NUMERICSERV’
Likewise, specifies that SERVICE is a numeric port string
(e.g., ‘"80"’).
‘AI_ADDRCONFIG’
Return only addresses configured on the local system It is
highly recommended to provide this flag when the returned
socket addresses are to be used to make connections;
otherwise, some of the returned addresses could be unreachable
or use a protocol that is not supported.
‘AI_V4MAPPED’
When looking up IPv6 addresses, return mapped IPv4 addresses
if there is no IPv6 address available at all.
‘AI_ALL’
If this flag is set along with ‘AI_V4MAPPED’ when looking up
IPv6 addresses, return all IPv6 addresses as well as all IPv4
addresses, the latter mapped to IPv6 format.
When given, HINT_FAMILY should specify the requested address
family, e.g., ‘AF_INET6’. Similarly, HINT_SOCKTYPE should specify
the requested socket type (e.g., ‘SOCK_DGRAM’), and HINT_PROTOCOL
should specify the requested protocol (its value is interpreted as
in calls to ‘socket’).
On error, an exception with key ‘getaddrinfo-error’ is thrown, with
an error code (an integer) as its argument:
(catch 'getaddrinfo-error
(lambda ()
(getaddrinfo "www.gnu.org" "gopher"))
(lambda (key errcode)
(cond ((= errcode EAI_SERVICE)
(display "doesn't know about Gopher!\n"))
((= errcode EAI_NONAME)
(display "www.gnu.org not found\\n"))
(else
(format #t "something wrong: ~a\n"
(gai-strerror errcode))))))
Error codes are:
‘EAI_AGAIN’
The name or service could not be resolved at this time.
Future attempts may succeed.
‘EAI_BADFLAGS’
HINT_FLAGS contains an invalid value.
‘EAI_FAIL’
A non-recoverable error occurred when attempting to resolve
the name.
‘EAI_FAMILY’
HINT_FAMILY was not recognized.
‘EAI_NONAME’
Either NAME does not resolve for the supplied parameters, or
neither NAME nor SERVICE were supplied.
‘EAI_NODATA’
This non-POSIX error code can be returned on some systems (GNU
and Darwin, at least), for example when NAME is known but
requests that were made turned out no data. Error handling
code should be prepared to handle it when it is defined.
‘EAI_SERVICE’
SERVICE was not recognized for the specified socket type.
‘EAI_SOCKTYPE’
HINT_SOCKTYPE was not recognized.
‘EAI_SYSTEM’
A system error occurred. In C, the error code can be found in
‘errno’; this value is not accessible from Scheme, but in
practice it provides little information about the actual error
cause.
Users are encouraged to read the "POSIX specification
(http://www.opengroup.org/onlinepubs/9699919799/functions/getaddrinfo.html)
for more details.
The following procedures take an ‘addrinfo’ object as returned by
‘getaddrinfo’:
-- Scheme Procedure: addrinfo:flags ai
Return flags for AI as a bitwise or of ‘AI_’ values (see above).
-- Scheme Procedure: addrinfo:fam ai
Return the address family of AI (a ‘AF_’ value).
-- Scheme Procedure: addrinfo:socktype ai
Return the socket type for AI (a ‘SOCK_’ value).
-- Scheme Procedure: addrinfo:protocol ai
Return the protocol of AI.
-- Scheme Procedure: addrinfo:addr ai
Return the socket address associated with AI as a ‘sockaddr’ object
(*note Network Socket Address::).
-- Scheme Procedure: addrinfo:canonname ai
Return a string for the canonical name associated with AI if the
‘AI_CANONNAME’ flag was supplied.
The Host Database
.................
A “host object” is a structure that represents what is known about a
network host, and is the usual way of representing a system’s network
identity inside software.
The following functions accept a host object and return a selected
component:
-- Scheme Procedure: hostent:name host
The “official” hostname for HOST.
-- Scheme Procedure: hostent:aliases host
A list of aliases for HOST.
-- Scheme Procedure: hostent:addrtype host
The host address type, one of the ‘AF’ constants, such as ‘AF_INET’
or ‘AF_INET6’.
-- Scheme Procedure: hostent:length host
The length of each address for HOST, in bytes.
-- Scheme Procedure: hostent:addr-list host
The list of network addresses associated with HOST. For ‘AF_INET’
these are integer IPv4 address (*note Network Address
Conversion::).
The following procedures can be used to search the host database.
However, ‘getaddrinfo’ should be preferred over them since it’s more
generic and thread-safe.
-- Scheme Procedure: gethost [host]
-- Scheme Procedure: gethostbyname hostname
-- Scheme Procedure: gethostbyaddr address
-- C Function: scm_gethost (host)
Look up a host by name or address, returning a host object. The
‘gethost’ procedure will accept either a string name or an integer
address; if given no arguments, it behaves like ‘gethostent’ (see
below). If a name or address is supplied but the address can not
be found, an error will be thrown to one of the keys:
‘host-not-found’, ‘try-again’, ‘no-recovery’ or ‘no-data’,
corresponding to the equivalent ‘h_error’ values. Unusual
conditions may result in errors thrown to the ‘system-error’ or
‘misc_error’ keys.
(gethost "www.gnu.org")
⇒ #("www.gnu.org" () 2 4 (3353880842))
(gethostbyname "www.emacs.org")
⇒ #("emacs.org" ("www.emacs.org") 2 4 (1073448978))
The following procedures may be used to step through the host
database from beginning to end.
-- Scheme Procedure: sethostent [stayopen]
Initialize an internal stream from which host objects may be read.
This procedure must be called before any calls to ‘gethostent’, and
may also be called afterward to reset the host entry stream. If
STAYOPEN is supplied and is not ‘#f’, the database is not closed by
subsequent ‘gethostbyname’ or ‘gethostbyaddr’ calls, possibly
giving an efficiency gain.
-- Scheme Procedure: gethostent
Return the next host object from the host database, or ‘#f’ if
there are no more hosts to be found (or an error has been
encountered). This procedure may not be used before ‘sethostent’
has been called.
-- Scheme Procedure: endhostent
Close the stream used by ‘gethostent’. The return value is
unspecified.
-- Scheme Procedure: sethost [stayopen]
-- C Function: scm_sethost (stayopen)
If STAYOPEN is omitted, this is equivalent to ‘endhostent’.
Otherwise it is equivalent to ‘sethostent stayopen’.
The Network Database
....................
The following functions accept an object representing a network and
return a selected component:
-- Scheme Procedure: netent:name net
The “official” network name.
-- Scheme Procedure: netent:aliases net
A list of aliases for the network.
-- Scheme Procedure: netent:addrtype net
The type of the network number. Currently, this returns only
‘AF_INET’.
-- Scheme Procedure: netent:net net
The network number.
The following procedures are used to search the network database:
-- Scheme Procedure: getnet [net]
-- Scheme Procedure: getnetbyname net-name
-- Scheme Procedure: getnetbyaddr net-number
-- C Function: scm_getnet (net)
Look up a network by name or net number in the network database.
The NET-NAME argument must be a string, and the NET-NUMBER argument
must be an integer. ‘getnet’ will accept either type of argument,
behaving like ‘getnetent’ (see below) if no arguments are given.
The following procedures may be used to step through the network
database from beginning to end.
-- Scheme Procedure: setnetent [stayopen]
Initialize an internal stream from which network objects may be
read. This procedure must be called before any calls to
‘getnetent’, and may also be called afterward to reset the net
entry stream. If STAYOPEN is supplied and is not ‘#f’, the
database is not closed by subsequent ‘getnetbyname’ or
‘getnetbyaddr’ calls, possibly giving an efficiency gain.
-- Scheme Procedure: getnetent
Return the next entry from the network database.
-- Scheme Procedure: endnetent
Close the stream used by ‘getnetent’. The return value is
unspecified.
-- Scheme Procedure: setnet [stayopen]
-- C Function: scm_setnet (stayopen)
If STAYOPEN is omitted, this is equivalent to ‘endnetent’.
Otherwise it is equivalent to ‘setnetent stayopen’.
The Protocol Database
.....................
The following functions accept an object representing a protocol and
return a selected component:
-- Scheme Procedure: protoent:name protocol
The “official” protocol name.
-- Scheme Procedure: protoent:aliases protocol
A list of aliases for the protocol.
-- Scheme Procedure: protoent:proto protocol
The protocol number.
The following procedures are used to search the protocol database:
-- Scheme Procedure: getproto [protocol]
-- Scheme Procedure: getprotobyname name
-- Scheme Procedure: getprotobynumber number
-- C Function: scm_getproto (protocol)
Look up a network protocol by name or by number. ‘getprotobyname’
takes a string argument, and ‘getprotobynumber’ takes an integer
argument. ‘getproto’ will accept either type, behaving like
‘getprotoent’ (see below) if no arguments are supplied.
The following procedures may be used to step through the protocol
database from beginning to end.
-- Scheme Procedure: setprotoent [stayopen]
Initialize an internal stream from which protocol objects may be
read. This procedure must be called before any calls to
‘getprotoent’, and may also be called afterward to reset the
protocol entry stream. If STAYOPEN is supplied and is not ‘#f’,
the database is not closed by subsequent ‘getprotobyname’ or
‘getprotobynumber’ calls, possibly giving an efficiency gain.
-- Scheme Procedure: getprotoent
Return the next entry from the protocol database.
-- Scheme Procedure: endprotoent
Close the stream used by ‘getprotoent’. The return value is
unspecified.
-- Scheme Procedure: setproto [stayopen]
-- C Function: scm_setproto (stayopen)
If STAYOPEN is omitted, this is equivalent to ‘endprotoent’.
Otherwise it is equivalent to ‘setprotoent stayopen’.
The Service Database
....................
The following functions accept an object representing a service and
return a selected component:
-- Scheme Procedure: servent:name serv
The “official” name of the network service.
-- Scheme Procedure: servent:aliases serv
A list of aliases for the network service.
-- Scheme Procedure: servent:port serv
The Internet port used by the service.
-- Scheme Procedure: servent:proto serv
The protocol used by the service. A service may be listed many
times in the database under different protocol names.
The following procedures are used to search the service database:
-- Scheme Procedure: getserv [name [protocol]]
-- Scheme Procedure: getservbyname name protocol
-- Scheme Procedure: getservbyport port protocol
-- C Function: scm_getserv (name, protocol)
Look up a network service by name or by service number, and return
a network service object. The PROTOCOL argument specifies the name
of the desired protocol; if the protocol found in the network
service database does not match this name, a system error is
signalled.
The ‘getserv’ procedure will take either a service name or number
as its first argument; if given no arguments, it behaves like
‘getservent’ (see below).
(getserv "imap" "tcp")
⇒ #("imap2" ("imap") 143 "tcp")
(getservbyport 88 "udp")
⇒ #("kerberos" ("kerberos5" "krb5") 88 "udp")
The following procedures may be used to step through the service
database from beginning to end.
-- Scheme Procedure: setservent [stayopen]
Initialize an internal stream from which service objects may be
read. This procedure must be called before any calls to
‘getservent’, and may also be called afterward to reset the service
entry stream. If STAYOPEN is supplied and is not ‘#f’, the
database is not closed by subsequent ‘getservbyname’ or
‘getservbyport’ calls, possibly giving an efficiency gain.
-- Scheme Procedure: getservent
Return the next entry from the services database.
-- Scheme Procedure: endservent
Close the stream used by ‘getservent’. The return value is
unspecified.
-- Scheme Procedure: setserv [stayopen]
-- C Function: scm_setserv (stayopen)
If STAYOPEN is omitted, this is equivalent to ‘endservent’.
Otherwise it is equivalent to ‘setservent stayopen’.
7.2.11.3 Network Socket Address
...............................
A “socket address” object identifies a socket endpoint for
communication. In the case of ‘AF_INET’ for instance, the socket
address object comprises the host address (or interface on the host) and
a port number which specifies a particular open socket in a running
client or server process. A socket address object can be created with,
-- Scheme Procedure: make-socket-address AF_INET ipv4addr port
-- Scheme Procedure: make-socket-address AF_INET6 ipv6addr port
[flowinfo [scopeid]]
-- Scheme Procedure: make-socket-address AF_UNIX path
-- C Function: scm_make_socket_address (family, address, arglist)
Return a new socket address object. The first argument is the
address family, one of the ‘AF’ constants, then the arguments vary
according to the family.
For ‘AF_INET’ the arguments are an IPv4 network address number
(*note Network Address Conversion::), and a port number.
For ‘AF_INET6’ the arguments are an IPv6 network address number and
a port number. Optional FLOWINFO and SCOPEID arguments may be
given (both integers, default 0).
For ‘AF_UNIX’ the argument is a filename (a string).
The C function ‘scm_make_socket_address’ takes the FAMILY and
ADDRESS arguments directly, then ARGLIST is a list of further
arguments, being the port for IPv4, port and optional flowinfo and
scopeid for IPv6, or the empty list ‘SCM_EOL’ for Unix domain.
The following functions access the fields of a socket address object,
-- Scheme Procedure: sockaddr:fam sa
Return the address family from socket address object SA. This is
one of the ‘AF’ constants (e.g. ‘AF_INET’).
-- Scheme Procedure: sockaddr:path sa
For an ‘AF_UNIX’ socket address object SA, return the filename.
-- Scheme Procedure: sockaddr:addr sa
For an ‘AF_INET’ or ‘AF_INET6’ socket address object SA, return the
network address number.
-- Scheme Procedure: sockaddr:port sa
For an ‘AF_INET’ or ‘AF_INET6’ socket address object SA, return the
port number.
-- Scheme Procedure: sockaddr:flowinfo sa
For an ‘AF_INET6’ socket address object SA, return the flowinfo
value.
-- Scheme Procedure: sockaddr:scopeid sa
For an ‘AF_INET6’ socket address object SA, return the scope ID
value.
The functions below convert to and from the C ‘struct sockaddr’
(*note (libc)Address Formats::). That structure is a generic type, an
application can cast to or from ‘struct sockaddr_in’, ‘struct
sockaddr_in6’ or ‘struct sockaddr_un’ according to the address family.
In a ‘struct sockaddr’ taken or returned, the byte ordering in the
fields follows the C conventions (*note Byte Order Conversion:
(libc)Byte Order.). This means network byte order for ‘AF_INET’ host
address (‘sin_addr.s_addr’) and port number (‘sin_port’), and ‘AF_INET6’
port number (‘sin6_port’). But at the Scheme level these values are
taken or returned in host byte order, so the port is an ordinary
integer, and the host address likewise is an ordinary integer (as
described in *note Network Address Conversion::).
-- C Function: struct sockaddr * scm_c_make_socket_address (SCM family,
SCM address, SCM args, size_t *outsize)
Return a newly-‘malloc’ed ‘struct sockaddr’ created from arguments
like those taken by ‘scm_make_socket_address’ above.
The size (in bytes) of the ‘struct sockaddr’ return is stored into
‘*OUTSIZE’. An application must call ‘free’ to release the
returned structure when no longer required.
-- C Function: SCM scm_from_sockaddr (const struct sockaddr *address,
unsigned address_size)
Return a Scheme socket address object from the C ADDRESS structure.
ADDRESS_SIZE is the size in bytes of ADDRESS.
-- C Function: struct sockaddr * scm_to_sockaddr (SCM address, size_t
*address_size)
Return a newly-‘malloc’ed ‘struct sockaddr’ from a Scheme level
socket address object.
The size (in bytes) of the ‘struct sockaddr’ return is stored into
‘*OUTSIZE’. An application must call ‘free’ to release the
returned structure when no longer required.
7.2.11.4 Network Sockets and Communication
..........................................
Socket ports can be created using ‘socket’ and ‘socketpair’. The ports
are initially unbuffered, to make reading and writing to the same port
more reliable. A buffer can be added to the port using ‘setvbuf’ (*note
Buffering::).
Most systems have limits on how many files and sockets can be open,
so it’s strongly recommended that socket ports be closed explicitly when
no longer required (*note Ports::).
Some of the underlying C functions take values in network byte order,
but the convention in Guile is that at the Scheme level everything is
ordinary host byte order and conversions are made automatically where
necessary.
-- Scheme Procedure: socket family style proto
-- C Function: scm_socket (family, style, proto)
Return a new socket port of the type specified by FAMILY, STYLE and
PROTO. All three parameters are integers. The possible values for
FAMILY are as follows, where supported by the system,
-- Variable: PF_UNIX
-- Variable: PF_INET
-- Variable: PF_INET6
The possible values for STYLE are as follows, again where supported
by the system,
-- Variable: SOCK_STREAM
-- Variable: SOCK_DGRAM
-- Variable: SOCK_RAW
-- Variable: SOCK_RDM
-- Variable: SOCK_SEQPACKET
PROTO can be obtained from a protocol name using ‘getprotobyname’
(*note Network Databases::). A value of zero means the default
protocol, which is usually right.
A socket cannot by used for communication until it has been
connected somewhere, usually with either ‘connect’ or ‘accept’
below.
-- Scheme Procedure: socketpair family style proto
-- C Function: scm_socketpair (family, style, proto)
Return a pair, the ‘car’ and ‘cdr’ of which are two unnamed socket
ports connected to each other. The connection is full-duplex, so
data can be transferred in either direction between the two.
FAMILY, STYLE and PROTO are as per ‘socket’ above. But many
systems only support socket pairs in the ‘PF_UNIX’ family. Zero is
likely to be the only meaningful value for PROTO.
-- Scheme Procedure: getsockopt sock level optname
-- Scheme Procedure: setsockopt sock level optname value
-- C Function: scm_getsockopt (sock, level, optname)
-- C Function: scm_setsockopt (sock, level, optname, value)
Get or set an option on socket port SOCK. ‘getsockopt’ returns the
current value. ‘setsockopt’ sets a value and the return is
unspecified.
LEVEL is an integer specifying a protocol layer, either
‘SOL_SOCKET’ for socket level options, or a protocol number from
the ‘IPPROTO’ constants or ‘getprotoent’ (*note Network
Databases::).
-- Variable: SOL_SOCKET
-- Variable: IPPROTO_IP
-- Variable: IPPROTO_IPV6
-- Variable: IPPROTO_TCP
-- Variable: IPPROTO_UDP
OPTNAME is an integer specifying an option within the protocol
layer.
For ‘SOL_SOCKET’ level the following OPTNAMEs are defined (when
provided by the system). For their meaning see *note
(libc)Socket-Level Options::, or ‘man 7 socket’.
-- Variable: SO_DEBUG
-- Variable: SO_REUSEADDR
-- Variable: SO_STYLE
-- Variable: SO_TYPE
-- Variable: SO_ERROR
-- Variable: SO_DONTROUTE
-- Variable: SO_BROADCAST
-- Variable: SO_SNDBUF
-- Variable: SO_RCVBUF
-- Variable: SO_KEEPALIVE
-- Variable: SO_OOBINLINE
-- Variable: SO_NO_CHECK
-- Variable: SO_PRIORITY
-- Variable: SO_REUSEPORT
-- Variable: SO_RCVTIMEO
-- Variable: SO_SNDTIMEO
The VALUE taken or returned is an integer.
-- Variable: SO_LINGER
The VALUE taken or returned is a pair of integers ‘(ENABLE .
TIMEOUT)’. On old systems without timeout support (ie.
without ‘struct linger’), only ENABLE has an effect but the
value in Guile is always a pair.
For IP level (‘IPPROTO_IP’) the following OPTNAMEs are defined
(when provided by the system). See ‘man ip’ for what they mean.
-- Variable: IP_MULTICAST_IF
This sets the source interface used by multicast traffic.
-- Variable: IP_MULTICAST_TTL
This sets the default TTL for multicast traffic. This
defaults to 1 and should be increased to allow traffic to pass
beyond the local network.
-- Variable: IP_ADD_MEMBERSHIP
-- Variable: IP_DROP_MEMBERSHIP
These can be used only with ‘setsockopt’, not ‘getsockopt’.
VALUE is a pair ‘(MULTIADDR . INTERFACEADDR)’ of integer IPv4
addresses (*note Network Address Conversion::). MULTIADDR is
a multicast address to be added to or dropped from the
interface INTERFACEADDR. INTERFACEADDR can be ‘INADDR_ANY’ to
have the system select the interface. INTERFACEADDR can also
be an interface index number, on systems supporting that.
Last, for IPv6 level (‘IPPROTO_IPV6’), the following OPTNAMEs are
defined. See ‘man 7 ipv6’ for details.
-- Variable: IPV6_V6ONLY
Determines whether an ‘AF_INET6’ socket is restricted to
transmitting IPv6 packets only, or whether it can also
transmit packets for an IPv4-mapped IPv6 address.
For ‘IPPROTO_TCP’ level the following OPTNAMEs are defined (when
provided by the system). For their meaning see ‘man 7 tcp’.
-- Variable: TCP_NODELAY
-- Variable: TCP_CORK
The VALUE taken or returned is an integer.
-- Scheme Procedure: shutdown sock how
-- C Function: scm_shutdown (sock, how)
Sockets can be closed simply by using ‘close-port’. The ‘shutdown’
procedure allows reception or transmission on a connection to be
shut down individually, according to the parameter HOW:
0
Stop receiving data for this socket. If further data arrives,
reject it.
1
Stop trying to transmit data from this socket. Discard any
data waiting to be sent. Stop looking for acknowledgement of
data already sent; don’t retransmit it if it is lost.
2
Stop both reception and transmission.
The return value is unspecified.
-- Scheme Procedure: connect sock sockaddr
-- Scheme Procedure: connect sock AF_INET ipv4addr port
-- Scheme Procedure: connect sock AF_INET6 ipv6addr port [flowinfo
[scopeid]]
-- Scheme Procedure: connect sock AF_UNIX path
-- C Function: scm_connect (sock, fam, address, args)
Initiate a connection on socket port SOCK to a given address. The
destination is either a socket address object, or arguments the
same as ‘make-socket-address’ would take to make such an object
(*note Network Socket Address::). Return true unless the socket
was configured as non-blocking and the connection could not be made
immediately.
(connect sock AF_INET INADDR_LOOPBACK 23)
(connect sock (make-socket-address AF_INET INADDR_LOOPBACK 23))
-- Scheme Procedure: bind sock sockaddr
-- Scheme Procedure: bind sock AF_INET ipv4addr port
-- Scheme Procedure: bind sock AF_INET6 ipv6addr port [flowinfo
[scopeid]]
-- Scheme Procedure: bind sock AF_UNIX path
-- C Function: scm_bind (sock, fam, address, args)
Bind socket port SOCK to the given address. The address is either
a socket address object, or arguments the same as
‘make-socket-address’ would take to make such an object (*note
Network Socket Address::). The return value is unspecified.
Generally a socket is only explicitly bound to a particular address
when making a server, i.e. to listen on a particular port. For an
outgoing connection the system will assign a local address
automatically, if not already bound.
(bind sock AF_INET INADDR_ANY 12345)
(bind sock (make-socket-address AF_INET INADDR_ANY 12345))
-- Scheme Procedure: listen sock backlog
-- C Function: scm_listen (sock, backlog)
Enable SOCK to accept connection requests. BACKLOG is an integer
specifying the maximum length of the queue for pending connections.
If the queue fills, new clients will fail to connect until the
server calls ‘accept’ to accept a connection from the queue.
The return value is unspecified.
-- Scheme Procedure: accept sock [flags]
-- C Function: scm_accept (sock)
Accept a connection from socket port SOCK which has been enabled
for listening with ‘listen’ above.
If there are no incoming connections in the queue, there are two
possible behaviors, depending on whether SOCK has been configured
for non-blocking operation or not:
• If there is no connection waiting and the socket was set to
non-blocking mode with the ‘O_NONBLOCK’ port option (*note
‘fcntl’: Ports and File Descriptors.), return ‘#f’ directly.
• Otherwise wait until a connection is available.
The return value is a pair. The ‘car’ is a new socket port,
connected and ready to communicate. The ‘cdr’ is a socket address
object (*note Network Socket Address::) which is where the remote
connection is from (like ‘getpeername’ below).
FLAGS, if given, may include ‘SOCK_CLOEXEC’ or ‘SOCK_NONBLOCK’,
which like ‘O_CLOEXEC’ and ‘O_NONBLOCK’ apply to the newly accepted
socket.
All communication takes place using the new socket returned. The
given SOCK remains bound and listening, and ‘accept’ may be called
on it again to get another incoming connection when desired.
-- Scheme Procedure: getsockname sock
-- C Function: scm_getsockname (sock)
Return a socket address object which is the where SOCK is bound
locally. SOCK may have obtained its local address from ‘bind’
(above), or if a ‘connect’ is done with an otherwise unbound socket
(which is usual) then the system will have assigned an address.
Note that on many systems the address of a socket in the ‘AF_UNIX’
namespace cannot be read.
-- Scheme Procedure: getpeername sock
-- C Function: scm_getpeername (sock)
Return a socket address object which is where SOCK is connected to,
i.e. the remote endpoint.
Note that on many systems the address of a socket in the ‘AF_UNIX’
namespace cannot be read.
-- Scheme Procedure: recv! sock buf [flags]
-- C Function: scm_recv (sock, buf, flags)
Receive data from a socket port. SOCK must already be bound to the
address from which data is to be received. BUF is a bytevector
into which the data will be written. The size of BUF limits the
amount of data which can be received: in the case of packet
protocols, if a packet larger than this limit is encountered then
some data will be irrevocably lost.
The optional FLAGS argument is a value or bitwise OR of ‘MSG_OOB’,
‘MSG_PEEK’, ‘MSG_DONTROUTE’ etc.
The value returned is the number of bytes read from the socket.
Note that the data is read directly from the socket file
descriptor: any unread buffered port data is ignored.
-- Scheme Procedure: send sock message [flags]
-- C Function: scm_send (sock, message, flags)
Transmit bytevector MESSAGE on socket port SOCK. SOCK must already
be bound to a destination address. The value returned is the
number of bytes transmitted—it’s possible for this to be less than
the length of MESSAGE if the socket is set to be non-blocking. The
optional FLAGS argument is a value or bitwise OR of ‘MSG_OOB’,
‘MSG_PEEK’, ‘MSG_DONTROUTE’ etc.
Note that the data is written directly to the socket file
descriptor: any unflushed buffered port data is ignored.
-- Scheme Procedure: recvfrom! sock buf [flags [start [end]]]
-- C Function: scm_recvfrom (sock, buf, flags, start, end)
Receive data from socket port SOCK, returning the originating
address as well as the data. This function is usually for datagram
sockets, but can be used on stream-oriented sockets too.
The data received is stored in bytevector BUF, using either the
whole bytevector or just the region between the optional START and
END positions. The size of BUF limits the amount of data that can
be received. For datagram protocols if a packet larger than this
is received then excess bytes are irrevocably lost.
The return value is a pair. The ‘car’ is the number of bytes read.
The ‘cdr’ is a socket address object (*note Network Socket
Address::) which is where the data came from, or ‘#f’ if the origin
is unknown.
The optional FLAGS argument is a or bitwise-OR (‘logior’) of
‘MSG_OOB’, ‘MSG_PEEK’, ‘MSG_DONTROUTE’ etc.
Data is read directly from the socket file descriptor, any buffered
port data is ignored.
On a GNU/Linux system ‘recvfrom!’ is not multi-threading, all
threads stop while a ‘recvfrom!’ call is in progress. An
application may need to use ‘select’, ‘O_NONBLOCK’ or
‘MSG_DONTWAIT’ to avoid this.
-- Scheme Procedure: sendto sock message sockaddr [flags]
-- Scheme Procedure: sendto sock message AF_INET ipv4addr port [flags]
-- Scheme Procedure: sendto sock message AF_INET6 ipv6addr port
[flowinfo [scopeid [flags]]]
-- Scheme Procedure: sendto sock message AF_UNIX path [flags]
-- C Function: scm_sendto (sock, message, fam, address, args_and_flags)
Transmit bytevector MESSAGE as a datagram socket port SOCK. The
destination is specified either as a socket address object, or as
arguments the same as would be taken by ‘make-socket-address’ to
create such an object (*note Network Socket Address::).
The destination address may be followed by an optional FLAGS
argument which is a ‘logior’ (*note Bitwise Operations::) of
‘MSG_OOB’, ‘MSG_PEEK’, ‘MSG_DONTROUTE’ etc.
The value returned is the number of bytes transmitted – it’s
possible for this to be less than the length of MESSAGE if the
socket is set to be non-blocking. Note that the data is written
directly to the socket file descriptor: any unflushed buffered port
data is ignored.
7.2.11.5 Network Socket Examples
................................
The following give examples of how to use network sockets.
Internet Socket Client Example
..............................
The following example demonstrates an Internet socket client. It
connects to the HTTP daemon running on the local machine and returns the
contents of the root index URL.
(let ((s (socket PF_INET SOCK_STREAM 0)))
(connect s AF_INET (inet-pton AF_INET "127.0.0.1") 80)
(display "GET / HTTP/1.0\r\n\r\n" s)
(do ((line (read-line s) (read-line s)))
((eof-object? line))
(display line)
(newline)))
Internet Socket Server Example
..............................
The following example shows a simple Internet server which listens on
port 2904 for incoming connections and sends a greeting back to the
client.
(let ((s (socket PF_INET SOCK_STREAM 0)))
(setsockopt s SOL_SOCKET SO_REUSEADDR 1)
;; Specific address?
;; (bind s AF_INET (inet-pton AF_INET "127.0.0.1") 2904)
(bind s AF_INET INADDR_ANY 2904)
(listen s 5)
(simple-format #t "Listening for clients in pid: ~S" (getpid))
(newline)
(while #t
(let* ((client-connection (accept s))
(client-details (cdr client-connection))
(client (car client-connection)))
(simple-format #t "Got new client connection: ~S"
client-details)
(newline)
(simple-format #t "Client address: ~S"
(gethostbyaddr
(sockaddr:addr client-details)))
(newline)
;; Send back the greeting to the client port
(display "Hello client\r\n" client)
(close client))))
7.2.12 System Identification
----------------------------
This section lists the various procedures Guile provides for accessing
information about the system it runs on.
-- Scheme Procedure: uname
-- C Function: scm_uname ()
Return an object with some information about the computer system
the program is running on.
The following procedures accept an object as returned by ‘uname’
and return a selected component (all of which are strings).
-- Scheme Procedure: utsname:sysname un
The name of the operating system.
-- Scheme Procedure: utsname:nodename un
The network name of the computer.
-- Scheme Procedure: utsname:release un
The current release level of the operating system
implementation.
-- Scheme Procedure: utsname:version un
The current version level within the release of the operating
system.
-- Scheme Procedure: utsname:machine un
A description of the hardware.
-- Scheme Procedure: gethostname
-- C Function: scm_gethostname ()
Return the host name of the current processor.
-- Scheme Procedure: sethostname name
-- C Function: scm_sethostname (name)
Set the host name of the current processor to NAME. May only be
used by the superuser. The return value is not specified.
7.2.13 Locales
--------------
-- Scheme Procedure: setlocale category [locale]
-- C Function: scm_setlocale (category, locale)
Get or set the current locale, used for various
internationalizations. Locales are strings, such as ‘sv_SE’.
If LOCALE is given then the locale for the given CATEGORY is set
and the new value returned. If LOCALE is not given then the
current value is returned. CATEGORY should be one of the following
values (*note Categories of Activities that Locales Affect:
(libc)Locale Categories.):
-- Variable: LC_ALL
-- Variable: LC_COLLATE
-- Variable: LC_CTYPE
-- Variable: LC_MESSAGES
-- Variable: LC_MONETARY
-- Variable: LC_NUMERIC
-- Variable: LC_TIME
A common usage is ‘(setlocale LC_ALL "")’, which initializes all
categories based on standard environment variables (‘LANG’ etc).
For full details on categories and locale names *note Locales and
Internationalization: (libc)Locales.
Note that ‘setlocale’ affects locale settings for the whole
process. *Note locale objects and ‘make-locale’: i18n
Introduction, for a thread-safe alternative.
7.2.14 Encryption
-----------------
Please note that the procedures in this section are not suited for
strong encryption, they are only interfaces to the well-known and common
system library functions of the same name. They are just as good (or
bad) as the underlying functions, so you should refer to your system
documentation before using them (*note Encrypting Passwords:
(libc)crypt.).
-- Scheme Procedure: crypt key salt
-- C Function: scm_crypt (key, salt)
Encrypt KEY, with the addition of SALT (both strings), using the
‘crypt’ C library call.
Although ‘getpass’ is not an encryption procedure per se, it appears
here because it is often used in combination with ‘crypt’:
-- Scheme Procedure: getpass prompt
-- C Function: scm_getpass (prompt)
Display PROMPT to the standard error output and read a password
from ‘/dev/tty’. If this file is not accessible, it reads from
standard input. The password may be up to 127 characters in
length. Additional characters and the terminating newline
character are discarded. While reading the password, echoing and
the generation of signals by special characters is disabled.
7.3 HTTP, the Web, and All That
===============================
It has always been possible to connect computers together and share
information between them, but the rise of the World Wide Web over the
last couple of decades has made it much easier to do so. The result is
a richly connected network of computation, in which Guile forms a part.
By “the web”, we mean the HTTP protocol(1) as handled by servers,
clients, proxies, caches, and the various kinds of messages and message
components that can be sent and received by that protocol, notably HTML.
On one level, the web is text in motion: the protocols themselves are
textual (though the payload may be binary), and it’s possible to create
a socket and speak text to the web. But such an approach is obviously
primitive. This section details the higher-level data types and
operations provided by Guile: URIs, HTTP request and response records,
and a conventional web server implementation.
The material in this section is arranged in ascending order, in which
later concepts build on previous ones. If you prefer to start with the
highest-level perspective, *note Web Examples::, and work your way back.
---------- Footnotes ----------
(1) Yes, the P is for protocol, but this phrase appears repeatedly in
RFC 2616.
7.3.1 Types and the Web
-----------------------
It is a truth universally acknowledged, that a program with good use of
data types, will be free from many common bugs. Unfortunately, the
common practice in web programming seems to ignore this maxim. This
subsection makes the case for expressive data types in web programming.
By “expressive data types”, we mean that the data types _say_
something about how a program solves a problem. For example, if we
choose to represent dates using SRFI 19 date records (*note SRFI-19::),
this indicates that there is a part of the program that will always have
valid dates. Error handling for a number of basic cases, like invalid
dates, occurs on the boundary in which we produce a SRFI 19 date record
from other types, like strings.
With regards to the web, data types are helpful in the two broad
phases of HTTP messages: parsing and generation.
Consider a server, which has to parse a request, and produce a
response. Guile will parse the request into an HTTP request object
(*note Requests::), with each header parsed into an appropriate Scheme
data type. This transition from an incoming stream of characters to
typed data is a state change in a program—the strings might parse, or
they might not, and something has to happen if they do not. (Guile
throws an error in this case.) But after you have the parsed request,
“client” code (code built on top of the Guile web framework) will not
have to check for syntactic validity. The types already make this
information manifest.
This state change on the parsing boundary makes programs more robust,
as they themselves are freed from the need to do a number of common
error checks, and they can use normal Scheme procedures to handle a
request instead of ad-hoc string parsers.
The need for types on the response generation side (in a server) is
more subtle, though not less important. Consider the example of a POST
handler, which prints out the text that a user submits from a form.
Such a handler might include a procedure like this:
;; First, a helper procedure
(define (para . contents)
(string-append "" (string-concatenate contents) "
"))
;; Now the meat of our simple web application
(define (you-said text)
(para "You said: " text))
(display (you-said "Hi!"))
⊣ You said: Hi!
This is a perfectly valid implementation, provided that the incoming
text does not contain the special HTML characters ‘<’, ‘>’, or ‘&’. But
this provision of a restricted character set is not reflected anywhere
in the program itself: we must _assume_ that the programmer understands
this, and performs the check elsewhere.
Unfortunately, the short history of the practice of programming does
not bear out this assumption. A “cross-site scripting” (XSS)
vulnerability is just such a common error in which unfiltered user input
is allowed into the output. A user could submit a crafted comment to
your web site which results in visitors running malicious Javascript,
within the security context of your domain:
(display (you-said ""))
⊣ You said:
The fundamental problem here is that both user data and the program
template are represented using strings. This identity means that types
can’t help the programmer to make a distinction between these two, so
they get confused.
There are a number of possible solutions, but perhaps the best is to
treat HTML not as strings, but as native s-expressions: as SXML. The
basic idea is that HTML is either text, represented by a string, or an
element, represented as a tagged list. So ‘foo’ becomes ‘"foo"’, and
‘foo’ becomes ‘(b "foo")’. Attributes, if present, go in a
tagged list headed by ‘@’, like ‘(img (@ (src
"http://example.com/foo.png")))’. *Note SXML::, for more information.
The good thing about SXML is that HTML elements cannot be confused
with text. Let’s make a new definition of ‘para’:
(define (para . contents)
`(p ,@contents))
(use-modules (sxml simple))
(sxml->xml (you-said "Hi!"))
⊣ You said: Hi!
(sxml->xml (you-said "Rats, foiled again!"))
⊣ You said: <i>Rats, foiled again!</i>
So we see in the second example that HTML elements cannot be
unwittingly introduced into the output. However it is now perfectly
acceptable to pass SXML to ‘you-said’; in fact, that is the big
advantage of SXML over everything-as-a-string.
(sxml->xml (you-said (you-said "")))
⊣ You said:
You said: <Hi!>
The SXML types allow procedures to _compose_. The types make
manifest which parts are HTML elements, and which are text. So you
needn’t worry about escaping user input; the type transition back to a
string handles that for you. XSS vulnerabilities are a thing of the
past.
Well. That’s all very nice and opinionated and such, but how do I
use the thing? Read on!
7.3.2 Universal Resource Identifiers
------------------------------------
Guile provides a standard data type for Universal Resource Identifiers
(URIs), as defined in RFC 3986.
The generic URI syntax is as follows:
URI-reference := [scheme ":"] ["//" [userinfo "@"] host [":" port]] path \
[ "?" query ] [ "#" fragment ]
For example, in the URI, ‘http://www.gnu.org/help/’, the scheme is
‘http’, the host is ‘www.gnu.org’, the path is ‘/help/’, and there is no
userinfo, port, query, or fragment.
Userinfo is something of an abstraction, as some legacy URI schemes
allowed userinfo of the form ‘USERNAME:PASSWD’. But since passwords do
not belong in URIs, the RFC does not want to condone this practice, so
it calls anything before the ‘@’ sign “userinfo”.
(use-modules (web uri))
The following procedures can be found in the ‘(web uri)’ module.
Load it into your Guile, using a form like the above, to have access to
them.
The most common way to build a URI from Scheme is with the
‘build-uri’ function.
-- Scheme Procedure: build-uri scheme [#:userinfo=#f] [#:host=#f]
[#:port=#f] [#:path=""] [#:query=#f] [#:fragment=#f]
[#:validate?=#t]
Construct a URI. SCHEME should be a symbol, PORT either a positive,
exact integer or ‘#f’, and the rest of the fields are either
strings or ‘#f’. If VALIDATE? is true, also run some consistency
checks to make sure that the constructed URI is valid.
-- Scheme Procedure: uri? obj
Return ‘#t’ if OBJ is a URI.
Guile, URIs are represented as URI records, with a number of
associated accessors.
-- Scheme Procedure: uri-scheme uri
-- Scheme Procedure: uri-userinfo uri
-- Scheme Procedure: uri-host uri
-- Scheme Procedure: uri-port uri
-- Scheme Procedure: uri-path uri
-- Scheme Procedure: uri-query uri
-- Scheme Procedure: uri-fragment uri
Field accessors for the URI record type. The URI scheme will be a
symbol, or ‘#f’ if the object is a relative-ref (see below). The
port will be either a positive, exact integer or ‘#f’, and the rest
of the fields will be either strings or ‘#f’ if not present.
-- Scheme Procedure: string->uri string
Parse STRING into a URI object. Return ‘#f’ if the string could
not be parsed.
-- Scheme Procedure: uri->string uri [#:include-fragment?=#t]
Serialize URI to a string. If the URI has a port that is the
default port for its scheme, the port is not included in the
serialization. If INCLUDE-FRAGMENT? is given as false, the
resulting string will omit the fragment (if any).
-- Scheme Procedure: declare-default-port! scheme port
Declare a default port for the given URI scheme.
-- Scheme Procedure: uri-decode str [#:encoding="utf-8"]
[#:decode-plus-to-space? #t]
Percent-decode the given STR, according to ENCODING, which should
be the name of a character encoding.
Note that this function should not generally be applied to a full
URI string. For paths, use ‘split-and-decode-uri-path’ instead.
For query strings, split the query on ‘&’ and ‘=’ boundaries, and
decode the components separately.
Note also that percent-encoded strings encode _bytes_, not
characters. There is no guarantee that a given byte sequence is a
valid string encoding. Therefore this routine may signal an error
if the decoded bytes are not valid for the given encoding. Pass
‘#f’ for ENCODING if you want decoded bytes as a bytevector
directly. *Note ‘set-port-encoding!’: Ports, for more information
on character encodings.
If DECODE-PLUS-TO-SPACE? is true, which is the default, also
replace instances of the plus character ‘+’ with a space character.
This is needed when parsing ‘application/x-www-form-urlencoded’
data.
Returns a string of the decoded characters, or a bytevector if
ENCODING was ‘#f’.
-- Scheme Procedure: uri-encode str [#:encoding="utf-8"]
[#:unescaped-chars]
Percent-encode any character not in the character set,
UNESCAPED-CHARS.
The default character set includes alphanumerics from ASCII, as
well as the special characters ‘-’, ‘.’, ‘_’, and ‘~’. Any other
character will be percent-encoded, by writing out the character to
a bytevector within the given ENCODING, then encoding each byte as
‘%HH’, where HH is the hexadecimal representation of the byte.
-- Scheme Procedure: split-and-decode-uri-path path
Split PATH into its components, and decode each component, removing
empty components.
For example, ‘"/foo/bar%20baz/"’ decodes to the two-element list,
‘("foo" "bar baz")’.
-- Scheme Procedure: encode-and-join-uri-path parts
URI-encode each element of PARTS, which should be a list of
strings, and join the parts together with ‘/’ as a delimiter.
For example, the list ‘("scrambled eggs" "biscuits&gravy")’ encodes
as ‘"scrambled%20eggs/biscuits%26gravy"’.
Subtypes of URI
...............
As we noted above, not all URI objects have a scheme. You might have
noted in the “generic URI syntax” example that the left-hand side of
that grammar definition was URI-reference, not URI. A “URI-reference” is
a generalization of a URI where the scheme is optional. If no scheme is
specified, it is taken to be relative to some other related URI. A
common use of URI references is when you want to be vague regarding the
choice of HTTP or HTTPS – serving a web page referring to ‘/foo.css’
will use HTTPS if loaded over HTTPS, or HTTP otherwise.
-- Scheme Procedure: build-uri-reference [#:scheme=#f] [#:userinfo=#f]
[#:host=#f] [#:port=#f] [#:path=""] [#:query=#f]
[#:fragment=#f] [#:validate?=#t]
Like ‘build-uri’, but with an optional scheme.
-- Scheme Procedure: uri-reference? obj
Return ‘#t’ if OBJ is a URI-reference. This is the most general
URI predicate, as it includes not only full URIs that have schemes
(those that match ‘uri?’) but also URIs without schemes.
It’s also possible to build a “relative-ref”: a URI-reference that
explicitly lacks a scheme.
-- Scheme Procedure: build-relative-ref [#:userinfo=#f] [#:host=#f]
[#:port=#f] [#:path=""] [#:query=#f] [#:fragment=#f]
[#:validate?=#t]
Like ‘build-uri’, but with no scheme.
-- Scheme Procedure: relative-ref? obj
Return ‘#t’ if OBJ is a “relative-ref”: a URI-reference that has no
scheme. Every URI-reference will either match ‘uri?’ or
‘relative-ref?’ (but not both).
In case it’s not clear from the above, the most general of these URI
types is the URI-reference, with ‘build-uri-reference’ as the most
general constructor. ‘build-uri’ and ‘build-relative-ref’ enforce
enforce specific restrictions on the URI-reference. The most generic
URI parser is then ‘string->uri-reference’, and there is also a parser
for when you know that you want a relative-ref.
Note that ‘uri?’ will only return ‘#t’ for URI objects that have
schemes; that is, it rejects relative-refs.
-- Scheme Procedure: string->uri-reference string
Parse STRING into a URI object, while not requiring a scheme.
Return ‘#f’ if the string could not be parsed.
-- Scheme Procedure: string->relative-ref string
Parse STRING into a URI object, while asserting that no scheme is
present. Return ‘#f’ if the string could not be parsed.
7.3.3 The Hyper-Text Transfer Protocol
--------------------------------------
The initial motivation for including web functionality in Guile, rather
than rely on an external package, was to establish a standard base on
which people can share code. To that end, we continue the focus on data
types by providing a number of low-level parsers and unparsers for
elements of the HTTP protocol.
If you are want to skip the low-level details for now and move on to
web pages, *note Web Client::, and *note Web Server::. Otherwise, load
the HTTP module, and read on.
(use-modules (web http))
The focus of the ‘(web http)’ module is to parse and unparse standard
HTTP headers, representing them to Guile as native data structures. For
example, a ‘Date:’ header will be represented as a SRFI-19 date record
(*note SRFI-19::), rather than as a string.
Guile tries to follow RFCs fairly strictly—the road to perdition
being paved with compatibility hacks—though some allowances are made for
not-too-divergent texts.
Header names are represented as lower-case symbols.
-- Scheme Procedure: string->header name
Parse NAME to a symbolic header name.
-- Scheme Procedure: header->string sym
Return the string form for the header named SYM.
For example:
(string->header "Content-Length")
⇒ content-length
(header->string 'content-length)
⇒ "Content-Length"
(string->header "FOO")
⇒ foo
(header->string 'foo)
⇒ "Foo"
Guile keeps a registry of known headers, their string names, and some
parsing and serialization procedures. If a header is unknown, its
string name is simply its symbol name in title-case.
-- Scheme Procedure: known-header? sym
Return ‘#t’ if SYM is a known header, with associated parsers and
serialization procedures, or ‘#f’ otherwise.
-- Scheme Procedure: header-parser sym
Return the value parser for headers named SYM. The result is a
procedure that takes one argument, a string, and returns the parsed
value. If the header isn’t known to Guile, a default parser is
returned that passes through the string unchanged.
-- Scheme Procedure: header-validator sym
Return a predicate which returns ‘#t’ if the given value is valid
for headers named SYM. The default validator for unknown headers
is ‘string?’.
-- Scheme Procedure: header-writer sym
Return a procedure that writes values for headers named SYM to a
port. The resulting procedure takes two arguments: a value and a
port. The default writer is ‘display’.
For more on the set of headers that Guile knows about out of the box,
*note HTTP Headers::. To add your own, use the ‘declare-header!’
procedure:
-- Scheme Procedure: declare-header! name parser validator writer
[#:multiple?=#f]
Declare a parser, validator, and writer for a given header.
For example, let’s say you are running a web server behind some sort
of proxy, and your proxy adds an ‘X-Client-Address’ header, indicating
the IPv4 address of the original client. You would like for the HTTP
request record to parse out this header to a Scheme value, instead of
leaving it as a string. You could register this header with Guile’s
HTTP stack like this:
(declare-header! "X-Client-Address"
(lambda (str)
(inet-pton AF_INET str))
(lambda (ip)
(and (integer? ip) (exact? ip) (<= 0 ip #xffffffff)))
(lambda (ip port)
(display (inet-ntop AF_INET ip) port)))
-- Scheme Procedure: declare-opaque-header! name
A specialised version of ‘declare-header!’ for the case in which
you want a header’s value to be returned/written “as-is”.
-- Scheme Procedure: valid-header? sym val
Return a true value if VAL is a valid Scheme value for the header
with name SYM, or ‘#f’ otherwise.
Now that we have a generic interface for reading and writing headers,
we do just that.
-- Scheme Procedure: read-header port
Read one HTTP header from PORT. Return two values: the header name
and the parsed Scheme value. May raise an exception if the header
was known but the value was invalid.
Returns the end-of-file object for both values if the end of the
message body was reached (i.e., a blank line).
-- Scheme Procedure: parse-header name val
Parse VAL, a string, with the parser for the header named NAME.
Returns the parsed value.
-- Scheme Procedure: write-header name val port
Write the given header name and value to PORT, using the writer
from ‘header-writer’.
-- Scheme Procedure: read-headers port
Read the headers of an HTTP message from PORT, returning them as an
ordered alist.
-- Scheme Procedure: write-headers headers port
Write the given header alist to PORT. Doesn’t write the final
‘\r\n’, as the user might want to add another header.
The ‘(web http)’ module also has some utility procedures to read and
write request and response lines.
-- Scheme Procedure: parse-http-method str [start] [end]
Parse an HTTP method from STR. The result is an upper-case symbol,
like ‘GET’.
-- Scheme Procedure: parse-http-version str [start] [end]
Parse an HTTP version from STR, returning it as a major–minor pair.
For example, ‘HTTP/1.1’ parses as the pair of integers, ‘(1 . 1)’.
-- Scheme Procedure: parse-request-uri str [start] [end]
Parse a URI from an HTTP request line. Note that URIs in requests
do not have to have a scheme or host name. The result is a URI
object.
-- Scheme Procedure: read-request-line port
Read the first line of an HTTP request from PORT, returning three
values: the method, the URI, and the version.
-- Scheme Procedure: write-request-line method uri version port
Write the first line of an HTTP request to PORT.
-- Scheme Procedure: read-response-line port
Read the first line of an HTTP response from PORT, returning three
values: the HTTP version, the response code, and the “reason
phrase”.
-- Scheme Procedure: write-response-line version code reason-phrase
port
Write the first line of an HTTP response to PORT.
7.3.4 HTTP Headers
------------------
In addition to defining the infrastructure to parse headers, the ‘(web
http)’ module defines specific parsers and unparsers for all headers
defined in the HTTP/1.1 standard.
For example, if you receive a header named ‘Accept-Language’ with a
value ‘en, es;q=0.8’, Guile parses it as a quality list (defined below):
(parse-header 'accept-language "en, es;q=0.8")
⇒ ((1000 . "en") (800 . "es"))
The format of the value for ‘Accept-Language’ headers is defined
below, along with all other headers defined in the HTTP standard. (If
the header were unknown, the value would have been returned as a
string.)
For brevity, the header definitions below are given in the form, TYPE
‘NAME’, indicating that values for the header ‘NAME’ will be of the
given TYPE. Since Guile internally treats header names in lower case,
in this document we give types title-cased names. A short description
of the each header’s purpose and an example follow.
For full details on the meanings of all of these headers, see the
HTTP 1.1 standard, RFC 2616.
7.3.4.1 HTTP Header Types
.........................
Here we define the types that are used below, when defining headers.
-- HTTP Header Type: Date
A SRFI-19 date.
-- HTTP Header Type: KVList
A list whose elements are keys or key-value pairs. Keys are parsed
to symbols. Values are strings by default. Non-string values are
the exception, and are mentioned explicitly below, as appropriate.
-- HTTP Header Type: SList
A list of strings.
-- HTTP Header Type: Quality
An exact integer between 0 and 1000. Qualities are used to express
preference, given multiple options. An option with a quality of
870, for example, is preferred over an option with quality 500.
(Qualities are written out over the wire as numbers between 0.0 and
1.0, but since the standard only allows three digits after the
decimal, it’s equivalent to integers between 0 and 1000, so that’s
what Guile uses.)
-- HTTP Header Type: QList
A quality list: a list of pairs, the car of which is a quality, and
the cdr a string. Used to express a list of options, along with
their qualities.
-- HTTP Header Type: ETag
An entity tag, represented as a pair. The car of the pair is an
opaque string, and the cdr is ‘#t’ if the entity tag is a “strong”
entity tag, and ‘#f’ otherwise.
7.3.4.2 General Headers
.......................
General HTTP headers may be present in any HTTP message.
-- HTTP Header: KVList cache-control
A key-value list of cache-control directives. See RFC 2616, for
more details.
If present, parameters to ‘max-age’, ‘max-stale’, ‘min-fresh’, and
‘s-maxage’ are all parsed as non-negative integers.
If present, parameters to ‘private’ and ‘no-cache’ are parsed as
lists of header names, as symbols.
(parse-header 'cache-control "no-cache,no-store"
⇒ (no-cache no-store)
(parse-header 'cache-control "no-cache=\"Authorization,Date\",no-store"
⇒ ((no-cache . (authorization date)) no-store)
(parse-header 'cache-control "no-cache=\"Authorization,Date\",max-age=10"
⇒ ((no-cache . (authorization date)) (max-age . 10))
-- HTTP Header: List connection
A list of header names that apply only to this HTTP connection, as
symbols. Additionally, the symbol ‘close’ may be present, to
indicate that the server should close the connection after
responding to the request.
(parse-header 'connection "close")
⇒ (close)
-- HTTP Header: Date date
The date that a given HTTP message was originated.
(parse-header 'date "Tue, 15 Nov 1994 08:12:31 GMT")
⇒ #
-- HTTP Header: KVList pragma
A key-value list of implementation-specific directives.
(parse-header 'pragma "no-cache, broccoli=tasty")
⇒ (no-cache (broccoli . "tasty"))
-- HTTP Header: List trailer
A list of header names which will appear after the message body,
instead of with the message headers.
(parse-header 'trailer "ETag")
⇒ (etag)
-- HTTP Header: List transfer-encoding
A list of transfer codings, expressed as key-value lists. The only
transfer coding defined by the specification is ‘chunked’.
(parse-header 'transfer-encoding "chunked")
⇒ ((chunked))
-- HTTP Header: List upgrade
A list of strings, indicating additional protocols that a server
could use in response to a request.
(parse-header 'upgrade "WebSocket")
⇒ ("WebSocket")
FIXME: parse out more fully?
-- HTTP Header: List via
A list of strings, indicating the protocol versions and hosts of
intermediate servers and proxies. There may be multiple ‘via’
headers in one message.
(parse-header 'via "1.0 venus, 1.1 mars")
⇒ ("1.0 venus" "1.1 mars")
-- HTTP Header: List warning
A list of warnings given by a server or intermediate proxy. Each
warning is a itself a list of four elements: a code, as an exact
integer between 0 and 1000, a host as a string, the warning text as
a string, and either ‘#f’ or a SRFI-19 date.
There may be multiple ‘warning’ headers in one message.
(parse-header 'warning "123 foo \"core breach imminent\"")
⇒ ((123 "foo" "core-breach imminent" #f))
7.3.4.3 Entity Headers
......................
Entity headers may be present in any HTTP message, and refer to the
resource referenced in the HTTP request or response.
-- HTTP Header: List allow
A list of allowed methods on a given resource, as symbols.
(parse-header 'allow "GET, HEAD")
⇒ (GET HEAD)
-- HTTP Header: List content-encoding
A list of content codings, as symbols.
(parse-header 'content-encoding "gzip")
⇒ (gzip)
-- HTTP Header: List content-language
The languages that a resource is in, as strings.
(parse-header 'content-language "en")
⇒ ("en")
-- HTTP Header: UInt content-length
The number of bytes in a resource, as an exact, non-negative
integer.
(parse-header 'content-length "300")
⇒ 300
-- HTTP Header: URI content-location
The canonical URI for a resource, in the case that it is also
accessible from a different URI.
(parse-header 'content-location "http://example.com/foo")
⇒ #< ...>
-- HTTP Header: String content-md5
The MD5 digest of a resource.
(parse-header 'content-md5 "ffaea1a79810785575e29e2bd45e2fa5")
⇒ "ffaea1a79810785575e29e2bd45e2fa5"
-- HTTP Header: List content-range
Range specification as a list of three elements: the symbol
‘bytes’, either the symbol ‘*’ or a pair of integers indicating the
byte range, and either ‘*’ or an integer indicating the instance
length. Used to indicate that a response only includes part of a
resource.
(parse-header 'content-range "bytes 10-20/*")
⇒ (bytes (10 . 20) *)
-- HTTP Header: List content-type
The MIME type of a resource, as a symbol, along with any
parameters.
(parse-header 'content-type "text/plain")
⇒ (text/plain)
(parse-header 'content-type "text/plain;charset=utf-8")
⇒ (text/plain (charset . "utf-8"))
Note that the ‘charset’ parameter is something of a misnomer, and
the HTTP specification admits this. It specifies the _encoding_ of
the characters, not the character set.
-- HTTP Header: Date expires
The date/time after which the resource given in a response is
considered stale.
(parse-header 'expires "Tue, 15 Nov 1994 08:12:31 GMT")
⇒ #
-- HTTP Header: Date last-modified
The date/time on which the resource given in a response was last
modified.
(parse-header 'expires "Tue, 15 Nov 1994 08:12:31 GMT")
⇒ #
7.3.4.4 Request Headers
.......................
Request headers may only appear in an HTTP request, not in a response.
-- HTTP Header: List accept
A list of preferred media types for a response. Each element of
the list is itself a list, in the same format as ‘content-type’.
(parse-header 'accept "text/html,text/plain;charset=utf-8")
⇒ ((text/html) (text/plain (charset . "utf-8")))
Preference is expressed with quality values:
(parse-header 'accept "text/html;q=0.8,text/plain;q=0.6")
⇒ ((text/html (q . 800)) (text/plain (q . 600)))
-- HTTP Header: QList accept-charset
A quality list of acceptable charsets. Note again that what HTTP
calls a “charset” is what Guile calls a “character encoding”.
(parse-header 'accept-charset "iso-8859-5, unicode-1-1;q=0.8")
⇒ ((1000 . "iso-8859-5") (800 . "unicode-1-1"))
-- HTTP Header: QList accept-encoding
A quality list of acceptable content codings.
(parse-header 'accept-encoding "gzip,identity=0.8")
⇒ ((1000 . "gzip") (800 . "identity"))
-- HTTP Header: QList accept-language
A quality list of acceptable languages.
(parse-header 'accept-language "cn,en=0.75")
⇒ ((1000 . "cn") (750 . "en"))
-- HTTP Header: Pair authorization
Authorization credentials. The car of the pair indicates the
authentication scheme, like ‘basic’. For basic authentication, the
cdr of the pair will be the base64-encoded ‘USER:PASS’ string. For
other authentication schemes, like ‘digest’, the cdr will be a
key-value list of credentials.
(parse-header 'authorization "Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ=="
⇒ (basic . "QWxhZGRpbjpvcGVuIHNlc2FtZQ==")
-- HTTP Header: List expect
A list of expectations that a client has of a server. The
expectations are key-value lists.
(parse-header 'expect "100-continue")
⇒ ((100-continue))
-- HTTP Header: String from
The email address of a user making an HTTP request.
(parse-header 'from "bob@example.com")
⇒ "bob@example.com"
-- HTTP Header: Pair host
The host for the resource being requested, as a hostname-port pair.
If no port is given, the port is ‘#f’.
(parse-header 'host "gnu.org:80")
⇒ ("gnu.org" . 80)
(parse-header 'host "gnu.org")
⇒ ("gnu.org" . #f)
-- HTTP Header: *|List if-match
A set of etags, indicating that the request should proceed if and
only if the etag of the resource is in that set. Either the symbol
‘*’, indicating any etag, or a list of entity tags.
(parse-header 'if-match "*")
⇒ *
(parse-header 'if-match "asdfadf")
⇒ (("asdfadf" . #t))
(parse-header 'if-match W/"asdfadf")
⇒ (("asdfadf" . #f))
-- HTTP Header: Date if-modified-since
Indicates that a response should proceed if and only if the
resource has been modified since the given date.
(parse-header 'if-modified-since "Tue, 15 Nov 1994 08:12:31 GMT")
⇒ #
-- HTTP Header: *|List if-none-match
A set of etags, indicating that the request should proceed if and
only if the etag of the resource is not in the set. Either the
symbol ‘*’, indicating any etag, or a list of entity tags.
(parse-header 'if-none-match "*")
⇒ *
-- HTTP Header: ETag|Date if-range
Indicates that the range request should proceed if and only if the
resource matches a modification date or an etag. Either an entity
tag, or a SRFI-19 date.
(parse-header 'if-range "\"original-etag\"")
⇒ ("original-etag" . #t)
-- HTTP Header: Date if-unmodified-since
Indicates that a response should proceed if and only if the
resource has not been modified since the given date.
(parse-header 'if-not-modified-since "Tue, 15 Nov 1994 08:12:31 GMT")
⇒ #
-- HTTP Header: UInt max-forwards
The maximum number of proxy or gateway hops that a request should
be subject to.
(parse-header 'max-forwards "10")
⇒ 10
-- HTTP Header: Pair proxy-authorization
Authorization credentials for a proxy connection. See the
documentation for ‘authorization’ above for more information on the
format.
(parse-header 'proxy-authorization "Digest foo=bar,baz=qux"
⇒ (digest (foo . "bar") (baz . "qux"))
-- HTTP Header: Pair range
A range request, indicating that the client wants only part of a
resource. The car of the pair is the symbol ‘bytes’, and the cdr
is a list of pairs. Each element of the cdr indicates a range; the
car is the first byte position and the cdr is the last byte
position, as integers, or ‘#f’ if not given.
(parse-header 'range "bytes=10-30,50-")
⇒ (bytes (10 . 30) (50 . #f))
-- HTTP Header: URI referer
The URI of the resource that referred the user to this resource.
The name of the header is a misspelling, but we are stuck with it.
(parse-header 'referer "http://www.gnu.org/")
⇒ #
-- HTTP Header: List te
A list of transfer codings, expressed as key-value lists. A common
transfer coding is ‘trailers’.
(parse-header 'te "trailers")
⇒ ((trailers))
-- HTTP Header: String user-agent
A string indicating the user agent making the request. The
specification defines a structured format for this header, but it
is widely disregarded, so Guile does not attempt to parse strictly.
(parse-header 'user-agent "Mozilla/5.0")
⇒ "Mozilla/5.0"
7.3.4.5 Response Headers
........................
-- HTTP Header: List accept-ranges
A list of range units that the server supports, as symbols.
(parse-header 'accept-ranges "bytes")
⇒ (bytes)
-- HTTP Header: UInt age
The age of a cached response, in seconds.
(parse-header 'age "3600")
⇒ 3600
-- HTTP Header: ETag etag
The entity-tag of the resource.
(parse-header 'etag "\"foo\"")
⇒ ("foo" . #t)
-- HTTP Header: URI-reference location
A URI reference on which a request may be completed. Used in
combination with a redirecting status code to perform client-side
redirection.
(parse-header 'location "http://example.com/other")
⇒ #
-- HTTP Header: List proxy-authenticate
A list of challenges to a proxy, indicating the need for
authentication.
(parse-header 'proxy-authenticate "Basic realm=\"foo\"")
⇒ ((basic (realm . "foo")))
-- HTTP Header: UInt|Date retry-after
Used in combination with a server-busy status code, like 503, to
indicate that a client should retry later. Either a number of
seconds, or a date.
(parse-header 'retry-after "60")
⇒ 60
-- HTTP Header: String server
A string identifying the server.
(parse-header 'server "My first web server")
⇒ "My first web server"
-- HTTP Header: *|List vary
A set of request headers that were used in computing this response.
Used to indicate that server-side content negotiation was
performed, for example in response to the ‘accept-language’ header.
Can also be the symbol ‘*’, indicating that all headers were
considered.
(parse-header 'vary "Accept-Language, Accept")
⇒ (accept-language accept)
-- HTTP Header: List www-authenticate
A list of challenges to a user, indicating the need for
authentication.
(parse-header 'www-authenticate "Basic realm=\"foo\"")
⇒ ((basic (realm . "foo")))
7.3.5 Transfer Codings
----------------------
HTTP 1.1 allows for various transfer codings to be applied to message
bodies. These include various types of compression, and HTTP chunked
encoding. Currently, only chunked encoding is supported by guile.
Chunked coding is an optional coding that may be applied to message
bodies, to allow messages whose length is not known beforehand to be
returned. Such messages can be split into chunks, terminated by a final
zero length chunk.
In order to make dealing with encodings more simple, guile provides
procedures to create ports that “wrap” existing ports, applying
transformations transparently under the hood.
These procedures are in the ‘(web http)’ module.
(use-modules (web http))
-- Scheme Procedure: make-chunked-input-port port [#:keep-alive?=#f]
Returns a new port, that transparently reads and decodes
chunk-encoded data from PORT. If no more chunk-encoded data is
available, it returns the end-of-file object. When the port is
closed, PORT will also be closed, unless KEEP-ALIVE? is true.
If the chunked input ends prematurely, a
‘&chunked-input-ended-promaturely’ exception will be raised.
(use-modules (ice-9 rdelim))
(define s "5\r\nFirst\r\nA\r\n line\n Sec\r\n8\r\nond line\r\n0\r\n")
(define p (make-chunked-input-port (open-input-string s)))
(read-line s)
⇒ "First line"
(read-line s)
⇒ "Second line"
-- Scheme Procedure: make-chunked-output-port port [#:keep-alive?=#f]
Returns a new port, which transparently encodes data as
chunk-encoded before writing it to PORT. Whenever a write occurs
on this port, it buffers it, until the port is flushed, at which
point it writes a chunk containing all the data written so far.
When the port is closed, the data remaining is written to PORT, as
is the terminating zero chunk. It also causes PORT to be closed,
unless KEEP-ALIVE? is true.
Note. Forcing a chunked output port when there is no data is
buffered does not write a zero chunk, as this would cause the data
to be interpreted incorrectly by the client.
(call-with-output-string
(lambda (out)
(define out* (make-chunked-output-port out #:keep-alive? #t))
(display "first chunk" out*)
(force-output out*)
(force-output out*) ; note this does not write a zero chunk
(display "second chunk" out*)
(close-port out*)))
⇒ "b\r\nfirst chunk\r\nc\r\nsecond chunk\r\n0\r\n"
7.3.6 HTTP Requests
-------------------
(use-modules (web request))
The request module contains a data type for HTTP requests.
7.3.6.1 An Important Note on Character Sets
...........................................
HTTP requests consist of two parts: the request proper, consisting of a
request line and a set of headers, and (optionally) a body. The body
might have a binary content-type, and even in the textual case its
length is specified in bytes, not characters.
Therefore, HTTP is a fundamentally binary protocol. However the
request line and headers are specified to be in a subset of ASCII, so
they can be treated as text, provided that the port’s encoding is set to
an ASCII-compatible one-byte-per-character encoding. ISO-8859-1
(latin-1) is just such an encoding, and happens to be very efficient for
Guile.
So what Guile does when reading requests from the wire, or writing
them out, is to set the port’s encoding to latin-1, and treating the
request headers as text.
The request body is another issue. For binary data, the data is
probably in a bytevector, so we use the R6RS binary output procedures to
write out the binary payload. Textual data usually has to be written
out to some character encoding, usually UTF-8, and then the resulting
bytevector is written out to the port.
In summary, Guile reads and writes HTTP over latin-1 sockets, without
any loss of generality.
7.3.6.2 Request API
...................
-- Scheme Procedure: request? obj
-- Scheme Procedure: request-method request
-- Scheme Procedure: request-uri request
-- Scheme Procedure: request-version request
-- Scheme Procedure: request-headers request
-- Scheme Procedure: request-meta request
-- Scheme Procedure: request-port request
A predicate and field accessors for the request type. The fields
are as follows:
‘method’
The HTTP method, for example, ‘GET’.
‘uri’
The URI as a URI record.
‘version’
The HTTP version pair, like ‘(1 . 1)’.
‘headers’
The request headers, as an alist of parsed values.
‘meta’
An arbitrary alist of other data, for example information
returned in the ‘sockaddr’ from ‘accept’ (*note Network
Sockets and Communication::).
‘port’
The port on which to read or write a request body, if any.
-- Scheme Procedure: read-request port [meta='()]
Read an HTTP request from PORT, optionally attaching the given
metadata, META.
As a side effect, sets the encoding on PORT to ISO-8859-1
(latin-1), so that reading one character reads one byte. See the
discussion of character sets above, for more information.
Note that the body is not part of the request. Once you have read
a request, you may read the body separately, and likewise for
writing requests.
-- Scheme Procedure: build-request uri [#:method='GET] [#:version='(1 .
1)] [#:headers='()] [#:port=#f] [#:meta='()]
[#:validate-headers?=#t]
Construct an HTTP request object. If VALIDATE-HEADERS? is true,
the headers are each run through their respective validators.
-- Scheme Procedure: write-request r port
Write the given HTTP request to PORT.
Return a new request, whose ‘request-port’ will continue writing on
PORT, perhaps using some transfer encoding.
-- Scheme Procedure: read-request-body r
Reads the request body from R, as a bytevector. Return ‘#f’ if
there was no request body.
-- Scheme Procedure: write-request-body r bv
Write BV, a bytevector, to the port corresponding to the HTTP
request R.
The various headers that are typically associated with HTTP requests
may be accessed with these dedicated accessors. *Note HTTP Headers::,
for more information on the format of parsed headers.
-- Scheme Procedure: request-accept request [default='()]
-- Scheme Procedure: request-accept-charset request [default='()]
-- Scheme Procedure: request-accept-encoding request [default='()]
-- Scheme Procedure: request-accept-language request [default='()]
-- Scheme Procedure: request-allow request [default='()]
-- Scheme Procedure: request-authorization request [default=#f]
-- Scheme Procedure: request-cache-control request [default='()]
-- Scheme Procedure: request-connection request [default='()]
-- Scheme Procedure: request-content-encoding request [default='()]
-- Scheme Procedure: request-content-language request [default='()]
-- Scheme Procedure: request-content-length request [default=#f]
-- Scheme Procedure: request-content-location request [default=#f]
-- Scheme Procedure: request-content-md5 request [default=#f]
-- Scheme Procedure: request-content-range request [default=#f]
-- Scheme Procedure: request-content-type request [default=#f]
-- Scheme Procedure: request-date request [default=#f]
-- Scheme Procedure: request-expect request [default='()]
-- Scheme Procedure: request-expires request [default=#f]
-- Scheme Procedure: request-from request [default=#f]
-- Scheme Procedure: request-host request [default=#f]
-- Scheme Procedure: request-if-match request [default=#f]
-- Scheme Procedure: request-if-modified-since request [default=#f]
-- Scheme Procedure: request-if-none-match request [default=#f]
-- Scheme Procedure: request-if-range request [default=#f]
-- Scheme Procedure: request-if-unmodified-since request [default=#f]
-- Scheme Procedure: request-last-modified request [default=#f]
-- Scheme Procedure: request-max-forwards request [default=#f]
-- Scheme Procedure: request-pragma request [default='()]
-- Scheme Procedure: request-proxy-authorization request [default=#f]
-- Scheme Procedure: request-range request [default=#f]
-- Scheme Procedure: request-referer request [default=#f]
-- Scheme Procedure: request-te request [default=#f]
-- Scheme Procedure: request-trailer request [default='()]
-- Scheme Procedure: request-transfer-encoding request [default='()]
-- Scheme Procedure: request-upgrade request [default='()]
-- Scheme Procedure: request-user-agent request [default=#f]
-- Scheme Procedure: request-via request [default='()]
-- Scheme Procedure: request-warning request [default='()]
Return the given request header, or DEFAULT if none was present.
-- Scheme Procedure: request-absolute-uri r [default-host=#f]
[default-port=#f] [default-scheme=#f]
A helper routine to determine the absolute URI of a request, using
the ‘host’ header and the default scheme, host and port. If there
is no default scheme and the URI is not itself absolute, an error
is signalled.
7.3.7 HTTP Responses
--------------------
(use-modules (web response))
As with requests (*note Requests::), Guile offers a data type for
HTTP responses. Again, the body is represented separately from the
request.
-- Scheme Procedure: response? obj
-- Scheme Procedure: response-version response
-- Scheme Procedure: response-code response
-- Scheme Procedure: response-reason-phrase response
-- Scheme Procedure: response-headers response
-- Scheme Procedure: response-port response
A predicate and field accessors for the response type. The fields
are as follows:
‘version’
The HTTP version pair, like ‘(1 . 1)’.
‘code’
The HTTP response code, like ‘200’.
‘reason-phrase’
The reason phrase, or the standard reason phrase for the
response’s code.
‘headers’
The response headers, as an alist of parsed values.
‘port’
The port on which to read or write a response body, if any.
-- Scheme Procedure: read-response port
Read an HTTP response from PORT.
As a side effect, sets the encoding on PORT to ISO-8859-1
(latin-1), so that reading one character reads one byte. See the
discussion of character sets in *note Responses::, for more
information.
-- Scheme Procedure: build-response [#:version='(1 . 1)] [#:code=200]
[#:reason-phrase=#f] [#:headers='()] [#:port=#f]
[#:validate-headers?=#t]
Construct an HTTP response object. If VALIDATE-HEADERS? is true,
the headers are each run through their respective validators.
-- Scheme Procedure: adapt-response-version response version
Adapt the given response to a different HTTP version. Return a new
HTTP response.
The idea is that many applications might just build a response for
the default HTTP version, and this method could handle a number of
programmatic transformations to respond to older HTTP versions (0.9
and 1.0). But currently this function is a bit heavy-handed, just
updating the version field.
-- Scheme Procedure: write-response r port
Write the given HTTP response to PORT.
Return a new response, whose ‘response-port’ will continue writing
on PORT, perhaps using some transfer encoding.
-- Scheme Procedure: response-must-not-include-body? r
Some responses, like those with status code 304, are specified as
never having bodies. This predicate returns ‘#t’ for those
responses.
Note also, though, that responses to ‘HEAD’ requests must also not
have a body.
-- Scheme Procedure: response-body-port r [#:decode?=#t]
[#:keep-alive?=#t]
Return an input port from which the body of R can be read. The
encoding of the returned port is set according to R’s
‘content-type’ header, when it’s textual, except if DECODE? is
‘#f’. Return ‘#f’ when no body is available.
When KEEP-ALIVE? is ‘#f’, closing the returned port also closes R’s
response port.
-- Scheme Procedure: read-response-body r
Read the response body from R, as a bytevector. Returns ‘#f’ if
there was no response body.
-- Scheme Procedure: write-response-body r bv
Write BV, a bytevector, to the port corresponding to the HTTP
response R.
As with requests, the various headers that are typically associated
with HTTP responses may be accessed with these dedicated accessors.
*Note HTTP Headers::, for more information on the format of parsed
headers.
-- Scheme Procedure: response-accept-ranges response [default=#f]
-- Scheme Procedure: response-age response [default='()]
-- Scheme Procedure: response-allow response [default='()]
-- Scheme Procedure: response-cache-control response [default='()]
-- Scheme Procedure: response-connection response [default='()]
-- Scheme Procedure: response-content-encoding response [default='()]
-- Scheme Procedure: response-content-language response [default='()]
-- Scheme Procedure: response-content-length response [default=#f]
-- Scheme Procedure: response-content-location response [default=#f]
-- Scheme Procedure: response-content-md5 response [default=#f]
-- Scheme Procedure: response-content-range response [default=#f]
-- Scheme Procedure: response-content-type response [default=#f]
-- Scheme Procedure: response-date response [default=#f]
-- Scheme Procedure: response-etag response [default=#f]
-- Scheme Procedure: response-expires response [default=#f]
-- Scheme Procedure: response-last-modified response [default=#f]
-- Scheme Procedure: response-location response [default=#f]
-- Scheme Procedure: response-pragma response [default='()]
-- Scheme Procedure: response-proxy-authenticate response [default=#f]
-- Scheme Procedure: response-retry-after response [default=#f]
-- Scheme Procedure: response-server response [default=#f]
-- Scheme Procedure: response-trailer response [default='()]
-- Scheme Procedure: response-transfer-encoding response [default='()]
-- Scheme Procedure: response-upgrade response [default='()]
-- Scheme Procedure: response-vary response [default='()]
-- Scheme Procedure: response-via response [default='()]
-- Scheme Procedure: response-warning response [default='()]
-- Scheme Procedure: response-www-authenticate response [default=#f]
Return the given response header, or DEFAULT if none was present.
-- Scheme Procedure: text-content-type? TYPE
Return ‘#t’ if TYPE, a symbol as returned by
‘response-content-type’, represents a textual type such as
‘text/plain’.
7.3.8 Web Client
----------------
‘(web client)’ provides a simple, synchronous HTTP client, built on the
lower-level HTTP, request, and response modules.
(use-modules (web client))
-- Scheme Procedure: open-socket-for-uri uri [#:verify-certificate? #t]
Return an open input/output port for a connection to URI. Guile
dynamically loads Guile-GnuTLS for HTTPS support.
See the Web site of Guile-GnuTLS
(https://gitlab.com/gnutls/guile/), and *note how to install the
GnuTLS bindings for Guile: (gnutls-guile)Guile Preparations, for
more information.
When VERIFY-CERTIFICATE? is true, verify the server’s X.509
certificates against those read from ‘x509-certificate-directory’.
When an error occurs—e.g., the server’s certificate has expired, or
its host name does not match—raise a ‘tls-certificate-error’
exception. The arguments to the ‘tls-certificate-error’ exception
are:
1. a symbol indicating the failure cause, ‘host-mismatch’ if the
certificate’s host name does not match the server’s host name,
and ‘invalid-certificate’ for other causes;
2. the server’s X.509 certificate (*note GnuTLS Guile reference:
(gnutls-guile)Guile Reference.);
3. the server’s host name (a string);
4. in the case of ‘invalid-certificate’ errors, a list of GnuTLS
certificate status values—one of the ‘certificate-status/’
constants, such as ‘certificate-status/signer-not-found’ or
‘certificate-status/revoked’.
-- Scheme Procedure: http-request URI ARG...
Connect to the server corresponding to URI and make a request over
HTTP, using METHOD (‘GET’, ‘HEAD’, ‘POST’, etc.).
The following keyword arguments allow you to modify the requests in
various ways, for example attaching a body to the request, or
setting specific headers. The following table lists the keyword
arguments and their default values.
‘#:method 'GET’
‘#:body #f’
‘#:verify-certificate? #t’
‘#:port (open-socket-for-uri URI #:verify-certificate? VERIFY-CERTIFICATE?)’
‘#:version '(1 . 1)’
‘#:keep-alive? #f’
‘#:headers '()’
‘#:decode-body? #t’
‘#:streaming? #f’
If you already have a port open, pass it as PORT. Otherwise, a
connection will be opened to the server corresponding to URI. Any
extra headers in the alist HEADERS will be added to the request.
If BODY is not ‘#f’, a message body will also be sent with the HTTP
request. If BODY is a string, it is encoded according to the
content-type in HEADERS, defaulting to UTF-8. Otherwise BODY
should be a bytevector, or ‘#f’ for no body. Although a message
body may be sent with any request, usually only ‘POST’ and ‘PUT’
requests have bodies.
If DECODE-BODY? is true, as is the default, the body of the
response will be decoded to string, if it is a textual
content-type. Otherwise it will be returned as a bytevector.
However, if STREAMING? is true, instead of eagerly reading the
response body from the server, this function only reads off the
headers. The response body will be returned as a port on which the
data may be read.
Unless KEEP-ALIVE? is true, the port will be closed after the full
response body has been read.
If PORT is false, URI denotes an HTTPS URL, and VERIFY-CERTIFICATE?
is true, verify X.509 certificates against those available in
‘x509-certificate-directory’.
Returns two values: the response read from the server, and the
response body as a string, bytevector, #f value, or as a port (if
STREAMING? is true).
-- Scheme Procedure: http-get URI ARG...
-- Scheme Procedure: http-head URI ARG...
-- Scheme Procedure: http-post URI ARG...
-- Scheme Procedure: http-put URI ARG...
-- Scheme Procedure: http-delete URI ARG...
-- Scheme Procedure: http-trace URI ARG...
-- Scheme Procedure: http-options URI ARG...
Connect to the server corresponding to URI and make a request over
HTTP, using the appropriate method (‘GET’, ‘HEAD’, ‘POST’, etc.).
These procedures are variants of ‘http-request’ specialized with a
specific METHOD argument, and have the same prototype: a URI
followed by an optional sequence of keyword arguments. *Note
http-request::, for full documentation on the various keyword
arguments.
-- Scheme Parameter: x509-certificate-directory
This parameter gives the name of the directory where X.509
certificates for HTTPS connections should be looked for.
Its default value is one of:
• the value of the ‘GUILE_TLS_CERTIFICATE_DIRECTORY’ environment
variable;
• or the value of the ‘SSL_CERT_DIR’ environment variable (also
honored by the OpenSSL library);
• or, as a last resort, ‘"/etc/ssl/certs"’.
X.509 certificates are used when authenticating the identity of a
remote site, when the ‘#:verify-certificate?’ argument to
‘open-socket-for-uri’, to ‘http-request’, or to related procedures
is true.
‘http-get’ is useful for making one-off requests to web sites. If
you are writing a web spider or some other client that needs to handle a
number of requests in parallel, it’s better to build an event-driven URL
fetcher, similar in structure to the web server (*note Web Server::).
Another option, good but not as performant, would be to use threads,
possibly via par-map or futures.
-- Scheme Parameter: current-http-proxy
-- Scheme Parameter: current-https-proxy
Either ‘#f’ or a non-empty string containing the URL of the HTTP or
HTTPS proxy server to be used by the procedures in the ‘(web
client)’ module, including ‘open-socket-for-uri’. Its initial
value is based on the ‘http_proxy’ and ‘https_proxy’ environment
variables.
(current-http-proxy) ⇒ "http://localhost:8123/"
(parameterize ((current-http-proxy #f))
(http-get "http://example.com/")) ; temporarily bypass proxy
(current-http-proxy) ⇒ "http://localhost:8123/"
7.3.9 Web Server
----------------
‘(web server)’ is a generic web server interface, along with a main loop
implementation for web servers controlled by Guile.
(use-modules (web server))
The lowest layer is the ‘’ object, which defines a set
of hooks to open a server, read a request from a client, write a
response to a client, and close a server. These hooks – ‘open’, ‘read’,
‘write’, and ‘close’, respectively – are bound together in a
‘’ object. Procedures in this module take a
‘’ object, if needed.
A ‘’ may also be looked up by name. If you pass the
‘http’ symbol to ‘run-server’, Guile looks for a variable named ‘http’
in the ‘(web server http)’ module, which should be bound to a
‘’ object. Such a binding is made by instantiation of the
‘define-server-impl’ syntax. In this way the run-server loop can
automatically load other backends if available.
The life cycle of a server goes as follows:
1. The ‘open’ hook is called, to open the server. ‘open’ takes zero
or more arguments, depending on the backend, and returns an opaque
server socket object, or signals an error.
2. The ‘read’ hook is called, to read a request from a new client.
The ‘read’ hook takes one argument, the server socket. It should
return three values: an opaque client socket, the request, and the
request body. The request should be a ‘’ object, from
‘(web request)’. The body should be a string or a bytevector, or
‘#f’ if there is no body.
If the read failed, the ‘read’ hook may return #f for the client
socket, request, and body.
3. A user-provided handler procedure is called, with the request and
body as its arguments. The handler should return two values: the
response, as a ‘’ record from ‘(web response)’, and the
response body as bytevector, or ‘#f’ if not present.
The respose and response body are run through ‘sanitize-response’,
documented below. This allows the handler writer to take some
convenient shortcuts: for example, instead of a ‘’, the
handler can simply return an alist of headers, in which case a
default response object is constructed with those headers. Instead
of a bytevector for the body, the handler can return a string,
which will be serialized into an appropriate encoding; or it can
return a procedure, which will be called on a port to write out the
data. See the ‘sanitize-response’ documentation, for more.
4. The ‘write’ hook is called with three arguments: the client socket,
the response, and the body. The ‘write’ hook returns no values.
5. At this point the request handling is complete. For a loop, we
loop back and try to read a new request.
6. If the user interrupts the loop, the ‘close’ hook is called on the
server socket.
A user may define a server implementation with the following form:
-- Scheme Syntax: define-server-impl name open read write close
Make a ‘’ object with the hooks OPEN, READ, WRITE, and
CLOSE, and bind it to the symbol NAME in the current module.
-- Scheme Procedure: lookup-server-impl impl
Look up a server implementation. If IMPL is a server
implementation already, it is returned directly. If it is a
symbol, the binding named IMPL in the ‘(web server IMPL)’ module is
looked up. Otherwise an error is signaled.
Currently a server implementation is a somewhat opaque type, useful
only for passing to other procedures in this module, like
‘read-client’.
The ‘(web server)’ module defines a number of routines that use
‘’ objects to implement parts of a web server. Given that
we don’t expose the accessors for the various fields of a
‘’, indeed these routines are the only procedures with any
access to the impl objects.
-- Scheme Procedure: open-server impl open-params
Open a server for the given implementation. Return one value, the
new server object. The implementation’s ‘open’ procedure is
applied to OPEN-PARAMS, which should be a list.
-- Scheme Procedure: read-client impl server
Read a new client from SERVER, by applying the implementation’s
‘read’ procedure to the server. If successful, return three
values: an object corresponding to the client, a request object,
and the request body. If any exception occurs, return ‘#f’ for all
three values.
-- Scheme Procedure: handle-request handler request body state
Handle a given request, returning the response and body.
The response and response body are produced by calling the given
HANDLER with REQUEST and BODY as arguments.
The elements of STATE are also passed to HANDLER as arguments, and
may be returned as additional values. The new STATE, collected
from the HANDLER’s return values, is then returned as a list. The
idea is that a server loop receives a handler from the user, along
with whatever state values the user is interested in, allowing the
user’s handler to explicitly manage its state.
-- Scheme Procedure: sanitize-response request response body
“Sanitize” the given response and body, making them appropriate for
the given request.
As a convenience to web handler authors, RESPONSE may be given as
an alist of headers, in which case it is used to construct a
default response. Ensures that the response version corresponds to
the request version. If BODY is a string, encodes the string to a
bytevector, in an encoding appropriate for RESPONSE. Adds a
‘content-length’ and ‘content-type’ header, as necessary.
If BODY is a procedure, it is called with a port as an argument,
and the output collected as a bytevector. In the future we might
try to instead use a compressing, chunk-encoded port, and call this
procedure later, in the write-client procedure. Authors are
advised not to rely on the procedure being called at any particular
time.
-- Scheme Procedure: write-client impl server client response body
Write an HTTP response and body to CLIENT. If the server and
client support persistent connections, it is the implementation’s
responsibility to keep track of the client thereafter, presumably
by attaching it to the SERVER argument somehow.
-- Scheme Procedure: close-server impl server
Release resources allocated by a previous invocation of
‘open-server’.
Given the procedures above, it is a small matter to make a web
server:
-- Scheme Procedure: serve-one-client handler impl server state
Read one request from SERVER, call HANDLER on the request and body,
and write the response to the client. Return the new state
produced by the handler procedure.
-- Scheme Procedure: run-server handler [impl='http] [open-params='()]
arg ...
Run Guile’s built-in web server.
HANDLER should be a procedure that takes two or more arguments, the
HTTP request and request body, and returns two or more values, the
response and response body.
For examples, skip ahead to the next section, *note Web Examples::.
The response and body will be run through ‘sanitize-response’
before sending back to the client.
Additional arguments to HANDLER are taken from ARG .... These
arguments comprise a “state”. Additional return values are
accumulated into a new state, which will be used for subsequent
requests. In this way a handler can explicitly manage its state.
The default web server implementation is ‘http’, which binds to a
socket, listening for request on that port.
-- HTTP Implementation: http [#:host=#f] [#:family=AF_INET]
[#:addr=INADDR_LOOPBACK] [#:port 8080] [#:socket]
The default HTTP implementation. We document it as a function with
keyword arguments, because that is precisely the way that it is –
all of the OPEN-PARAMS to ‘run-server’ get passed to the
implementation’s open function.
;; The defaults: localhost:8080
(run-server handler)
;; Same thing
(run-server handler 'http '())
;; On a different port
(run-server handler 'http '(#:port 8081))
;; IPv6
(run-server handler 'http '(#:family AF_INET6 #:port 8081))
;; Custom socket
(run-server handler 'http `(#:socket ,(sudo-make-me-a-socket)))
7.3.10 Web Examples
-------------------
Well, enough about the tedious internals. Let’s make a web application!
7.3.10.1 Hello, World!
......................
The first program we have to write, of course, is “Hello, World!”. This
means that we have to implement a web handler that does what we want.
Now we define a handler, a function of two arguments and two return
values:
(define (handler request request-body)
(values RESPONSE RESPONSE-BODY))
In this first example, we take advantage of a short-cut, returning an
alist of headers instead of a proper response object. The response body
is our payload:
(define (hello-world-handler request request-body)
(values '((content-type . (text/plain)))
"Hello World!"))
Now let’s test it, by running a server with this handler. Load up
the web server module if you haven’t yet done so, and run a server with
this handler:
(use-modules (web server))
(run-server hello-world-handler)
By default, the web server listens for requests on ‘localhost:8080’.
Visit that address in your web browser to test. If you see the string,
‘Hello World!’, sweet!
7.3.10.2 Inspecting the Request
...............................
The Hello World program above is a general greeter, responding to all
URIs. To make a more exclusive greeter, we need to inspect the request
object, and conditionally produce different results. So let’s load up
the request, response, and URI modules, and do just that.
(use-modules (web server)) ; you probably did this already
(use-modules (web request)
(web response)
(web uri))
(define (request-path-components request)
(split-and-decode-uri-path (uri-path (request-uri request))))
(define (hello-hacker-handler request body)
(if (equal? (request-path-components request)
'("hacker"))
(values '((content-type . (text/plain)))
"Hello hacker!")
(not-found request)))
(run-server hello-hacker-handler)
Here we see that we have defined a helper to return the components of
the URI path as a list of strings, and used that to check for a request
to ‘/hacker/’. Then the success case is just as before – visit
‘http://localhost:8080/hacker/’ in your browser to check.
You should always match against URI path components as decoded by
‘split-and-decode-uri-path’. The above example will work for
‘/hacker/’, ‘//hacker///’, and ‘/h%61ck%65r’.
But we forgot to define ‘not-found’! If you are pasting these
examples into a REPL, accessing any other URI in your web browser will
drop your Guile console into the debugger:
:38:7: In procedure module-lookup:
:38:7: Unbound variable: not-found
Entering a new prompt. Type `,bt' for a backtrace or `,q' to continue.
scheme@(guile-user) [1]>
So let’s define the function, right there in the debugger. As you
probably know, we’ll want to return a 404 response.
;; Paste this in your REPL
(define (not-found request)
(values (build-response #:code 404)
(string-append "Resource not found: "
(uri->string (request-uri request)))))
;; Now paste this to let the web server keep going:
,continue
Now if you access ‘http://localhost/foo/’, you get this error
message. (Note that some popular web browsers won’t show
server-generated 404 messages, showing their own instead, unless the 404
message body is long enough.)
7.3.10.3 Higher-Level Interfaces
................................
The web handler interface is a common baseline that all kinds of Guile
web applications can use. You will usually want to build something on
top of it, however, especially when producing HTML. Here is a simple
example that builds up HTML output using SXML (*note SXML::).
First, load up the modules:
(use-modules (web server)
(web request)
(web response)
(sxml simple))
Now we define a simple templating function that takes a list of HTML
body elements, as SXML, and puts them in our super template:
(define (templatize title body)
`(html (head (title ,title))
(body ,@body)))
For example, the simplest Hello HTML can be produced like this:
(sxml->xml (templatize "Hello!" '((b "Hi!"))))
⊣
Hello! Hi!
Much better to work with Scheme data types than to work with HTML as
strings. Now we define a little response helper:
(define* (respond #:optional body #:key
(status 200)
(title "Hello hello!")
(doctype "\n")
(content-type-params '((charset . "utf-8")))
(content-type 'text/html)
(extra-headers '())
(sxml (and body (templatize title body))))
(values (build-response
#:code status
#:headers `((content-type
. (,content-type ,@content-type-params))
,@extra-headers))
(lambda (port)
(if sxml
(begin
(if doctype (display doctype port))
(sxml->xml sxml port))))))
Here we see the power of keyword arguments with default initializers.
By the time the arguments are fully parsed, the ‘sxml’ local variable
will hold the templated SXML, ready for sending out to the client.
Also, instead of returning the body as a string, ‘respond’ gives a
procedure, which will be called by the web server to write out the
response to the client.
Now, a simple example using this responder, which lays out the
incoming headers in an HTML table.
(define (debug-page request body)
(respond
`((h1 "hello world!")
(table
(tr (th "header") (th "value"))
,@(map (lambda (pair)
`(tr (td (tt ,(with-output-to-string
(lambda () (display (car pair))))))
(td (tt ,(with-output-to-string
(lambda ()
(write (cdr pair))))))))
(request-headers request))))))
(run-server debug-page)
Now if you visit any local address in your web browser, we actually
see some HTML, finally.
7.3.10.4 Conclusion
...................
Well, this is about as far as Guile’s built-in web support goes, for
now. There are many ways to make a web application, but hopefully by
standardizing the most fundamental data types, users will be able to
choose the approach that suits them best, while also being able to
switch between implementations of the server. This is a relatively new
part of Guile, so if you have feedback, let us know, and we can take it
into account. Happy hacking on the web!
7.4 The (ice-9 getopt-long) Module
==================================
The ‘(ice-9 getopt-long)’ facility is designed to help parse arguments
that are passed to Guile programs on the command line, and is modelled
after the C library’s facility of the same name (*note (libc)Getopt::).
For a more low-level interface to command-line argument parsing, *Note
SRFI-37::.
The ‘(ice-9 getopt-long)’ module exports two procedures:
‘getopt-long’ and ‘option-ref’.
• ‘getopt-long’ takes a list of strings — the command line arguments
— an “option specification”, and some optional keyword parameters.
It parses the command line arguments according to the option
specification and keyword parameters, and returns a data structure
that encapsulates the results of the parsing.
• ‘option-ref’ then takes the parsed data structure and a specific
option’s name, and returns information about that option in
particular.
To make these procedures available to your Guile script, include the
expression ‘(use-modules (ice-9 getopt-long))’ somewhere near the top,
before the first usage of ‘getopt-long’ or ‘option-ref’.
7.4.1 A Short getopt-long Example
---------------------------------
This section illustrates how ‘getopt-long’ is used by presenting and
dissecting a simple example. The first thing that we need is an “option
specification” that tells ‘getopt-long’ how to parse the command line.
This specification is an association list with the long option name as
the key. Here is how such a specification might look:
(define option-spec
'((version (single-char #\v) (value #f))
(help (single-char #\h) (value #f))))
This alist tells ‘getopt-long’ that it should accept two long
options, called _version_ and _help_, and that these options can also be
selected by the single-letter abbreviations _v_ and _h_, respectively.
The ‘(value #f)’ clauses indicate that neither of the options accepts a
value.
With this specification we can use ‘getopt-long’ to parse a given
command line:
(define options (getopt-long (command-line) option-spec))
After this call, ‘options’ contains the parsed command line and is
ready to be examined by ‘option-ref’. ‘option-ref’ is called like this:
(option-ref options 'help #f)
It expects the parsed command line, a symbol indicating the option to
examine, and a default value. The default value is returned if the
option was not present in the command line, or if the option was present
but without a value; otherwise the value from the command line is
returned. Usually ‘option-ref’ is called once for each possible option
that a script supports.
The following example shows a main program which puts all this
together to parse its command line and figure out what the user wanted.
(define (main args)
(let* ((option-spec '((version (single-char #\v) (value #f))
(help (single-char #\h) (value #f))))
(options (getopt-long args option-spec))
(help-wanted (option-ref options 'help #f))
(version-wanted (option-ref options 'version #f)))
(if (or version-wanted help-wanted)
(begin
(if version-wanted
(display "getopt-long-example version 0.3\n"))
(if help-wanted
(display "\
getopt-long-example [options]
-v, --version Display version
-h, --help Display this help
")))
(begin
(display "Hello, World!") (newline)))))
7.4.2 How to Write an Option Specification
------------------------------------------
An option specification is an association list (*note Association
Lists::) with one list element for each supported option. The key of
each list element is a symbol that names the option, while the value is
a list of option properties:
OPTION-SPEC ::= '( (OPT-NAME1 (PROP-NAME PROP-VALUE) ...)
(OPT-NAME2 (PROP-NAME PROP-VALUE) ...)
(OPT-NAME3 (PROP-NAME PROP-VALUE) ...)
...
)
Each OPT-NAME specifies the long option name for that option. For
example, a list element with OPT-NAME ‘background’ specifies an option
that can be specified on the command line using the long option
‘--background’. Further information about the option — whether it takes
a value, whether it is required to be present in the command line, and
so on — is specified by the option properties.
In the example of the preceding section, we already saw that a long
option name can have a equivalent “short option” character. The
equivalent short option character can be set for an option by specifying
a ‘single-char’ property in that option’s property list. For example, a
list element like ‘'(output (single-char #\o) ...)’ specifies an option
with long name ‘--output’ that can also be specified by the equivalent
short name ‘-o’.
The ‘value’ property specifies whether an option requires or accepts
a value. If the ‘value’ property is set to ‘#t’, the option requires a
value: ‘getopt-long’ will signal an error if the option name is present
without a corresponding value. If set to ‘#f’, the option does not take
a value; in this case, a non-option word that follows the option name in
the command line will be treated as a non-option argument. If set to
the symbol ‘optional’, the option accepts a value but does not require
one: a non-option word that follows the option name in the command line
will be interpreted as that option’s value. If the option name for an
option with ‘'(value optional)’ is immediately followed in the command
line by _another_ option name, the value for the first option is
implicitly ‘#t’.
The ‘required?’ property indicates whether an option is required to
be present in the command line. If the ‘required?’ property is set to
‘#t’, ‘getopt-long’ will signal an error if the option is not specified.
Finally, the ‘predicate’ property can be used to constrain the
possible values of an option. If used, the ‘predicate’ property should
be set to a procedure that takes one argument — the proposed option
value as a string — and returns either ‘#t’ or ‘#f’ according as the
proposed value is or is not acceptable. If the predicate procedure
returns ‘#f’, ‘getopt-long’ will signal an error.
By default, options do not have single-character equivalents, are not
required, and do not take values. Where the list element for an option
includes a ‘value’ property but no ‘predicate’ property, the option
values are unconstrained.
7.4.3 Expected Command Line Format
----------------------------------
In order for ‘getopt-long’ to correctly parse a command line, that
command line must conform to a standard set of rules for how command
line options are specified. This section explains what those rules are.
‘getopt-long’ splits a given command line into several pieces. All
elements of the argument list are classified to be either options or
normal arguments. Options consist of two dashes and an option name
(so-called “long” options), or of one dash followed by a single letter
(“short” options).
Options can behave as switches, when they are given without a value,
or they can be used to pass a value to the program. The value for an
option may be specified using an equals sign, or else is simply the next
word in the command line, so the following two invocations are
equivalent:
$ ./foo.scm --output=bar.txt
$ ./foo.scm --output bar.txt
Short options can be used instead of their long equivalents and can
be grouped together after a single dash. For example, the following
commands are equivalent.
$ ./foo.scm --version --help
$ ./foo.scm -v --help
$ ./foo.scm -vh
If an option requires a value, it can only be grouped together with
other short options if it is the last option in the group; the value is
the next argument. So, for example, with the following option
specification —
((apples (single-char #\a))
(blimps (single-char #\b) (value #t))
(catalexis (single-char #\c) (value #t)))
— the following command lines would all be acceptable:
$ ./foo.scm -a -b bang -c couth
$ ./foo.scm -ab bang -c couth
$ ./foo.scm -ac couth -b bang
But the next command line is an error, because ‘-b’ is not the last
option in its combination, and because a group of short options cannot
include two options that both require values:
$ ./foo.scm -abc couth bang
If an option’s value is optional, ‘getopt-long’ decides whether the
option has a value by looking at what follows it in the argument list.
If the next element is a string, and it does not appear to be an option
itself, then that string is the option’s value.
If the option ‘--’ appears in the argument list, argument parsing
stops there and subsequent arguments are returned as ordinary arguments,
even if they resemble options. So, with the command line
$ ./foo.scm --apples "Granny Smith" -- --blimp Goodyear
‘getopt-long’ will recognize the ‘--apples’ option as having the value
"Granny Smith", but will not treat ‘--blimp’ as an option. The strings
‘--blimp’ and ‘Goodyear’ will be returned as ordinary argument strings.
7.4.4 Reference Documentation for ‘getopt-long’
-----------------------------------------------
-- Scheme Procedure: getopt-long args grammar
[#:stop-at-first-non-option #f]
Parse the command line given in ARGS (which must be a list of
strings) according to the option specification GRAMMAR.
The GRAMMAR argument is expected to be a list of this form:
‘((OPTION (PROPERTY VALUE) ...) ...)’
where each OPTION is a symbol denoting the long option, but without
the two leading dashes (e.g. ‘version’ if the option is called
‘--version’).
For each option, there may be list of arbitrarily many
property/value pairs. The order of the pairs is not important, but
every property may only appear once in the property list. The
following table lists the possible properties:
‘(single-char CHAR)’
Accept ‘-CHAR’ as a single-character equivalent to ‘--OPTION’.
This is how to specify traditional Unix-style flags.
‘(required? BOOL)’
If BOOL is true, the option is required. ‘getopt-long’ will
raise an error if it is not found in ARGS.
‘(value BOOL)’
If BOOL is ‘#t’, the option accepts a value; if it is ‘#f’, it
does not; and if it is the symbol ‘optional’, the option may
appear in ARGS with or without a value.
‘(predicate FUNC)’
If the option accepts a value (i.e. you specified ‘(value #t)’
for this option), then ‘getopt-long’ will apply FUNC to the
value, and throw an exception if it returns ‘#f’. FUNC should
be a procedure which accepts a string and returns a boolean
value; you may need to use quasiquotes to get it into GRAMMAR.
The ‘#:stop-at-first-non-option’ keyword, if specified with any
true value, tells ‘getopt-long’ to stop when it gets to the first
non-option in the command line. That is, at the first word which
is neither an option itself, nor the value of an option.
Everything in the command line from that word onwards will be
returned as non-option arguments.
‘getopt-long’’s ARGS parameter is expected to be a list of strings
like the one returned by ‘command-line’, with the first element being
the name of the command. Therefore ‘getopt-long’ ignores the first
element in ARGS and starts argument interpretation with the second
element.
‘getopt-long’ signals an error if any of the following conditions
hold.
• The option grammar has an invalid syntax.
• One of the options in the argument list was not specified by the
grammar.
• A required option is omitted.
• An option which requires an argument did not get one.
• An option that doesn’t accept an argument does get one (this can
only happen using the long option ‘--opt=VALUE’ syntax).
• An option predicate fails.
‘#:stop-at-first-non-option’ is useful for command line invocations
like ‘guild [--help | --version] [script [script-options]]’ and ‘cvs
[general-options] command [command-options]’, where there are options at
two levels: some generic and understood by the outer command, and some
that are specific to the particular script or command being invoked. To
use ‘getopt-long’ in such cases, you would call it twice: firstly with
‘#:stop-at-first-non-option #t’, so as to parse any generic options and
identify the wanted script or sub-command; secondly, and after trimming
off the initial generic command words, with a script- or
sub-command-specific option grammar, so as to process those specific
options.
7.4.5 Reference Documentation for ‘option-ref’
----------------------------------------------
-- Scheme Procedure: option-ref options key default
Search OPTIONS for a command line option named KEY and return its
value, if found. If the option has no value, but was given, return
‘#t’. If the option was not given, return DEFAULT. OPTIONS must
be the result of a call to ‘getopt-long’.
‘option-ref’ always succeeds, either by returning the requested
option value from the command line, or the default value.
The special key ‘'()’ can be used to get a list of all non-option
arguments.
7.5 SRFI Support Modules
========================
SRFI is an acronym for Scheme Request For Implementation. The SRFI
documents define a lot of syntactic and procedure extensions to standard
Scheme as defined in R5RS.
Guile has support for a number of SRFIs. This chapter gives an
overview over the available SRFIs and some usage hints. For complete
documentation, design rationales and further examples, we advise you to
get the relevant SRFI documents from the SRFI home page
--> (rec )
--> (rec (+) )
The first syntax can be used to create self-referential expressions,
for example:
guile> (define tmp (rec ones (cons 1 (delay ones))))
The second syntax can be used to create anonymous recursive
functions:
guile> (define tmp (rec (display-n item n)
(if (positive? n)
(begin (display n) (display-n (- n 1))))))
guile> (tmp 42 3)
424242
guile>
7.5.23 SRFI-34 - Exception handling for programs
------------------------------------------------
Guile provides an implementation of SRFI-34’s exception handling
mechanisms (http://srfi.schemers.org/srfi-34/srfi-34.html) as an
alternative to its own built-in mechanisms (*note Exceptions::). It can
be made available as follows:
(use-modules (srfi srfi-34))
*Note Raising and Handling Exceptions::, for more on
‘with-exception-handler’ and ‘raise’ (known as ‘raise-exception’ in core
Guile).
SRFI-34’s ‘guard’ form is syntactic sugar over
‘with-exception-handler’:
-- Syntax: guard (var clause ...) body ...
Evaluate BODY with an exception handler that binds the raised
object to VAR and within the scope of that binding evaluates
CLAUSE... as if they were the clauses of a cond expression. That
implicit cond expression is evaluated with the continuation and
dynamic environment of the guard expression.
If every CLAUSE’s test evaluates to false and there is no ‘else’
clause, then ‘raise’ is re-invoked on the raised object within the
dynamic environment of the original call to ‘raise’ except that the
current exception handler is that of the ‘guard’ expression.
7.5.24 SRFI-35 - Conditions
---------------------------
SRFI-35 (http://srfi.schemers.org/srfi-35/srfi-35.html) defines
“conditions”, a data structure akin to records designed to convey
information about exceptional conditions between parts of a program. It
is normally used in conjunction with SRFI-34’s ‘raise’:
(raise (condition (&message
(message "An error occurred"))))
Users can define “condition types” containing arbitrary information.
Condition types may inherit from one another. This allows the part of
the program that handles (or “catches”) conditions to get accurate
information about the exceptional condition that arose.
SRFI-35 conditions are made available using:
(use-modules (srfi srfi-35))
The procedures available to manipulate condition types are the
following:
-- Scheme Procedure: make-condition-type id parent field-names
Return a new condition type named ID, inheriting from PARENT, and
with the fields whose names are listed in FIELD-NAMES. FIELD-NAMES
must be a list of symbols and must not contain names already used
by PARENT or one of its supertypes.
-- Scheme Procedure: condition-type? obj
Return true if OBJ is a condition type.
Conditions can be created and accessed with the following procedures:
-- Scheme Procedure: make-condition type . field+value
Return a new condition of type TYPE with fields initialized as
specified by FIELD+VALUE, a sequence of field names (symbols) and
values as in the following example:
(let ((&ct (make-condition-type 'foo &condition '(a b c))))
(make-condition &ct 'a 1 'b 2 'c 3))
Note that all fields of TYPE and its supertypes must be specified.
-- Scheme Procedure: make-compound-condition condition1 condition2 ...
Return a new compound condition composed of CONDITION1 CONDITION2
.... The returned condition has the type of each condition of
condition1 condition2 ... (per ‘condition-has-type?’).
-- Scheme Procedure: condition-has-type? c type
Return true if condition C has type TYPE.
-- Scheme Procedure: condition-ref c field-name
Return the value of the field named FIELD-NAME from condition C.
If C is a compound condition and several underlying condition types
contain a field named FIELD-NAME, then the value of the first such
field is returned, using the order in which conditions were passed
to ‘make-compound-condition’.
-- Scheme Procedure: extract-condition c type
Return a condition of condition type TYPE with the field values
specified by C.
If C is a compound condition, extract the field values from the
subcondition belonging to TYPE that appeared first in the call to
‘make-compound-condition’ that created the condition.
Convenience macros are also available to create condition types and
conditions.
-- library syntax: define-condition-type type supertype predicate
field-spec...
Define a new condition type named TYPE that inherits from
SUPERTYPE. In addition, bind PREDICATE to a type predicate that
returns true when passed a condition of type TYPE or any of its
subtypes. FIELD-SPEC must have the form ‘(field accessor)’ where
FIELD is the name of field of TYPE and ACCESSOR is the name of a
procedure to access field FIELD in conditions of type TYPE.
The example below defines condition type ‘&foo’, inheriting from
‘&condition’ with fields ‘a’, ‘b’ and ‘c’:
(define-condition-type &foo &condition
foo-condition?
(a foo-a)
(b foo-b)
(c foo-c))
-- library syntax: condition type-field-binding1 type-field-binding2
...
Return a new condition or compound condition, initialized according
to TYPE-FIELD-BINDING1 TYPE-FIELD-BINDING2 .... Each
TYPE-FIELD-BINDING must have the form ‘(type field-specs...)’,
where TYPE is the name of a variable bound to a condition type;
each FIELD-SPEC must have the form ‘(field-name value)’ where
FIELD-NAME is a symbol denoting the field being initialized to
VALUE. As for ‘make-condition’, all fields must be specified.
The following example returns a simple condition:
(condition (&message (message "An error occurred")))
The one below returns a compound condition:
(condition (&message (message "An error occurred"))
(&serious))
Finally, SRFI-35 defines a several standard condition types.
-- Variable: &condition
This condition type is the root of all condition types. It has no
fields.
-- Variable: &message
A condition type that carries a message describing the nature of
the condition to humans.
-- Scheme Procedure: message-condition? c
Return true if C is of type ‘&message’ or one of its subtypes.
-- Scheme Procedure: condition-message c
Return the message associated with message condition C.
-- Variable: &serious
This type describes conditions serious enough that they cannot
safely be ignored. It has no fields.
-- Scheme Procedure: serious-condition? c
Return true if C is of type ‘&serious’ or one of its subtypes.
-- Variable: &error
This condition describes errors, typically caused by something that
has gone wrong in the interaction of the program with the external
world or the user.
-- Scheme Procedure: error? c
Return true if C is of type ‘&error’ or one of its subtypes.
As an implementation note, condition objects in Guile are the same as
“exception objects”. *Note Exception Objects::. The ‘&condition’,
‘&serious’, and ‘&error’ condition types are known in core Guile as
‘&exception’, ‘&error’, and ‘&external-error’, respectively.
7.5.25 SRFI-37 - args-fold
--------------------------
This is a processor for GNU ‘getopt_long’-style program arguments. It
provides an alternative, less declarative interface than ‘getopt-long’
in ‘(ice-9 getopt-long)’ (*note The (ice-9 getopt-long) Module:
getopt-long.). Unlike ‘getopt-long’, it supports repeated options and
any number of short and long names per option. Access it with:
(use-modules (srfi srfi-37))
SRFI-37 principally provides an ‘option’ type and the ‘args-fold’
function. To use the library, create a set of options with ‘option’ and
use it as a specification for invoking ‘args-fold’.
Here is an example of a simple argument processor for the typical
‘--version’ and ‘--help’ options, which returns a backwards list of
files given on the command line:
(args-fold (cdr (program-arguments))
(let ((display-and-exit-proc
(lambda (msg)
(lambda (opt name arg loads)
(display msg) (quit)))))
(list (option '(#\v "version") #f #f
(display-and-exit-proc "Foo version 42.0\n"))
(option '(#\h "help") #f #f
(display-and-exit-proc
"Usage: foo scheme-file ..."))))
(lambda (opt name arg loads)
(error "Unrecognized option `~A'" name))
(lambda (op loads) (cons op loads))
'())
-- Scheme Procedure: option names required-arg? optional-arg? processor
Return an object that specifies a single kind of program option.
NAMES is a list of command-line option names, and should consist of
characters for traditional ‘getopt’ short options and strings for
‘getopt_long’-style long options.
REQUIRED-ARG? and OPTIONAL-ARG? are mutually exclusive; one or both
must be ‘#f’. If REQUIRED-ARG?, the option must be followed by an
argument on the command line, such as ‘--opt=value’ for long
options, or an error will be signalled. If OPTIONAL-ARG?, an
argument will be taken if available.
PROCESSOR is a procedure that takes at least 3 arguments, called
when ‘args-fold’ encounters the option: the containing option
object, the name used on the command line, and the argument given
for the option (or ‘#f’ if none). The rest of the arguments are
‘args-fold’ “seeds”, and the PROCESSOR should return seeds as well.
-- Scheme Procedure: option-names opt
-- Scheme Procedure: option-required-arg? opt
-- Scheme Procedure: option-optional-arg? opt
-- Scheme Procedure: option-processor opt
Return the specified field of OPT, an option object, as described
above for ‘option’.
-- Scheme Procedure: args-fold args options unrecognized-option-proc
operand-proc seed ...
Process ARGS, a list of program arguments such as that returned by
‘(cdr (program-arguments))’, in order against OPTIONS, a list of
option objects as described above. All functions called take the
“seeds”, or the last multiple-values as multiple arguments,
starting with SEED ..., and must return the new seeds. Return the
final seeds.
Call ‘unrecognized-option-proc’, which is like an option object’s
processor, for any options not found in OPTIONS.
Call ‘operand-proc’ with any items on the command line that are not
named options. This includes arguments after ‘--’. It is called
with the argument in question, as well as the seeds.
7.5.26 SRFI-38 - External Representation for Data With Shared Structure
-----------------------------------------------------------------------
This subsection is based on the specification of SRFI-38
(http://srfi.schemers.org/srfi-38/srfi-38.html) written by Ray
Dillinger.
This SRFI creates an alternative external representation for data
written and read using ‘write-with-shared-structure’ and
‘read-with-shared-structure’. It is identical to the grammar for
external representation for data written and read with ‘write’ and
‘read’ given in section 7 of R5RS, except that the single production
--> |
is replaced by the following five productions:
--> | |
--> #=
--> ##
--> |
--> +
-- Scheme procedure: write-with-shared-structure obj
-- Scheme procedure: write-with-shared-structure obj port
-- Scheme procedure: write-with-shared-structure obj port optarg
Writes an external representation of OBJ to the given port.
Strings that appear in the written representation are enclosed in
doublequotes, and within those strings backslash and doublequote
characters are escaped by backslashes. Character objects are
written using the ‘#\’ notation.
Objects which denote locations rather than values (cons cells,
vectors, and non-zero-length strings in R5RS scheme; also Guile’s
structs, bytevectors and ports and hash-tables), if they appear at
more than one point in the data being written, are preceded by
‘#N=’ the first time they are written and replaced by ‘#N#’ all
subsequent times they are written, where N is a natural number used
to identify that particular object. If objects which denote
locations occur only once in the structure, then
‘write-with-shared-structure’ must produce the same external
representation for those objects as ‘write’.
‘write-with-shared-structure’ terminates in finite time and
produces a finite representation when writing finite data.
‘write-with-shared-structure’ returns an unspecified value. The
PORT argument may be omitted, in which case it defaults to the
value returned by ‘(current-output-port)’. The OPTARG argument may
also be omitted. If present, its effects on the output and return
value are unspecified but ‘write-with-shared-structure’ must still
write a representation that can be read by
‘read-with-shared-structure’. Some implementations may wish to use
OPTARG to specify formatting conventions, numeric radixes, or
return values. Guile’s implementation ignores OPTARG.
For example, the code
(begin (define a (cons 'val1 'val2))
(set-cdr! a a)
(write-with-shared-structure a))
should produce the output ‘#1=(val1 . #1#)’. This shows a cons
cell whose ‘cdr’ contains itself.
-- Scheme procedure: read-with-shared-structure
-- Scheme procedure: read-with-shared-structure port
‘read-with-shared-structure’ converts the external representations
of Scheme objects produced by ‘write-with-shared-structure’ into
Scheme objects. That is, it is a parser for the nonterminal
‘’ in the augmented external representation grammar defined
above. ‘read-with-shared-structure’ returns the next object
parsable from the given input port, updating PORT to point to the
first character past the end of the external representation of the
object.
If an end-of-file is encountered in the input before any characters
are found that can begin an object, then an end-of-file object is
returned. The port remains open, and further attempts to read it
(by ‘read-with-shared-structure’ or ‘read’ will also return an
end-of-file object. If an end of file is encountered after the
beginning of an object’s external representation, but the external
representation is incomplete and therefore not parsable, an error
is signalled.
The PORT argument may be omitted, in which case it defaults to the
value returned by ‘(current-input-port)’. It is an error to read
from a closed port.
7.5.27 SRFI-39 - Parameters
---------------------------
This SRFI adds support for dynamically-scoped parameters. SRFI 39 is
implemented in the Guile core; there’s no module needed to get SRFI-39
itself. Parameters are documented in *note Parameters::.
This module does export one extra function: ‘with-parameters*’. This
is a Guile-specific addition to the SRFI, similar to the core
‘with-fluids*’ (*note Fluids and Dynamic States::).
-- Function: with-parameters* param-list value-list thunk
Establish a new dynamic scope, as per ‘parameterize’ above, taking
parameters from PARAM-LIST and corresponding values from
VALUE-LIST. A call ‘(THUNK)’ is made in the new scope and the
result from that THUNK is the return from ‘with-parameters*’.
7.5.28 SRFI-41 - Streams
------------------------
This subsection is based on the specification of SRFI-41
(http://srfi.schemers.org/srfi-41/srfi-41.html) by Philip L. Bewig.
This SRFI implements streams, sometimes called lazy lists, a sequential
data structure containing elements computed only on demand. A stream is
either null or is a pair with a stream in its cdr. Since elements of a
stream are computed only when accessed, streams can be infinite. Once
computed, the value of a stream element is cached in case it is needed
again. SRFI-41 can be made available with:
(use-modules (srfi srfi-41))
7.5.28.1 SRFI-41 Stream Fundamentals
....................................
SRFI-41 Streams are based on two mutually-recursive abstract data types:
An object of the ‘stream’ abstract data type is a promise that, when
forced, is either ‘stream-null’ or is an object of type ‘stream-pair’.
An object of the ‘stream-pair’ abstract data type contains a
‘stream-car’ and a ‘stream-cdr’, which must be a ‘stream’. The
essential feature of streams is the systematic suspensions of the
recursive promises between the two data types.
The object stored in the ‘stream-car’ of a ‘stream-pair’ is a promise
that is forced the first time the ‘stream-car’ is accessed; its value is
cached in case it is needed again. The object may have any type, and
different stream elements may have different types. If the ‘stream-car’
is never accessed, the object stored there is never evaluated.
Likewise, the ‘stream-cdr’ is a promise to return a stream, and is only
forced on demand.
7.5.28.2 SRFI-41 Stream Primitives
..................................
This library provides eight operators: constructors for ‘stream-null’
and ‘stream-pair’s, type predicates for streams and the two kinds of
streams, accessors for both fields of a ‘stream-pair’, and a lambda that
creates procedures that return streams.
-- Scheme Variable: stream-null
A promise that, when forced, is a single object, distinguishable
from all other objects, that represents the null stream.
‘stream-null’ is immutable and unique.
-- Scheme Syntax: stream-cons object-expr stream-expr
Creates a newly-allocated stream containing a promise that, when
forced, is a ‘stream-pair’ with OBJECT-EXPR in its ‘stream-car’ and
STREAM-EXPR in its ‘stream-cdr’. Neither OBJECT-EXPR nor
STREAM-EXPR is evaluated when ‘stream-cons’ is called.
Once created, a ‘stream-pair’ is immutable; there is no
‘stream-set-car!’ or ‘stream-set-cdr!’ that modifies an existing
stream-pair. There is no dotted-pair or improper stream as with
lists.
-- Scheme Procedure: stream? object
Returns true if OBJECT is a stream, otherwise returns false. If
OBJECT is a stream, its promise will not be forced. If ‘(stream?
obj)’ returns true, then one of ‘(stream-null? obj)’ or
‘(stream-pair? obj)’ will return true and the other will return
false.
-- Scheme Procedure: stream-null? object
Returns true if OBJECT is the distinguished null stream, otherwise
returns false. If OBJECT is a stream, its promise will be forced.
-- Scheme Procedure: stream-pair? object
Returns true if OBJECT is a ‘stream-pair’ constructed by
‘stream-cons’, otherwise returns false. If OBJECT is a stream, its
promise will be forced.
-- Scheme Procedure: stream-car stream
Returns the object stored in the ‘stream-car’ of STREAM. An error
is signalled if the argument is not a ‘stream-pair’. This causes
the OBJECT-EXPR passed to ‘stream-cons’ to be evaluated if it had
not yet been; the value is cached in case it is needed again.
-- Scheme Procedure: stream-cdr stream
Returns the stream stored in the ‘stream-cdr’ of STREAM. An error
is signalled if the argument is not a ‘stream-pair’.
-- Scheme Syntax: stream-lambda formals body ...
Creates a procedure that returns a promise to evaluate the BODY of
the procedure. The last BODY expression to be evaluated must yield
a stream. As with normal ‘lambda’, FORMALS may be a single
variable name, in which case all the formal arguments are collected
into a single list, or a list of variable names, which may be null
if there are no arguments, proper if there are an exact number of
arguments, or dotted if a fixed number of arguments is to be
followed by zero or more arguments collected into a list. BODY
must contain at least one expression, and may contain internal
definitions preceding any expressions to be evaluated.
(define strm123
(stream-cons 1
(stream-cons 2
(stream-cons 3
stream-null))))
(stream-car strm123) ⇒ 1
(stream-car (stream-cdr strm123) ⇒ 2
(stream-pair?
(stream-cdr
(stream-cons (/ 1 0) stream-null))) ⇒ #f
(stream? (list 1 2 3)) ⇒ #f
(define iter
(stream-lambda (f x)
(stream-cons x (iter f (f x)))))
(define nats (iter (lambda (x) (+ x 1)) 0))
(stream-car (stream-cdr nats)) ⇒ 1
(define stream-add
(stream-lambda (s1 s2)
(stream-cons
(+ (stream-car s1) (stream-car s2))
(stream-add (stream-cdr s1)
(stream-cdr s2)))))
(define evens (stream-add nats nats))
(stream-car evens) ⇒ 0
(stream-car (stream-cdr evens)) ⇒ 2
(stream-car (stream-cdr (stream-cdr evens))) ⇒ 4
7.5.28.3 SRFI-41 Stream Library
...............................
-- Scheme Syntax: define-stream (name args ...) body ...
Creates a procedure that returns a stream, and may appear anywhere
a normal ‘define’ may appear, including as an internal definition.
It may contain internal definitions of its own. The defined
procedure takes arguments in the same way as ‘stream-lambda’.
‘define-stream’ is syntactic sugar on ‘stream-lambda’; see also
‘stream-let’, which is also a sugaring of ‘stream-lambda’.
A simple version of ‘stream-map’ that takes only a single input
stream calls itself recursively:
(define-stream (stream-map proc strm)
(if (stream-null? strm)
stream-null
(stream-cons
(proc (stream-car strm))
(stream-map proc (stream-cdr strm))))))
-- Scheme Procedure: list->stream list
Returns a newly-allocated stream containing the elements from LIST.
-- Scheme Procedure: port->stream [port]
Returns a newly-allocated stream containing in its elements the
characters on the port. If PORT is not given it defaults to the
current input port. The returned stream has finite length and is
terminated by ‘stream-null’.
It looks like one use of ‘port->stream’ would be this:
(define s ;wrong!
(with-input-from-file filename
(lambda () (port->stream))))
But that fails, because ‘with-input-from-file’ is eager, and closes
the input port prematurely, before the first character is read. To
read a file into a stream, say:
(define-stream (file->stream filename)
(let ((p (open-input-file filename)))
(stream-let loop ((c (read-char p)))
(if (eof-object? c)
(begin (close-input-port p)
stream-null)
(stream-cons c
(loop (read-char p)))))))
-- Scheme Syntax: stream object-expr ...
Creates a newly-allocated stream containing in its elements the
objects, in order. The OBJECT-EXPRs are evaluated when they are
accessed, not when the stream is created. If no objects are given,
as in (stream), the null stream is returned. See also
‘list->stream’.
(define strm123 (stream 1 2 3))
; (/ 1 0) not evaluated when stream is created
(define s (stream 1 (/ 1 0) -1))
-- Scheme Procedure: stream->list [n] stream
Returns a newly-allocated list containing in its elements the first
N items in STREAM. If STREAM has less than N items, all the items
in the stream will be included in the returned list. If N is not
given it defaults to infinity, which means that unless STREAM is
finite ‘stream->list’ will never return.
(stream->list 10
(stream-map (lambda (x) (* x x))
(stream-from 0)))
⇒ (0 1 4 9 16 25 36 49 64 81)
-- Scheme Procedure: stream-append stream ...
Returns a newly-allocated stream containing in its elements those
elements contained in its input STREAMs, in order of input. If any
of the input streams is infinite, no elements of any of the
succeeding input streams will appear in the output stream. See
also ‘stream-concat’.
-- Scheme Procedure: stream-concat stream
Takes a STREAM consisting of one or more streams and returns a
newly-allocated stream containing all the elements of the input
streams. If any of the streams in the input STREAM is infinite,
any remaining streams in the input stream will never appear in the
output stream. See also ‘stream-append’.
-- Scheme Procedure: stream-constant object ...
Returns a newly-allocated stream containing in its elements the
OBJECTs, repeating in succession forever.
(stream-constant 1) ⇒ 1 1 1 ...
(stream-constant #t #f) ⇒ #t #f #t #f #t #f ...
-- Scheme Procedure: stream-drop n stream
Returns the suffix of the input STREAM that starts at the next
element after the first N elements. The output stream shares
structure with the input STREAM; thus, promises forced in one
instance of the stream are also forced in the other instance of the
stream. If the input STREAM has less than N elements,
‘stream-drop’ returns the null stream. See also ‘stream-take’.
-- Scheme Procedure: stream-drop-while pred stream
Returns the suffix of the input STREAM that starts at the first
element X for which ‘(pred x)’ returns false. The output stream
shares structure with the input STREAM. See also
‘stream-take-while’.
-- Scheme Procedure: stream-filter pred stream
Returns a newly-allocated stream that contains only those elements
X of the input STREAM which satisfy the predicate ‘pred’.
(stream-filter odd? (stream-from 0))
⇒ 1 3 5 7 9 ...
-- Scheme Procedure: stream-fold proc base stream
Applies a binary procedure PROC to BASE and the first element of
STREAM to compute a new BASE, then applies the procedure to the new
BASE and the next element of STREAM to compute a succeeding BASE,
and so on, accumulating a value that is finally returned as the
value of ‘stream-fold’ when the end of the stream is reached.
STREAM must be finite, or ‘stream-fold’ will enter an infinite
loop. See also ‘stream-scan’, which is similar to ‘stream-fold’,
but useful for infinite streams. For readers familiar with other
functional languages, this is a left-fold; there is no
corresponding right-fold, since right-fold relies on finite streams
that are fully-evaluated, in which case they may as well be
converted to a list.
-- Scheme Procedure: stream-for-each proc stream ...
Applies PROC element-wise to corresponding elements of the input
STREAMs for side-effects; it returns nothing. ‘stream-for-each’
stops as soon as any of its input streams is exhausted.
-- Scheme Procedure: stream-from first [step]
Creates a newly-allocated stream that contains FIRST as its first
element and increments each succeeding element by STEP. If STEP is
not given it defaults to 1. FIRST and STEP may be of any numeric
type. ‘stream-from’ is frequently useful as a generator in
‘stream-of’ expressions. See also ‘stream-range’ for a similar
procedure that creates finite streams.
-- Scheme Procedure: stream-iterate proc base
Creates a newly-allocated stream containing BASE in its first
element and applies PROC to each element in turn to determine the
succeeding element. See also ‘stream-unfold’ and ‘stream-unfolds’.
-- Scheme Procedure: stream-length stream
Returns the number of elements in the STREAM; it does not evaluate
its elements. ‘stream-length’ may only be used on finite streams;
it enters an infinite loop with infinite streams.
-- Scheme Syntax: stream-let tag ((var expr) ...) body ...
Creates a local scope that binds each variable to the value of its
corresponding expression. It additionally binds TAG to a procedure
which takes the bound variables as arguments and BODY as its
defining expressions, binding the TAG with ‘stream-lambda’. TAG is
in scope within body, and may be called recursively. When the
expanded expression defined by the ‘stream-let’ is evaluated,
‘stream-let’ evaluates the expressions in its BODY in an
environment containing the newly-bound variables, returning the
value of the last expression evaluated, which must yield a stream.
‘stream-let’ provides syntactic sugar on ‘stream-lambda’, in the
same manner as normal ‘let’ provides syntactic sugar on normal
‘lambda’. However, unlike normal ‘let’, the TAG is required, not
optional, because unnamed ‘stream-let’ is meaningless.
For example, ‘stream-member’ returns the first ‘stream-pair’ of the
input STRM with a ‘stream-car’ X that satisfies ‘(eql? obj x)’, or
the null stream if X is not present in STRM.
(define-stream (stream-member eql? obj strm)
(stream-let loop ((strm strm))
(cond ((stream-null? strm) strm)
((eql? obj (stream-car strm)) strm)
(else (loop (stream-cdr strm))))))
-- Scheme Procedure: stream-map proc stream ...
Applies PROC element-wise to corresponding elements of the input
STREAMs, returning a newly-allocated stream containing elements
that are the results of those procedure applications. The output
stream has as many elements as the minimum-length input stream, and
may be infinite.
-- Scheme Syntax: stream-match stream clause ...
Provides pattern-matching for streams. The input STREAM is an
expression that evaluates to a stream. Clauses are of the form
‘(pattern [fender] expression)’, consisting of a PATTERN that
matches a stream of a particular shape, an optional FENDER that
must succeed if the pattern is to match, and an EXPRESSION that is
evaluated if the pattern matches. There are four types of
patterns:
• () matches the null stream.
• (PAT0 PAT1 ...) matches a finite stream with length exactly
equal to the number of pattern elements.
• (PAT0 PAT1 ... ‘.’ PAT-REST) matches an infinite stream, or a
finite stream with length at least as great as the number of
pattern elements before the literal dot.
• PAT matches an entire stream. Should always appear last in
the list of clauses; it’s not an error to appear elsewhere,
but subsequent clauses could never match.
Each pattern element may be either:
• An identifier, which matches any stream element.
Additionally, the value of the stream element is bound to the
variable named by the identifier, which is in scope in the
FENDER and EXPRESSION of the corresponding CLAUSE. Each
identifier in a single pattern must be unique.
• A literal underscore (‘_’), which matches any stream element
but creates no bindings.
The PATTERNs are tested in order, left-to-right, until a matching
pattern is found; if FENDER is present, it must evaluate to a true
value for the match to be successful. Pattern variables are bound
in the corresponding FENDER and EXPRESSION. Once the matching
PATTERN is found, the corresponding EXPRESSION is evaluated and
returned as the result of the match. An error is signaled if no
pattern matches the input STREAM.
‘stream-match’ is often used to distinguish null streams from
non-null streams, binding HEAD and TAIL:
(define (len strm)
(stream-match strm
(() 0)
((head . tail) (+ 1 (len tail)))))
Fenders can test the common case where two stream elements must be
identical; the ‘else’ pattern is an identifier bound to the entire
stream, not a keyword as in ‘cond’.
(stream-match strm
((x y . _) (equal? x y) 'ok)
(else 'error))
A more complex example uses two nested matchers to match two
different stream arguments; ‘(stream-merge lt? . strms)’ stably
merges two or more streams ordered by the ‘lt?’ predicate:
(define-stream (stream-merge lt? . strms)
(define-stream (merge xx yy)
(stream-match xx (() yy) ((x . xs)
(stream-match yy (() xx) ((y . ys)
(if (lt? y x)
(stream-cons y (merge xx ys))
(stream-cons x (merge xs yy))))))))
(stream-let loop ((strms strms))
(cond ((null? strms) stream-null)
((null? (cdr strms)) (car strms))
(else (merge (car strms)
(apply stream-merge lt?
(cdr strms)))))))
-- Scheme Syntax: stream-of expr clause ...
Provides the syntax of stream comprehensions, which generate
streams by means of looping expressions. The result is a stream of
objects of the type returned by EXPR. There are four types of
clauses:
• (VAR ‘in’ STREAM-EXPR) loops over the elements of STREAM-EXPR,
in order from the start of the stream, binding each element of
the stream in turn to VAR. ‘stream-from’ and ‘stream-range’
are frequently useful as generators for STREAM-EXPR.
• (VAR ‘is’ EXPR) binds VAR to the value obtained by evaluating
EXPR.
• (PRED EXPR) includes in the output stream only those elements
X which satisfy the predicate PRED.
The scope of variables bound in the stream comprehension is the
clauses to the right of the binding clause (but not the binding
clause itself) plus the result expression.
When two or more generators are present, the loops are processed as
if they are nested from left to right; that is, the rightmost
generator varies fastest. A consequence of this is that only the
first generator may be infinite and all subsequent generators must
be finite. If no generators are present, the result of a stream
comprehension is a stream containing the result expression; thus,
‘(stream-of 1)’ produces a finite stream containing only the
element 1.
(stream-of (* x x)
(x in (stream-range 0 10))
(even? x))
⇒ 0 4 16 36 64
(stream-of (list a b)
(a in (stream-range 1 4))
(b in (stream-range 1 3)))
⇒ (1 1) (1 2) (2 1) (2 2) (3 1) (3 2)
(stream-of (list i j)
(i in (stream-range 1 5))
(j in (stream-range (+ i 1) 5)))
⇒ (1 2) (1 3) (1 4) (2 3) (2 4) (3 4)
-- Scheme Procedure: stream-range first past [step]
Creates a newly-allocated stream that contains FIRST as its first
element and increments each succeeding element by STEP. The stream
is finite and ends before PAST, which is not an element of the
stream. If STEP is not given it defaults to 1 if FIRST is less
than past and -1 otherwise. FIRST, PAST and STEP may be of any
real numeric type. ‘stream-range’ is frequently useful as a
generator in ‘stream-of’ expressions. See also ‘stream-from’ for a
similar procedure that creates infinite streams.
(stream-range 0 10) ⇒ 0 1 2 3 4 5 6 7 8 9
(stream-range 0 10 2) ⇒ 0 2 4 6 8
Successive elements of the stream are calculated by adding STEP to
FIRST, so if any of FIRST, PAST or STEP are inexact, the length of
the output stream may differ from ‘(ceiling (- (/ (- past first)
step) 1)’.
-- Scheme Procedure: stream-ref stream n
Returns the Nth element of stream, counting from zero. An error is
signaled if N is greater than or equal to the length of stream.
(define (fact n)
(stream-ref
(stream-scan * 1 (stream-from 1))
n))
-- Scheme Procedure: stream-reverse stream
Returns a newly-allocated stream containing the elements of the
input STREAM but in reverse order. ‘stream-reverse’ may only be
used with finite streams; it enters an infinite loop with infinite
streams. ‘stream-reverse’ does not force evaluation of the
elements of the stream.
-- Scheme Procedure: stream-scan proc base stream
Accumulates the partial folds of an input STREAM into a
newly-allocated output stream. The output stream is the BASE
followed by ‘(stream-fold proc base (stream-take i stream))’ for
each of the first I elements of STREAM.
(stream-scan + 0 (stream-from 1))
⇒ (stream 0 1 3 6 10 15 ...)
(stream-scan * 1 (stream-from 1))
⇒ (stream 1 1 2 6 24 120 ...)
-- Scheme Procedure: stream-take n stream
Returns a newly-allocated stream containing the first N elements of
the input STREAM. If the input STREAM has less than N elements, so
does the output stream. See also ‘stream-drop’.
-- Scheme Procedure: stream-take-while pred stream
Takes a predicate and a ‘stream’ and returns a newly-allocated
stream containing those elements ‘x’ that form the maximal prefix
of the input stream which satisfy PRED. See also
‘stream-drop-while’.
-- Scheme Procedure: stream-unfold map pred gen base
The fundamental recursive stream constructor. It constructs a
stream by repeatedly applying GEN to successive values of BASE, in
the manner of ‘stream-iterate’, then applying MAP to each of the
values so generated, appending each of the mapped values to the
output stream as long as ‘(pred? base)’ returns a true value. See
also ‘stream-iterate’ and ‘stream-unfolds’.
The expression below creates the finite stream ‘0 1 4 9 16 25 36 49
64 81’. Initially the BASE is 0, which is less than 10, so MAP
squares the BASE and the mapped value becomes the first element of
the output stream. Then GEN increments the BASE by 1, so it
becomes 1; this is less than 10, so MAP squares the new BASE and 1
becomes the second element of the output stream. And so on, until
the base becomes 10, when PRED stops the recursion and stream-null
ends the output stream.
(stream-unfold
(lambda (x) (expt x 2)) ; map
(lambda (x) (< x 10)) ; pred?
(lambda (x) (+ x 1)) ; gen
0) ; base
-- Scheme Procedure: stream-unfolds proc seed
Returns N newly-allocated streams containing those elements
produced by successive calls to the generator PROC, which takes the
current SEED as its argument and returns N+1 values
(PROC SEED) ⇒ SEED RESULT_0 ... RESULT_N-1
where the returned SEED is the input SEED to the next call to the
generator and RESULT_I indicates how to produce the next element of
the Ith result stream:
• (VALUE): VALUE is the next car of the result stream.
• ‘#f’: no value produced by this iteration of the generator
PROC for the result stream.
• (): the end of the result stream.
It may require multiple calls of PROC to produce the next element
of any particular result stream. See also ‘stream-iterate’ and
‘stream-unfold’.
(define (stream-partition pred? strm)
(stream-unfolds
(lambda (s)
(if (stream-null? s)
(values s '() '())
(let ((a (stream-car s))
(d (stream-cdr s)))
(if (pred? a)
(values d (list a) #f)
(values d #f (list a))))))
strm))
(call-with-values
(lambda ()
(stream-partition odd?
(stream-range 1 6)))
(lambda (odds evens)
(list (stream->list odds)
(stream->list evens))))
⇒ ((1 3 5) (2 4))
-- Scheme Procedure: stream-zip stream ...
Returns a newly-allocated stream in which each element is a list
(not a stream) of the corresponding elements of the input STREAMs.
The output stream is as long as the shortest input STREAM, if any
of the input STREAMs is finite, or is infinite if all the input
STREAMs are infinite.
7.5.29 SRFI-42 - Eager Comprehensions
-------------------------------------
See the specification of SRFI-42
(http://srfi.schemers.org/srfi-42/srfi-42.html).
7.5.30 SRFI-43 - Vector Library
-------------------------------
This subsection is based on the specification of SRFI-43
(http://srfi.schemers.org/srfi-43/srfi-43.html) by Taylor Campbell.
SRFI-43 implements a comprehensive library of vector operations. It can
be made available with:
(use-modules (srfi srfi-43))
7.5.30.1 SRFI-43 Constructors
.............................
-- Scheme Procedure: make-vector size [fill]
Create and return a vector of size SIZE, optionally filling it with
FILL. The default value of FILL is unspecified.
(make-vector 5 3) ⇒ #(3 3 3 3 3)
-- Scheme Procedure: vector x ...
Create and return a vector whose elements are X ....
(vector 0 1 2 3 4) ⇒ #(0 1 2 3 4)
-- Scheme Procedure: vector-unfold f length initial-seed ...
The fundamental vector constructor. Create a vector whose length
is LENGTH and iterates across each index k from 0 up to LENGTH - 1,
applying F at each iteration to the current index and current
seeds, in that order, to receive n + 1 values: the element to put
in the kth slot of the new vector, and n new seeds for the next
iteration. It is an error for the number of seeds to vary between
iterations.
(vector-unfold (lambda (i x) (values x (- x 1)))
10 0)
⇒ #(0 -1 -2 -3 -4 -5 -6 -7 -8 -9)
(vector-unfold values 10)
⇒ #(0 1 2 3 4 5 6 7 8 9)
-- Scheme Procedure: vector-unfold-right f length initial-seed ...
Like ‘vector-unfold’, but it uses F to generate elements from
right-to-left, rather than left-to-right.
(vector-unfold-right (lambda (i x) (values x (+ x 1)))
10 0)
⇒ #(9 8 7 6 5 4 3 2 1 0)
-- Scheme Procedure: vector-copy vec [start [end [fill]]]
Allocate a new vector whose length is END - START and fills it with
elements from VEC, taking elements from VEC starting at index START
and stopping at index END. START defaults to 0 and END defaults to
the value of ‘(vector-length vec)’. If END extends beyond the
length of VEC, the slots in the new vector that obviously cannot be
filled by elements from VEC are filled with FILL, whose default
value is unspecified.
(vector-copy '#(a b c d e f g h i))
⇒ #(a b c d e f g h i)
(vector-copy '#(a b c d e f g h i) 6)
⇒ #(g h i)
(vector-copy '#(a b c d e f g h i) 3 6)
⇒ #(d e f)
(vector-copy '#(a b c d e f g h i) 6 12 'x)
⇒ #(g h i x x x)
-- Scheme Procedure: vector-reverse-copy vec [start [end]]
Like ‘vector-copy’, but it copies the elements in the reverse order
from VEC.
(vector-reverse-copy '#(5 4 3 2 1 0) 1 5)
⇒ #(1 2 3 4)
-- Scheme Procedure: vector-append vec ...
Return a newly allocated vector that contains all elements in order
from the subsequent locations in VEC ....
(vector-append '#(a) '#(b c d))
⇒ #(a b c d)
-- Scheme Procedure: vector-concatenate list-of-vectors
Append each vector in LIST-OF-VECTORS. Equivalent to ‘(apply
vector-append list-of-vectors)’.
(vector-concatenate '(#(a b) #(c d)))
⇒ #(a b c d)
7.5.30.2 SRFI-43 Predicates
...........................
-- Scheme Procedure: vector? obj
Return true if OBJ is a vector, else return false.
-- Scheme Procedure: vector-empty? vec
Return true if VEC is empty, i.e. its length is 0, else return
false.
-- Scheme Procedure: vector= elt=? vec ...
Return true if the vectors VEC ... have equal lengths and equal
elements according to ELT=?. ELT=? is always applied to two
arguments. Element comparison must be consistent with ‘eq?’ in the
following sense: if ‘(eq? a b)’ returns true, then ‘(elt=? a b)’
must also return true. The order in which comparisons are
performed is unspecified.
7.5.30.3 SRFI-43 Selectors
..........................
-- Scheme Procedure: vector-ref vec i
Return the element at index I in VEC. Indexing is based on zero.
-- Scheme Procedure: vector-length vec
Return the length of VEC.
7.5.30.4 SRFI-43 Iteration
..........................
-- Scheme Procedure: vector-fold kons knil vec1 vec2 ...
The fundamental vector iterator. KONS is iterated over each index
in all of the vectors, stopping at the end of the shortest; KONS is
applied as
(kons i state (vector-ref vec1 i) (vector-ref vec2 i) ...)
where STATE is the current state value, and I is the current index.
The current state value begins with KNIL, and becomes whatever KONS
returned at the respective iteration. The iteration is strictly
left-to-right.
-- Scheme Procedure: vector-fold-right kons knil vec1 vec2 ...
Similar to ‘vector-fold’, but it iterates right-to-left instead of
left-to-right.
-- Scheme Procedure: vector-map f vec1 vec2 ...
Return a new vector of the shortest size of the vector arguments.
Each element at index i of the new vector is mapped from the old
vectors by
(f i (vector-ref vec1 i) (vector-ref vec2 i) ...)
The dynamic order of application of F is unspecified.
-- Scheme Procedure: vector-map! f vec1 vec2 ...
Similar to ‘vector-map’, but rather than mapping the new elements
into a new vector, the new mapped elements are destructively
inserted into VEC1. The dynamic order of application of F is
unspecified.
-- Scheme Procedure: vector-for-each f vec1 vec2 ...
Call ‘(f i (vector-ref vec1 i) (vector-ref vec2 i) ...)’ for each
index i less than the length of the shortest vector passed. The
iteration is strictly left-to-right.
-- Scheme Procedure: vector-count pred? vec1 vec2 ...
Count the number of parallel elements in the vectors that satisfy
PRED?, which is applied, for each index i less than the length of
the smallest vector, to i and each parallel element in the vectors
at that index, in order.
(vector-count (lambda (i elt) (even? elt))
'#(3 1 4 1 5 9 2 5 6))
⇒ 3
(vector-count (lambda (i x y) (< x y))
'#(1 3 6 9) '#(2 4 6 8 10 12))
⇒ 2
7.5.30.5 SRFI-43 Searching
..........................
-- Scheme Procedure: vector-index pred? vec1 vec2 ...
Find and return the index of the first elements in VEC1 VEC2 ...
that satisfy PRED?. If no matching element is found by the end of
the shortest vector, return ‘#f’.
(vector-index even? '#(3 1 4 1 5 9))
⇒ 2
(vector-index < '#(3 1 4 1 5 9 2 5 6) '#(2 7 1 8 2))
⇒ 1
(vector-index = '#(3 1 4 1 5 9 2 5 6) '#(2 7 1 8 2))
⇒ #f
-- Scheme Procedure: vector-index-right pred? vec1 vec2 ...
Like ‘vector-index’, but it searches right-to-left, rather than
left-to-right. Note that the SRFI 43 specification requires that
all the vectors must have the same length, but both the SRFI 43
reference implementation and Guile’s implementation allow vectors
with unequal lengths, and start searching from the last index of
the shortest vector.
-- Scheme Procedure: vector-skip pred? vec1 vec2 ...
Find and return the index of the first elements in VEC1 VEC2 ...
that do not satisfy PRED?. If no matching element is found by the
end of the shortest vector, return ‘#f’. Equivalent to
‘vector-index’ but with the predicate inverted.
(vector-skip number? '#(1 2 a b 3 4 c d)) ⇒ 2
-- Scheme Procedure: vector-skip-right pred? vec1 vec2 ...
Like ‘vector-skip’, but it searches for a non-matching element
right-to-left, rather than left-to-right. Note that the SRFI 43
specification requires that all the vectors must have the same
length, but both the SRFI 43 reference implementation and Guile’s
implementation allow vectors with unequal lengths, and start
searching from the last index of the shortest vector.
-- Scheme Procedure: vector-binary-search vec value cmp [start [end]]
Find and return an index of VEC between START and END whose value
is VALUE using a binary search. If no matching element is found,
return ‘#f’. The default START is 0 and the default END is the
length of VEC.
CMP must be a procedure of two arguments such that ‘(cmp a b)’
returns a negative integer if a < b, a positive integer if a > b,
or zero if a = b. The elements of VEC must be sorted in
non-decreasing order according to CMP.
Note that SRFI 43 does not document the START and END arguments,
but both its reference implementation and Guile’s implementation
support them.
(define (char-cmp c1 c2)
(cond ((char? c1 c2) 1)
(else 0)))
(vector-binary-search '#(#\a #\b #\c #\d #\e #\f #\g #\h)
#\g
char-cmp)
⇒ 6
-- Scheme Procedure: vector-any pred? vec1 vec2 ...
Find the first parallel set of elements from VEC1 VEC2 ... for
which PRED? returns a true value. If such a parallel set of
elements exists, ‘vector-any’ returns the value that PRED? returned
for that set of elements. The iteration is strictly left-to-right.
-- Scheme Procedure: vector-every pred? vec1 vec2 ...
If, for every index i between 0 and the length of the shortest
vector argument, the set of elements ‘(vector-ref vec1 i)’
‘(vector-ref vec2 i)’ ... satisfies PRED?, ‘vector-every’ returns
the value that PRED? returned for the last set of elements, at the
last index of the shortest vector. Otherwise it returns ‘#f’. The
iteration is strictly left-to-right.
7.5.30.6 SRFI-43 Mutators
.........................
-- Scheme Procedure: vector-set! vec i value
Assign the contents of the location at I in VEC to VALUE.
-- Scheme Procedure: vector-swap! vec i j
Swap the values of the locations in VEC at I and J.
-- Scheme Procedure: vector-fill! vec fill [start [end]]
Assign the value of every location in VEC between START and END to
FILL. START defaults to 0 and END defaults to the length of VEC.
-- Scheme Procedure: vector-reverse! vec [start [end]]
Destructively reverse the contents of VEC between START and END.
START defaults to 0 and END defaults to the length of VEC.
-- Scheme Procedure: vector-copy! target tstart source [sstart [send]]
Copy a block of elements from SOURCE to TARGET, both of which must
be vectors, starting in TARGET at TSTART and starting in SOURCE at
SSTART, ending when (SEND - SSTART) elements have been copied. It
is an error for TARGET to have a length less than (TSTART + SEND -
SSTART). SSTART defaults to 0 and SEND defaults to the length of
SOURCE.
-- Scheme Procedure: vector-reverse-copy! target tstart source [sstart
[send]]
Like ‘vector-copy!’, but this copies the elements in the reverse
order. It is an error if TARGET and SOURCE are identical vectors
and the TARGET and SOURCE ranges overlap; however, if TSTART =
SSTART, ‘vector-reverse-copy!’ behaves as ‘(vector-reverse! target
tstart send)’ would.
7.5.30.7 SRFI-43 Conversion
...........................
-- Scheme Procedure: vector->list vec [start [end]]
Return a newly allocated list containing the elements in VEC
between START and END. START defaults to 0 and END defaults to the
length of VEC.
-- Scheme Procedure: reverse-vector->list vec [start [end]]
Like ‘vector->list’, but the resulting list contains the specified
range of elements of VEC in reverse order.
-- Scheme Procedure: list->vector proper-list [start [end]]
Return a newly allocated vector of the elements from PROPER-LIST
with indices between START and END. START defaults to 0 and END
defaults to the length of PROPER-LIST. Note that SRFI 43 does not
document the START and END arguments, but both its reference
implementation and Guile’s implementation support them.
-- Scheme Procedure: reverse-list->vector proper-list [start [end]]
Like ‘list->vector’, but the resulting vector contains the
specified range of elements of PROPER-LIST in reverse order. Note
that SRFI 43 does not document the START and END arguments, but
both its reference implementation and Guile’s implementation
support them.
7.5.31 SRFI-45 - Primitives for Expressing Iterative Lazy Algorithms
--------------------------------------------------------------------
This subsection is based on the specification of SRFI-45
(http://srfi.schemers.org/srfi-45/srfi-45.html) written by André van
Tonder.
Lazy evaluation is traditionally simulated in Scheme using ‘delay’
and ‘force’. However, these primitives are not powerful enough to
express a large class of lazy algorithms that are iterative. Indeed, it
is folklore in the Scheme community that typical iterative lazy
algorithms written using delay and force will often require unbounded
memory.
This SRFI provides set of three operations: {‘lazy’, ‘delay’,
‘force’}, which allow the programmer to succinctly express lazy
algorithms while retaining bounded space behavior in cases that are
properly tail-recursive. A general recipe for using these primitives is
provided. An additional procedure ‘eager’ is provided for the
construction of eager promises in cases where efficiency is a concern.
Although this SRFI redefines ‘delay’ and ‘force’, the extension is
conservative in the sense that the semantics of the subset {‘delay’,
‘force’} in isolation (i.e., as long as the program does not use ‘lazy’)
agrees with that in R5RS. In other words, no program that uses the R5RS
definitions of delay and force will break if those definition are
replaced by the SRFI-45 definitions of delay and force.
Guile also adds ‘promise?’ to the list of exports, which is not part
of the official SRFI-45.
-- Scheme Procedure: promise? obj
Return true if OBJ is an SRFI-45 promise, otherwise return false.
-- Scheme Syntax: delay expression
Takes an expression of arbitrary type A and returns a promise of
type ‘(Promise A)’ which at some point in the future may be asked
(by the ‘force’ procedure) to evaluate the expression and deliver
the resulting value.
-- Scheme Syntax: lazy expression
Takes an expression of type ‘(Promise A)’ and returns a promise of
type ‘(Promise A)’ which at some point in the future may be asked
(by the ‘force’ procedure) to evaluate the expression and deliver
the resulting promise.
-- Scheme Procedure: force expression
Takes an argument of type ‘(Promise A)’ and returns a value of type
A as follows: If a value of type A has been computed for the
promise, this value is returned. Otherwise, the promise is first
evaluated, then overwritten by the obtained promise or value, and
then force is again applied (iteratively) to the promise.
-- Scheme Procedure: eager expression
Takes an argument of type A and returns a value of type ‘(Promise
A)’. As opposed to ‘delay’, the argument is evaluated eagerly.
Semantically, writing ‘(eager expression)’ is equivalent to writing
(let ((value expression)) (delay value)).
However, the former is more efficient since it does not require
unnecessary creation and evaluation of thunks. We also have the
equivalence
(delay expression) = (lazy (eager expression))
The following reduction rules may be helpful for reasoning about
these primitives. However, they do not express the memoization and
memory usage semantics specified above:
(force (delay expression)) -> expression
(force (lazy expression)) -> (force expression)
(force (eager value)) -> value
Correct usage
.............
We now provide a general recipe for using the primitives {‘lazy’,
‘delay’, ‘force’} to express lazy algorithms in Scheme. The
transformation is best described by way of an example: Consider the
stream-filter algorithm, expressed in a hypothetical lazy language as
(define (stream-filter p? s)
(if (null? s) '()
(let ((h (car s))
(t (cdr s)))
(if (p? h)
(cons h (stream-filter p? t))
(stream-filter p? t)))))
This algorithm can be expressed as follows in Scheme:
(define (stream-filter p? s)
(lazy
(if (null? (force s)) (delay '())
(let ((h (car (force s)))
(t (cdr (force s))))
(if (p? h)
(delay (cons h (stream-filter p? t)))
(stream-filter p? t))))))
In other words, we
• wrap all constructors (e.g., ‘'()’, ‘cons’) with ‘delay’,
• apply ‘force’ to arguments of deconstructors (e.g., ‘car’, ‘cdr’
and ‘null?’),
• wrap procedure bodies with ‘(lazy ...)’.
7.5.32 SRFI-46 Basic syntax-rules Extensions
--------------------------------------------
Guile’s core ‘syntax-rules’ supports the extensions specified by
SRFI-46/R7RS. Tail patterns have been supported since at least Guile
2.0, and custom ellipsis identifiers have been supported since Guile
2.0.10. *Note Syntax Rules::.
7.5.33 SRFI-55 - Requiring Features
-----------------------------------
SRFI-55 provides ‘require-extension’ which is a portable mechanism to
load selected SRFI modules. This is implemented in the Guile core,
there’s no module needed to get SRFI-55 itself.
-- library syntax: require-extension clause1 clause2 ...
Require the features of CLAUSE1 CLAUSE2 ... , throwing an error if
any are unavailable.
A CLAUSE is of the form ‘(IDENTIFIER arg...)’. The only IDENTIFIER
currently supported is ‘srfi’ and the arguments are SRFI numbers.
For example to get SRFI-1 and SRFI-6,
(require-extension (srfi 1 6))
‘require-extension’ can only be used at the top-level.
A Guile-specific program can simply ‘use-modules’ to load SRFIs not
already in the core, ‘require-extension’ is for programs designed
to be portable to other Scheme implementations.
7.5.34 SRFI-60 - Integers as Bits
---------------------------------
This SRFI provides various functions for treating integers as bits and
for bitwise manipulations. These functions can be obtained with,
(use-modules (srfi srfi-60))
Integers are treated as infinite precision twos-complement, the same
as in the core logical functions (*note Bitwise Operations::). And
likewise bit indexes start from 0 for the least significant bit. The
following functions in this SRFI are already in the Guile core,
‘logand’, ‘logior’, ‘logxor’, ‘lognot’, ‘logtest’, ‘logcount’,
‘integer-length’, ‘logbit?’, ‘ash’
-- Function: bitwise-and n1 ...
-- Function: bitwise-ior n1 ...
-- Function: bitwise-xor n1 ...
-- Function: bitwise-not n
-- Function: any-bits-set? j k
-- Function: bit-set? index n
-- Function: arithmetic-shift n count
-- Function: bit-field n start end
-- Function: bit-count n
Aliases for ‘logand’, ‘logior’, ‘logxor’, ‘lognot’, ‘logtest’,
‘logbit?’, ‘ash’, ‘bit-extract’ and ‘logcount’ respectively.
Note that the name ‘bit-count’ conflicts with ‘bit-count’ in the
core (*note Bit Vectors::).
-- Function: bitwise-if mask n1 n0
-- Function: bitwise-merge mask n1 n0
Return an integer with bits selected from N1 and N0 according to
MASK. Those bits where MASK has 1s are taken from N1, and those
where MASK has 0s are taken from N0.
(bitwise-if 3 #b0101 #b1010) ⇒ 9
-- Function: log2-binary-factors n
-- Function: first-set-bit n
Return a count of how many factors of 2 are present in N. This is
also the bit index of the lowest 1 bit in N. If N is 0, the return
is -1.
(log2-binary-factors 6) ⇒ 1
(log2-binary-factors -8) ⇒ 3
-- Function: copy-bit index n newbit
Return N with the bit at INDEX set according to NEWBIT. NEWBIT
should be ‘#t’ to set the bit to 1, or ‘#f’ to set it to 0. Bits
other than at INDEX are unchanged in the return.
(copy-bit 1 #b0101 #t) ⇒ 7
-- Function: copy-bit-field n newbits start end
Return N with the bits from START (inclusive) to END (exclusive)
changed to the value NEWBITS.
The least significant bit in NEWBITS goes to START, the next to
START+1, etc. Anything in NEWBITS past the END given is ignored.
(copy-bit-field #b10000 #b11 1 3) ⇒ #b10110
-- Function: rotate-bit-field n count start end
Return N with the bit field from START (inclusive) to END
(exclusive) rotated upwards by COUNT bits.
COUNT can be positive or negative, and it can be more than the
field width (it’ll be reduced modulo the width).
(rotate-bit-field #b0110 2 1 4) ⇒ #b1010
-- Function: reverse-bit-field n start end
Return N with the bits from START (inclusive) to END (exclusive)
reversed.
(reverse-bit-field #b101001 2 4) ⇒ #b100101
-- Function: integer->list n [len]
Return bits from N in the form of a list of ‘#t’ for 1 and ‘#f’ for
0. The least significant LEN bits are returned, and the first list
element is the most significant of those bits. If LEN is not
given, the default is ‘(integer-length N)’ (*note Bitwise
Operations::).
(integer->list 6) ⇒ (#t #t #f)
(integer->list 1 4) ⇒ (#f #f #f #t)
-- Function: list->integer lst
-- Function: booleans->integer bool...
Return an integer formed bitwise from the given LST list of
booleans, or for ‘booleans->integer’ from the BOOL arguments.
Each boolean is ‘#t’ for a 1 and ‘#f’ for a 0. The first element
becomes the most significant bit in the return.
(list->integer '(#t #f #t #f)) ⇒ 10
7.5.35 SRFI-61 - A more general ‘cond’ clause
---------------------------------------------
This SRFI extends RnRS ‘cond’ to support test expressions that return
multiple values, as well as arbitrary definitions of test success. SRFI
61 is implemented in the Guile core; there’s no module needed to get
SRFI-61 itself. Extended ‘cond’ is documented in *note Simple
Conditional Evaluation: Conditionals.
7.5.36 SRFI-62 - S-expression comments.
---------------------------------------
Starting from version 2.0, Guile’s ‘read’ supports SRFI-62/R7RS
S-expression comments by default.
7.5.37 SRFI-64 - A Scheme API for test suites.
----------------------------------------------
See the specification of SRFI-64
(http://srfi.schemers.org/srfi-64/srfi-64.html).
7.5.38 SRFI-67 - Compare procedures
-----------------------------------
See the specification of SRFI-67
(http://srfi.schemers.org/srfi-67/srfi-67.html).
7.5.39 SRFI-69 - Basic hash tables
----------------------------------
This is a portable wrapper around Guile’s built-in hash table and weak
table support. *Note Hash Tables::, for information on that built-in
support. Above that, this hash-table interface provides association of
equality and hash functions with tables at creation time, so variants of
each function are not required, as well as a procedure that takes care
of most uses for Guile hash table handles, which this SRFI does not
provide as such.
Access it with:
(use-modules (srfi srfi-69))
7.5.39.1 Creating hash tables
.............................
-- Scheme Procedure: make-hash-table [equal-proc hash-proc #:weak
weakness start-size]
Create and answer a new hash table with EQUAL-PROC as the equality
function and HASH-PROC as the hashing function.
By default, EQUAL-PROC is ‘equal?’. It can be any two-argument
procedure, and should answer whether two keys are the same for this
table’s purposes.
By default HASH-PROC assumes that ‘equal-proc’ is no coarser than
‘equal?’ unless it is literally ‘string-ci=?’. If provided,
HASH-PROC should be a two-argument procedure that takes a key and
the current table size, and answers a reasonably good hash integer
between 0 (inclusive) and the size (exclusive).
WEAKNESS should be ‘#f’ or a symbol indicating how “weak” the hash
table is:
‘#f’
An ordinary non-weak hash table. This is the default.
‘key’
When the key has no more non-weak references at GC, remove
that entry.
‘value’
When the value has no more non-weak references at GC, remove
that entry.
‘key-or-value’
When either has no more non-weak references at GC, remove the
association.
As a legacy of the time when Guile couldn’t grow hash tables,
START-SIZE is an optional integer argument that specifies the
approximate starting size for the hash table, which will be rounded
to an algorithmically-sounder number.
By “coarser” than ‘equal?’, we mean that for all X and Y values where
‘(EQUAL-PROC X Y)’, ‘(equal? X Y)’ as well. If that does not hold for
your EQUAL-PROC, you must provide a HASH-PROC.
In the case of weak tables, remember that “references” above always
refers to ‘eq?’-wise references. Just because you have a reference to
some string ‘"foo"’ doesn’t mean that an association with key ‘"foo"’ in
a weak-key table _won’t_ be collected; it only counts as a reference if
the two ‘"foo"’s are ‘eq?’, regardless of EQUAL-PROC. As such, it is
usually only sensible to use ‘eq?’ and ‘hashq’ as the equivalence and
hash functions for a weak table. *Note Weak References::, for more
information on Guile’s built-in weak table support.
-- Scheme Procedure: alist->hash-table alist [equal-proc hash-proc
#:weak weakness start-size]
As with ‘make-hash-table’, but initialize it with the associations
in ALIST. Where keys are repeated in ALIST, the leftmost
association takes precedence.
7.5.39.2 Accessing table items
..............................
-- Scheme Procedure: hash-table-ref table key [default-thunk]
-- Scheme Procedure: hash-table-ref/default table key default
Answer the value associated with KEY in TABLE. If KEY is not
present, answer the result of invoking the thunk DEFAULT-THUNK,
which signals an error instead by default.
‘hash-table-ref/default’ is a variant that requires a third
argument, DEFAULT, and answers DEFAULT itself instead of invoking
it.
-- Scheme Procedure: hash-table-set! table key new-value
Set KEY to NEW-VALUE in TABLE.
-- Scheme Procedure: hash-table-delete! table key
Remove the association of KEY in TABLE, if present. If absent, do
nothing.
-- Scheme Procedure: hash-table-exists? table key
Answer whether KEY has an association in TABLE.
-- Scheme Procedure: hash-table-update! table key modifier
[default-thunk]
-- Scheme Procedure: hash-table-update!/default table key modifier
default
Replace KEY’s associated value in TABLE by invoking MODIFIER with
one argument, the old value.
If KEY is not present, and DEFAULT-THUNK is provided, invoke it
with no arguments to get the “old value” to be passed to MODIFIER
as above. If DEFAULT-THUNK is not provided in such a case, signal
an error.
‘hash-table-update!/default’ is a variant that requires the fourth
argument, which is used directly as the “old value” rather than as
a thunk to be invoked to retrieve the “old value”.
7.5.39.3 Table properties
.........................
-- Scheme Procedure: hash-table-size table
Answer the number of associations in TABLE. This is guaranteed to
run in constant time for non-weak tables.
-- Scheme Procedure: hash-table-keys table
Answer an unordered list of the keys in TABLE.
-- Scheme Procedure: hash-table-values table
Answer an unordered list of the values in TABLE.
-- Scheme Procedure: hash-table-walk table proc
Invoke PROC once for each association in TABLE, passing the key and
value as arguments.
-- Scheme Procedure: hash-table-fold table proc init
Invoke ‘(PROC KEY VALUE PREVIOUS)’ for each KEY and VALUE in TABLE,
where PREVIOUS is the result of the previous invocation, using INIT
as the first PREVIOUS value. Answer the final PROC result.
-- Scheme Procedure: hash-table->alist table
Answer an alist where each association in TABLE is an association
in the result.
7.5.39.4 Hash table algorithms
..............................
Each hash table carries an “equivalence function” and a “hash function”,
used to implement key lookups. Beginning users should follow the rules
for consistency of the default HASH-PROC specified above. Advanced
users can use these to implement their own equivalence and hash
functions for specialized lookup semantics.
-- Scheme Procedure: hash-table-equivalence-function hash-table
-- Scheme Procedure: hash-table-hash-function hash-table
Answer the equivalence and hash function of HASH-TABLE,
respectively.
-- Scheme Procedure: hash obj [size]
-- Scheme Procedure: string-hash obj [size]
-- Scheme Procedure: string-ci-hash obj [size]
-- Scheme Procedure: hash-by-identity obj [size]
Answer a hash value appropriate for equality predicate ‘equal?’,
‘string=?’, ‘string-ci=?’, and ‘eq?’, respectively.
‘hash’ is a backwards-compatible replacement for Guile’s built-in
‘hash’.
7.5.40 SRFI-71 - Extended let-syntax for multiple values
--------------------------------------------------------
This SRFI shadows the forms for ‘let’, ‘let*’, and ‘letrec’ so that they
may accept multiple values. For example:
(use-modules (srfi srfi-71))
(let* ((x y (values 1 2))
(z (+ x y)))
(* z 2))
⇒ 6
See the specification of SRFI-71
(http://srfi.schemers.org/srfi-71/srfi-71.html).
7.5.41 SRFI-87 => in case clauses
---------------------------------
Starting from version 2.0.6, Guile’s core ‘case’ syntax supports ‘=>’ in
clauses, as specified by SRFI-87/R7RS. *Note Conditionals::.
7.5.42 SRFI-88 Keyword Objects
------------------------------
SRFI-88 (http://srfi.schemers.org/srfi-88/srfi-88.html) provides
“keyword objects”, which are equivalent to Guile’s keywords (*note
Keywords::). SRFI-88 keywords can be entered using the “postfix keyword
syntax”, which consists of an identifier followed by ‘:’ (*note
‘postfix’ keyword syntax: Scheme Read.). SRFI-88 can be made available
with:
(use-modules (srfi srfi-88))
Doing so installs the right reader option for keyword syntax, using
‘(read-set! keywords 'postfix)’. It also provides the procedures
described below.
-- Scheme Procedure: keyword? obj
Return ‘#t’ if OBJ is a keyword. This is the same procedure as the
same-named built-in procedure (*note ‘keyword?’: Keyword
Procedures.).
(keyword? foo:) ⇒ #t
(keyword? 'foo:) ⇒ #t
(keyword? "foo") ⇒ #f
-- Scheme Procedure: keyword->string kw
Return the name of KW as a string, i.e., without the trailing
colon. The returned string may not be modified, e.g., with
‘string-set!’.
(keyword->string foo:) ⇒ "foo"
-- Scheme Procedure: string->keyword str
Return the keyword object whose name is STR.
(keyword->string (string->keyword "a b c")) ⇒ "a b c"
7.5.43 SRFI-98 Accessing environment variables.
-----------------------------------------------
This is a portable wrapper around Guile’s built-in support for
interacting with the current environment, *Note Runtime Environment::.
-- Scheme Procedure: get-environment-variable name
Returns a string containing the value of the environment variable
given by the string ‘name’, or ‘#f’ if the named environment
variable is not found. This is equivalent to ‘(getenv name)’.
-- Scheme Procedure: get-environment-variables
Returns the names and values of all the environment variables as an
association list in which both the keys and the values are strings.
7.5.44 SRFI-105 Curly-infix expressions.
----------------------------------------
Guile’s built-in reader includes support for SRFI-105 curly-infix
expressions. See the specification of SRFI-105
(http://srfi.schemers.org/srfi-105/srfi-105.html). Some examples:
{n <= 5} ⇒ (<= n 5)
{a + b + c} ⇒ (+ a b c)
{a * {b + c}} ⇒ (* a (+ b c))
{(- a) / b} ⇒ (/ (- a) b)
{-(a) / b} ⇒ (/ (- a) b) as well
{(f a b) + (g h)} ⇒ (+ (f a b) (g h))
{f(a b) + g(h)} ⇒ (+ (f a b) (g h)) as well
{f[a b] + g(h)} ⇒ (+ ($bracket-apply$ f a b) (g h))
'{a + f(b) + x} ⇒ '(+ a (f b) x)
{length(x) >= 6} ⇒ (>= (length x) 6)
{n-1 + n-2} ⇒ (+ n-1 n-2)
{n * factorial{n - 1}} ⇒ (* n (factorial (- n 1)))
{{a > 0} and {b >= 1}} ⇒ (and (> a 0) (>= b 1))
{f{n - 1}(x)} ⇒ ((f (- n 1)) x)
{a . z} ⇒ ($nfx$ a . z)
{a + b - c} ⇒ ($nfx$ a + b - c)
To enable curly-infix expressions within a file, place the reader
directive ‘#!curly-infix’ before the first use of curly-infix notation.
To globally enable curly-infix expressions in Guile’s reader, set the
‘curly-infix’ read option.
Guile also implements the following non-standard extension to
SRFI-105: if ‘curly-infix’ is enabled and there is no other meaning
assigned to square brackets (i.e. the ‘square-brackets’ read option is
turned off), then lists within square brackets are read as normal lists
but with the special symbol ‘$bracket-list$’ added to the front. To
enable this combination of read options within a file, use the reader
directive ‘#!curly-infix-and-bracket-lists’. For example:
[a b] ⇒ ($bracket-list$ a b)
[a . b] ⇒ ($bracket-list$ a . b)
For more information on reader options, *Note Scheme Read::.
7.5.45 SRFI-111 Boxes.
----------------------
SRFI-111 (http://srfi.schemers.org/srfi-111/srfi-111.html) provides
boxes: objects with a single mutable cell.
-- Scheme Procedure: box value
Return a newly allocated box whose contents is initialized to
VALUE.
-- Scheme Procedure: box? obj
Return true if OBJ is a box, otherwise return false.
-- Scheme Procedure: unbox box
Return the current contents of BOX.
-- Scheme Procedure: set-box! box value
Set the contents of BOX to VALUE.
7.5.46 Transducers
------------------
Some of the most common operations used in the Scheme language are those
transforming lists: map, filter, take and so on. They work well, are
well understood, and are used daily by most Scheme programmers. They
are however not general because they only work on lists, and they do not
compose very well since combining N of them builds ‘(- N 1)’
intermediate lists.
Transducers are oblivious to what kind of process they are used in,
and are composable without building intermediate collections. This
means we can create a transducer that squares all odd numbers:
(compose (tfilter odd?) (tmap (lambda (x) (* x x))))
and reuse it with lists, vectors, or in just about any context where
data flows in one direction. We could use it as a processing step for
asynchronous channels, with an event framework as a pre-processing step,
or even in lazy contexts where you pass a lazy collection and a
transducer to a function and get a new lazy collection back.
The traditional Scheme approach of having collection-specific
procedures is not changed. We instead specify a general form of
transformations that complement these procedures. The benefits are
obvious: a clear, well-understood way of describing common
transformations in a way that is faster than just chaining the
collection-specific counterparts. For guile in particular this means a
lot better GC performance.
Notice however that ‘(compose ...)’ composes transducers
left-to-right, due to how transducers are initiated.
7.5.46.1 SRFI-171 General Discussion
....................................
The concept of reducers
-----------------------
The central part of transducers are 3-arity reducing procedures.
• no arguments: Produces the identity of the reducer.
• (result-so-far): completion. Returns ‘result-so-far’ either with
or without transforming it first.
• (result-so-far input) combines ‘result-so-far’ and ‘input’ to
produce a new ‘result-so-far’.
In the case of a summing ‘+’ reducer, the reducer would produce, in
arity order: ‘0’, ‘result-so-far’, ‘(+ result-so-far input)’. This
happens to be exactly what the regular ‘+’ does.
The concept of transducers
--------------------------
A transducer is a one-arity procedure that takes a reducer and produces
a reducing function that behaves as follows:
• no arguments: calls reducer with no arguments (producing its
identity)
• (result-so-far): Maybe transform the result-so-far and call reducer
with it.
• (result-so-far input) Maybe do something to input and maybe call
the reducer with result-so-far and the maybe-transformed input.
A simple example is as following:
(list-transduce (tfilter odd?) + '(1 2 3 4 5)).
This first returns a transducer filtering all odd elements, then it
runs ‘+’ without arguments to retrieve its identity. It then starts the
transduction by passing ‘+’ to the transducer returned by ‘(tfilter
odd?)’ which returns a reducing function. It works not unlike reduce
from SRFI 1, but also checks whether one of the intermediate transducers
returns a "reduced" value (implemented as a SRFI 9 record), which means
the reduction finished early.
Because transducers compose and the final reduction is only executed
in the last step, composed transducers will not build any intermediate
result or collections. Although the normal way of thinking about
application of composed functions is right to left, due to how the
transduction is built it is applied left to right. ‘(compose (tfilter
odd?) (tmap sqrt))’ will create a transducer that first filters out any
odd values and then computes the square root of the rest.
State
-----
Even though transducers appear to be somewhat of a generalisation of
‘map’ and friends, this is not really true. Since transducers don’t
know in which context they are being used, some transducers must keep
state where their collection-specific counterparts do not. The
transducers that keep state do so using hidden mutable state, and as
such all the caveats of mutation, parallelism, and multi-shot
continuations apply. Each transducer keeping state is clearly described
as doing so in the documentation.
Naming
------
Reducers exported from the transducers module are named as in their
SRFI-1 counterpart, but prepended with an r. Transducers also follow
that naming, but are prepended with a t.
7.5.46.2 Applying Transducers
.............................
-- Scheme Procedure: list-transduce xform f lst
-- Scheme Procedure: list-transduce xform f identity lst
Initialize the transducer XFORM by passing the reducer F to it. If
no identity is provided, F runs without arguments to return the
reducer identity. It then reduces over LST using the identity as
the seed.
If one of the transducers finishes early (such as ‘ttake’ or
‘tdrop’), it communicates this by returning a reduced value, which
in the guile implementation is just a value wrapped in a SRFI 9
record type named “reduced”. If such a value is returned by the
transducer, ‘list-transduce’ must stop execution and return an
unreduced value immediately.
-- Scheme Procedure: vector-transduce xform f vec
-- Scheme Procedure: vector-transduce xform f identity vec
-- Scheme Procedure: string-transduce xform f str
-- Scheme Procedure: string-transduce xform f identity str
-- Scheme Procedure: bytevector-u8-transduce xform f bv
-- Scheme Procedure: bytevector-u8-transduce xform f identity bv
-- Scheme Procedure: generator-transduce xform f gen
-- Scheme Procedure: generator-transduce xform f identity gen
Same as ‘list-transduce’, but for vectors, strings, u8-bytevectors
and SRFI-158-styled generators respectively.
-- Scheme Procedure: port-transduce xform f reader
-- Scheme Procedure: port-transduce xform f reader port
-- Scheme Procedure: port-transduce xform f identity reader port
Same as ‘list-transduce’ but for ports. Called without a port, it
reduces over the results of applying READER until the EOF-object is
returned, presumably to read from ‘current-input-port’. With a
port READER is applied to PORT instead of without any arguments.
If IDENTITY is provided, that is used as the initial identity in
the reduction.
7.5.46.3 Reducers
.................
-- Scheme Procedure: rcons
a simple consing reducer. When called without values, it returns
its identity, ‘'()’. With one value, which will be a list, it
reverses the list (using ‘reverse!’). When called with two values,
it conses the second value to the first.
(list-transduce (tmap (lambda (x) (+ x 1)) rcons (list 0 1 2 3))
⇒ (1 2 3 4)
-- Scheme Procedure: reverse-rcons
same as rcons, but leaves the values in their reversed order.
(list-transduce (tmap (lambda (x) (+ x 1))) reverse-rcons (list 0 1 2 3))
⇒ (4 3 2 1)
-- Scheme Procedure: rany pred?
The reducer version of any. Returns ‘(reduced (pred? value))’ if
any ‘(pred? value)’ returns non-#f. The identity is #f.
(list-transduce (tmap (lambda (x) (+ x 1))) (rany odd?) (list 1 3 5))
⇒ #f
(list-transduce (tmap (lambda (x) (+ x 1))) (rany odd?) (list 1 3 4 5))
⇒ #t
-- Scheme Procedure: revery pred?
The reducer version of every. Stops the transduction and returns
‘(reduced #f)’ if any ‘(pred? value)’ returns #f. If every ‘(pred?
value)’ returns true, it returns the result of the last invocation
of ‘(pred? value)’. The identity is #t.
(list-transduce
(tmap (lambda (x) (+ x 1)))
(revery (lambda (v) (if (odd? v) v #f)))
(list 2 4 6))
⇒ 7
(list-transduce (tmap (lambda (x) (+ x 1)) (revery odd?) (list 2 4 5 6))
⇒ #f
-- Scheme Procedure: rcount
A simple counting reducer. Counts the values that pass through the
transduction.
(list-transduce (tfilter odd?) rcount (list 1 2 3 4)) ⇒ 2.
7.5.46.4 Transducers
....................
-- Scheme Procedure: tmap proc
Returns a transducer that applies PROC to all values. Stateless.
-- Scheme Procedure: tfilter pred?
Returns a transducer that removes values for which PRED? returns
#f.
Stateless.
-- Scheme Procedure: tremove pred?
Returns a transducer that removes values for which PRED? returns
non-#f.
Stateless
-- Scheme Procedure: tfilter-map proc
The same as ‘(compose (tmap proc) (tfilter values))’. Stateless.
-- Scheme Procedure: treplace mapping
The argument MAPPING is an association list (using ‘equal?’ to
compare keys), a hash-table, a one-argument procedure taking one
argument and either producing that same argument or a replacement
value.
Returns a transducer which checks for the presence of any value
passed through it in mapping. If a mapping is found, the value of
that mapping is returned, otherwise it just returns the original
value.
Does not keep internal state, but modifying the mapping while it’s
in use by treplace is an error.
-- Scheme Procedure: tdrop n
Returns a transducer that discards the first N values.
Stateful.
-- Scheme Procedure: ttake n
Returns a transducer that discards all values and stops the
transduction after the first N values have been let through. Any
subsequent values are ignored.
Stateful.
-- Scheme Procedure: tdrop-while pred?
Returns a transducer that discards the first values for which PRED?
returns true.
Stateful.
-- Scheme Procedure: ttake-while pred?
-- Scheme Procedure: ttake-while pred? retf
Returns a transducer that stops the transduction after PRED? has
returned #f. Any subsequent values are ignored and the last
successful value is returned. RETF is a function that gets called
whenever PRED? returns false. The arguments passed are the result
so far and the input for which pred? returns ‘#f’. The default
function is ‘(lambda (result input) result)’.
Stateful.
-- Scheme Procedure: tconcatenate
tconcatenate _is_ a transducer that concatenates the content of
each value (that must be a list) into the reduction.
(list-transduce tconcatenate rcons '((1 2) (3 4 5) (6 (7 8) 9)))
⇒ (1 2 3 4 5 6 (7 8) 9)
-- Scheme Procedure: tappend-map proc
The same as ‘(compose (tmap proc) tconcatenate)’.
-- Scheme Procedure: tflatten
tflatten _is_ a transducer that flattens an input consisting of
lists.
(list-transduce tflatten rcons '((1 2) 3 (4 (5 6) 7 8) 9)
⇒ (1 2 3 4 5 6 7 8 9)
-- Scheme Procedure: tdelete-neighbor-duplicates
-- Scheme Procedure: tdelete-neighbor-duplicates equality-predicate
Returns a transducer that removes any directly following duplicate
elements. The default EQUALITY-PREDICATE is ‘equal?’.
Stateful.
-- Scheme Procedure: tdelete-duplicates
-- Scheme Procedure: tdelete-duplicates equality-predicate
Returns a transducer that removes any subsequent duplicate elements
compared using EQUALITY-PREDICATE. The default EQUALITY-PREDICATE
is ‘equal?’.
Stateful.
-- Scheme Procedure: tsegment n
Returns a transducer that groups inputs into lists of N elements.
When the transduction stops, it flushes any remaining collection,
even if it contains fewer than N elements.
Stateful.
-- Scheme Procedure: tpartition pred?
Returns a transducer that groups inputs in lists by whenever
‘(pred? input)’ changes value.
Stateful.
-- Scheme Procedure: tadd-between value
Returns a transducer which interposes VALUE between each value and
the next. This does not compose gracefully with transducers like
‘ttake’, as you might end up ending the transduction on ‘value’.
Stateful.
-- Scheme Procedure: tenumerate
-- Scheme Procedure: tenumerate start
Returns a transducer that indexes values passed through it,
starting at START, which defaults to 0. The indexing is done
through cons pairs like ‘(index . input)’.
(list-transduce (tenumerate 1) rcons (list 'first 'second 'third))
⇒ ((1 . first) (2 . second) (3 . third))
Stateful.
-- Scheme Procedure: tlog
-- Scheme Procedure: tlog logger
Returns a transducer that can be used to log or print values and
results. The result of the LOGGER procedure is discarded. The
default LOGGER is ‘(lambda (result input) (write input)
(newline))’.
Stateless.
Guile-specific transducers
--------------------------
These transducers are available in the ‘(srfi srfi-171 gnu)’ library,
and are provided outside the standard described by the SRFI-171
document.
-- Scheme Procedure: tbatch reducer
-- Scheme Procedure: tbatch transducer reducer
A batching transducer that accumulates results using REDUCER or
‘((transducer) reducer)’ until it returns a reduced value. This
can be used to generalize something like ‘tsegment’:
;; This behaves exactly like (tsegment 4).
(list-transduce (tbatch (ttake 4) rcons) rcons (iota 10))
⇒ ((0 1 2 3) (4 5 6 7) (8 9))
-- Scheme Procedure: tfold reducer
-- Scheme Procedure: tfold reducer seed
A folding transducer that yields the result of ‘(reducer seed
value)’, saving its result between iterations.
(list-transduce (tfold +) rcons (iota 10))
⇒ (0 1 3 6 10 15 21 28 36 45)
7.5.46.5 Helper functions for writing transducers
.................................................
These functions are in the ‘(srfi srfi-171 meta)’ module and are only
usable when you want to write your own transducers.
-- Scheme Procedure: reduced value
Wraps a value in a ‘’ container, signalling that the
reduction should stop.
-- Scheme Procedure: reduced? value
Returns #t if value is a ‘’ record.
-- Scheme Procedure: unreduce reduced-container
Returns the value in reduced-container.
-- Scheme Procedure: ensure-reduced value
Wraps value in a ‘’ container if it is not already
reduced.
-- Scheme Procedure: preserving-reduced reducer
Wraps ‘reducer’ in another reducer that encapsulates any returned
reduced value in another reduced container. This is useful in
places where you re-use a reducer with [collection]-reduce. If the
reducer returns a reduced value, [collection]-reduce unwraps it.
Unless handled, this leads to the reduction continuing.
-- Scheme Procedure: list-reduce f identity lst
The reducing function used internally by ‘list-transduce’. F is a
reducer as returned by a transducer. IDENTITY is the identity
(sometimes called "seed") of the reduction. LST is a list. If F
returns a reduced value, the reduction stops immediately and the
unreduced value is returned.
-- Scheme Procedure: vector-reduce f identity vec
The vector version of list-reduce.
-- Scheme Procedure: string-reduce f identity str
The string version of list-reduce.
-- Scheme Procedure: bytevector-u8-reduce f identity bv
The bytevector-u8 version of list-reduce.
-- Scheme Procedure: port-reduce f identity reader port
The port version of list-reduce. It reduces over port using reader
until reader returns the EOF object.
-- Scheme Procedure: generator-reduce f identity gen
The generator version of list-reduce. It reduces over ‘gen’ until
it returns the EOF object
7.6 R6RS Support
================
*Note R6RS Libraries::, for more information on how to define R6RS
libraries, and their integration with Guile modules.
7.6.1 Incompatibilities with the R6RS
-------------------------------------
There are some incompatibilities between Guile and the R6RS. Some of
them are intentional, some of them are bugs, and some are simply
unimplemented features. Please let the Guile developers know if you
find one that is not on this list.
• The R6RS specifies many situations in which a conforming
implementation must signal a specific error. Guile doesn’t really
care about that too much—if a correct R6RS program would not hit
that error, we don’t bother checking for it.
• Multiple ‘library’ forms in one file are not yet supported. This
is because the expansion of ‘library’ sets the current module, but
does not restore it. This is a bug.
• R6RS unicode escapes within strings are disabled by default,
because they conflict with Guile’s already-existing escapes. The
same is the case for R6RS treatment of escaped newlines in strings.
R6RS behavior can be turned on via a reader option. *Note String
Syntax::, for more information.
• Guile does not yet support Unicode escapes in symbols, such as
‘H\x65;llo’ (the same as ‘Hello’), or ‘\x3BB;’ (the same as ‘λ’).
• A ‘set!’ to a variable transformer may only expand to an
expression, not a definition—even if the original ‘set!’ expression
was in definition context.
• Instead of using the algorithm detailed in chapter 10 of the R6RS,
expansion of toplevel forms happens sequentially.
For example, while the expansion of the following set of toplevel
definitions does the correct thing:
(begin
(define even?
(lambda (x)
(or (= x 0) (odd? (- x 1)))))
(define-syntax odd?
(syntax-rules ()
((odd? x) (not (even? x)))))
(even? 10))
⇒ #t
The same definitions outside of the ‘begin’ wrapper do not:
(define even?
(lambda (x)
(or (= x 0) (odd? (- x 1)))))
(define-syntax odd?
(syntax-rules ()
((odd? x) (not (even? x)))))
(even? 10)
:4:18: In procedure even?:
:4:18: Wrong type to apply: #
This is because when expanding the right-hand-side of ‘even?’, the
reference to ‘odd?’ is not yet marked as a syntax transformer, so
it is assumed to be a function.
This bug will only affect top-level programs, not code in ‘library’
forms. Fixing it for toplevel forms seems doable, but tricky to
implement in a backward-compatible way. Suggestions and/or patches
would be appreciated.
• The ‘(rnrs io ports)’ module is incomplete. Work is ongoing to fix
this.
• Guile does not prevent use of textual I/O procedures on binary
ports, or vice versa. All ports in Guile support both binary and
textual I/O. *Note Encoding::, for full details.
• Guile’s implementation of ‘equal?’ may fail to terminate when
applied to arguments containing cycles.
Guile exposes a procedure in the root module to choose R6RS defaults
over Guile’s historical defaults.
-- Scheme Procedure: install-r6rs!
Alter Guile’s default settings to better conform to the R6RS.
While Guile’s defaults may evolve over time, the current changes
that this procedure imposes are to add ‘.sls’ and ‘.guile.sls’ to
the set of supported ‘%load-extensions’, to better support R6RS
conventions. *Note Load Paths::. Also, enable R6RS unicode
escapes in strings; see the discussion above.
Finally, note that the ‘--r6rs’ command-line argument will call
‘install-r6rs!’ before calling user code. R6RS users probably want to
pass this argument to their Guile.
7.6.2 R6RS Standard Libraries
-----------------------------
In contrast with earlier versions of the Revised Report, the R6RS
organizes the procedures and syntactic forms required of conforming
implementations into a set of “standard libraries” which can be imported
as necessary by user programs and libraries. Here we briefly list the
libraries that have been implemented for Guile.
We do not attempt to document these libraries fully here, as most of
their functionality is already available in Guile itself. The
expectation is that most Guile users will use the well-known and
well-documented Guile modules. These R6RS libraries are mostly useful
to users who want to port their code to other R6RS systems.
The documentation in the following sections reproduces some of the
content of the library section of the Report, but is mostly intended to
provide supplementary information about Guile’s implementation of the
R6RS standard libraries. For complete documentation, design rationales
and further examples, we advise you to consult the “Standard Libraries”
section of the Report (*note R6RS Standard Libraries: (r6rs)Standard
Libraries.).
7.6.2.1 Library Usage
.....................
Guile implements the R6RS ‘library’ form as a transformation to a native
Guile module definition. As a consequence of this, all of the libraries
described in the following subsections, in addition to being available
for use by R6RS libraries and top-level programs, can also be imported
as if they were normal Guile modules—via a ‘use-modules’ form, say. For
example, the R6RS “composite” library can be imported by:
(import (rnrs (6)))
(use-modules ((rnrs) :version (6)))
For more information on Guile’s library implementation, see (*note
R6RS Libraries::).
7.6.2.2 rnrs base
.................
The ‘(rnrs base (6))’ library exports the procedures and syntactic forms
described in the main section of the Report (*note R6RS Base library:
(r6rs)Base library.). They are grouped below by the existing manual
sections to which they correspond.
-- Scheme Procedure: boolean? obj
-- Scheme Procedure: not x
*Note Booleans::, for documentation.
-- Scheme Procedure: symbol? obj
-- Scheme Procedure: symbol->string sym
-- Scheme Procedure: string->symbol str
*Note Symbol Primitives::, for documentation.
-- Scheme Procedure: char? obj
-- Scheme Procedure: char=?
-- Scheme Procedure: char?
-- Scheme Procedure: char<=?
-- Scheme Procedure: char>=?
-- Scheme Procedure: integer->char n
-- Scheme Procedure: char->integer chr
*Note Characters::, for documentation.
-- Scheme Procedure: list? x
-- Scheme Procedure: null? x
*Note List Predicates::, for documentation.
-- Scheme Procedure: pair? x
-- Scheme Procedure: cons x y
-- Scheme Procedure: car pair
-- Scheme Procedure: cdr pair
-- Scheme Procedure: caar pair
-- Scheme Procedure: cadr pair
-- Scheme Procedure: cdar pair
-- Scheme Procedure: cddr pair
-- Scheme Procedure: caaar pair
-- Scheme Procedure: caadr pair
-- Scheme Procedure: cadar pair
-- Scheme Procedure: cdaar pair
-- Scheme Procedure: caddr pair
-- Scheme Procedure: cdadr pair
-- Scheme Procedure: cddar pair
-- Scheme Procedure: cdddr pair
-- Scheme Procedure: caaaar pair
-- Scheme Procedure: caaadr pair
-- Scheme Procedure: caadar pair
-- Scheme Procedure: cadaar pair
-- Scheme Procedure: cdaaar pair
-- Scheme Procedure: cddaar pair
-- Scheme Procedure: cdadar pair
-- Scheme Procedure: cdaadr pair
-- Scheme Procedure: cadadr pair
-- Scheme Procedure: caaddr pair
-- Scheme Procedure: caddar pair
-- Scheme Procedure: cadddr pair
-- Scheme Procedure: cdaddr pair
-- Scheme Procedure: cddadr pair
-- Scheme Procedure: cdddar pair
-- Scheme Procedure: cddddr pair
*Note Pairs::, for documentation.
-- Scheme Procedure: number? obj
*Note Numerical Tower::, for documentation.
-- Scheme Procedure: string? obj
*Note String Predicates::, for documentation.
-- Scheme Procedure: procedure? obj
*Note Procedure Properties::, for documentation.
-- Scheme Syntax: define name value
-- Scheme Syntax: set! variable-name value
*Note Definition::, for documentation.
-- Scheme Syntax: define-syntax keyword expression
-- Scheme Syntax: let-syntax ((keyword transformer) ...) exp1 exp2 ...
-- Scheme Syntax: letrec-syntax ((keyword transformer) ...) exp1 exp2
...
*Note Defining Macros::, for documentation.
-- Scheme Syntax: identifier-syntax exp
*Note Identifier Macros::, for documentation.
-- Scheme Syntax: syntax-rules literals (pattern template) ...
*Note Syntax Rules::, for documentation.
-- Scheme Syntax: lambda formals body
*Note Lambda::, for documentation.
-- Scheme Syntax: let bindings body
-- Scheme Syntax: let* bindings body
-- Scheme Syntax: letrec bindings body
-- Scheme Syntax: letrec* bindings body
*Note Local Bindings::, for documentation.
-- Scheme Syntax: let-values bindings body
-- Scheme Syntax: let*-values bindings body
*Note SRFI-11::, for documentation.
-- Scheme Syntax: begin expr1 expr2 ...
*Note begin::, for documentation.
-- Scheme Syntax: quote expr
-- Scheme Syntax: quasiquote expr
-- Scheme Syntax: unquote expr
-- Scheme Syntax: unquote-splicing expr
*Note Expression Syntax::, for documentation.
-- Scheme Syntax: if test consequence [alternate]
-- Scheme Syntax: cond clause1 clause2 ...
-- Scheme Syntax: case key clause1 clause2 ...
*Note Conditionals::, for documentation.
-- Scheme Syntax: and expr ...
-- Scheme Syntax: or expr ...
*Note and or::, for documentation.
-- Scheme Procedure: eq? x y
-- Scheme Procedure: eqv? x y
-- Scheme Procedure: equal? x y
-- Scheme Procedure: symbol=? symbol1 symbol2 ...
*Note Equality::, for documentation.
‘symbol=?’ is identical to ‘eq?’.
-- Scheme Procedure: complex? z
*Note Complex Numbers::, for documentation.
-- Scheme Procedure: real-part z
-- Scheme Procedure: imag-part z
-- Scheme Procedure: make-rectangular real_part imaginary_part
-- Scheme Procedure: make-polar x y
-- Scheme Procedure: magnitude z
-- Scheme Procedure: angle z
*Note Complex::, for documentation.
-- Scheme Procedure: sqrt z
-- Scheme Procedure: exp z
-- Scheme Procedure: expt z1 z2
-- Scheme Procedure: log z
-- Scheme Procedure: sin z
-- Scheme Procedure: cos z
-- Scheme Procedure: tan z
-- Scheme Procedure: asin z
-- Scheme Procedure: acos z
-- Scheme Procedure: atan z
*Note Scientific::, for documentation.
-- Scheme Procedure: real? x
-- Scheme Procedure: rational? x
-- Scheme Procedure: numerator x
-- Scheme Procedure: denominator x
-- Scheme Procedure: rationalize x eps
*Note Reals and Rationals::, for documentation.
-- Scheme Procedure: exact? x
-- Scheme Procedure: inexact? x
-- Scheme Procedure: exact z
-- Scheme Procedure: inexact z
*Note Exactness::, for documentation. The ‘exact’ and ‘inexact’
procedures are identical to the ‘inexact->exact’ and
‘exact->inexact’ procedures provided by Guile’s code library.
-- Scheme Procedure: integer? x
*Note Integers::, for documentation.
-- Scheme Procedure: odd? n
-- Scheme Procedure: even? n
-- Scheme Procedure: gcd x ...
-- Scheme Procedure: lcm x ...
-- Scheme Procedure: exact-integer-sqrt k
*Note Integer Operations::, for documentation.
-- Scheme Procedure: =
-- Scheme Procedure: <
-- Scheme Procedure: >
-- Scheme Procedure: <=
-- Scheme Procedure: >=
-- Scheme Procedure: zero? x
-- Scheme Procedure: positive? x
-- Scheme Procedure: negative? x
*Note Comparison::, for documentation.
-- Scheme Procedure: for-each f lst1 lst2 ...
*Note SRFI-1 Fold and Map::, for documentation.
-- Scheme Procedure: list elem ...
*Note List Constructors::, for documentation.
-- Scheme Procedure: length lst
-- Scheme Procedure: list-ref lst k
-- Scheme Procedure: list-tail lst k
*Note List Selection::, for documentation.
-- Scheme Procedure: append lst ... obj
-- Scheme Procedure: append
-- Scheme Procedure: reverse lst
*Note Append/Reverse::, for documentation.
-- Scheme Procedure: number->string n [radix]
-- Scheme Procedure: string->number str [radix]
*Note Conversion::, for documentation.
-- Scheme Procedure: string char ...
-- Scheme Procedure: make-string k [chr]
-- Scheme Procedure: list->string lst
*Note String Constructors::, for documentation.
-- Scheme Procedure: string->list str [start [end]]
*Note List/String Conversion::, for documentation.
-- Scheme Procedure: string-length str
-- Scheme Procedure: string-ref str k
-- Scheme Procedure: string-copy str [start [end]]
-- Scheme Procedure: substring str start [end]
*Note String Selection::, for documentation.
-- Scheme Procedure: string=? s1 s2 s3 ...
-- Scheme Procedure: string? s1 s2 s3 ...
-- Scheme Procedure: string<=? s1 s2 s3 ...
-- Scheme Procedure: string>=? s1 s2 s3 ...
*Note String Comparison::, for documentation.
-- Scheme Procedure: string-append arg ...
*Note Reversing and Appending Strings::, for documentation.
-- Scheme Procedure: string-for-each proc s [start [end]]
*Note Mapping Folding and Unfolding::, for documentation.
-- Scheme Procedure: + z1 ...
-- Scheme Procedure: - z1 z2 ...
-- Scheme Procedure: * z1 ...
-- Scheme Procedure: / z1 z2 ...
-- Scheme Procedure: max x1 x2 ...
-- Scheme Procedure: min x1 x2 ...
-- Scheme Procedure: abs x
-- Scheme Procedure: truncate x
-- Scheme Procedure: floor x
-- Scheme Procedure: ceiling x
-- Scheme Procedure: round x
*Note Arithmetic::, for documentation.
-- Scheme Procedure: div x y
-- Scheme Procedure: mod x y
-- Scheme Procedure: div-and-mod x y
These procedures accept two real numbers X and Y, where the divisor
Y must be non-zero. ‘div’ returns the integer Q and ‘mod’ returns
the real number R such that X = Q*Y + R and 0 <= R < abs(Y).
‘div-and-mod’ returns both Q and R, and is more efficient than
computing each separately. Note that when Y > 0, ‘div’ returns
floor(X/Y), otherwise it returns ceiling(X/Y).
(div 123 10) ⇒ 12
(mod 123 10) ⇒ 3
(div-and-mod 123 10) ⇒ 12 and 3
(div-and-mod 123 -10) ⇒ -12 and 3
(div-and-mod -123 10) ⇒ -13 and 7
(div-and-mod -123 -10) ⇒ 13 and 7
(div-and-mod -123.2 -63.5) ⇒ 2.0 and 3.8
(div-and-mod 16/3 -10/7) ⇒ -3 and 22/21
-- Scheme Procedure: div0 x y
-- Scheme Procedure: mod0 x y
-- Scheme Procedure: div0-and-mod0 x y
These procedures accept two real numbers X and Y, where the divisor
Y must be non-zero. ‘div0’ returns the integer Q and ‘mod0’
returns the real number R such that X = Q*Y + R and -abs(Y/2) <= R
< abs(Y/2). ‘div0-and-mod0’ returns both Q and R, and is more
efficient than computing each separately.
Note that ‘div0’ returns X/Y rounded to the nearest integer. When
X/Y lies exactly half-way between two integers, the tie is broken
according to the sign of Y. If Y > 0, ties are rounded toward
positive infinity, otherwise they are rounded toward negative
infinity. This is a consequence of the requirement that -abs(Y/2)
<= R < abs(Y/2).
(div0 123 10) ⇒ 12
(mod0 123 10) ⇒ 3
(div0-and-mod0 123 10) ⇒ 12 and 3
(div0-and-mod0 123 -10) ⇒ -12 and 3
(div0-and-mod0 -123 10) ⇒ -12 and -3
(div0-and-mod0 -123 -10) ⇒ 12 and -3
(div0-and-mod0 -123.2 -63.5) ⇒ 2.0 and 3.8
(div0-and-mod0 16/3 -10/7) ⇒ -4 and -8/21
-- Scheme Procedure: real-valued? obj
-- Scheme Procedure: rational-valued? obj
-- Scheme Procedure: integer-valued? obj
These procedures return ‘#t’ if and only if their arguments can,
respectively, be coerced to a real, rational, or integer value
without a loss of numerical precision.
‘real-valued?’ will return ‘#t’ for complex numbers whose imaginary
parts are zero.
-- Scheme Procedure: nan? x
-- Scheme Procedure: infinite? x
-- Scheme Procedure: finite? x
‘nan?’ returns ‘#t’ if X is a NaN value, ‘#f’ otherwise.
‘infinite?’ returns ‘#t’ if X is an infinite value, ‘#f’ otherwise.
‘finite?’ returns ‘#t’ if X is neither infinite nor a NaN value,
otherwise it returns ‘#f’. Every real number satisfies exactly one
of these predicates. An exception is raised if X is not real.
-- Scheme Syntax: assert expr
Raises an ‘&assertion’ condition if EXPR evaluates to ‘#f’;
otherwise evaluates to the value of EXPR.
-- Scheme Procedure: error who message irritant1 ...
-- Scheme Procedure: assertion-violation who message irritant1 ...
These procedures raise compound conditions based on their
arguments: If WHO is not ‘#f’, the condition will include a ‘&who’
condition whose ‘who’ field is set to WHO; a ‘&message’ condition
will be included with a ‘message’ field equal to MESSAGE; an
‘&irritants’ condition will be included with its ‘irritants’ list
given by ‘irritant1 ...’.
‘error’ produces a compound condition with the simple conditions
described above, as well as an ‘&error’ condition;
‘assertion-violation’ produces one that includes an ‘&assertion’
condition.
-- Scheme Procedure: vector-map proc v
-- Scheme Procedure: vector-for-each proc v
These procedures implement the ‘map’ and ‘for-each’ contracts over
vectors.
-- Scheme Procedure: vector arg ...
-- Scheme Procedure: vector? obj
-- Scheme Procedure: make-vector len
-- Scheme Procedure: make-vector len fill
-- Scheme Procedure: list->vector l
-- Scheme Procedure: vector->list v
*Note Vector Creation::, for documentation.
-- Scheme Procedure: vector-length vector
-- Scheme Procedure: vector-ref vector k
-- Scheme Procedure: vector-set! vector k obj
-- Scheme Procedure: vector-fill! v fill
*Note Vector Accessors::, for documentation.
-- Scheme Procedure: call-with-current-continuation proc
-- Scheme Procedure: call/cc proc
*Note Continuations::, for documentation.
-- Scheme Procedure: values arg ...
-- Scheme Procedure: call-with-values producer consumer
*Note Multiple Values::, for documentation.
-- Scheme Procedure: dynamic-wind in_guard thunk out_guard
*Note Dynamic Wind::, for documentation.
-- Scheme Procedure: apply proc arg ... arglst
*Note Fly Evaluation::, for documentation.
7.6.2.3 rnrs unicode
....................
The ‘(rnrs unicode (6))’ library provides procedures for manipulating
Unicode characters and strings.
-- Scheme Procedure: char-upcase char
-- Scheme Procedure: char-downcase char
-- Scheme Procedure: char-titlecase char
-- Scheme Procedure: char-foldcase char
These procedures translate their arguments from one Unicode
character set to another. ‘char-upcase’, ‘char-downcase’, and
‘char-titlecase’ are identical to their counterparts in the Guile
core library; *Note Characters::, for documentation.
‘char-foldcase’ returns the result of applying ‘char-upcase’ to its
argument, followed by ‘char-downcase’—except in the case of the
Turkic characters ‘U+0130’ and ‘U+0131’, for which the procedure
acts as the identity function.
-- Scheme Procedure: char-ci=? char1 char2 char3 ...
-- Scheme Procedure: char-ci? char1 char2 char3 ...
-- Scheme Procedure: char-ci<=? char1 char2 char3 ...
-- Scheme Procedure: char-ci>=? char1 char2 char3 ...
These procedures facilitate case-insensitive comparison of Unicode
characters. They are identical to the procedures provided by
Guile’s core library. *Note Characters::, for documentation.
-- Scheme Procedure: char-alphabetic? char
-- Scheme Procedure: char-numeric? char
-- Scheme Procedure: char-whitespace? char
-- Scheme Procedure: char-upper-case? char
-- Scheme Procedure: char-lower-case? char
-- Scheme Procedure: char-title-case? char
These procedures implement various Unicode character set
predicates. They are identical to the procedures provided by
Guile’s core library. *Note Characters::, for documentation.
-- Scheme Procedure: char-general-category char
*Note Characters::, for documentation.
-- Scheme Procedure: string-upcase string
-- Scheme Procedure: string-downcase string
-- Scheme Procedure: string-titlecase string
-- Scheme Procedure: string-foldcase string
These procedures perform Unicode case folding operations on their
input. *Note Alphabetic Case Mapping::, for documentation.
-- Scheme Procedure: string-ci=? string1 string2 string3 ...
-- Scheme Procedure: string-ci? string1 string2 string3 ...
-- Scheme Procedure: string-ci<=? string1 string2 string3 ...
-- Scheme Procedure: string-ci>=? string1 string2 string3 ...
These procedures perform case-insensitive comparison on their
input. *Note String Comparison::, for documentation.
-- Scheme Procedure: string-normalize-nfd string
-- Scheme Procedure: string-normalize-nfkd string
-- Scheme Procedure: string-normalize-nfc string
-- Scheme Procedure: string-normalize-nfkc string
These procedures perform Unicode string normalization operations on
their input. *Note String Comparison::, for documentation.
7.6.2.4 rnrs bytevectors
........................
The ‘(rnrs bytevectors (6))’ library provides procedures for working
with blocks of binary data. This functionality is documented in its own
section of the manual; *Note Bytevectors::.
7.6.2.5 rnrs lists
..................
The ‘(rnrs lists (6))’ library provides procedures additional procedures
for working with lists.
-- Scheme Procedure: find proc list
This procedure is identical to the one defined in Guile’s SRFI-1
implementation. *Note SRFI-1 Searching::, for documentation.
-- Scheme Procedure: for-all proc list1 list2 ...
-- Scheme Procedure: exists proc list1 list2 ...
The ‘for-all’ procedure is identical to the ‘every’ procedure
defined by SRFI-1; the ‘exists’ procedure is identical to SRFI-1’s
‘any’. *Note SRFI-1 Searching::, for documentation.
-- Scheme Procedure: filter proc list
-- Scheme Procedure: partition proc list
These procedures are identical to the ones provided by SRFI-1.
*Note List Modification::, for a description of ‘filter’; *Note
SRFI-1 Filtering and Partitioning::, for ‘partition’.
-- Scheme Procedure: fold-right combine nil list1 list2 ...
This procedure is identical the ‘fold-right’ procedure provided by
SRFI-1. *Note SRFI-1 Fold and Map::, for documentation.
-- Scheme Procedure: fold-left combine nil list1 list2 ...
This procedure is like ‘fold’ from SRFI-1, but COMBINE is called
with the seed as the first argument. *Note SRFI-1 Fold and Map::,
for documentation.
-- Scheme Procedure: remp proc list
-- Scheme Procedure: remove obj list
-- Scheme Procedure: remv obj list
-- Scheme Procedure: remq obj list
‘remove’, ‘remv’, and ‘remq’ are identical to the ‘delete’, ‘delv’,
and ‘delq’ procedures provided by Guile’s core library, (*note List
Modification::). ‘remp’ is identical to the alternate ‘remove’
procedure provided by SRFI-1; *Note SRFI-1 Deleting::.
-- Scheme Procedure: memp proc list
-- Scheme Procedure: member obj list
-- Scheme Procedure: memv obj list
-- Scheme Procedure: memq obj list
‘member’, ‘memv’, and ‘memq’ are identical to the procedures
provided by Guile’s core library; *Note List Searching::, for their
documentation. ‘memp’ uses the specified predicate function ‘proc’
to test elements of the list LIST—it behaves similarly to ‘find’,
except that it returns the first sublist of LIST whose ‘car’
satisfies PROC.
-- Scheme Procedure: assp proc alist
-- Scheme Procedure: assoc obj alist
-- Scheme Procedure: assv obj alist
-- Scheme Procedure: assq obj alist
‘assoc’, ‘assv’, and ‘assq’ are identical to the procedures
provided by Guile’s core library; *Note Alist Key Equality::, for
their documentation. ‘assp’ uses the specified predicate function
‘proc’ to test keys in the association list ALIST.
-- Scheme Procedure: cons* obj1 ... obj
-- Scheme Procedure: cons* obj
This procedure is identical to the one exported by Guile’s core
library. *Note List Constructors::, for documentation.
7.6.2.6 rnrs sorting
....................
The ‘(rnrs sorting (6))’ library provides procedures for sorting lists
and vectors.
-- Scheme Procedure: list-sort proc list
-- Scheme Procedure: vector-sort proc vector
These procedures return their input sorted in ascending order,
without modifying the original data. PROC must be a procedure that
takes two elements from the input list or vector as arguments, and
returns a true value if the first is “less” than the second, ‘#f’
otherwise. ‘list-sort’ returns a list; ‘vector-sort’ returns a
vector.
Both ‘list-sort’ and ‘vector-sort’ are implemented in terms of the
‘stable-sort’ procedure from Guile’s core library. *Note
Sorting::, for a discussion of the behavior of that procedure.
-- Scheme Procedure: vector-sort! proc vector
Performs a destructive, “in-place” sort of VECTOR, using PROC as
described above to determine an ascending ordering of elements.
‘vector-sort!’ returns an unspecified value.
This procedure is implemented in terms of the ‘sort!’ procedure
from Guile’s core library. *Note Sorting::, for more information.
7.6.2.7 rnrs control
....................
The ‘(rnrs control (6))’ library provides syntactic forms useful for
constructing conditional expressions and controlling the flow of
execution.
-- Scheme Syntax: when test expression1 expression2 ...
-- Scheme Syntax: unless test expression1 expression2 ...
The ‘when’ form is evaluated by evaluating the specified TEST
expression; if the result is a true value, the EXPRESSIONs that
follow it are evaluated in order, and the value of the final
EXPRESSION becomes the value of the entire ‘when’ expression.
The ‘unless’ form behaves similarly, with the exception that the
specified EXPRESSIONs are only evaluated if the value of TEST is
false.
-- Scheme Syntax: do ((variable init step) ...) (test expression ...)
command ...
This form is identical to the one provided by Guile’s core library.
*Note while do::, for documentation.
-- Scheme Syntax: case-lambda clause ...
This form is identical to the one provided by Guile’s core library.
*Note Case-lambda::, for documentation.
7.6.2.8 R6RS Records
....................
The manual sections below describe Guile’s implementation of R6RS
records, which provide support for user-defined data types. The R6RS
records API provides a superset of the features provided by Guile’s
“native” records, as well as those of the SRFI-9 records API; *Note
Records::, and *note SRFI-9 Records::, for a description of those
interfaces.
As with SRFI-9 and Guile’s native records, R6RS records are
constructed using a record-type descriptor that specifies attributes
like the record’s name, its fields, and the mutability of those fields.
R6RS records extend this framework to support single inheritance via
the specification of a “parent” type for a record type at definition
time. Accessors and mutator procedures for the fields of a parent type
may be applied to records of a subtype of this parent. A record type
may be “sealed”, in which case it cannot be used as the parent of
another record type.
The inheritance mechanism for record types also informs the process
of initializing the fields of a record and its parents. Constructor
procedures that generate new instances of a record type are obtained
from a record constructor descriptor, which encapsulates the record-type
descriptor of the record to be constructed along with a “protocol”
procedure that defines how constructors for record subtypes delegate to
the constructors of their parent types.
A protocol is a procedure used by the record system at construction
time to bind arguments to the fields of the record being constructed.
The protocol procedure is passed a procedure N that accepts the
arguments required to construct the record’s parent type; this
procedure, when invoked, will return a procedure P that accepts the
arguments required to construct a new instance of the record type itself
and returns a new instance of the record type.
The protocol should in turn return a procedure that uses N and P to
initialize the fields of the record type and its parent type(s). This
procedure will be the constructor returned by
As a trivial example, consider the hypothetical record type ‘pixel’,
which encapsulates an x-y location on a screen, and ‘voxel’, which has
‘pixel’ as its parent type and stores an additional coordinate. The
following protocol produces a constructor procedure that accepts all
three coordinates, uses the first two to initialize the fields of
‘pixel’, and binds the third to the single field of ‘voxel’.
(lambda (n)
(lambda (x y z)
(let ((p (n x y)))
(p z))))
It may be helpful to think of protocols as “constructor factories”
that produce chains of delegating constructors glued together by the
helper procedure N.
An R6RS record type may be declared to be “nongenerative” via the use
of a unique generated or user-supplied symbol—or “uid”—such that
subsequent record type declarations with the same uid and attributes
will return the previously-declared record-type descriptor.
R6RS record types may also be declared to be “opaque”, in which case
the various predicates and introspection procedures defined in ‘(rnrs
records introspection)’ will behave as if records of this type are not
records at all.
Note that while the R6RS records API shares much of its namespace
with both the SRFI-9 and native Guile records APIs, it is not currently
compatible with either.
7.6.2.9 rnrs records syntactic
..............................
The ‘(rnrs records syntactic (6))’ library exports the syntactic API for
working with R6RS records.
-- Scheme Syntax: define-record-type name-spec record-clause ...
Defines a new record type, introducing bindings for a record-type
descriptor, a record constructor descriptor, a constructor
procedure, a record predicate, and accessor and mutator procedures
for the new record type’s fields.
NAME-SPEC must either be an identifier or must take the form
‘(record-name constructor-name predicate-name)’, where RECORD-NAME,
CONSTRUCTOR-NAME, and PREDICATE-NAME are all identifiers and
specify the names to which, respectively, the record-type
descriptor, constructor, and predicate procedures will be bound.
If NAME-SPEC is only an identifier, it specifies the name to which
the generated record-type descriptor will be bound.
Each RECORD-CLAUSE must be one of the following:
• ‘(fields field-spec*)’, where each FIELD-SPEC specifies a
field of the new record type and takes one of the following
forms:
• ‘(immutable field-name accessor-name)’, which specifies
an immutable field with the name FIELD-NAME and binds an
accessor procedure for it to the name given by
ACCESSOR-NAME
• ‘(mutable field-name accessor-name mutator-name)’, which
specifies a mutable field with the name FIELD-NAME and
binds accessor and mutator procedures to ACCESSOR-NAME
and MUTATOR-NAME, respectively
• ‘(immutable field-name)’, which specifies an immutable
field with the name FIELD-NAME; an accessor procedure for
it will be created and named by appending record name and
FIELD-NAME with a hyphen separator
• ‘(mutable field-name’), which specifies a mutable field
with the name FIELD-NAME; an accessor procedure for it
will be created and named as described above; a mutator
procedure will also be created and named by appending
‘-set!’ to the accessor name
• ‘field-name’, which specifies an immutable field with the
name FIELD-NAME; an access procedure for it will be
created and named as described above
• ‘(parent parent-name)’, where PARENT-NAME is a symbol giving
the name of the record type to be used as the parent of the
new record type
• ‘(protocol expression)’, where EXPRESSION evaluates to a
protocol procedure which behaves as described above, and is
used to create a record constructor descriptor for the new
record type
• ‘(sealed sealed?)’, where SEALED? is a boolean value that
specifies whether or not the new record type is sealed
• ‘(opaque opaque?)’, where OPAQUE? is a boolean value that
specifies whether or not the new record type is opaque
• ‘(nongenerative [uid])’, which specifies that the record type
is nongenerative via the optional uid UID. If UID is not
specified, a unique uid will be generated at expansion time
• ‘(parent-rtd parent-rtd parent-cd)’, a more explicit form of
the ‘parent’ form above; PARENT-RTD and PARENT-CD should
evaluate to a record-type descriptor and a record constructor
descriptor, respectively
-- Scheme Syntax: record-type-descriptor record-name
Evaluates to the record-type descriptor associated with the type
specified by RECORD-NAME.
-- Scheme Syntax: record-constructor-descriptor record-name
Evaluates to the record-constructor descriptor associated with the
type specified by RECORD-NAME.
7.6.2.10 rnrs records procedural
................................
The ‘(rnrs records procedural (6))’ library exports the procedural API
for working with R6RS records.
-- Scheme Procedure: make-record-type-descriptor name parent uid
sealed? opaque? fields
Returns a new record-type descriptor with the specified
characteristics: NAME must be a symbol giving the name of the new
record type; PARENT must be either ‘#f’ or a non-sealed record-type
descriptor for the returned record type to extend; UID must be
either ‘#f’, indicating that the record type is generative, or a
symbol giving the type’s nongenerative uid; SEALED? and OPAQUE?
must be boolean values that specify the sealedness and opaqueness
of the record type; FIELDS must be a vector of zero or more field
specifiers of the form ‘(mutable name)’ or ‘(immutable name)’,
where name is a symbol giving a name for the field.
If UID is not ‘#f’, it must be a symbol
-- Scheme Procedure: record-type-descriptor? obj
Returns ‘#t’ if OBJ is a record-type descriptor, ‘#f’ otherwise.
-- Scheme Procedure: make-record-constructor-descriptor rtd
parent-constructor-descriptor protocol
Returns a new record constructor descriptor that can be used to
produce constructors for the record type specified by the
record-type descriptor RTD and whose delegation and binding
behavior are specified by the protocol procedure PROTOCOL.
PARENT-CONSTRUCTOR-DESCRIPTOR specifies a record constructor
descriptor for the parent type of RTD, if one exists. If RTD
represents a base type, then PARENT-CONSTRUCTOR-DESCRIPTOR must be
‘#f’. If RTD is an extension of another type,
PARENT-CONSTRUCTOR-DESCRIPTOR may still be ‘#f’, but protocol must
also be ‘#f’ in this case.
-- Scheme Procedure: record-constructor rcd
Returns a record constructor procedure by invoking the protocol
defined by the record-constructor descriptor RCD.
-- Scheme Procedure: record-predicate rtd
Returns the record predicate procedure for the record-type
descriptor RTD.
-- Scheme Procedure: record-accessor rtd k
Returns the record field accessor procedure for the Kth field of
the record-type descriptor RTD.
-- Scheme Procedure: record-mutator rtd k
Returns the record field mutator procedure for the Kth field of the
record-type descriptor RTD. An ‘&assertion’ condition will be
raised if this field is not mutable.
7.6.2.11 rnrs records inspection
................................
The ‘(rnrs records inspection (6))’ library provides procedures useful
for accessing metadata about R6RS records.
-- Scheme Procedure: record? obj
Return ‘#t’ if the specified object is a non-opaque R6RS record,
‘#f’ otherwise.
-- Scheme Procedure: record-rtd record
Returns the record-type descriptor for RECORD. An ‘&assertion’ is
raised if RECORD is opaque.
-- Scheme Procedure: record-type-name rtd
Returns the name of the record-type descriptor RTD.
-- Scheme Procedure: record-type-parent rtd
Returns the parent of the record-type descriptor RTD, or ‘#f’ if it
has none.
-- Scheme Procedure: record-type-uid rtd
Returns the uid of the record-type descriptor RTD, or ‘#f’ if it
has none.
-- Scheme Procedure: record-type-generative? rtd
Returns ‘#t’ if the record-type descriptor RTD is generative, ‘#f’
otherwise.
-- Scheme Procedure: record-type-sealed? rtd
Returns ‘#t’ if the record-type descriptor RTD is sealed, ‘#f’
otherwise.
-- Scheme Procedure: record-type-opaque? rtd
Returns ‘#t’ if the record-type descriptor RTD is opaque, ‘#f’
otherwise.
-- Scheme Procedure: record-type-field-names rtd
Returns a vector of symbols giving the names of the fields defined
by the record-type descriptor RTD (and not any of its sub- or
supertypes).
-- Scheme Procedure: record-field-mutable? rtd k
Returns ‘#t’ if the field at index K of the record-type descriptor
RTD (and not any of its sub- or supertypes) is mutable.
7.6.2.12 rnrs exceptions
........................
The ‘(rnrs exceptions (6))’ library provides functionality related to
signaling and handling exceptional situations. This functionality
re-exports Guile’s core exception-handling primitives. *Note
Exceptions::, for a full discussion. *Note SRFI-34::, for a similar
pre-R6RS facility. In Guile, SRFI-34, SRFI-35, and R6RS exception
handling are all built on the same core facilities, and so are
interoperable.
-- Scheme Procedure: with-exception-handler handler thunk
*Note Raising and Handling Exceptions::, for more information on
‘with-exception-handler’.
-- Scheme Syntax: guard (variable clause1 clause2 ...) body
Evaluates the expression given by BODY, first creating an ad hoc
exception handler that binds a raised exception to VARIABLE and
then evaluates the specified CLAUSEs as if they were part of a
‘cond’ expression, with the value of the first matching clause
becoming the value of the ‘guard’ expression (*note
Conditionals::). If none of the clause’s test expressions
evaluates to ‘#t’, the exception is re-raised, with the exception
handler that was current before the evaluation of the ‘guard’ form.
For example, the expression
(guard (ex ((eq? ex 'foo) 'bar) ((eq? ex 'bar) 'baz))
(raise 'bar))
evaluates to ‘baz’.
-- Scheme Procedure: raise obj
Equivalent to core Guile ‘(raise-exception OBJ)’. *Note Raising
and Handling Exceptions::. (Unfortunately, ‘raise’ is already
bound to a different function in core Guile. *Note Signals::.)
-- Scheme Procedure: raise-continuable obj
Equivalent to core Guile ‘(raise-exception OBJ #:continuable? #t)’.
*Note Raising and Handling Exceptions::.
7.6.2.13 rnrs conditions
........................
The ‘(rnrs condition (6))’ library provides forms and procedures for
constructing new condition types, as well as a library of pre-defined
condition types that represent a variety of common exceptional
situations. Conditions are records of a subtype of the ‘&condition’
record type, which is neither sealed nor opaque. *Note R6RS Records::.
Conditions may be manipulated singly, as “simple conditions”, or when
composed with other conditions to form “compound conditions”. Compound
conditions do not “nest”—constructing a new compound condition out of
existing compound conditions will “flatten” them into their component
simple conditions. For example, making a new condition out of a
‘&message’ condition and a compound condition that contains an
‘&assertion’ condition and another ‘&message’ condition will produce a
compound condition that contains two ‘&message’ conditions and one
‘&assertion’ condition.
The record type predicates and field accessors described below can
operate on either simple or compound conditions. In the latter case,
the predicate returns ‘#t’ if the compound condition contains a
component simple condition of the appropriate type; the field accessors
return the requisite fields from the first component simple condition
found to be of the appropriate type.
Guile’s R6RS layer uses core exception types from the ‘(ice-9
exceptions)’ module as the basis for its R6RS condition system. Guile
prefers to use the term “exception object” and “exception type” rather
than “condition” or “condition type”, but that’s just a naming
difference. Guile also has different names for the types in the
condition hierarchy. *Note Exception Objects::, for full details.
This library is quite similar to the SRFI-35 conditions module (*note
SRFI-35::). Among other minor differences, the ‘(rnrs conditions)’
library features slightly different semantics around condition field
accessors, and comes with a larger number of pre-defined condition
types. The two APIs are compatible; the ‘condition?’ predicate from one
API will return ‘#t’ when applied to a condition object created in the
other. of the condition types are the same, also.
-- Condition Type: &condition
-- Scheme Procedure: condition? obj
The base record type for conditions. Known as ‘&exception’ in core
Guile.
-- Scheme Procedure: condition condition1 ...
-- Scheme Procedure: simple-conditions condition
The ‘condition’ procedure creates a new compound condition out of
its condition arguments, flattening any specified compound
conditions into their component simple conditions as described
above.
‘simple-conditions’ returns a list of the component simple
conditions of the compound condition ‘condition’, in the order in
which they were specified at construction time.
-- Scheme Procedure: condition-predicate rtd
-- Scheme Procedure: condition-accessor rtd proc
These procedures return condition predicate and accessor procedures
for the specified condition record type RTD.
-- Scheme Syntax: define-condition-type condition-type supertype
constructor predicate field-spec ...
Evaluates to a new record type definition for a condition type with
the name CONDITION-TYPE that has the condition type SUPERTYPE as
its parent. A default constructor, which binds its arguments to
the fields of this type and its parent types, will be bound to the
identifier CONSTRUCTOR; a condition predicate will be bound to
PREDICATE. The fields of the new type, which are immutable, are
specified by the FIELD-SPECs, each of which must be of the form:
(field accessor)
where FIELD gives the name of the field and ACCESSOR gives the name
for a binding to an accessor procedure created for this field.
-- Condition Type: &message
-- Scheme Procedure: make-message-condition message
-- Scheme Procedure: message-condition? obj
-- Scheme Procedure: condition-message condition
A type that includes a message describing the condition that
occurred.
-- Condition Type: &warning
-- Scheme Procedure: make-warning
-- Scheme Procedure: warning? obj
A base type for representing non-fatal conditions during execution.
-- Condition Type: &serious
-- Scheme Procedure: make-serious-condition
-- Scheme Procedure: serious-condition? obj
A base type for conditions representing errors serious enough that
cannot be ignored. Known as ‘&error’ in core Guile.
-- Condition Type: &error
-- Scheme Procedure: make-error
-- Scheme Procedure: error? obj
A base type for conditions representing errors. Known as
‘&external-error’ in core Guile.
-- Condition Type: &violation
-- Scheme Procedure: make-violation
-- Scheme Procedure: violation?
A subtype of ‘&serious’ that can be used to represent violations of
a language or library standard. Known as ‘&programming-error’ in
core Guile.
-- Condition Type: &assertion
-- Scheme Procedure: make-assertion-violation
-- Scheme Procedure: assertion-violation? obj
A subtype of ‘&violation’ that indicates an invalid call to a
procedure. Known as ‘&assertion-failure’ in core Guile.
-- Condition Type: &irritants
-- Scheme Procedure: make-irritants-condition irritants
-- Scheme Procedure: irritants-condition? obj
-- Scheme Procedure: condition-irritants condition
A base type used for storing information about the causes of
another condition in a compound condition.
-- Condition Type: &who
-- Scheme Procedure: make-who-condition who
-- Scheme Procedure: who-condition? obj
-- Scheme Procedure: condition-who condition
A base type used for storing the identity, a string or symbol, of
the entity responsible for another condition in a compound
condition.
-- Condition Type: &non-continuable
-- Scheme Procedure: make-non-continuable-violation
-- Scheme Procedure: non-continuable-violation? obj
A subtype of ‘&violation’ used to indicate that an exception
handler invoked by ‘raise’ has returned locally.
-- Condition Type: &implementation-restriction
-- Scheme Procedure: make-implementation-restriction-violation
-- Scheme Procedure: implementation-restriction-violation? obj
A subtype of ‘&violation’ used to indicate a violation of an
implementation restriction.
-- Condition Type: &lexical
-- Scheme Procedure: make-lexical-violation
-- Scheme Procedure: lexical-violation? obj
A subtype of ‘&violation’ used to indicate a syntax violation at
the level of the datum syntax.
-- Condition Type: &syntax
-- Scheme Procedure: make-syntax-violation form subform
-- Scheme Procedure: syntax-violation? obj
-- Scheme Procedure: syntax-violation-form condition
-- Scheme Procedure: syntax-violation-subform condition
A subtype of ‘&violation’ that indicates a syntax violation. The
FORM and SUBFORM fields, which must be datum values, indicate the
syntactic form responsible for the condition.
-- Condition Type: &undefined
-- Scheme Procedure: make-undefined-violation
-- Scheme Procedure: undefined-violation? obj
A subtype of ‘&violation’ that indicates a reference to an unbound
identifier. Known as ‘&undefined-variable’ in core Guile.
7.6.2.14 I/O Conditions
.......................
These condition types are exported by both the ‘(rnrs io ports (6))’ and
‘(rnrs io simple (6))’ libraries.
-- Condition Type: &i/o
-- Scheme Procedure: make-i/o-error
-- Scheme Procedure: i/o-error? obj
A condition supertype for more specific I/O errors.
-- Condition Type: &i/o-read
-- Scheme Procedure: make-i/o-read-error
-- Scheme Procedure: i/o-read-error? obj
A subtype of ‘&i/o’; represents read-related I/O errors.
-- Condition Type: &i/o-write
-- Scheme Procedure: make-i/o-write-error
-- Scheme Procedure: i/o-write-error? obj
A subtype of ‘&i/o’; represents write-related I/O errors.
-- Condition Type: &i/o-invalid-position
-- Scheme Procedure: make-i/o-invalid-position-error position
-- Scheme Procedure: i/o-invalid-position-error? obj
-- Scheme Procedure: i/o-error-position condition
A subtype of ‘&i/o’; represents an error related to an attempt to
set the file position to an invalid position.
-- Condition Type: &i/o-filename
-- Scheme Procedure: make-io-filename-error filename
-- Scheme Procedure: i/o-filename-error? obj
-- Scheme Procedure: i/o-error-filename condition
A subtype of ‘&i/o’; represents an error related to an operation on
a named file.
-- Condition Type: &i/o-file-protection
-- Scheme Procedure: make-i/o-file-protection-error filename
-- Scheme Procedure: i/o-file-protection-error? obj
A subtype of ‘&i/o-filename’; represents an error resulting from an
attempt to access a named file for which the caller had
insufficient permissions.
-- Condition Type: &i/o-file-is-read-only
-- Scheme Procedure: make-i/o-file-is-read-only-error filename
-- Scheme Procedure: i/o-file-is-read-only-error? obj
A subtype of ‘&i/o-file-protection’; represents an error related to
an attempt to write to a read-only file.
-- Condition Type: &i/o-file-already-exists
-- Scheme Procedure: make-i/o-file-already-exists-error filename
-- Scheme Procedure: i/o-file-already-exists-error? obj
A subtype of ‘&i/o-filename’; represents an error related to an
operation on an existing file that was assumed not to exist.
-- Condition Type: &i/o-file-does-not-exist
-- Scheme Procedure: make-i/o-file-does-not-exist-error
-- Scheme Procedure: i/o-file-does-not-exist-error? obj
A subtype of ‘&i/o-filename’; represents an error related to an
operation on a non-existent file that was assumed to exist.
-- Condition Type: &i/o-port
-- Scheme Procedure: make-i/o-port-error port
-- Scheme Procedure: i/o-port-error? obj
-- Scheme Procedure: i/o-error-port condition
A subtype of ‘&i/o’; represents an error related to an operation on
the port PORT.
7.6.2.15 Transcoders
....................
The transcoder facilities are exported by ‘(rnrs io ports)’.
Several different Unicode encoding schemes describe standard ways to
encode characters and strings as byte sequences and to decode those
sequences. Within this document, a “codec” is an immutable Scheme
object that represents a Unicode or similar encoding scheme.
An “end-of-line style” is a symbol that, if it is not ‘none’,
describes how a textual port transcodes representations of line endings.
A “transcoder” is an immutable Scheme object that combines a codec
with an end-of-line style and a method for handling decoding errors.
Each transcoder represents some specific bidirectional (but not
necessarily lossless), possibly stateful translation between byte
sequences and Unicode characters and strings. Every transcoder can
operate in the input direction (bytes to characters) or in the output
direction (characters to bytes). A TRANSCODER parameter name means that
the corresponding argument must be a transcoder.
A “binary port” is a port that supports binary I/O, does not have an
associated transcoder and does not support textual I/O. A “textual port”
is a port that supports textual I/O, and does not support binary I/O. A
textual port may or may not have an associated transcoder.
-- Scheme Procedure: latin-1-codec
-- Scheme Procedure: utf-8-codec
-- Scheme Procedure: utf-16-codec
These are predefined codecs for the ISO 8859-1, UTF-8, and UTF-16
encoding schemes.
A call to any of these procedures returns a value that is equal in
the sense of ‘eqv?’ to the result of any other call to the same
procedure.
-- Scheme Syntax: eol-style EOL-STYLE-SYMBOL
EOL-STYLE-SYMBOL should be a symbol whose name is one of ‘lf’,
‘cr’, ‘crlf’, ‘nel’, ‘crnel’, ‘ls’, and ‘none’.
The form evaluates to the corresponding symbol. If the name of
EOL-STYLE-SYMBOL is not one of these symbols, the effect and result
are implementation-dependent; in particular, the result may be an
eol-style symbol acceptable as an EOL-STYLE argument to
‘make-transcoder’. Otherwise, an exception is raised.
All eol-style symbols except ‘none’ describe a specific line-ending
encoding:
‘lf’
linefeed
‘cr’
carriage return
‘crlf’
carriage return, linefeed
‘nel’
next line
‘crnel’
carriage return, next line
‘ls’
line separator
For a textual port with a transcoder, and whose transcoder has an
eol-style symbol ‘none’, no conversion occurs. For a textual input
port, any eol-style symbol other than ‘none’ means that all of the
above line-ending encodings are recognized and are translated into
a single linefeed. For a textual output port, ‘none’ and ‘lf’ are
equivalent. Linefeed characters are encoded according to the
specified eol-style symbol, and all other characters that
participate in possible line endings are encoded as is.
Note: Only the name of EOL-STYLE-SYMBOL is significant.
-- Scheme Procedure: native-eol-style
Returns the default end-of-line style of the underlying platform,
e.g., ‘lf’ on Unix and ‘crlf’ on Windows.
-- Condition Type: &i/o-decoding
-- Scheme Procedure: make-i/o-decoding-error port
-- Scheme Procedure: i/o-decoding-error? obj
This condition type could be defined by
(define-condition-type &i/o-decoding &i/o-port
make-i/o-decoding-error i/o-decoding-error?)
An exception with this type is raised when one of the operations
for textual input from a port encounters a sequence of bytes that
cannot be translated into a character or string by the input
direction of the port’s transcoder.
When such an exception is raised, the port’s position is past the
invalid encoding.
-- Condition Type: &i/o-encoding
-- Scheme Procedure: make-i/o-encoding-error port char
-- Scheme Procedure: i/o-encoding-error? obj
-- Scheme Procedure: i/o-encoding-error-char condition
This condition type could be defined by
(define-condition-type &i/o-encoding &i/o-port
make-i/o-encoding-error i/o-encoding-error?
(char i/o-encoding-error-char))
An exception with this type is raised when one of the operations
for textual output to a port encounters a character that cannot be
translated into bytes by the output direction of the port’s
transcoder. CHAR is the character that could not be encoded.
-- Scheme Syntax: error-handling-mode ERROR-HANDLING-MODE-SYMBOL
ERROR-HANDLING-MODE-SYMBOL should be a symbol whose name is one of
‘ignore’, ‘raise’, and ‘replace’. The form evaluates to the
corresponding symbol. If ERROR-HANDLING-MODE-SYMBOL is not one of
these identifiers, effect and result are implementation-dependent:
The result may be an error-handling-mode symbol acceptable as a
HANDLING-MODE argument to ‘make-transcoder’. If it is not
acceptable as a HANDLING-MODE argument to ‘make-transcoder’, an
exception is raised.
Note: Only the name of ERROR-HANDLING-MODE-SYMBOL is
significant.
The error-handling mode of a transcoder specifies the behavior of
textual I/O operations in the presence of encoding or decoding
errors.
If a textual input operation encounters an invalid or incomplete
character encoding, and the error-handling mode is ‘ignore’, an
appropriate number of bytes of the invalid encoding are ignored and
decoding continues with the following bytes.
If the error-handling mode is ‘replace’, the replacement character
U+FFFD is injected into the data stream, an appropriate number of
bytes are ignored, and decoding continues with the following bytes.
If the error-handling mode is ‘raise’, an exception with condition
type ‘&i/o-decoding’ is raised.
If a textual output operation encounters a character it cannot
encode, and the error-handling mode is ‘ignore’, the character is
ignored and encoding continues with the next character. If the
error-handling mode is ‘replace’, a codec-specific replacement
character is emitted by the transcoder, and encoding continues with
the next character. The replacement character is U+FFFD for
transcoders whose codec is one of the Unicode encodings, but is the
‘?’ character for the Latin-1 encoding. If the error-handling mode
is ‘raise’, an exception with condition type ‘&i/o-encoding’ is
raised.
-- Scheme Procedure: make-transcoder codec
-- Scheme Procedure: make-transcoder codec eol-style
-- Scheme Procedure: make-transcoder codec eol-style handling-mode
CODEC must be a codec; EOL-STYLE, if present, an eol-style symbol;
and HANDLING-MODE, if present, an error-handling-mode symbol.
EOL-STYLE may be omitted, in which case it defaults to the native
end-of-line style of the underlying platform. HANDLING-MODE may be
omitted, in which case it defaults to ‘replace’. The result is a
transcoder with the behavior specified by its arguments.
-- Scheme procedure: native-transcoder
Returns an implementation-dependent transcoder that represents a
possibly locale-dependent “native” transcoding.
-- Scheme Procedure: transcoder-codec transcoder
-- Scheme Procedure: transcoder-eol-style transcoder
-- Scheme Procedure: transcoder-error-handling-mode transcoder
These are accessors for transcoder objects; when applied to a
transcoder returned by ‘make-transcoder’, they return the CODEC,
EOL-STYLE, and HANDLING-MODE arguments, respectively.
-- Scheme Procedure: bytevector->string bytevector transcoder
Returns the string that results from transcoding the BYTEVECTOR
according to the input direction of the transcoder.
-- Scheme Procedure: string->bytevector string transcoder
Returns the bytevector that results from transcoding the STRING
according to the output direction of the transcoder.
7.6.2.16 rnrs io ports
......................
Guile’s binary and textual port interface was heavily inspired by R6RS,
so many R6RS port interfaces are documented elsewhere. Note that R6RS
ports are not disjoint from Guile’s native ports, so Guile-specific
procedures will work on ports created using the R6RS API, and vice
versa. Also note that in Guile, all ports are both textual and binary.
*Note Input and Output::, for more on Guile’s core port API. The R6RS
ports module wraps Guile’s I/O routines in a helper that will translate
native Guile exceptions to R6RS conditions; *Note R6RS I/O Conditions::,
for more. *Note R6RS File Ports::, for documentation on the R6RS file
port interface.
_Note_: The implementation of this R6RS API is not complete yet.
-- Scheme Procedure: eof-object? obj
*Note Binary I/O::, for documentation.
-- Scheme Procedure: eof-object
Return the end-of-file (EOF) object.
(eof-object? (eof-object))
⇒ #t
-- Scheme Procedure: port? obj
-- Scheme Procedure: input-port? obj
-- Scheme Procedure: output-port? obj
-- Scheme Procedure: call-with-port port proc
*Note Ports::, for documentation.
-- Scheme Procedure: port-transcoder port
Return a transcoder associated with the encoding of PORT. *Note
Encoding::, and *Note R6RS Transcoders::.
-- Scheme Procedure: binary-port? port
Return ‘#t’ if PORT appears to be a binary port, else return ‘#f’.
Note that Guile does not currently distinguish between binary and
textual ports, so this predicate is not a reliable indicator of
whether the port was created as a binary port. Currently, it
returns ‘#t’ if and only if the port encoding is “ISO-8859-1”,
because Guile uses this encoding when creating a binary port.
*Note Encoding::, for more details.
-- Scheme Procedure: textual-port? port
Return ‘#t’ if PORT appears to be a textual port, else return ‘#f’.
Note that Guile does not currently distinguish between binary and
textual ports, so this predicate is not a reliable indicator of
whether the port was created as a textual port. Currently, it
always returns ‘#t’, because all ports can be used for textual I/O
in Guile. *Note Encoding::, for more details.
-- Scheme Procedure: transcoded-port binary-port transcoder
The ‘transcoded-port’ procedure returns a new textual port with the
specified TRANSCODER. Otherwise the new textual port’s state is
largely the same as that of BINARY-PORT. If BINARY-PORT is an
input port, the new textual port will be an input port and will
transcode the bytes that have not yet been read from BINARY-PORT.
If BINARY-PORT is an output port, the new textual port will be an
output port and will transcode output characters into bytes that
are written to the byte sink represented by BINARY-PORT.
As a side effect, however, ‘transcoded-port’ closes BINARY-PORT in
a special way that allows the new textual port to continue to use
the byte source or sink represented by BINARY-PORT, even though
BINARY-PORT itself is closed and cannot be used by the input and
output operations described in this chapter.
-- Scheme Procedure: port-position port
Equivalent to ‘(seek PORT 0 SEEK_CUR)’. *Note Random Access::.
-- Scheme Procedure: port-has-port-position? port
Return ‘#t’ is PORT supports ‘port-position’.
-- Scheme Procedure: set-port-position! port offset
Equivalent to ‘(seek PORT OFFSET SEEK_SET)’. *Note Random
Access::.
-- Scheme Procedure: port-has-set-port-position!? port
Return ‘#t’ is PORT supports ‘set-port-position!’.
-- Scheme Procedure: port-eof? input-port
Equivalent to ‘(eof-object? (lookahead-u8 INPUT-PORT))’.
-- Scheme Procedure: standard-input-port
-- Scheme Procedure: standard-output-port
-- Scheme Procedure: standard-error-port
Returns a fresh binary input port connected to standard input, or a
binary output port connected to the standard output or standard
error, respectively. Whether the port supports the ‘port-position’
and ‘set-port-position!’ operations is implementation-dependent.
-- Scheme Procedure: current-input-port
-- Scheme Procedure: current-output-port
-- Scheme Procedure: current-error-port
*Note Default Ports::.
-- Scheme Procedure: open-bytevector-input-port bv [transcoder]
-- Scheme Procedure: open-bytevector-output-port [transcoder]
*Note Bytevector Ports::.
-- Scheme Procedure: make-custom-binary-input-port id read!
get-position set-position! close
-- Scheme Procedure: make-custom-binary-output-port id write!
get-position set-position! close
-- Scheme Procedure: make-custom-binary-input/output-port id read!
write! get-position set-position! close
*Note Custom Ports::.
-- Scheme Procedure: get-u8 port
-- Scheme Procedure: lookahead-u8 port
-- Scheme Procedure: get-bytevector-n port count
-- Scheme Procedure: get-bytevector-n! port bv start count
-- Scheme Procedure: get-bytevector-some port
-- Scheme Procedure: get-bytevector-all port
-- Scheme Procedure: put-u8 port octet
-- Scheme Procedure: put-bytevector port bv [start [count]]
*Note Binary I/O::.
-- Scheme Procedure: get-char textual-input-port
-- Scheme Procedure: lookahead-char textual-input-port
-- Scheme Procedure: get-string-n textual-input-port count
-- Scheme Procedure: get-string-n! textual-input-port string start
count
-- Scheme Procedure: get-string-all textual-input-port
-- Scheme Procedure: get-line textual-input-port
-- Scheme Procedure: put-char port char
-- Scheme Procedure: put-string port string [start [count]]
*Note Textual I/O::.
-- Scheme Procedure: get-datum textual-input-port count
Reads an external representation from TEXTUAL-INPUT-PORT and
returns the datum it represents. The ‘get-datum’ procedure returns
the next datum that can be parsed from the given
TEXTUAL-INPUT-PORT, updating TEXTUAL-INPUT-PORT to point exactly
past the end of the external representation of the object.
Any _interlexeme space_ (comment or whitespace, *note Scheme
Syntax::) in the input is first skipped. If an end of file occurs
after the interlexeme space, the end-of-file object is returned.
If a character inconsistent with an external representation is
encountered in the input, an exception with condition types
‘&lexical’ and ‘&i/o-read’ is raised. Also, if the end of file is
encountered after the beginning of an external representation, but
the external representation is incomplete and therefore cannot be
parsed, an exception with condition types ‘&lexical’ and
‘&i/o-read’ is raised.
-- Scheme Procedure: put-datum textual-output-port datum
DATUM should be a datum value. The ‘put-datum’ procedure writes an
external representation of DATUM to TEXTUAL-OUTPUT-PORT. The
specific external representation is implementation-dependent.
However, whenever possible, an implementation should produce a
representation for which ‘get-datum’, when reading the
representation, will return an object equal (in the sense of
‘equal?’) to DATUM.
Note: Not all datums may allow producing an external
representation for which ‘get-datum’ will produce an object
that is equal to the original. Specifically, NaNs contained
in DATUM may make this impossible.
Note: The ‘put-datum’ procedure merely writes the external
representation, but no trailing delimiter. If ‘put-datum’ is
used to write several subsequent external representations to
an output port, care should be taken to delimit them properly
so they can be read back in by subsequent calls to
‘get-datum’.
-- Scheme Procedure: flush-output-port port
*Note Buffering::, for documentation on ‘force-output’.
7.6.2.17 R6RS File Ports
........................
The facilities described in this section are exported by the ‘(rnrs io
ports)’ module.
-- Scheme Syntax: buffer-mode BUFFER-MODE-SYMBOL
BUFFER-MODE-SYMBOL must be a symbol whose name is one of ‘none’,
‘line’, and ‘block’. The result is the corresponding symbol, and
specifies the associated buffer mode. *Note Buffering::, for a
discussion of these different buffer modes. To control the amount
of buffering, use ‘setvbuf’ instead. Note that only the name of
BUFFER-MODE-SYMBOL is significant.
*Note Buffering::, for a discussion of port buffering.
-- Scheme Procedure: buffer-mode? obj
Returns ‘#t’ if the argument is a valid buffer-mode symbol, and
returns ‘#f’ otherwise.
When opening a file, the various procedures accept a ‘file-options’
object that encapsulates flags to specify how the file is to be opened.
A ‘file-options’ object is an enum-set (*note rnrs enums::) over the
symbols constituting valid file options.
A FILE-OPTIONS parameter name means that the corresponding argument
must be a file-options object.
-- Scheme Syntax: file-options FILE-OPTIONS-SYMBOL ...
Each FILE-OPTIONS-SYMBOL must be a symbol.
The ‘file-options’ syntax returns a file-options object that
encapsulates the specified options.
When supplied to an operation that opens a file for output, the
file-options object returned by ‘(file-options)’ specifies that the
file is created if it does not exist and an exception with
condition type ‘&i/o-file-already-exists’ is raised if it does
exist. The following standard options can be included to modify
the default behavior.
‘no-create’
If the file does not already exist, it is not created;
instead, an exception with condition type
‘&i/o-file-does-not-exist’ is raised. If the file already
exists, the exception with condition type
‘&i/o-file-already-exists’ is not raised and the file is
truncated to zero length.
‘no-fail’
If the file already exists, the exception with condition type
‘&i/o-file-already-exists’ is not raised, even if ‘no-create’
is not included, and the file is truncated to zero length.
‘no-truncate’
If the file already exists and the exception with condition
type ‘&i/o-file-already-exists’ has been inhibited by
inclusion of ‘no-create’ or ‘no-fail’, the file is not
truncated, but the port’s current position is still set to the
beginning of the file.
These options have no effect when a file is opened only for input.
Symbols other than those listed above may be used as
FILE-OPTIONS-SYMBOLs; they have implementation-specific meaning, if
any.
Note: Only the name of FILE-OPTIONS-SYMBOL is significant.
-- Scheme Procedure: open-file-input-port filename
-- Scheme Procedure: open-file-input-port filename file-options
-- Scheme Procedure: open-file-input-port filename file-options
buffer-mode
-- Scheme Procedure: open-file-input-port filename file-options
buffer-mode maybe-transcoder
MAYBE-TRANSCODER must be either a transcoder or ‘#f’.
The ‘open-file-input-port’ procedure returns an input port for the
named file. The FILE-OPTIONS and MAYBE-TRANSCODER arguments are
optional.
The FILE-OPTIONS argument, which may determine various aspects of
the returned port, defaults to the value of ‘(file-options)’.
The BUFFER-MODE argument, if supplied, must be one of the symbols
that name a buffer mode. The BUFFER-MODE argument defaults to
‘block’.
If MAYBE-TRANSCODER is a transcoder, it becomes the transcoder
associated with the returned port.
If MAYBE-TRANSCODER is ‘#f’ or absent, the port will be a binary
port and will support the ‘port-position’ and ‘set-port-position!’
operations. Otherwise the port will be a textual port, and whether
it supports the ‘port-position’ and ‘set-port-position!’ operations
is implementation-dependent (and possibly transcoder-dependent).
-- Scheme Procedure: open-file-output-port filename
-- Scheme Procedure: open-file-output-port filename file-options
-- Scheme Procedure: open-file-output-port filename file-options
buffer-mode
-- Scheme Procedure: open-file-output-port filename file-options
buffer-mode maybe-transcoder
MAYBE-TRANSCODER must be either a transcoder or ‘#f’.
The ‘open-file-output-port’ procedure returns an output port for
the named file.
The FILE-OPTIONS argument, which may determine various aspects of
the returned port, defaults to the value of ‘(file-options)’.
The BUFFER-MODE argument, if supplied, must be one of the symbols
that name a buffer mode. The BUFFER-MODE argument defaults to
‘block’.
If MAYBE-TRANSCODER is a transcoder, it becomes the transcoder
associated with the port.
If MAYBE-TRANSCODER is ‘#f’ or absent, the port will be a binary
port and will support the ‘port-position’ and ‘set-port-position!’
operations. Otherwise the port will be a textual port, and whether
it supports the ‘port-position’ and ‘set-port-position!’ operations
is implementation-dependent (and possibly transcoder-dependent).
7.6.2.18 rnrs io simple
.......................
The ‘(rnrs io simple (6))’ library provides convenience functions for
performing textual I/O on ports. This library also exports all of the
condition types and associated procedures described in (*note R6RS I/O
Conditions::). In the context of this section, when stating that a
procedure behaves “identically” to the corresponding procedure in
Guile’s core library, this is modulo the behavior wrt. conditions: such
procedures raise the appropriate R6RS conditions in case of error, but
otherwise behave identically.
Note: There are still known issues regarding condition-correctness;
some errors may still be thrown as native Guile exceptions instead
of the appropriate R6RS conditions.
-- Scheme Procedure: eof-object
-- Scheme Procedure: eof-object? obj
These procedures are identical to the ones provided by the ‘(rnrs
io ports (6))’ library. *Note rnrs io ports::, for documentation.
-- Scheme Procedure: input-port? obj
-- Scheme Procedure: output-port? obj
These procedures are identical to the ones provided by Guile’s core
library. *Note Ports::, for documentation.
-- Scheme Procedure: call-with-input-file filename proc
-- Scheme Procedure: call-with-output-file filename proc
-- Scheme Procedure: open-input-file filename
-- Scheme Procedure: open-output-file filename
-- Scheme Procedure: with-input-from-file filename thunk
-- Scheme Procedure: with-output-to-file filename thunk
These procedures are identical to the ones provided by Guile’s core
library. *Note File Ports::, for documentation.
-- Scheme Procedure: close-input-port input-port
-- Scheme Procedure: close-output-port output-port
Closes the given INPUT-PORT or OUTPUT-PORT. These are legacy
interfaces; just use ‘close-port’.
-- Scheme Procedure: peek-char
-- Scheme Procedure: peek-char textual-input-port
-- Scheme Procedure: read-char
-- Scheme Procedure: read-char textual-input-port
These procedures are identical to the ones provided by Guile’s core
library. *Note Venerable Port Interfaces::, for documentation.
-- Scheme Procedure: read
-- Scheme Procedure: read textual-input-port
This procedure is identical to the one provided by Guile’s core
library. *Note Scheme Read::, for documentation.
-- Scheme Procedure: display obj
-- Scheme Procedure: display obj textual-output-port
-- Scheme Procedure: newline
-- Scheme Procedure: newline textual-output-port
-- Scheme Procedure: write obj
-- Scheme Procedure: write obj textual-output-port
-- Scheme Procedure: write-char char
-- Scheme Procedure: write-char char textual-output-port
These procedures are identical to the ones provided by Guile’s core
library. *Note Venerable Port Interfaces::, and *Note Scheme
Write::, for documentation.
7.6.2.19 rnrs files
...................
The ‘(rnrs files (6))’ library provides the ‘file-exists?’ and
‘delete-file’ procedures, which test for the existence of a file and
allow the deletion of files from the file system, respectively.
These procedures are identical to the ones provided by Guile’s core
library. *Note File System::, for documentation.
7.6.2.20 rnrs programs
......................
The ‘(rnrs programs (6))’ library provides procedures for process
management and introspection.
-- Scheme Procedure: command-line
This procedure is identical to the one provided by Guile’s core
library. *Note Runtime Environment::, for documentation.
-- Scheme Procedure: exit [status]
This procedure is identical to the one provided by Guile’s core
library. *Note Processes::, for documentation.
7.6.2.21 rnrs arithmetic fixnums
................................
The ‘(rnrs arithmetic fixnums (6))’ library provides procedures for
performing arithmetic operations on an implementation-dependent range of
exact integer values, which R6RS refers to as “fixnums”. In Guile, the
size of a fixnum is determined by the size of the ‘SCM’ type; a single
SCM struct is guaranteed to be able to hold an entire fixnum, making
fixnum computations particularly efficient—(*note The SCM Type::). On
32-bit systems, the most negative and most positive fixnum values are,
respectively, -536870912 and 536870911.
Unless otherwise specified, all of the procedures below take fixnums
as arguments, and will raise an ‘&assertion’ condition if passed a
non-fixnum argument or an ‘&implementation-restriction’ condition if
their result is not itself a fixnum.
-- Scheme Procedure: fixnum? obj
Returns ‘#t’ if OBJ is a fixnum, ‘#f’ otherwise.
-- Scheme Procedure: fixnum-width
-- Scheme Procedure: least-fixnum
-- Scheme Procedure: greatest-fixnum
These procedures return, respectively, the maximum number of bits
necessary to represent a fixnum value in Guile, the minimum fixnum
value, and the maximum fixnum value.
-- Scheme Procedure: fx=? fx1 fx2 fx3 ...
-- Scheme Procedure: fx>? fx1 fx2 fx3 ...
-- Scheme Procedure: fx=? fx1 fx2 fx3 ...
-- Scheme Procedure: fx<=? fx1 fx2 fx3 ...
These procedures return ‘#t’ if their fixnum arguments are
(respectively): equal, monotonically increasing, monotonically
decreasing, monotonically nondecreasing, or monotonically
nonincreasing; ‘#f’ otherwise.
-- Scheme Procedure: fxzero? fx
-- Scheme Procedure: fxpositive? fx
-- Scheme Procedure: fxnegative? fx
-- Scheme Procedure: fxodd? fx
-- Scheme Procedure: fxeven? fx
These numerical predicates return ‘#t’ if FX is, respectively,
zero, greater than zero, less than zero, odd, or even; ‘#f’
otherwise.
-- Scheme Procedure: fxmax fx1 fx2 ...
-- Scheme Procedure: fxmin fx1 fx2 ...
These procedures return the maximum or minimum of their arguments.
-- Scheme Procedure: fx+ fx1 fx2
-- Scheme Procedure: fx* fx1 fx2
These procedures return the sum or product of their arguments.
-- Scheme Procedure: fx- fx1 fx2
-- Scheme Procedure: fx- fx
Returns the difference of FX1 and FX2, or the negation of FX, if
called with a single argument.
An ‘&assertion’ condition is raised if the result is not itself a
fixnum.
-- Scheme Procedure: fxdiv-and-mod fx1 fx2
-- Scheme Procedure: fxdiv fx1 fx2
-- Scheme Procedure: fxmod fx1 fx2
-- Scheme Procedure: fxdiv0-and-mod0 fx1 fx2
-- Scheme Procedure: fxdiv0 fx1 fx2
-- Scheme Procedure: fxmod0 fx1 fx2
These procedures implement number-theoretic division on fixnums;
*Note (rnrs base)::, for a description of their semantics.
-- Scheme Procedure: fx+/carry fx1 fx2 fx3
Returns the two fixnum results of the following computation:
(let* ((s (+ fx1 fx2 fx3))
(s0 (mod0 s (expt 2 (fixnum-width))))
(s1 (div0 s (expt 2 (fixnum-width)))))
(values s0 s1))
-- Scheme Procedure: fx-/carry fx1 fx2 fx3
Returns the two fixnum results of the following computation:
(let* ((d (- fx1 fx2 fx3))
(d0 (mod0 d (expt 2 (fixnum-width))))
(d1 (div0 d (expt 2 (fixnum-width)))))
(values d0 d1))
-- Scheme Procedure: fx*/carry fx1 fx2 fx3
Returns the two fixnum results of the following computation:
(let* ((s (+ (* fx1 fx2) fx3))
(s0 (mod0 s (expt 2 (fixnum-width))))
(s1 (div0 s (expt 2 (fixnum-width)))))
(values s0 s1))
-- Scheme Procedure: fxnot fx
-- Scheme Procedure: fxand fx1 ...
-- Scheme Procedure: fxior fx1 ...
-- Scheme Procedure: fxxor fx1 ...
These procedures are identical to the ‘lognot’, ‘logand’, ‘logior’,
and ‘logxor’ procedures provided by Guile’s core library. *Note
Bitwise Operations::, for documentation.
-- Scheme Procedure: fxif fx1 fx2 fx3
Returns the bitwise “if” of its fixnum arguments. The bit at
position ‘i’ in the return value will be the ‘i’th bit from FX2 if
the ‘i’th bit of FX1 is 1, the ‘i’th bit from FX3.
-- Scheme Procedure: fxbit-count fx
Returns the number of 1 bits in the two’s complement representation
of FX.
-- Scheme Procedure: fxlength fx
Returns the number of bits necessary to represent FX.
-- Scheme Procedure: fxfirst-bit-set fx
Returns the index of the least significant 1 bit in the two’s
complement representation of FX.
-- Scheme Procedure: fxbit-set? fx1 fx2
Returns ‘#t’ if the FX2th bit in the two’s complement
representation of FX1 is 1, ‘#f’ otherwise.
-- Scheme Procedure: fxcopy-bit fx1 fx2 fx3
Returns the result of setting the FX2th bit of FX1 to the FX2th bit
of FX3.
-- Scheme Procedure: fxbit-field fx1 fx2 fx3
Returns the integer representation of the contiguous sequence of
bits in FX1 that starts at position FX2 (inclusive) and ends at
position FX3 (exclusive).
-- Scheme Procedure: fxcopy-bit-field fx1 fx2 fx3 fx4
Returns the result of replacing the bit field in FX1 with start and
end positions FX2 and FX3 with the corresponding bit field from
FX4.
-- Scheme Procedure: fxarithmetic-shift fx1 fx2
-- Scheme Procedure: fxarithmetic-shift-left fx1 fx2
-- Scheme Procedure: fxarithmetic-shift-right fx1 fx2
Returns the result of shifting the bits of FX1 right or left by the
FX2 positions. ‘fxarithmetic-shift’ is identical to
‘fxarithmetic-shift-left’.
-- Scheme Procedure: fxrotate-bit-field fx1 fx2 fx3 fx4
Returns the result of cyclically permuting the bit field in FX1
with start and end positions FX2 and FX3 by FX4 bits in the
direction of more significant bits.
-- Scheme Procedure: fxreverse-bit-field fx1 fx2 fx3
Returns the result of reversing the order of the bits of FX1
between position FX2 (inclusive) and position FX3 (exclusive).
7.6.2.22 rnrs arithmetic flonums
................................
The ‘(rnrs arithmetic flonums (6))’ library provides procedures for
performing arithmetic operations on inexact representations of real
numbers, which R6RS refers to as “flonums”.
Unless otherwise specified, all of the procedures below take flonums
as arguments, and will raise an ‘&assertion’ condition if passed a
non-flonum argument.
-- Scheme Procedure: flonum? obj
Returns ‘#t’ if OBJ is a flonum, ‘#f’ otherwise.
-- Scheme Procedure: real->flonum x
Returns the flonum that is numerically closest to the real number
X.
-- Scheme Procedure: fl=? fl1 fl2 fl3 ...
-- Scheme Procedure: fl? fl1 fl2 fl3 ...
-- Scheme Procedure: fl>=? fl1 fl2 fl3 ...
These procedures return ‘#t’ if their flonum arguments are
(respectively): equal, monotonically increasing, monotonically
decreasing, monotonically nondecreasing, or monotonically
nonincreasing; ‘#f’ otherwise.
-- Scheme Procedure: flinteger? fl
-- Scheme Procedure: flzero? fl
-- Scheme Procedure: flpositive? fl
-- Scheme Procedure: flnegative? fl
-- Scheme Procedure: flodd? fl
-- Scheme Procedure: fleven? fl
These numerical predicates return ‘#t’ if FL is, respectively, an
integer, zero, greater than zero, less than zero, odd, even, ‘#f’
otherwise. In the case of ‘flodd?’ and ‘fleven?’, FL must be an
integer-valued flonum.
-- Scheme Procedure: flfinite? fl
-- Scheme Procedure: flinfinite? fl
-- Scheme Procedure: flnan? fl
These numerical predicates return ‘#t’ if FL is, respectively, not
infinite, infinite, or a ‘NaN’ value.
-- Scheme Procedure: flmax fl1 fl2 ...
-- Scheme Procedure: flmin fl1 fl2 ...
These procedures return the maximum or minimum of their arguments.
-- Scheme Procedure: fl+ fl1 ...
-- Scheme Procedure: fl* fl ...
These procedures return the sum or product of their arguments.
-- Scheme Procedure: fl- fl1 fl2 ...
-- Scheme Procedure: fl- fl
-- Scheme Procedure: fl/ fl1 fl2 ...
-- Scheme Procedure: fl/ fl
These procedures return, respectively, the difference or quotient
of their arguments when called with two arguments; when called with
a single argument, they return the additive or multiplicative
inverse of FL.
-- Scheme Procedure: flabs fl
Returns the absolute value of FL.
-- Scheme Procedure: fldiv-and-mod fl1 fl2
-- Scheme Procedure: fldiv fl1 fl2
-- Scheme Procedure: fldmod fl1 fl2
-- Scheme Procedure: fldiv0-and-mod0 fl1 fl2
-- Scheme Procedure: fldiv0 fl1 fl2
-- Scheme Procedure: flmod0 fl1 fl2
These procedures implement number-theoretic division on flonums;
*Note (rnrs base)::, for a description for their semantics.
-- Scheme Procedure: flnumerator fl
-- Scheme Procedure: fldenominator fl
These procedures return the numerator or denominator of FL as a
flonum.
-- Scheme Procedure: flfloor fl1
-- Scheme Procedure: flceiling fl
-- Scheme Procedure: fltruncate fl
-- Scheme Procedure: flround fl
These procedures are identical to the ‘floor’, ‘ceiling’,
‘truncate’, and ‘round’ procedures provided by Guile’s core
library. *Note Arithmetic::, for documentation.
-- Scheme Procedure: flexp fl
-- Scheme Procedure: fllog fl
-- Scheme Procedure: fllog fl1 fl2
-- Scheme Procedure: flsin fl
-- Scheme Procedure: flcos fl
-- Scheme Procedure: fltan fl
-- Scheme Procedure: flasin fl
-- Scheme Procedure: flacos fl
-- Scheme Procedure: flatan fl
-- Scheme Procedure: flatan fl1 fl2
These procedures, which compute the usual transcendental functions,
are the flonum variants of the procedures provided by the R6RS base
library (*note (rnrs base)::).
-- Scheme Procedure: flsqrt fl
Returns the square root of FL. If FL is ‘-0.0’, -0.0 is returned;
for other negative values, a ‘NaN’ value is returned.
-- Scheme Procedure: flexpt fl1 fl2
Returns the value of FL1 raised to the power of FL2.
The following condition types are provided to allow Scheme
implementations that do not support infinities or ‘NaN’ values to
indicate that a computation resulted in such a value. Guile supports
both of these, so these conditions will never be raised by Guile’s
standard libraries implementation.
-- Condition Type: &no-infinities
-- Scheme Procedure: make-no-infinities-violation obj
-- Scheme Procedure: no-infinities-violation?
A condition type indicating that a computation resulted in an
infinite value on a Scheme implementation incapable of representing
infinities.
-- Condition Type: &no-nans
-- Scheme Procedure: make-no-nans-violation obj
-- Scheme Procedure: no-nans-violation? obj
A condition type indicating that a computation resulted in a ‘NaN’
value on a Scheme implementation incapable of representing ‘NaN’s.
-- Scheme Procedure: fixnum->flonum fx
Returns the flonum that is numerically closest to the fixnum FX.
7.6.2.23 rnrs arithmetic bitwise
................................
The ‘(rnrs arithmetic bitwise (6))’ library provides procedures for
performing bitwise arithmetic operations on the two’s complement
representations of fixnums.
This library and the procedures it exports share functionality with
SRFI-60, which provides support for bitwise manipulation of integers
(*note SRFI-60::).
-- Scheme Procedure: bitwise-not ei
-- Scheme Procedure: bitwise-and ei1 ...
-- Scheme Procedure: bitwise-ior ei1 ...
-- Scheme Procedure: bitwise-xor ei1 ...
These procedures are identical to the ‘lognot’, ‘logand’, ‘logior’,
and ‘logxor’ procedures provided by Guile’s core library. *Note
Bitwise Operations::, for documentation.
-- Scheme Procedure: bitwise-if ei1 ei2 ei3
Returns the bitwise “if” of its arguments. The bit at position ‘i’
in the return value will be the ‘i’th bit from EI2 if the ‘i’th bit
of EI1 is 1, the ‘i’th bit from EI3.
-- Scheme Procedure: bitwise-bit-count ei
Returns the number of 1 bits in the two’s complement representation
of EI.
-- Scheme Procedure: bitwise-length ei
Returns the number of bits necessary to represent EI.
-- Scheme Procedure: bitwise-first-bit-set ei
Returns the index of the least significant 1 bit in the two’s
complement representation of EI.
-- Scheme Procedure: bitwise-bit-set? ei1 ei2
Returns ‘#t’ if the EI2th bit in the two’s complement
representation of EI1 is 1, ‘#f’ otherwise.
-- Scheme Procedure: bitwise-copy-bit ei1 ei2 ei3
Returns the result of setting the EI2th bit of EI1 to the EI2th bit
of EI3.
-- Scheme Procedure: bitwise-bit-field ei1 ei2 ei3
Returns the integer representation of the contiguous sequence of
bits in EI1 that starts at position EI2 (inclusive) and ends at
position EI3 (exclusive).
-- Scheme Procedure: bitwise-copy-bit-field ei1 ei2 ei3 ei4
Returns the result of replacing the bit field in EI1 with start and
end positions EI2 and EI3 with the corresponding bit field from
EI4.
-- Scheme Procedure: bitwise-arithmetic-shift ei1 ei2
-- Scheme Procedure: bitwise-arithmetic-shift-left ei1 ei2
-- Scheme Procedure: bitwise-arithmetic-shift-right ei1 ei2
Returns the result of shifting the bits of EI1 right or left by the
EI2 positions. ‘bitwise-arithmetic-shift’ is identical to
‘bitwise-arithmetic-shift-left’.
-- Scheme Procedure: bitwise-rotate-bit-field ei1 ei2 ei3 ei4
Returns the result of cyclically permuting the bit field in EI1
with start and end positions EI2 and EI3 by EI4 bits in the
direction of more significant bits.
-- Scheme Procedure: bitwise-reverse-bit-field ei1 ei2 ei3
Returns the result of reversing the order of the bits of EI1
between position EI2 (inclusive) and position EI3 (exclusive).
7.6.2.24 rnrs syntax-case
.........................
The ‘(rnrs syntax-case (6))’ library provides access to the
‘syntax-case’ system for writing hygienic macros. With one exception,
all of the forms and procedures exported by this library are
“re-exports” of Guile’s native support for ‘syntax-case’; *Note Syntax
Case::, for documentation, examples, and rationale.
-- Scheme Procedure: make-variable-transformer proc
Creates a new variable transformer out of PROC, a procedure that
takes a syntax object as input and returns a syntax object. If an
identifier to which the result of this procedure is bound appears
on the left-hand side of a ‘set!’ expression, PROC will be called
with a syntax object representing the entire ‘set!’ expression, and
its return value will replace that ‘set!’ expression.
-- Scheme Syntax: syntax-case expression (literal ...) clause ...
The ‘syntax-case’ pattern matching form.
-- Scheme Syntax: syntax template
-- Scheme Syntax: quasisyntax template
-- Scheme Syntax: unsyntax template
-- Scheme Syntax: unsyntax-splicing template
These forms allow references to be made in the body of a
syntax-case output expression subform to datum and non-datum
values. They are identical to the forms provided by Guile’s core
library; *Note Syntax Case::, for documentation.
-- Scheme Procedure: identifier? obj
-- Scheme Procedure: bound-identifier=? id1 id2
-- Scheme Procedure: free-identifier=? id1 id2
These predicate procedures operate on syntax objects representing
Scheme identifiers. ‘identifier?’ returns ‘#t’ if OBJ represents
an identifier, ‘#f’ otherwise. ‘bound-identifier=?’ returns ‘#t’
if and only if a binding for ID1 would capture a reference to ID2
in the transformer’s output, or vice-versa. ‘free-identifier=?’
returns ‘#t’ if and only ID1 and ID2 would refer to the same
binding in the output of the transformer, independent of any
bindings introduced by the transformer.
-- Scheme Procedure: generate-temporaries l
Returns a list, of the same length as L, which must be a list or a
syntax object representing a list, of globally unique symbols.
-- Scheme Procedure: syntax->datum syntax-object
-- Scheme Procedure: datum->syntax template-id datum
These procedures convert wrapped syntax objects to and from Scheme
datum values. The syntax object returned by ‘datum->syntax’ shares
contextual information with the syntax object TEMPLATE-ID.
-- Scheme Procedure: syntax-violation whom message form
-- Scheme Procedure: syntax-violation whom message form subform
Constructs a new compound condition that includes the following
simple conditions:
• If WHOM is not ‘#f’, a ‘&who’ condition with the WHOM as its
field
• A ‘&message’ condition with the specified MESSAGE
• A ‘&syntax’ condition with the specified FORM and optional
SUBFORM fields
7.6.2.25 rnrs hashtables
........................
The ‘(rnrs hashtables (6))’ library provides structures and procedures
for creating and accessing hash tables. The hash tables API defined by
R6RS is substantially similar to both Guile’s native hash tables
implementation as well as the one provided by SRFI-69; *Note Hash
Tables::, and *note SRFI-69::, respectively. Note that you can write
portable R6RS library code that manipulates SRFI-69 hash tables (by
importing the ‘(srfi :69)’ library); however, hash tables created by one
API cannot be used by another.
Like SRFI-69 hash tables—and unlike Guile’s native ones—R6RS hash
tables associate hash and equality functions with a hash table at the
time of its creation. Additionally, R6RS allows for the creation (via
‘hashtable-copy’; see below) of immutable hash tables.
-- Scheme Procedure: make-eq-hashtable
-- Scheme Procedure: make-eq-hashtable k
Returns a new hash table that uses ‘eq?’ to compare keys and
Guile’s ‘hashq’ procedure as a hash function. If K is given, it
specifies the initial capacity of the hash table.
-- Scheme Procedure: make-eqv-hashtable
-- Scheme Procedure: make-eqv-hashtable k
Returns a new hash table that uses ‘eqv?’ to compare keys and
Guile’s ‘hashv’ procedure as a hash function. If K is given, it
specifies the initial capacity of the hash table.
-- Scheme Procedure: make-hashtable hash-function equiv
-- Scheme Procedure: make-hashtable hash-function equiv k
Returns a new hash table that uses EQUIV to compare keys and
HASH-FUNCTION as a hash function. EQUIV must be a procedure that
accepts two arguments and returns a true value if they are
equivalent, ‘#f’ otherwise; HASH-FUNCTION must be a procedure that
accepts one argument and returns a non-negative integer.
If K is given, it specifies the initial capacity of the hash table.
-- Scheme Procedure: hashtable? obj
Returns ‘#t’ if OBJ is an R6RS hash table, ‘#f’ otherwise.
-- Scheme Procedure: hashtable-size hashtable
Returns the number of keys currently in the hash table HASHTABLE.
-- Scheme Procedure: hashtable-ref hashtable key default
Returns the value associated with KEY in the hash table HASHTABLE,
or DEFAULT if none is found.
-- Scheme Procedure: hashtable-set! hashtable key obj
Associates the key KEY with the value OBJ in the hash table
HASHTABLE, and returns an unspecified value. An ‘&assertion’
condition is raised if HASHTABLE is immutable.
-- Scheme Procedure: hashtable-delete! hashtable key
Removes any association found for the key KEY in the hash table
HASHTABLE, and returns an unspecified value. An ‘&assertion’
condition is raised if HASHTABLE is immutable.
-- Scheme Procedure: hashtable-contains? hashtable key
Returns ‘#t’ if the hash table HASHTABLE contains an association
for the key KEY, ‘#f’ otherwise.
-- Scheme Procedure: hashtable-update! hashtable key proc default
Associates with KEY in the hash table HASHTABLE the result of
calling PROC, which must be a procedure that takes one argument, on
the value currently associated KEY in HASHTABLE—or on DEFAULT if no
such association exists. An ‘&assertion’ condition is raised if
HASHTABLE is immutable.
-- Scheme Procedure: hashtable-copy hashtable
-- Scheme Procedure: hashtable-copy hashtable mutable
Returns a copy of the hash table HASHTABLE. If the optional
argument MUTABLE is provided and is a true value, the new hash
table will be mutable.
-- Scheme Procedure: hashtable-clear! hashtable
-- Scheme Procedure: hashtable-clear! hashtable k
Removes all of the associations from the hash table HASHTABLE. The
optional argument K, which specifies a new capacity for the hash
table, is accepted by Guile’s ‘(rnrs hashtables)’ implementation,
but is ignored.
-- Scheme Procedure: hashtable-keys hashtable
Returns a vector of the keys with associations in the hash table
HASHTABLE, in an unspecified order.
-- Scheme Procedure: hashtable-entries hashtable
Return two values—a vector of the keys with associations in the
hash table HASHTABLE, and a vector of the values to which these
keys are mapped, in corresponding but unspecified order.
-- Scheme Procedure: hashtable-equivalence-function hashtable
Returns the equivalence predicated use by HASHTABLE. This
procedure returns ‘eq?’ and ‘eqv?’, respectively, for hash tables
created by ‘make-eq-hashtable’ and ‘make-eqv-hashtable’.
-- Scheme Procedure: hashtable-hash-function hashtable
Returns the hash function used by HASHTABLE. For hash tables
created by ‘make-eq-hashtable’ or ‘make-eqv-hashtable’, ‘#f’ is
returned.
-- Scheme Procedure: hashtable-mutable? hashtable
Returns ‘#t’ if HASHTABLE is mutable, ‘#f’ otherwise.
A number of hash functions are provided for convenience:
-- Scheme Procedure: equal-hash obj
Returns an integer hash value for OBJ, based on its structure and
current contents. This hash function is suitable for use with
‘equal?’ as an equivalence function.
-- Scheme Procedure: string-hash string
-- Scheme Procedure: symbol-hash symbol
These procedures are identical to the ones provided by Guile’s core
library. *Note Hash Table Reference::, for documentation.
-- Scheme Procedure: string-ci-hash string
Returns an integer hash value for STRING based on its contents,
ignoring case. This hash function is suitable for use with
‘string-ci=?’ as an equivalence function.
7.6.2.26 rnrs enums
...................
The ‘(rnrs enums (6))’ library provides structures and procedures for
working with enumerable sets of symbols. Guile’s implementation defines
an “enum-set” record type that encapsulates a finite set of distinct
symbols, the “universe”, and a subset of these symbols, which define the
enumeration set.
The SRFI-1 list library provides a number of procedures for
performing set operations on lists; Guile’s ‘(rnrs enums)’
implementation makes use of several of them. *Note SRFI-1 Set
Operations::, for more information.
-- Scheme Procedure: make-enumeration symbol-list
Returns a new enum-set whose universe and enumeration set are both
equal to SYMBOL-LIST, a list of symbols.
-- Scheme Procedure: enum-set-universe enum-set
Returns an enum-set representing the universe of ENUM-SET, an
enum-set.
-- Scheme Procedure: enum-set-indexer enum-set
Returns a procedure that takes a single argument and returns the
zero-indexed position of that argument in the universe of ENUM-SET,
or ‘#f’ if its argument is not a member of that universe.
-- Scheme Procedure: enum-set-constructor enum-set
Returns a procedure that takes a single argument, a list of symbols
from the universe of ENUM-SET, an enum-set, and returns a new
enum-set with the same universe that represents a subset containing
the specified symbols.
-- Scheme Procedure: enum-set->list enum-set
Returns a list containing the symbols of the set represented by
ENUM-SET, an enum-set, in the order that they appear in the
universe of ENUM-SET.
-- Scheme Procedure: enum-set-member? symbol enum-set
-- Scheme Procedure: enum-set-subset? enum-set1 enum-set2
-- Scheme Procedure: enum-set=? enum-set1 enum-set2
These procedures test for membership of symbols and enum-sets in
other enum-sets. ‘enum-set-member?’ returns ‘#t’ if and only if
SYMBOL is a member of the subset specified by ENUM-SET.
‘enum-set-subset?’ returns ‘#t’ if and only if the universe of
ENUM-SET1 is a subset of the universe of ENUM-SET2 and every symbol
in ENUM-SET1 is present in ENUM-SET2. ‘enum-set=?’ returns ‘#t’ if
and only if ENUM-SET1 is a subset, as per ‘enum-set-subset?’ of
ENUM-SET2 and vice versa.
-- Scheme Procedure: enum-set-union enum-set1 enum-set2
-- Scheme Procedure: enum-set-intersection enum-set1 enum-set2
-- Scheme Procedure: enum-set-difference enum-set1 enum-set2
These procedures return, respectively, the union, intersection, and
difference of their enum-set arguments.
-- Scheme Procedure: enum-set-complement enum-set
Returns ENUM-SET’s complement (an enum-set), with regard to its
universe.
-- Scheme Procedure: enum-set-projection enum-set1 enum-set2
Returns the projection of the enum-set ENUM-SET1 onto the universe
of the enum-set ENUM-SET2.
-- Scheme Syntax: define-enumeration type-name (symbol ...)
constructor-syntax
Evaluates to two new definitions: A constructor bound to
CONSTRUCTOR-SYNTAX that behaves similarly to constructors created
by ‘enum-set-constructor’, above, and creates new ENUM-SETs in the
universe specified by ‘(symbol ...)’; and a “predicate macro” bound
to TYPE-NAME, which has the following form:
(TYPE-NAME sym)
If SYM is a member of the universe specified by the SYMBOLs above,
this form evaluates to SYM. Otherwise, a ‘&syntax’ condition is
raised.
7.6.2.27 rnrs
.............
The ‘(rnrs (6))’ library is a composite of all of the other R6RS
standard libraries—it imports and re-exports all of their exported
procedures and syntactic forms—with the exception of the following
libraries:
• ‘(rnrs eval (6))’
• ‘(rnrs mutable-pairs (6))’
• ‘(rnrs mutable-strings (6))’
• ‘(rnrs r5rs (6))’
7.6.2.28 rnrs eval
..................
The ‘(rnrs eval (6)’ library provides procedures for performing
“on-the-fly” evaluation of expressions.
-- Scheme Procedure: eval expression environment
Evaluates EXPRESSION, which must be a datum representation of a
valid Scheme expression, in the environment specified by
ENVIRONMENT. This procedure is identical to the one provided by
Guile’s code library; *Note Fly Evaluation::, for documentation.
-- Scheme Procedure: environment import-spec ...
Constructs and returns a new environment based on the specified
IMPORT-SPECs, which must be datum representations of the import
specifications used with the ‘import’ form. *Note R6RS
Libraries::, for documentation.
7.6.2.29 rnrs mutable-pairs
...........................
The ‘(rnrs mutable-pairs (6))’ library provides the ‘set-car!’ and
‘set-cdr!’ procedures, which allow the ‘car’ and ‘cdr’ fields of a pair
to be modified.
These procedures are identical to the ones provide by Guile’s core
library. *Note Pairs::, for documentation. All pairs in Guile are
mutable; consequently, these procedures will never throw the
‘&assertion’ condition described in the R6RS libraries specification.
7.6.2.30 rnrs mutable-strings
.............................
The ‘(rnrs mutable-strings (6))’ library provides the ‘string-set!’ and
‘string-fill!’ procedures, which allow the content of strings to be
modified “in-place.”
These procedures are identical to the ones provided by Guile’s core
library. *Note String Modification::, for documentation. All strings
in Guile are mutable; consequently, these procedures will never throw
the ‘&assertion’ condition described in the R6RS libraries
specification.
7.6.2.31 rnrs r5rs
..................
The ‘(rnrs r5rs (6))’ library exports bindings for some procedures
present in R5RS but omitted from the R6RS base library specification.
-- Scheme Procedure: exact->inexact z
-- Scheme Procedure: inexact->exact z
These procedures are identical to the ones provided by Guile’s core
library. *Note Exactness::, for documentation.
-- Scheme Procedure: quotient n1 n2
-- Scheme Procedure: remainder n1 n2
-- Scheme Procedure: modulo n1 n2
These procedures are identical to the ones provided by Guile’s core
library. *Note Integer Operations::, for documentation.
-- Scheme Syntax: delay expr
-- Scheme Procedure: force promise
The ‘delay’ form and the ‘force’ procedure are identical to their
counterparts in Guile’s core library. *Note Delayed Evaluation::,
for documentation.
-- Scheme Procedure: null-environment n
-- Scheme Procedure: scheme-report-environment n
These procedures are identical to the ones provided by the ‘(ice-9
r5rs)’ Guile module. *Note Environments::, for documentation.
7.7 R7RS Support
================
The R7RS (https://small.r7rs.org/) standard is essentially R5RS
(directly supported by Guile), plus a module facility, plus an
organization of bindings into a standard set of modules.
Happily, the syntax for R7RS modules was chosen to be compatible with
R6RS, and so Guile’s documentation there applies. *Note R6RS
Libraries::, for more information on how to define R6RS libraries, and
their integration with Guile modules. *Note Library Usage::, also.
7.7.1 Incompatibilities with the R7RS
-------------------------------------
As the R7RS is a much less ambitious standard than the R6RS (*note Guile
and Scheme::), it is very easy for Guile to support. As such, Guile is
a fully conforming implementation of R7RS, with the exception of the
occasional bug and a couple of unimplemented features:
• The R7RS specifies a syntax for reading circular data structures
using “datum labels”, such as ‘#0=(1 2 3 . #0#)’. Guile’s reader
does not support this syntax currently;
.
• As with R6RS, a number of lexical features of R7RS conflict with
Guile’s historical syntax. In addition to ‘r6rs-hex-escapes’ and
‘hungry-eol-escapes’ (*note R6RS Incompatibilities::), the
‘r7rs-symbols’ reader feature needs to be explicitly enabled.
Guile exposes a procedure in the root module to choose R7RS defaults
over Guile’s historical defaults.
-- Scheme Procedure: install-r7rs!
Alter Guile’s default settings to better conform to the R7RS.
While Guile’s defaults may evolve over time, the current changes
that this procedure imposes are to add ‘.sls’ and ‘.guile.sls’ to
the set of supported ‘%load-extensions’, to better support R7RS
conventions. *Note Load Paths::. ‘install-r7rs!’ will also enable
the reader options mentioned above.
Finally, note that the ‘--r7rs’ command-line argument will call
‘install-r7rs!’ before calling user code. R7RS users probably want to
pass this argument to their Guile.
7.7.2 R7RS Standard Libraries
-----------------------------
The R7RS organizes the definitions from R5RS into modules, and also adds
a few new definitions.
We do not attempt to document these libraries fully here, as unlike
R6RS, there are few new definitions in R7RS relative to R5RS. Most of
their functionality is already in Guile’s standard environment. Again,
the expectation is that most Guile users will use the well-known and
well-documented Guile modules; these R7RS libraries are mostly useful to
users who want to port their code to other R7RS systems.
As a brief overview, we note that the libraries defined by the R7RS
are as follows:
‘(scheme base)’
The core functions, mostly corresponding to R5RS minus the elements
listed separately below, but plus SRFI-34 error handling (*note
SRFI-34::), bytevectors and bytevector ports (*note Bytevectors::),
and some miscellaneous other new procedures.
‘(scheme case-lambda)’
‘case-lambda’.
‘(scheme char)’
Converting strings and characters to upper or lower case,
predicates for if a characer is numeric, and so on.
‘(scheme complex)’
Constructors and accessors for complex numbers.
‘(scheme cxr)’
‘cddr’, ‘cadadr’, and all that.
‘(scheme eval)’
‘eval’, but also an ‘environment’ routine allowing a user to
specify an environment using a module import set.
‘(scheme file)’
‘call-with-input-file’ and so on.
‘(scheme inexact)’
Routines that operate on inexact numbers: ‘sin’, ‘finite?’, and so
on.
‘(scheme lazy)’
Promises.
‘(scheme load)’
The ‘load’ procedure.
‘(scheme process-context)’
Environment variables. *Note SRFI-98::. Also, ‘commmand-line’,
‘emergency-exit’ (like Guile’s ‘primitive-_exit’), and ‘exit’.
‘(scheme r5rs)’
The precise set of bindings exported by ‘r5rs’, but without
‘transcript-off’ / ‘transcript-on’, and also with the auxiliary
syntax definitions like ‘_’ or ‘else’. *Note Syntax Rules::, for
more on auxiliary syntax.
‘(scheme read)’
The ‘read’ procedure.
‘(scheme repl)’
The ‘interaction-environment’ procedure.
‘(scheme time)’
‘current-second’, as well as ‘current-jiffy’ and
‘jiffies-per-second’. Guile uses the term “internal time unit” for
what R7RS calls “jiffies”.
‘(scheme write)’
‘display’, ‘write’, as well as ‘write-shared’ and ‘write-simple’.
For complete documentation, we advise the interested user to consult
the R7RS directly (*note (r7rs)R7RS::).
7.8 Pattern Matching
====================
The ‘(ice-9 match)’ module provides a “pattern matcher”, written by Alex
Shinn, and compatible with Andrew K. Wright’s pattern matcher found in
many Scheme implementations.
A pattern matcher can match an object against several patterns and
extract the elements that make it up. Patterns can represent any Scheme
object: lists, strings, symbols, records, etc. They can optionally
contain “pattern variables”. When a matching pattern is found, an
expression associated with the pattern is evaluated, optionally with all
pattern variables bound to the corresponding elements of the object:
(let ((l '(hello (world))))
(match l ;; <- the input object
(('hello (who)) ;; <- the pattern
who))) ;; <- the expression evaluated upon matching
⇒ world
In this example, list L matches the pattern ‘('hello (who))’, because
it is a two-element list whose first element is the symbol ‘hello’ and
whose second element is a one-element list. Here WHO is a pattern
variable. ‘match’, the pattern matcher, locally binds WHO to the value
contained in this one-element list—i.e., the symbol ‘world’. An error
would be raised if L did not match the pattern.
The same object can be matched against a simpler pattern:
(let ((l '(hello (world))))
(match l
((x y)
(values x y))))
⇒ hello
⇒ (world)
Here pattern ‘(x y)’ matches any two-element list, regardless of the
types of these elements. Pattern variables X and Y are bound to,
respectively, the first and second element of L.
Patterns can be composed, and nested. For instance, ‘...’ (ellipsis)
means that the previous pattern may be matched zero or more times in a
list:
(match lst
(((heads tails ...) ...)
heads))
This expression returns the first element of each list within LST. For
proper lists of proper lists, it is equivalent to ‘(map car lst)’.
However, it performs additional checks to make sure that LST and the
lists therein are proper lists, as prescribed by the pattern, raising an
error if they are not.
Compared to hand-written code, pattern matching noticeably improves
clarity and conciseness—no need to resort to series of ‘car’ and ‘cdr’
calls when matching lists, for instance. It also improves robustness,
by making sure the input _completely_ matches the pattern—conversely,
hand-written code often trades robustness for conciseness. And of
course, ‘match’ is a macro, and the code it expands to is just as
efficient as equivalent hand-written code.
The pattern matcher is defined as follows:
-- Scheme Syntax: match exp clause1 clause2 ...
Match object EXP against the patterns in CLAUSE1 CLAUSE2 ... in the
order in which they appear. Return the value produced by the first
matching clause. If no clause matches, throw an exception with key
‘match-error’.
Each clause has the form ‘(pattern body1 body2 ...)’. Each PATTERN
must follow the syntax described below. Each body is an arbitrary
Scheme expression, possibly referring to pattern variables of
PATTERN.
The syntax and interpretation of patterns is as follows:
patterns: matches:
pat ::= identifier anything, and binds identifier
| _ anything
| () the empty list
| #t #t
| #f #f
| string a string
| number a number
| character a character
| 'sexp an s-expression
| 'symbol a symbol (special case of s-expr)
| (pat_1 ... pat_n) list of n elements
| (pat_1 ... pat_n . pat_{n+1}) list of n or more
| (pat_1 ... pat_n pat_n+1 ooo) list of n or more, each element
of remainder must match pat_n+1
| #(pat_1 ... pat_n) vector of n elements
| #(pat_1 ... pat_n pat_n+1 ooo) vector of n or more, each element
of remainder must match pat_n+1
| #&pat box
| ($ record-name pat_1 ... pat_n) a record
| (= field pat) a ``field'' of an object
| (and pat_1 ... pat_n) if all of pat_1 thru pat_n match
| (or pat_1 ... pat_n) if any of pat_1 thru pat_n match
| (not pat_1 ... pat_n) if all pat_1 thru pat_n don't match
| (? predicate pat_1 ... pat_n) if predicate true and all of
pat_1 thru pat_n match
| (set! identifier) anything, and binds setter
| (get! identifier) anything, and binds getter
| `qp a quasi-pattern
| (identifier *** pat) matches pat in a tree and binds
identifier to the path leading
to the object that matches pat
ooo ::= ... zero or more
| ___ zero or more
| ..1 1 or more
quasi-patterns: matches:
qp ::= () the empty list
| #t #t
| #f #f
| string a string
| number a number
| character a character
| identifier a symbol
| (qp_1 ... qp_n) list of n elements
| (qp_1 ... qp_n . qp_{n+1}) list of n or more
| (qp_1 ... qp_n qp_n+1 ooo) list of n or more, each element
of remainder must match qp_n+1
| #(qp_1 ... qp_n) vector of n elements
| #(qp_1 ... qp_n qp_n+1 ooo) vector of n or more, each element
of remainder must match qp_n+1
| #&qp box
| ,pat a pattern
| ,@pat a pattern
The names ‘quote’, ‘quasiquote’, ‘unquote’, ‘unquote-splicing’, ‘?’,
‘_’, ‘$’, ‘and’, ‘or’, ‘not’, ‘set!’, ‘get!’, ‘...’, and ‘___’ cannot be
used as pattern variables.
Here is a more complex example:
(use-modules (srfi srfi-9))
(let ()
(define-record-type person
(make-person name friends)
person?
(name person-name)
(friends person-friends))
(letrec ((alice (make-person "Alice" (delay (list bob))))
(bob (make-person "Bob" (delay (list alice)))))
(match alice
(($ person name (= force (($ person "Bob"))))
(list 'friend-of-bob name))
(_ #f))))
⇒ (friend-of-bob "Alice")
Here the ‘$’ pattern is used to match a SRFI-9 record of type PERSON
containing two or more slots. The value of the first slot is bound to
NAME. The ‘=’ pattern is used to apply ‘force’ on the second slot, and
then checking that the result matches the given pattern. In other
words, the complete pattern matches any PERSON whose second slot is a
promise that evaluates to a one-element list containing a PERSON whose
first slot is ‘"Bob"’.
The ‘(ice-9 match)’ module also provides the following convenient
syntactic sugar macros wrapping around ‘match’.
-- Scheme Syntax: match-lambda clause1 clause2 ...
Create a procedure of one argument that matches its argument
against each clause, and returns the result of evaluating the
corresponding expressions.
(match-lambda clause1 clause2 ...)
≡
(lambda (arg) (match arg clause1 clause2 ...))
((match-lambda
(('hello (who))
who))
'(hello (world)))
⇒ world
-- Scheme Syntax: match-lambda* clause1 clause2 ...
Create a procedure of any number of arguments that matches its
argument list against each clause, and returns the result of
evaluating the corresponding expressions.
(match-lambda* clause1 clause2 ...)
≡
(lambda args (match args clause1 clause2 ...))
((match-lambda*
(('hello (who))
who))
'hello '(world))
⇒ world
-- Scheme Syntax: match-let ((pattern expression) ...) body
Match each pattern to the corresponding expression, and evaluate
the body with all matched variables in scope. Raise an error if
any of the expressions fail to match. ‘match-let’ is analogous to
named let and can also be used for recursive functions which match
on their arguments as in ‘match-lambda*’.
(match-let (((x y) (list 1 2))
((a b) (list 3 4)))
(list a b x y))
⇒
(3 4 1 2)
-- Scheme Syntax: match-let variable ((pattern init) ...) body
Similar to ‘match-let’, but analogously to “named let”, locally
bind VARIABLE to a new procedure which accepts as many arguments as
there are INIT expressions. The procedure is initially applied to
the results of evaluating the INIT expressions. When called, the
procedure matches each argument against the corresponding PATTERN,
and returns the result(s) of evaluating the BODY expressions.
*Note Iteration: while do, for more on “named let”.
-- Scheme Syntax: match-let* ((variable expression) ...) body
Similar to ‘match-let’, but analogously to ‘let*’, match and bind
the variables in sequence, with preceding match variables in scope.
(match-let* (((x y) (list 1 2))
((a b) (list x 4)))
(list a b x y))
≡
(match-let (((x y) (list 1 2)))
(match-let (((a b) (list x 4)))
(list a b x y)))
⇒
(1 4 1 2)
-- Scheme Syntax: match-letrec ((variable expression) ...) body
Similar to ‘match-let’, but analogously to ‘letrec’, match and bind
the variables with all match variables in scope.
Guile also comes with a pattern matcher specifically tailored to SXML
trees, *Note sxml-match::.
7.9 Readline Support
====================
Guile comes with an interface module to the readline library (*note
(readline)Top::). This makes interactive use much more convenient,
because of the command-line editing features of readline. Using ‘(ice-9
readline)’, you can navigate through the current input line with the
cursor keys, retrieve older command lines from the input history and
even search through the history entries.
7.9.1 Loading Readline Support
------------------------------
The module is not loaded by default and so has to be loaded and
activated explicitly. This is done with two simple lines of code:
(use-modules (ice-9 readline))
(activate-readline)
The first line will load the necessary code, and the second will
activate readline’s features for the REPL. If you plan to use this
module often, you should save these to lines to your ‘.guile’ personal
startup file.
You will notice that the REPL’s behaviour changes a bit when you have
loaded the readline module. For example, when you press Enter before
typing in the closing parentheses of a list, you will see the
“continuation” prompt, three dots: ‘...’ This gives you a nice visual
feedback when trying to match parentheses. To make this even easier,
“bouncing parentheses” are implemented. That means that when you type
in a closing parentheses, the cursor will jump to the corresponding
opening parenthesis for a short time, making it trivial to make them
match.
Once the readline module is activated, all lines entered
interactively will be stored in a history and can be recalled later
using the cursor-up and -down keys. Readline also understands the Emacs
keys for navigating through the command line and history.
When you quit your Guile session by evaluating ‘(quit)’ or pressing
Ctrl-D, the history will be saved to the file ‘.guile_history’ and read
in when you start Guile for the next time. Thus you can start a new
Guile session and still have the (probably long-winded) definition
expressions available.
You can specify a different history file by setting the environment
variable ‘GUILE_HISTORY’. And you can make Guile specific
customizations to your ‘.inputrc’ by testing for application ‘Guile’
(*note (readline)Conditional Init Constructs::). For instance to define
a key inserting a matched pair of parentheses,
$if Guile
"\C-o": "()\C-b"
$endif
7.9.2 Readline Options
----------------------
The readline interface module can be tweaked in a few ways to better
suit the user’s needs. Configuration is done via the readline module’s
options interface, in a similar way to the evaluator and debugging
options (*note Runtime Options::).
-- Scheme Procedure: readline-options
-- Scheme Procedure: readline-enable option-name
-- Scheme Procedure: readline-disable option-name
-- Scheme Syntax: readline-set! option-name value
Accessors for the readline options. Note that unlike the
enable/disable procedures, ‘readline-set!’ is syntax, which expects
an unquoted option name.
Here is the list of readline options generated by typing
‘(readline-options 'help)’ in Guile. You can also see the default
values.
history-file yes Use history file.
history-length 200 History length.
bounce-parens 500 Time (ms) to show matching opening parenthesis
(0 = off).
bracketed-paste yes Disable interpretation of control characters
in pastes.
The readline options interface can only be used _after_ loading the
readline module, because it is defined in that module.
7.9.3 Readline Functions
------------------------
The following functions are provided by
(use-modules (ice-9 readline))
There are two ways to use readline from Scheme code, either make
calls to ‘readline’ directly to get line by line input, or use the
readline port below with all the usual reading functions.
-- Function: readline [prompt]
Read a line of input from the user and return it as a string
(without a newline at the end). PROMPT is the prompt to show, or
the default is the string set in ‘set-readline-prompt!’ below.
(readline "Type something: ") ⇒ "hello"
-- Function: set-readline-input-port! port
-- Function: set-readline-output-port! port
Set the input and output port the readline function should read
from and write to. PORT must be a file port (*note File Ports::),
and should usually be a terminal.
The default is the ‘current-input-port’ and ‘current-output-port’
(*note Default Ports::) when ‘(ice-9 readline)’ loads, which in an
interactive user session means the Unix “standard input” and
“standard output”.
7.9.3.1 Readline Port
.....................
-- Function: readline-port
Return a buffered input port (*note Buffered Input::) which calls
the ‘readline’ function above to get input. This port can be used
with all the usual reading functions (‘read’, ‘read-char’, etc),
and the user gets the interactive editing features of readline.
There’s only a single readline port created. ‘readline-port’
creates it when first called, and on subsequent calls just returns
what it previously made.
-- Function: activate-readline
If the ‘current-input-port’ is a terminal (*note ‘isatty?’:
Terminals and Ptys.) then enable readline for all reading from
‘current-input-port’ (*note Default Ports::) and enable readline
features in the interactive REPL (*note The REPL::).
(activate-readline)
(read-char)
‘activate-readline’ enables readline on ‘current-input-port’ simply
by a ‘set-current-input-port’ to the ‘readline-port’ above. An
application can do that directly if the extra REPL features that
‘activate-readline’ adds are not wanted.
-- Function: set-readline-prompt! prompt1 [prompt2]
Set the prompt string to print when reading input. This is used
when reading through ‘readline-port’, and is also the default
prompt for the ‘readline’ function above.
PROMPT1 is the initial prompt shown. If a user might enter an
expression across multiple lines, then PROMPT2 is a different
prompt to show further input required. In the Guile REPL for
instance this is an ellipsis (‘...’).
See ‘set-buffered-input-continuation?!’ (*note Buffered Input::)
for an application to indicate the boundaries of logical
expressions (assuming of course an application has such a notion).
7.9.3.2 Completion
..................
-- Function: with-readline-completion-function completer thunk
Call ‘(THUNK)’ with COMPLETER as the readline tab completion
function to be used in any readline calls within that THUNK.
COMPLETER can be ‘#f’ for no completion.
COMPLETER will be called as ‘(COMPLETER text state)’, as described
in (*note (readline)How Completing Works::). TEXT is a partial
word to be completed, and each COMPLETER call should return a
possible completion string or ‘#f’ when no more. STATE is ‘#f’ for
the first call asking about a new TEXT then ‘#t’ while getting
further completions of that TEXT.
Here’s an example COMPLETER for user login names from the password
file (*note User Information::), much like readline’s own
‘rl_username_completion_function’,
(define (username-completer-function text state)
(if (not state)
(setpwent)) ;; new, go to start of database
(let more ((pw (getpwent)))
(if pw
(if (string-prefix? text (passwd:name pw))
(passwd:name pw) ;; this name matches, return it
(more (getpwent))) ;; doesn't match, look at next
(begin
;; end of database, close it and return #f
(endpwent)
#f))))
-- Function: apropos-completion-function text state
A completion function offering completions for Guile functions and
variables (all ‘define’s). This is the default completion
function.
-- Function: filename-completion-function text state
A completion function offering filename completions. This is
readline’s ‘rl_filename_completion_function’ (*note
(readline)Completion Functions::).
-- Function: make-completion-function string-list
Return a completion function which offers completions from the
possibilities in STRING-LIST. Matching is case-sensitive.
7.10 Pretty Printing
====================
The module ‘(ice-9 pretty-print)’ provides the procedure ‘pretty-print’,
which provides nicely formatted output of Scheme objects. This is
especially useful for deeply nested or complex data structures, such as
lists and vectors.
The module is loaded by entering the following:
(use-modules (ice-9 pretty-print))
This makes the procedure ‘pretty-print’ available. As an example how
‘pretty-print’ will format the output, see the following:
(pretty-print '(define (foo) (lambda (x)
(cond ((zero? x) #t) ((negative? x) -x) (else
(if (= x 1) 2 (* x x x)))))))
⊣
(define (foo)
(lambda (x)
(cond ((zero? x) #t)
((negative? x) -x)
(else (if (= x 1) 2 (* x x x))))))
-- Scheme Procedure: pretty-print obj [port] [keyword-options]
Print the textual representation of the Scheme object OBJ to PORT.
PORT defaults to the current output port, if not given.
The further KEYWORD-OPTIONS are keywords and parameters as follows,
#:display? FLAG
If FLAG is true then print using ‘display’. The default is
‘#f’ which means use ‘write’ style. *Note Scheme Write::.
#:per-line-prefix STRING
Print the given STRING as a prefix on each line. The default
is no prefix.
#:width COLUMNS
Print within the given COLUMNS. The default is 79.
#:max-expr-width COLUMNS
The maximum width of an expression. The default is 50.
Also exported by the ‘(ice-9 pretty-print)’ module is
‘truncated-print’, a procedure to print Scheme datums, truncating the
output to a certain number of characters. This is useful when you need
to present an arbitrary datum to the user, but you only have one line in
which to do so.
(define exp '(a b #(c d e) f . g))
(truncated-print exp #:width 10) (newline)
⊣ (a b . #)
(truncated-print exp #:width 15) (newline)
⊣ (a b # f . g)
(truncated-print exp #:width 18) (newline)
⊣ (a b #(c ...) . #)
(truncated-print exp #:width 20) (newline)
⊣ (a b #(c d e) f . g)
(truncated-print "The quick brown fox" #:width 20) (newline)
⊣ "The quick brown..."
(truncated-print (current-module) #:width 20) (newline)
⊣ #
‘truncated-print’ will not output a trailing newline. If an
expression does not fit in the given width, it will be truncated –
possibly ellipsized(1), or in the worst case, displayed as #.
-- Scheme Procedure: truncated-print obj [port] [keyword-options]
Print OBJ, truncating the output, if necessary, to make it fit into
WIDTH characters. By default, OBJ will be printed using ‘write’,
though that behavior can be overridden via the DISPLAY? keyword
argument.
The default behaviour is to print depth-first, meaning that the
entire remaining width will be available to each sub-expression of
OBJ – e.g., if OBJ is a vector, each member of OBJ. One can
attempt to “ration” the available width, trying to allocate it
equally to each sub-expression, via the BREADTH-FIRST? keyword
argument.
The further KEYWORD-OPTIONS are keywords and parameters as follows,
#:display? FLAG
If FLAG is true then print using ‘display’. The default is
‘#f’ which means use ‘write’ style. *note Scheme Write::.
#:width COLUMNS
Print within the given COLUMNS. The default is 79.
#:breadth-first? FLAG
If FLAG is true, then allocate the available width
breadth-first among elements of a compound data structure
(list, vector, pair, etc.). The default is ‘#f’ which means
that any element is allowed to consume all of the available
width.
---------- Footnotes ----------
(1) On Unicode-capable ports, the ellipsis is represented by
character ‘HORIZONTAL ELLIPSIS’ (U+2026), otherwise it is represented by
three dots.
7.11 Formatted Output
=====================
The ‘format’ function is a powerful way to print numbers, strings and
other objects together with literal text under the control of a format
string. This function is available from
(use-modules (ice-9 format))
A format string is generally more compact and easier than using just
the standard procedures like ‘display’, ‘write’ and ‘newline’.
Parameters in the output string allow various output styles, and
parameters can be taken from the arguments for runtime flexibility.
‘format’ is similar to the Common Lisp procedure of the same name,
but it’s not identical and doesn’t have quite all the features found in
Common Lisp.
C programmers will note the similarity between ‘format’ and ‘printf’,
though escape sequences are marked with ~ instead of %, and are more
powerful.
-- Scheme Procedure: format dest fmt arg ...
Write output specified by the FMT string to DEST. DEST can be an
output port, ‘#t’ for ‘current-output-port’ (*note Default
Ports::), or ‘#f’ to return the output as a string.
FMT can contain literal text to be output, and ~ escapes. Each
escape has the form
~ [param [, param...] [:] [@] code
code is a character determining the escape sequence. The : and @
characters are optional modifiers, one or both of which change the
way various codes operate. Optional parameters are accepted by
some codes too. Parameters have the following forms,
[+/-]number
An integer, with optional + or -.
’ (apostrophe)
The following character in the format string, for instance ’z
for z.
v
The next function argument as the parameter. v stands for
“variable”, a parameter can be calculated at runtime and
included in the arguments. Upper case V can be used too.
#
The number of arguments remaining. (See ~* below for some
usages.)
Parameters are separated by commas (,). A parameter can be left
empty to keep its default value when supplying later parameters.
The following escapes are available. The code letters are not
case-sensitive, upper and lower case are the same.
~a
~s
Object output. Parameters: MINWIDTH, PADINC, MINPAD, PADCHAR.
~a outputs an argument like ‘display’, ~s outputs an argument
like ‘write’ (*note Scheme Write::).
(format #t "~a" "foo") ⊣ foo
(format #t "~s" "foo") ⊣ "foo"
~:a and ~:s put objects that don’t have an external
representation in quotes like a string.
(format #t "~:a" car) ⊣ "#"
If the output is less than MINWIDTH characters (default 0),
it’s padded on the right with PADCHAR (default space). ~@a
and ~@s put the padding on the left instead.
(format #f "~5a" 'abc) ⇒ "abc "
(format #f "~5,,,'-@a" 'abc) ⇒ "--abc"
MINPAD is a minimum for the padding then plus a multiple of
PADINC. Ie. the padding is MINPAD + N * PADINC, where N is
the smallest integer making the total object plus padding
greater than or equal to MINWIDTH. The default MINPAD is 0
and the default PADINC is 1 (imposing no minimum or multiple).
(format #f "~5,1,4a" 'abc) ⇒ "abc "
~c
Character. Parameter: CHARNUM.
Output a character. The default is to simply output, as per
‘write-char’ (*note Venerable Port Interfaces::). ~@c prints
in ‘write’ style. ~:c prints control characters (ASCII 0 to
31) in ^X form.
(format #t "~c" #\z) ⊣ z
(format #t "~@c" #\z) ⊣ #\z
(format #t "~:c" #\newline) ⊣ ^J
If the CHARNUM parameter is given then an argument is not
taken but instead the character is ‘(integer->char CHARNUM)’
(*note Characters::). This can be used for instance to output
characters given by their ASCII code.
(format #t "~65c") ⊣ A
~d
~x
~o
~b
Integer. Parameters: MINWIDTH, PADCHAR, COMMACHAR,
COMMAWIDTH.
Output an integer argument as a decimal, hexadecimal, octal or
binary integer (respectively), in a locale-independent way.
(format #t "~d" 123) ⊣ 123
~@d etc shows a + sign is shown on positive numbers.
(format #t "~@b" 12) ⊣ +1100
If the output is less than the MINWIDTH parameter (default no
minimum), it’s padded on the left with the PADCHAR parameter
(default space).
(format #t "~5,'*d" 12) ⊣ ***12
(format #t "~5,'0d" 12) ⊣ 00012
(format #t "~3d" 1234) ⊣ 1234
~:d adds commas (or the COMMACHAR parameter) every three
digits (or the COMMAWIDTH parameter many). However, when your
intent is to write numbers in a way that follows typographical
conventions, using ~h is recommended.
(format #t "~:d" 1234567) ⊣ 1,234,567
(format #t "~10,'*,'/,2:d" 12345) ⊣ ***1/23/45
Hexadecimal ~x output is in lower case, but the ~( and ~) case
conversion directives described below can be used to get upper
case.
(format #t "~x" 65261) ⊣ feed
(format #t "~:@(~x~)" 65261) ⊣ FEED
~r
Integer in words, roman numerals, or a specified radix.
Parameters: RADIX, MINWIDTH, PADCHAR, COMMACHAR, COMMAWIDTH.
With no parameters output is in words as a cardinal like
“ten”, or ~:r prints an ordinal like “tenth”.
(format #t "~r" 9) ⊣ nine ;; cardinal
(format #t "~r" -9) ⊣ minus nine ;; cardinal
(format #t "~:r" 9) ⊣ ninth ;; ordinal
And also with no parameters, ~@r gives roman numerals and ~:@r
gives old roman numerals. In old roman numerals there’s no
“subtraction”, so 9 is VIIII instead of IX. In both cases only
positive numbers can be output.
(format #t "~@r" 89) ⊣ LXXXIX ;; roman
(format #t "~:@r" 89) ⊣ LXXXVIIII ;; old roman
When a parameter is given it means numeric output in the
specified RADIX. The modifiers and parameters following the
radix are the same as described for ~d etc above.
(format #f "~3r" 27) ⇒ "1000" ;; base 3
(format #f "~3,5r" 26) ⇒ " 222" ;; base 3 width 5
~f
Fixed-point float. Parameters: WIDTH, DECIMALS, SCALE,
OVERFLOWCHAR, PADCHAR.
Output a number or number string in fixed-point format, ie.
with a decimal point.
(format #t "~f" 5) ⊣ 5.0
(format #t "~f" "123") ⊣ 123.0
(format #t "~f" "1e-1") ⊣ 0.1
~@f prints a + sign on positive numbers (including zero).
(format #t "~@f" 0) ⊣ +0.0
If the output is less than WIDTH characters it’s padded on the
left with PADCHAR (space by default). If the output equals or
exceeds WIDTH then there’s no padding. The default for WIDTH
is no padding.
(format #f "~6f" -1.5) ⇒ " -1.5"
(format #f "~6,,,,'*f" 23) ⇒ "**23.0"
(format #f "~6f" 1234567.0) ⇒ "1234567.0"
DECIMALS is how many digits to print after the decimal point,
with the value rounded or padded with zeros as necessary.
(The default is to output as many decimals as required.)
(format #t "~1,2f" 3.125) ⊣ 3.13
(format #t "~1,2f" 1.5) ⊣ 1.50
SCALE is a power of 10 applied to the value, moving the
decimal point that many places. A positive SCALE increases
the value shown, a negative decreases it.
(format #t "~,,2f" 1234) ⊣ 123400.0
(format #t "~,,-2f" 1234) ⊣ 12.34
If OVERFLOWCHAR and WIDTH are both given and if the output
would exceed WIDTH, then that many OVERFLOWCHARs are printed
instead of the value.
(format #t "~6,,,'xf" 12345) ⊣ 12345.
(format #t "~5,,,'xf" 12345) ⊣ xxxxx
~h
Localized number(1). Parameters: WIDTH, DECIMALS, PADCHAR.
Like ~f, output an exact or floating point number, but do so
according to the current locale, or according to the given
locale object when the ‘:’ modifier is used (*note
‘number->locale-string’: Number Input and Output.).
(format #t "~h" 12345.5678) ; with "C" as the current locale
⊣ 12345.5678
(format #t "~14,,'*:h" 12345.5678
(make-locale LC_ALL "en_US"))
⊣ ***12,345.5678
(format #t "~,2:h" 12345.5678
(make-locale LC_NUMERIC "fr_FR"))
⊣ 12 345,56
~e
Exponential float. Parameters: WIDTH, MANTDIGITS, EXPDIGITS,
INTDIGITS, OVERFLOWCHAR, PADCHAR, EXPCHAR.
Output a number or number string in exponential notation.
(format #t "~e" 5000.25) ⊣ 5.00025E+3
(format #t "~e" "123.4") ⊣ 1.234E+2
(format #t "~e" "1e4") ⊣ 1.0E+4
~@e prints a + sign on positive numbers (including zero).
(This is for the mantissa, a + or - sign is always shown on
the exponent.)
(format #t "~@e" 5000.0) ⊣ +5.0E+3
If the output is less than WIDTH characters it’s padded on the
left with PADCHAR (space by default). The default for WIDTH
is to output with no padding.
(format #f "~10e" 1234.0) ⇒ " 1.234E+3"
(format #f "~10,,,,,'*e" 0.5) ⇒ "****5.0E-1"
MANTDIGITS is the number of digits shown in the mantissa after
the decimal point. The value is rounded or trailing zeros are
added as necessary. The default MANTDIGITS is to show as much
as needed by the value.
(format #f "~,3e" 11111.0) ⇒ "1.111E+4"
(format #f "~,8e" 123.0) ⇒ "1.23000000E+2"
EXPDIGITS is the minimum number of digits shown for the
exponent, with leading zeros added if necessary. The default
for EXPDIGITS is to show only as many digits as required. At
least 1 digit is always shown.
(format #f "~,,1e" 1.0e99) ⇒ "1.0E+99"
(format #f "~,,6e" 1.0e99) ⇒ "1.0E+000099"
INTDIGITS (default 1) is the number of digits to show before
the decimal point in the mantissa. INTDIGITS can be zero, in
which case the integer part is a single 0, or it can be
negative, in which case leading zeros are shown after the
decimal point.
(format #t "~,,,3e" 12345.0) ⊣ 123.45E+2
(format #t "~,,,0e" 12345.0) ⊣ 0.12345E+5
(format #t "~,,,-3e" 12345.0) ⊣ 0.00012345E+8
If OVERFLOWCHAR is given then WIDTH is a hard limit. If the
output would exceed WIDTH then instead that many OVERFLOWCHARs
are printed.
(format #f "~6,,,,'xe" 100.0) ⇒ "1.0E+2"
(format #f "~3,,,,'xe" 100.0) ⇒ "xxx"
EXPCHAR is the exponent marker character (default E).
(format #t "~,,,,,,'ee" 100.0) ⊣ 1.0e+2
~g
General float. Parameters: WIDTH, MANTDIGITS, EXPDIGITS,
INTDIGITS, OVERFLOWCHAR, PADCHAR, EXPCHAR.
Output a number or number string in either exponential format
the same as ~e, or fixed-point format like ~f but aligned
where the mantissa would have been and followed by padding
where the exponent would have been.
Fixed-point is used when the absolute value is 0.1 or more and
it takes no more space than the mantissa in exponential
format, ie. basically up to MANTDIGITS digits.
(format #f "~12,4,2g" 999.0) ⇒ " 999.0 "
(format #f "~12,4,2g" "100000") ⇒ " 1.0000E+05"
The parameters are interpreted as per ~e above. When
fixed-point is used, the DECIMALS parameter to ~f is
established from MANTDIGITS, so as to give a total
MANTDIGITS+1 figures.
~$
Monetary style fixed-point float. Parameters: DECIMALS,
INTDIGITS, WIDTH, PADCHAR.
Output a number or number string in fixed-point format, ie.
with a decimal point. DECIMALS is the number of decimal
places to show, default 2.
(format #t "~$" 5) ⊣ 5.00
(format #t "~4$" "2.25") ⊣ 2.2500
(format #t "~4$" "1e-2") ⊣ 0.0100
~@$ prints a + sign on positive numbers (including zero).
(format #t "~@$" 0) ⊣ +0.00
INTDIGITS is a minimum number of digits to show in the integer
part of the value (default 1).
(format #t "~,3$" 9.5) ⊣ 009.50
(format #t "~,0$" 0.125) ⊣ .13
If the output is less than WIDTH characters (default 0), it’s
padded on the left with PADCHAR (default space). ~:$ puts the
padding after the sign.
(format #f "~,,8$" -1.5) ⇒ " -1.50"
(format #f "~,,8:$" -1.5) ⇒ "- 1.50"
(format #f "~,,8,'.:@$" 3) ⇒ "+...3.00"
Note that floating point for dollar amounts is generally not a
good idea, because a cent 0.01 cannot be represented exactly
in the binary floating point Guile uses, which leads to slowly
accumulating rounding errors. Keeping values as cents (or
fractions of a cent) in integers then printing with the scale
option in ~f may be a better approach.
~i
Complex fixed-point float. Parameters: WIDTH, DECIMALS,
SCALE, OVERFLOWCHAR, PADCHAR.
Output the argument as a complex number, with both real and
imaginary part shown (even if one or both are zero).
The parameters and modifiers are the same as for fixed-point
~f described above. The real and imaginary parts are both
output with the same given parameters and modifiers, except
that for the imaginary part the @ modifier is always enabled,
so as to print a + sign between the real and imaginary parts.
(format #t "~i" 1) ⊣ 1.0+0.0i
~p
Plural. No parameters.
Output nothing if the argument is 1, or ‘s’ for any other
value.
(format #t "enter name~p" 1) ⊣ enter name
(format #t "enter name~p" 2) ⊣ enter names
~@p prints ‘y’ for 1 or ‘ies’ otherwise.
(format #t "pupp~@p" 1) ⊣ puppy
(format #t "pupp~@p" 2) ⊣ puppies
~:p re-uses the preceding argument instead of taking a new
one, which can be convenient when printing some sort of count.
(format #t "~d cat~:p" 9) ⊣ 9 cats
(format #t "~d pupp~:@p" 5) ⊣ 5 puppies
~p is designed for English plurals and there’s no attempt to
support other languages. ~[ conditionals (below) may be able
to help. When using ‘gettext’ to translate messages
‘ngettext’ is probably best though (*note
Internationalization::).
~y
Structured printing. Parameters: WIDTH.
~y outputs an argument using ‘pretty-print’ (*note Pretty
Printing::). The result will be formatted to fit within WIDTH
columns (79 by default), consuming multiple lines if
necessary.
~@y outputs an argument using ‘truncated-print’ (*note Pretty
Printing::). The resulting code will be formatted to fit
within WIDTH columns (79 by default), on a single line. The
output will be truncated if necessary.
~:@y is like ~@y, except the WIDTH parameter is interpreted to
be the maximum column to which to output. That is to say, if
you are at column 10, and ~60:@y is seen, the datum will be
truncated to 50 columns.
~?
~k
Sub-format. No parameters.
Take a format string argument and a second argument which is a
list of arguments for that string, and output the result.
(format #t "~?" "~d ~d" '(1 2)) ⊣ 1 2
~@? takes arguments for the sub-format directly rather than
in a list.
(format #t "~@? ~s" "~d ~d" 1 2 "foo") ⊣ 1 2 "foo"
~? and ~k are the same, ~k is provided for T-Scheme
compatibility.
~*
Argument jumping. Parameter: N.
Move forward N arguments (default 1) in the argument list.
~:* moves backwards. (N cannot be negative.)
(format #f "~d ~2*~d" 1 2 3 4) ⇒ "1 4"
(format #f "~d ~:*~d" 6) ⇒ "6 6"
~@* moves to argument number N. The first argument is number
0 (and that’s the default for N).
(format #f "~d~d again ~@*~d~d" 1 2) ⇒ "12 again 12"
(format #f "~d~d~d ~1@*~d~d" 1 2 3) ⇒ "123 23"
A # move to the end followed by a : modifier move back can be
used for an absolute position relative to the end of the
argument list, a reverse of what the @ modifier does.
(format #t "~#*~2:*~a" 'a 'b 'c 'd) ⊣ c
At the end of the format string the current argument position
doesn’t matter, any further arguments are ignored.
~t
Advance to a column position. Parameters: COLNUM, COLINC,
PADCHAR.
Output PADCHAR (space by default) to move to the given COLNUM
column. The start of the line is column 0, the default for
COLNUM is 1.
(format #f "~tX") ⇒ " X"
(format #f "~3tX") ⇒ " X"
If the current column is already past COLNUM, then the move is
to there plus a multiple of COLINC, ie. column COLNUM + N *
COLINC for the smallest N which makes that value greater than
or equal to the current column. The default COLINC is 1
(which means no further move).
(format #f "abcd~2,5,'.tx") ⇒ "abcd...x"
~@t takes COLNUM as an offset from the current column. COLNUM
many pad characters are output, then further padding to make
the current column a multiple of COLINC, if it isn’t already
so.
(format #f "a~3,5'*@tx") ⇒ "a****x"
~t is implemented using ‘port-column’ (*note Textual I/O::),
so it works even there has been other output before ‘format’.
~~
Tilde character. Parameter: N.
Output a tilde character ~, or N many if a parameter is given.
Normally ~ introduces an escape sequence, ~~ is the way to
output a literal tilde.
~%
Newline. Parameter: N.
Output a newline character, or N many if a parameter is given.
A newline (or a few newlines) can of course be output just by
including them in the format string.
~&
Start a new line. Parameter: N.
Output a newline if not already at the start of a line. With
a parameter, output that many newlines, but with the first
only if not already at the start of a line. So for instance 3
would be a newline if not already at the start of a line, and
2 further newlines.
~_
Space character. Parameter: N.
Output a space character, or N many if a parameter is given.
With a variable parameter this is one way to insert runtime
calculated padding (~t or the various field widths can do
similar things).
(format #f "~v_foo" 4) ⇒ " foo"
~/
Tab character. Parameter: N.
Output a tab character, or N many if a parameter is given.
~|
Formfeed character. Parameter: N.
Output a formfeed character, or N many if a parameter is
given.
~!
Force output. No parameters.
At the end of output, call ‘force-output’ to flush any buffers
on the destination (*note Buffering::). ~! can occur
anywhere in the format string, but the force is done at the
end of output.
When output is to a string (destination ‘#f’), ~! does
nothing.
~newline (ie. newline character)
Continuation line. No parameters.
Skip this newline and any following whitespace in the format
string, ie. don’t send it to the output. This can be used to
break up a long format string for readability, but not print
the extra whitespace.
(format #f "abc~
~d def~
~d" 1 2) ⇒ "abc1 def2"
~:newline skips the newline but leaves any further whitespace
to be printed normally.
~@newline prints the newline then skips following whitespace.
~( ~)
Case conversion. No parameters.
Between ~( and ~) the case of all output is changed. The
modifiers on ~( control the conversion.
~( — lower case.
~:@( — upper case.
For example,
(format #t "~(Hello~)") ⊣ hello
(format #t "~:@(Hello~)") ⊣ HELLO
In the future it’s intended the modifiers : and @ alone will
capitalize the first letters of words, as per Common Lisp
‘format’, but the current implementation of this is flawed and
not recommended for use.
Case conversions do not nest, currently. This might change in
the future, but if it does then it will be to Common Lisp
style where the outermost conversion has priority, overriding
inner ones (making those fairly pointless).
~{ ~}
Iteration. Parameter: MAXREPS (for ~{).
The format between ~{ and ~} is iterated. The modifiers to ~{
determine how arguments are taken. The default is a list
argument with each iteration successively consuming elements
from it. This is a convenient way to output a whole list.
(format #t "~{~d~}" '(1 2 3)) ⊣ 123
(format #t "~{~s=~d ~}" '("x" 1 "y" 2)) ⊣ "x"=1 "y"=2
~:{ takes a single argument which is a list of lists, each of
those contained lists gives the arguments for the iterated
format.
(format #t "~:{~dx~d ~}" '((1 2) (3 4) (5 6)))
⊣ 1x2 3x4 5x6
~@{ takes arguments directly, with each iteration successively
consuming arguments.
(format #t "~@{~d~}" 1 2 3) ⊣ 123
(format #t "~@{~s=~d ~}" "x" 1 "y" 2) ⊣ "x"=1 "y"=2
~:@{ takes list arguments, one argument for each iteration,
using that list for the format.
(format #t "~:@{~dx~d ~}" '(1 2) '(3 4) '(5 6))
⊣ 1x2 3x4 5x6
Iterating stops when there are no more arguments or when the
MAXREPS parameter to ~{ is reached (default no maximum).
(format #t "~2{~d~}" '(1 2 3 4)) ⊣ 12
If the format between ~{ and ~} is empty, then a format string
argument is taken (before iteration argument(s)) and used
instead. This allows a sub-format (like ~? above) to be
iterated.
(format #t "~{~}" "~d" '(1 2 3)) ⊣ 123
Iterations can be nested, an inner iteration operates in the
same way as described, but of course on the arguments the
outer iteration provides it. This can be used to work into
nested list structures. For example in the following the
inner ~{~d~}x is applied to ‘(1 2)’ then ‘(3 4 5)’ etc.
(format #t "~{~{~d~}x~}" '((1 2) (3 4 5))) ⊣ 12x345x
See also ~^ below for escaping from iteration.
~[ ~; ~]
Conditional. Parameter: SELECTOR.
A conditional block is delimited by ~[ and ~], and ~;
separates clauses within the block. ~[ takes an integer
argument and that number clause is used. The first clause is
number 0.
(format #f "~[peach~;banana~;mango~]" 1) ⇒ "banana"
The SELECTOR parameter can be used for the clause number,
instead of taking an argument.
(format #f "~2[peach~;banana~;mango~]") ⇒ "mango"
If the clause number is out of range then nothing is output.
Or the last clause can be ~:; to use that for a number out of
range.
(format #f "~[banana~;mango~]" 99) ⇒ ""
(format #f "~[banana~;mango~:;fruit~]" 99) ⇒ "fruit"
~:[ treats the argument as a flag, and expects two clauses.
The first is used if the argument is ‘#f’ or the second
otherwise.
(format #f "~:[false~;not false~]" #f) ⇒ "false"
(format #f "~:[false~;not false~]" 'abc) ⇒ "not false"
(let ((n 3))
(format #t "~d gnu~:[s are~; is~] here" n (= 1 n)))
⊣ 3 gnus are here
~@[ also treats the argument as a flag, and expects one
clause. If the argument is ‘#f’ then no output is produced
and the argument is consumed, otherwise the clause is used and
the argument is not consumed, it’s left for the clause. This
can be used for instance to suppress output if ‘#f’ means
something not available.
(format #f "~@[temperature=~d~]" 27) ⇒ "temperature=27"
(format #f "~@[temperature=~d~]" #f) ⇒ ""
~^
Escape. Parameters: VAL1, VAL2, VAL3.
Stop formatting if there are no more arguments. This can be
used for instance to have a format string adapt to a variable
number of arguments.
(format #t "~d~^ ~d" 1) ⊣ 1
(format #t "~d~^ ~d" 1 2) ⊣ 1 2
Within a ~{ ~} iteration, ~^ stops the current iteration step
if there are no more arguments to that step, but continuing
with possible further steps and the rest of the format. This
can be used for instance to avoid a separator on the last
iteration, or to adapt to variable length argument lists.
(format #f "~{~d~^/~} go" '(1 2 3)) ⇒ "1/2/3 go"
(format #f "~:{ ~d~^~d~} go" '((1) (2 3))) ⇒ " 1 23 go"
Within a ~? sub-format, ~^ operates just on that sub-format.
If it terminates the sub-format then the originating format
will still continue.
(format #t "~? items" "~d~^ ~d" '(1)) ⊣ 1 items
(format #t "~? items" "~d~^ ~d" '(1 2)) ⊣ 1 2 items
The parameters to ~^ (which are numbers) change the condition
used to terminate. For a single parameter, termination is
when that value is zero (notice this makes plain ~^ equivalent
to ~#^). For two parameters, termination is when those two
are equal. For three parameters, termination is when VAL1 <=
VAL2 and VAL2 <= VAL3.
~q
Inquiry message. Insert a copyright message into the output.
~:q inserts the format implementation version.
It’s an error if there are not enough arguments for the escapes in
the format string, but any excess arguments are ignored.
Iterations ~{ ~} and conditionals ~[ ~; ~] can be nested, but must
be properly nested, meaning the inner form must be entirely within
the outer form. So it’s not possible, for instance, to try to
conditionalize the endpoint of an iteration.
(format #t "~{ ~[ ... ~] ~}" ...) ;; good
(format #t "~{ ~[ ... ~} ... ~]" ...) ;; bad
The same applies to case conversions ~( ~), they must properly nest
with respect to iterations and conditionals (though currently a
case conversion cannot nest within another case conversion).
When a sub-format (~?) is used, that sub-format string must be
self-contained. It cannot for instance give a ~{ to begin an
iteration form and have the ~} up in the originating format, or
similar.
Guile contains a ‘format’ procedure even when the module ‘(ice-9
format)’ is not loaded. The default ‘format’ is ‘simple-format’ (*note
Simple Output::), it doesn’t support all escape sequences documented in
this section, and will signal an error if you try to use one of them.
The reason for two versions is that the full ‘format’ is fairly large
and requires some time to load. ‘simple-format’ is often adequate too.
Note: Beware that when ‘(ice-9 format)’ is loaded, it replaces the
binding for ‘format’ on the toplevel. If your module loads another
module that loads ‘(ice-9 format)’, then your module will see the
‘format’ function from ‘(ice-9 format)’, even if it does not itself
import ‘(ice-9 format)’.
_This is legacy behavior and may be removed in a future Guile
version._
---------- Footnotes ----------
(1) The ~h format specifier first appeared in Guile version 2.0.6.
7.12 File Tree Walk
===================
The functions in this section traverse a tree of files and directories.
They come in two flavors: the first one is a high-level functional
interface, and the second one is similar to the C ‘ftw’ and ‘nftw’
routines (*note (libc)Working with Directory Trees::).
(use-modules (ice-9 ftw))
-- Scheme Procedure: file-system-tree file-name [enter? [stat]]
Return a tree of the form ‘(FILE-NAME STAT CHILDREN ...)’ where
STAT is the result of ‘(STAT FILE-NAME)’ and CHILDREN are similar
structures for each file contained in FILE-NAME when it designates
a directory.
The optional ENTER? predicate is invoked as ‘(ENTER? NAME STAT)’
and should return true to allow recursion into directory NAME; the
default value is a procedure that always returns ‘#t’. When a
directory does not match ENTER?, it nonetheless appears in the
resulting tree, only with zero children.
The STAT argument is optional and defaults to ‘lstat’, as for
‘file-system-fold’ (see below.)
The example below shows how to obtain a hierarchical listing of the
files under the ‘module/language’ directory in the Guile source
tree, discarding their ‘stat’ info:
(use-modules (ice-9 match))
(define remove-stat
;; Remove the `stat' object the `file-system-tree' provides
;; for each file in the tree.
(match-lambda
((name stat) ; flat file
name)
((name stat children ...) ; directory
(list name (map remove-stat children)))))
(let ((dir (string-append (assq-ref %guile-build-info 'top_srcdir)
"/module/language")))
(remove-stat (file-system-tree dir)))
⇒
("language"
(("value" ("spec.go" "spec.scm"))
("scheme"
("spec.go"
"spec.scm"
"compile-tree-il.scm"
"decompile-tree-il.scm"
"decompile-tree-il.go"
"compile-tree-il.go"))
("tree-il"
("spec.go"
"fix-letrec.go"
"inline.go"
"fix-letrec.scm"
"compile-glil.go"
"spec.scm"
"optimize.scm"
"primitives.scm"
...))
...))
It is often desirable to process directories entries directly, rather
than building up a tree of entries in memory, like ‘file-system-tree’
does. The following procedure, a “combinator”, is designed to allow
directory entries to be processed directly as a directory tree is
traversed; in fact, ‘file-system-tree’ is implemented in terms of it.
-- Scheme Procedure: file-system-fold enter? leaf down up skip error
init file-name [stat]
Traverse the directory at FILE-NAME, recursively, and return the
result of the successive applications of the LEAF, DOWN, UP, and
SKIP procedures as described below.
Enter sub-directories only when ‘(ENTER? PATH STAT RESULT)’ returns
true. When a sub-directory is entered, call ‘(DOWN PATH STAT
RESULT)’, where PATH is the path of the sub-directory and STAT the
result of ‘(false-if-exception (STAT PATH))’; when it is left, call
‘(UP PATH STAT RESULT)’.
For each file in a directory, call ‘(LEAF PATH STAT RESULT)’.
When ENTER? returns ‘#f’, or when an unreadable directory is
encountered, call ‘(SKIP PATH STAT RESULT)’.
When FILE-NAME names a flat file, ‘(LEAF PATH STAT INIT)’ is
returned.
When an ‘opendir’ or STAT call fails, call ‘(ERROR PATH STAT ERRNO
RESULT)’, with ERRNO being the operating system error number that
was raised—e.g., ‘EACCES’—and STAT either ‘#f’ or the result of the
STAT call for that entry, when available.
The special ‘.’ and ‘..’ entries are not passed to these
procedures. The PATH argument to the procedures is a full file
name—e.g., ‘"../foo/bar/gnu"’; if FILE-NAME is an absolute file
name, then PATH is also an absolute file name. Files and
directories, as identified by their device/inode number pair, are
traversed only once.
The optional STAT argument defaults to ‘lstat’, which means that
symbolic links are not followed; the ‘stat’ procedure can be used
instead when symbolic links are to be followed (*note stat: File
System.).
The example below illustrates the use of ‘file-system-fold’:
(define (total-file-size file-name)
"Return the size in bytes of the files under FILE-NAME (similar
to `du --apparent-size' with GNU Coreutils.)"
(define (enter? name stat result)
;; Skip version control directories.
(not (member (basename name) '(".git" ".svn" "CVS"))))
(define (leaf name stat result)
;; Return RESULT plus the size of the file at NAME.
(+ result (stat:size stat)))
;; Count zero bytes for directories.
(define (down name stat result) result)
(define (up name stat result) result)
;; Likewise for skipped directories.
(define (skip name stat result) result)
;; Ignore unreadable files/directories but warn the user.
(define (error name stat errno result)
(format (current-error-port) "warning: ~a: ~a~%"
name (strerror errno))
result)
(file-system-fold enter? leaf down up skip error
0 ; initial counter is zero bytes
file-name))
(total-file-size ".")
⇒ 8217554
(total-file-size "/dev/null")
⇒ 0
The alternative C-like functions are described below.
-- Scheme Procedure: scandir name [select? [entrystream list
-- Scheme Procedure: vector->stream vector
Return a stream with the contents of LIST or VECTOR.
LIST or VECTOR should not be modified subsequently, since it’s
unspecified whether changes there will be reflected in the stream
returned.
-- Scheme Procedure: port->stream port readproc
Return a stream which is the values obtained by reading from PORT
using READPROC. Each read call is ‘(READPROC PORT)’, and it should
return an EOF object (*note Binary I/O::) at the end of input.
For example a stream of characters from a file,
(port->stream (open-input-file "/foo/bar.txt") read-char)
-- Scheme Procedure: stream->list stream
Return a list which is the entire contents of STREAM.
-- Scheme Procedure: stream->reversed-list stream
Return a list which is the entire contents of STREAM, but in
reverse order.
-- Scheme Procedure: stream->list&length stream
Return two values (*note Multiple Values::), being firstly a list
which is the entire contents of STREAM, and secondly the number of
elements in that list.
-- Scheme Procedure: stream->reversed-list&length stream
Return two values (*note Multiple Values::) being firstly a list
which is the entire contents of STREAM, but in reverse order, and
secondly the number of elements in that list.
-- Scheme Procedure: stream->vector stream
Return a vector which is the entire contents of STREAM.
-- Function: stream-fold proc init stream1 stream2 ...
Apply PROC successively over the elements of the given streams,
from first to last until the end of the shortest stream is reached.
Return the result from the last PROC call.
Each call is ‘(PROC elem1 elem2 ... prev)’, where each ELEM is from
the corresponding STREAM. PREV is the return from the previous
PROC call, or the given INIT for the first call.
-- Function: stream-for-each proc stream1 stream2 ...
Call PROC on the elements from the given STREAMs. The return value
is unspecified.
Each call is ‘(PROC elem1 elem2 ...)’, where each ELEM is from the
corresponding STREAM. ‘stream-for-each’ stops when it reaches the
end of the shortest STREAM.
-- Function: stream-map proc stream1 stream2 ...
Return a new stream which is the results of applying PROC to the
elements of the given STREAMs.
Each call is ‘(PROC elem1 elem2 ...)’, where each ELEM is from the
corresponding STREAM. The new stream ends when the end of the
shortest given STREAM is reached.
7.15 Buffered Input
===================
The following functions are provided by
(use-modules (ice-9 buffered-input))
A buffered input port allows a reader function to return chunks of
characters which are to be handed out on reading the port. A notion of
further input for an application level logical expression is maintained
too, and passed through to the reader.
-- Scheme Procedure: make-buffered-input-port reader
Create an input port which returns characters obtained from the
given READER function. READER is called (READER cont), and should
return a string or an EOF object.
The new port gives precisely the characters returned by READER,
nothing is added, so if any newline characters or other separators
are desired they must come from the reader function.
The CONT parameter to READER is ‘#f’ for initial input, or ‘#t’
when continuing an expression. This is an application level
notion, set with ‘set-buffered-input-continuation?!’ below. If the
user has entered a partial expression then it allows READER for
instance to give a different prompt to show more is required.
-- Scheme Procedure: make-line-buffered-input-port reader
Create an input port which returns characters obtained from the
specified READER function, similar to ‘make-buffered-input-port’
above, but where READER is expected to be a line-oriented.
READER is called (READER cont), and should return a string or an
EOF object as above. Each string is a line of input without a
newline character, the port code inserts a newline after each
string.
-- Scheme Procedure: set-buffered-input-continuation?! port cont
Set the input continuation flag for a given buffered input PORT.
An application uses this by calling with a CONT flag of ‘#f’ when
beginning to read a new logical expression. For example with the
Scheme ‘read’ function (*note Scheme Read::),
(define my-port (make-buffered-input-port my-reader))
(set-buffered-input-continuation?! my-port #f)
(let ((obj (read my-port)))
...
7.16 Expect
===========
The macros in this section are made available with:
(use-modules (ice-9 expect))
‘expect’ is a macro for selecting actions based on the output from a
port. The name comes from a tool of similar functionality by Don Libes.
Actions can be taken when a particular string is matched, when a timeout
occurs, or when end-of-file is seen on the port. The ‘expect’ macro is
described below; ‘expect-strings’ is a front-end to ‘expect’ based on
regexec (see the regular expression documentation).
-- Macro: expect-strings clause ...
By default, ‘expect-strings’ will read from the current input port.
The first term in each clause consists of an expression evaluating
to a string pattern (regular expression). As characters are read
one-by-one from the port, they are accumulated in a buffer string
which is matched against each of the patterns. When a pattern
matches, the remaining expression(s) in the clause are evaluated
and the value of the last is returned. For example:
(with-input-from-file "/etc/passwd"
(lambda ()
(expect-strings
("^nobody" (display "Got a nobody user.\n")
(display "That's no problem.\n"))
("^daemon" (display "Got a daemon user.\n")))))
The regular expression is compiled with the ‘REG_NEWLINE’ flag, so
that the ^ and $ anchors will match at any newline, not just at the
start and end of the string.
There are two other ways to write a clause:
The expression(s) to evaluate can be omitted, in which case the
result of the regular expression match (converted to strings, as
obtained from regexec with match-pick set to "") will be returned
if the pattern matches.
The symbol ‘=>’ can be used to indicate that the expression is a
procedure which will accept the result of a successful regular
expression match. E.g.,
("^daemon" => write)
("^d(aemon)" => (lambda args (for-each write args)))
("^da(em)on" => (lambda (all sub)
(write all) (newline)
(write sub) (newline)))
The order of the substrings corresponds to the order in which the
opening brackets occur.
A number of variables can be used to control the behaviour of
‘expect’ (and ‘expect-strings’). Most have default top-level
bindings to the value ‘#f’, which produces the default behaviour.
They can be redefined at the top level or locally bound in a form
enclosing the expect expression.
‘expect-port’
A port to read characters from, instead of the current input
port.
‘expect-timeout’
‘expect’ will terminate after this number of seconds,
returning ‘#f’ or the value returned by expect-timeout-proc.
‘expect-timeout-proc’
A procedure called if timeout occurs. The procedure takes a
single argument: the accumulated string.
‘expect-eof-proc’
A procedure called if end-of-file is detected on the input
port. The procedure takes a single argument: the accumulated
string.
‘expect-char-proc’
A procedure to be called every time a character is read from
the port. The procedure takes a single argument: the
character which was read.
‘expect-strings-compile-flags’
Flags to be used when compiling a regular expression, which
are passed to ‘make-regexp’ *Note Regexp Functions::. The
default value is ‘regexp/newline’.
‘expect-strings-exec-flags’
Flags to be used when executing a regular expression, which
are passed to regexp-exec *Note Regexp Functions::. The
default value is ‘regexp/noteol’, which prevents ‘$’ from
matching the end of the string while it is still accumulating,
but still allows it to match after a line break or at the end
of file.
Here’s an example using all of the variables:
(let ((expect-port (open-input-file "/etc/passwd"))
(expect-timeout 1)
(expect-timeout-proc
(lambda (s) (display "Times up!\n")))
(expect-eof-proc
(lambda (s) (display "Reached the end of the file!\n")))
(expect-char-proc display)
(expect-strings-compile-flags (logior regexp/newline regexp/icase))
(expect-strings-exec-flags 0))
(expect-strings
("^nobody" (display "Got a nobody user\n"))))
-- Macro: expect clause ...
‘expect’ is used in the same way as ‘expect-strings’, but tests are
specified not as patterns, but as procedures. The procedures are
called in turn after each character is read from the port, with two
arguments: the value of the accumulated string and a flag to
indicate whether end-of-file has been reached. The flag will
usually be ‘#f’, but if end-of-file is reached, the procedures are
called an additional time with the final accumulated string and
‘#t’.
The test is successful if the procedure returns a non-false value.
If the ‘=>’ syntax is used, then if the test succeeds it must
return a list containing the arguments to be provided to the
corresponding expression.
In the following example, a string will only be matched at the
beginning of the file:
(let ((expect-port (open-input-file "/etc/passwd")))
(expect
((lambda (s eof?) (string=? s "fnord!"))
(display "Got a nobody user!\n"))))
The control variables described for ‘expect-strings’ also influence
the behaviour of ‘expect’, with the exception of variables whose
names begin with ‘expect-strings-’.
7.17 ‘sxml-match’: Pattern Matching of SXML
===========================================
The ‘(sxml match)’ module provides syntactic forms for pattern matching
of SXML trees, in a “by example” style reminiscent of the pattern
matching of the ‘syntax-rules’ and ‘syntax-case’ macro systems. *Note
SXML::, for more information on SXML.
The following example(1) provides a brief illustration, transforming
a music album catalog language into HTML.
(define (album->html x)
(sxml-match x
((album (@ (title ,t)) (catalog (num ,n) (fmt ,f)) ...)
`(ul (li ,t)
(li (b ,n) (i ,f)) ...))))
Three macros are provided: ‘sxml-match’, ‘sxml-match-let’, and
‘sxml-match-let*’.
Compared to a standard s-expression pattern matcher (*note Pattern
Matching::), ‘sxml-match’ provides the following benefits:
• matching of SXML elements does not depend on any degree of
normalization of the SXML;
• matching of SXML attributes (within an element) is under-ordered;
the order of the attributes specified within the pattern need not
match the ordering with the element being matched;
• all attributes specified in the pattern must be present in the
element being matched; in the spirit that XML is ’extensible’, the
element being matched may include additional attributes not
specified in the pattern.
The present module is a descendant of WebIt!, and was inspired by an
s-expression pattern matcher developed by Erik Hilsdale, Dan Friedman,
and Kent Dybvig at Indiana University.
Syntax
------
‘sxml-match’ provides ‘case’-like form for pattern matching of XML
nodes.
-- Scheme Syntax: sxml-match input-expression clause1 clause2 ...
Match INPUT-EXPRESSION, an SXML tree, according to the given
CLAUSEs (one or more), each consisting of a pattern and one or more
expressions to be evaluated if the pattern match succeeds.
Optionally, each CLAUSE within ‘sxml-match’ may include a “guard
expression”.
The pattern notation is based on that of Scheme’s ‘syntax-rules’ and
‘syntax-case’ macro systems. The grammar for the ‘sxml-match’ syntax is
given below:
match-form ::= (sxml-match input-expression
clause+)
clause ::= [node-pattern action-expression+]
| [node-pattern (guard expression*) action-expression+]
node-pattern ::= literal-pattern
| pat-var-or-cata
| element-pattern
| list-pattern
literal-pattern ::= string
| character
| number
| #t
| #f
attr-list-pattern ::= (@ attribute-pattern*)
| (@ attribute-pattern* . pat-var-or-cata)
attribute-pattern ::= (tag-symbol attr-val-pattern)
attr-val-pattern ::= literal-pattern
| pat-var-or-cata
| (pat-var-or-cata default-value-expr)
element-pattern ::= (tag-symbol attr-list-pattern?)
| (tag-symbol attr-list-pattern? nodeset-pattern)
| (tag-symbol attr-list-pattern?
nodeset-pattern? . pat-var-or-cata)
list-pattern ::= (list nodeset-pattern)
| (list nodeset-pattern? . pat-var-or-cata)
| (list)
nodeset-pattern ::= node-pattern
| node-pattern ...
| node-pattern nodeset-pattern
| node-pattern ... nodeset-pattern
pat-var-or-cata ::= (unquote var-symbol)
| (unquote [var-symbol*])
| (unquote [cata-expression -> var-symbol*])
Within a list or element body pattern, ellipses may appear only once,
but may be followed by zero or more node patterns.
Guard expressions cannot refer to the return values of catamorphisms.
Ellipses in the output expressions must appear only in an expression
context; ellipses are not allowed in a syntactic form.
The sections below illustrate specific aspects of the ‘sxml-match’
pattern matcher.
Matching XML Elements
---------------------
The example below illustrates the pattern matching of an XML element:
(sxml-match '(e (@ (i 1)) 3 4 5)
((e (@ (i ,d)) ,a ,b ,c) (list d a b c))
(,otherwise #f))
Each clause in ‘sxml-match’ contains two parts: a pattern and one or
more expressions which are evaluated if the pattern is successfully
match. The example above matches an element ‘e’ with an attribute ‘i’
and three children.
Pattern variables must be “unquoted” in the pattern. The above
expression binds D to ‘1’, A to ‘3’, B to ‘4’, and C to ‘5’.
Ellipses in Patterns
--------------------
As in ‘syntax-rules’, ellipses may be used to specify a repeated
pattern. Note that the pattern ‘item ...’ specifies zero-or-more
matches of the pattern ‘item’.
The use of ellipses in a pattern is illustrated in the code fragment
below, where nested ellipses are used to match the children of repeated
instances of an ‘a’ element, within an element ‘d’.
(define x '(d (a 1 2 3) (a 4 5) (a 6 7 8) (a 9 10)))
(sxml-match x
((d (a ,b ...) ...)
(list (list b ...) ...)))
The above expression returns a value of ‘((1 2 3) (4 5) (6 7 8) (9
10))’.
Ellipses in Quasiquote’d Output
-------------------------------
Within the body of an ‘sxml-match’ form, a slightly extended version of
quasiquote is provided, which allows the use of ellipses. This is
illustrated in the example below.
(sxml-match '(e 3 4 5 6 7)
((e ,i ... 6 7) `("start" ,(list 'wrap i) ... "end"))
(,otherwise #f))
The general pattern is that ‘`(something ,i ...)’ is rewritten as
‘`(something ,@i)’.
Matching Nodesets
-----------------
A nodeset pattern is designated by a list in the pattern, beginning the
identifier list. The example below illustrates matching a nodeset.
(sxml-match '("i" "j" "k" "l" "m")
((list ,a ,b ,c ,d ,e)
`((p ,a) (p ,b) (p ,c) (p ,d) (p ,e))))
This example wraps each nodeset item in an HTML paragraph element.
This example can be rewritten and simplified through using ellipsis:
(sxml-match '("i" "j" "k" "l" "m")
((list ,i ...)
`((p ,i) ...)))
This version will match nodesets of any length, and wrap each item in
the nodeset in an HTML paragraph element.
Matching the “Rest” of a Nodeset
--------------------------------
Matching the “rest” of a nodeset is achieved by using a ‘. rest)’
pattern at the end of an element or nodeset pattern.
This is illustrated in the example below:
(sxml-match '(e 3 (f 4 5 6) 7)
((e ,a (f . ,y) ,d)
(list a y d)))
The above expression returns ‘(3 (4 5 6) 7)’.
Matching the Unmatched Attributes
---------------------------------
Sometimes it is useful to bind a list of attributes present in the
element being matched, but which do not appear in the pattern. This is
achieved by using a ‘. rest)’ pattern at the end of the attribute list
pattern. This is illustrated in the example below:
(sxml-match '(a (@ (z 1) (y 2) (x 3)) 4 5 6)
((a (@ (y ,www) . ,qqq) ,t ,u ,v)
(list www qqq t u v)))
The above expression matches the attribute ‘y’ and binds a list of
the remaining attributes to the variable QQQ. The result of the above
expression is ‘(2 ((z 1) (x 3)) 4 5 6)’.
This type of pattern also allows the binding of all attributes:
(sxml-match '(a (@ (z 1) (y 2) (x 3)))
((a (@ . ,qqq))
qqq))
Default Values in Attribute Patterns
------------------------------------
It is possible to specify a default value for an attribute which is used
if the attribute is not present in the element being matched. This is
illustrated in the following example:
(sxml-match '(e 3 4 5)
((e (@ (z (,d 1))) ,a ,b ,c) (list d a b c)))
The value ‘1’ is used when the attribute ‘z’ is absent from the
element ‘e’.
Guards in Patterns
------------------
Guards may be added to a pattern clause via the ‘guard’ keyword. A
guard expression may include zero or more expressions which are
evaluated only if the pattern is matched. The body of the clause is
only evaluated if the guard expressions evaluate to ‘#t’.
The use of guard expressions is illustrated below:
(sxml-match '(a 2 3)
((a ,n) (guard (number? n)) n)
((a ,m ,n) (guard (number? m) (number? n)) (+ m n)))
Catamorphisms
-------------
The example below illustrates the use of explicit recursion within an
‘sxml-match’ form. This example implements a simple calculator for the
basic arithmetic operations, which are represented by the XML elements
‘plus’, ‘minus’, ‘times’, and ‘div’.
(define simple-eval
(lambda (x)
(sxml-match x
(,i (guard (integer? i)) i)
((plus ,x ,y) (+ (simple-eval x) (simple-eval y)))
((times ,x ,y) (* (simple-eval x) (simple-eval y)))
((minus ,x ,y) (- (simple-eval x) (simple-eval y)))
((div ,x ,y) (/ (simple-eval x) (simple-eval y)))
(,otherwise (error "simple-eval: invalid expression" x)))))
Using the catamorphism feature of ‘sxml-match’, a more concise
version of ‘simple-eval’ can be written. The pattern ‘,(x)’ recursively
invokes the pattern matcher on the value bound in this position.
(define simple-eval
(lambda (x)
(sxml-match x
(,i (guard (integer? i)) i)
((plus ,(x) ,(y)) (+ x y))
((times ,(x) ,(y)) (* x y))
((minus ,(x) ,(y)) (- x y))
((div ,(x) ,(y)) (/ x y))
(,otherwise (error "simple-eval: invalid expression" x)))))
Named-Catamorphisms
-------------------
It is also possible to explicitly name the operator in the “cata”
position. Where ‘,(id*)’ recurs to the top of the current ‘sxml-match’,
‘,(cata -> id*)’ recurs to ‘cata’. ‘cata’ must evaluate to a procedure
which takes one argument, and returns as many values as there are
identifiers following ‘->’.
Named catamorphism patterns allow processing to be split into
multiple, mutually recursive procedures. This is illustrated in the
example below: a transformation that formats a “TV Guide” into HTML.
(define (tv-guide->html g)
(define (cast-list cl)
(sxml-match cl
((CastList (CastMember (Character (Name ,ch)) (Actor (Name ,a))) ...)
`(div (ul (li ,ch ": " ,a) ...)))))
(define (prog p)
(sxml-match p
((Program (Start ,start-time) (Duration ,dur) (Series ,series-title)
(Description ,desc ...))
`(div (p ,start-time
(br) ,series-title
(br) ,desc ...)))
((Program (Start ,start-time) (Duration ,dur) (Series ,series-title)
(Description ,desc ...)
,(cast-list -> cl))
`(div (p ,start-time
(br) ,series-title
(br) ,desc ...)
,cl))))
(sxml-match g
((TVGuide (@ (start ,start-date)
(end ,end-date))
(Channel (Name ,nm) ,(prog -> p) ...) ...)
`(html (head (title "TV Guide"))
(body (h1 "TV Guide")
(div (h2 ,nm) ,p ...) ...)))))
‘sxml-match-let’ and ‘sxml-match-let*’
--------------------------------------
-- Scheme Syntax: sxml-match-let ((pat expr) ...) expression0
expression ...
-- Scheme Syntax: sxml-match-let* ((pat expr) ...) expression0
expression ...
These forms generalize the ‘let’ and ‘let*’ forms of Scheme to
allow an XML pattern in the binding position, rather than a simple
variable.
For example, the expression below:
(sxml-match-let (((a ,i ,j) '(a 1 2)))
(+ i j))
binds the variables I and J to ‘1’ and ‘2’ in the XML value given.
---------- Footnotes ----------
(1) This example is taken from a paper by Krishnamurthi et al. Their
paper was the first to show the usefulness of the ‘syntax-rules’ style
of pattern matching for transformation of XML, though the language
described, XT3D, is an XML language.
7.18 The Scheme shell (scsh)
============================
An incomplete port of the Scheme shell (scsh) was once available for
Guile as a separate package. However this code has bitrotten somewhat.
The pieces are available in Guile’s legacy CVS repository, which may be
browsed at
.
For information about scsh see :2:10
---
Sample count: 7
Total time: 0.123490713 seconds (0.201983993 seconds in GC)
All of the numerical data with the exception of the calls column is
statistically approximate. In the following column descriptions, and in
all of statprof, “time” refers to execution time (both user and system),
not wall clock time.
The ‘% time’ column indicates the percentage of the run-time time
spent inside the procedure itself (not counting children). It is
calculated as ‘self seconds’, measuring the amount of time spent in the
procedure, divided by the total run-time.
‘cumulative seconds’ also counts time spent in children of a
function. For recursive functions, this can exceed the total time, as
in our example above, because each activation on the stack adds to the
cumulative time.
Finally, the GC time measures the time spent in the garbage
collector. On systems with multiple cores, this time can be larger than
the run time, because it counts time spent in all threads, and will run
the “marking” phase of GC in parallel. If GC time is a significant
fraction of the run time, that means that most time in your program is
spent allocating objects and cleaning up after those allocations. To
speed up your program, one good place to start would be to look at how
to reduce the allocation rate.
Statprof’s main mode of operation is as a statistical profiler.
However statprof can also run in a “precise” mode as well. Pass the
‘#:count-calls? #t’ keyword argument to ‘statprof’ to record all calls:
(use-modules (statprof))
(statprof (lambda ()
(map 1+ (iota 1000000))
#f)
#:count-calls? #t)
The result has an additional ‘calls’ column:
% cumulative self
time seconds seconds calls procedure
82.26 0.73 0.73 1000000 1+
11.29 420925.80 0.10 1000001 ice-9/boot-9.scm:249:5:map1
4.84 0.06 0.04 1 ice-9/boot-9.scm:1165:0:iota
[...]
---
Sample count: 62
Total time: 0.893098065 seconds (1.222796536 seconds in GC)
As you can see, the profile is perturbed: ‘1+’ ends up on top,
whereas it was not marked as hot in the earlier profile. This is
because the overhead of call-counting unfairly penalizes calls. Still,
this precise mode can be useful at times to do algorithmic optimizations
based on the precise call counts.
Implementation notes
====================
The profiler works by setting the unix profiling signal ‘ITIMER_PROF’ to
go off after the interval you define in the call to ‘statprof-reset’.
When the signal fires, a sampling routine runs which crawls up the
stack, recording all instruction pointers into a buffer. After the
sample is complete, the profiler resets profiling timer to fire again
after the appropriate interval.
Later, when profiling stops, that log buffer is analyzed to produce
the “self seconds” and “cumulative seconds” statistics. A procedure at
the top of the stack counts toward “self” samples, and everything on the
stack counts towards “cumulative” samples.
While the profiler is running it measures how much CPU time (system
and user – which is also what ‘ITIMER_PROF’ tracks) has elapsed while
code has been executing within the profiler. Only run time counts
towards the profile, not wall-clock time. For example, sleeping and
waiting for input or output do not cause the timer clock to advance.
Usage
=====
-- Scheme Procedure: statprof thunk [#:loop loop=1] [#:hz hz=100]
[#:port port=(current-output-port)] [#:count-calls?
count-calls?=#f] [#:display-style display-style='flat]
Profile the execution of THUNK, and return its return values.
The stack will be sampled HZ times per second, and the thunk itself
will be called LOOP times.
If COUNT-CALLS? is true, all procedure calls will be recorded.
This operation is somewhat expensive.
After the THUNK has been profiled, print out a profile to PORT. If
DISPLAY-STYLE is ‘flat’, the results will be printed as a flat
profile. Otherwise if DISPLAY-STYLE is ‘tree’, print the results
as a tree profile.
Note that ‘statprof’ requires a working profiling timer. Some
platforms do not support profiling timers. ‘(provided?
'ITIMER_PROF)’ can be used to check for support of profiling
timers.
Profiling can also be enabled and disabled manually.
-- Scheme Procedure: statprof-active?
Returns ‘#t’ if ‘statprof-start’ has been called more times than
‘statprof-stop’, ‘#f’ otherwise.
-- Scheme Procedure: statprof-start
-- Scheme Procedure: statprof-stop
Start or stop the profiler.
-- Scheme Procedure: statprof-reset sample-seconds sample-microseconds
count-calls?
Reset the profiling sample interval to SAMPLE-SECONDS and
SAMPLE-MICROSECONDS. If COUNT-CALLS? is true, arrange to
instrument procedure calls as well as collecting statistical
profiling data.
If you use the manual ‘statprof-start’/‘statprof-stop’ interface, an
implicit statprof state will persist starting from the last call to
‘statprof-reset’, or the first call to ‘statprof-start’. There are a
number of accessors to fetch statistics from this implicit state.
-- Scheme Procedure: statprof-accumulated-time
Returns the time accumulated during the last statprof run.
-- Scheme Procedure: statprof-sample-count
Returns the number of samples taken during the last statprof run.
-- Scheme Procedure: statprof-fold-call-data proc init
Fold PROC over the call-data accumulated by statprof. This
procedure cannot be called while statprof is active.
PROC will be called with arguments, CALL-DATA and PRIOR-RESULT.
-- Scheme Procedure: statprof-proc-call-data proc
Returns the call-data associated with PROC, or ‘#f’ if none is
available.
-- Scheme Procedure: statprof-call-data-name cd
-- Scheme Procedure: statprof-call-data-calls cd
-- Scheme Procedure: statprof-call-data-cum-samples cd
-- Scheme Procedure: statprof-call-data-self-samples cd
Accessors for the fields in a statprof call-data object.
-- Scheme Procedure: statprof-call-data->stats call-data
Returns an object of type ‘statprof-stats’.
-- Scheme Procedure: statprof-stats-proc-name stats
-- Scheme Procedure: statprof-stats-%-time-in-proc stats
-- Scheme Procedure: statprof-stats-cum-secs-in-proc stats
-- Scheme Procedure: statprof-stats-self-secs-in-proc stats
-- Scheme Procedure: statprof-stats-calls stats
-- Scheme Procedure: statprof-stats-self-secs-per-call stats
-- Scheme Procedure: statprof-stats-cum-secs-per-call stats
Accessors for the fields in a ‘statprof-stats’ object.
-- Scheme Procedure: statprof-display [port=(current-output-port)]
[#:style style=flat]
Displays a summary of the statistics collected. Possible values
for STYLE include:
‘flat’
Display a traditional gprof-style flat profile.
‘anomalies’
Find statistical anomalies in the data.
‘tree’
Display a tree profile.
-- Scheme Procedure: statprof-fetch-stacks
Returns a list of stacks, as they were captured since the last call
to ‘statprof-reset’.
-- Scheme Procedure: statprof-fetch-call-tree [#:precise precise?=#f]
Return a call tree for the previous statprof run.
The return value is a list of nodes. A node is a list of the form:
@code
node ::= (@var{proc} @var{count} . @var{nodes})
@end code
The @var{proc} is a printable representation of a procedure, as a
string. If @var{precise?} is false, which is the default, then a node
corresponds to a procedure invocation. If it is true, then a node
corresponds to a return point in a procedure. Passing @code{#:precise?
#t} allows a user to distinguish different source lines in a procedure,
but usually it is too much detail, so it is off by default.
-- Scheme Procedure: gcprof thunk [#:loop]
Like the ‘statprof’ procedure, but instead of profiling CPU time,
we profile garbage collection.
The stack will be sampled soon after every garbage collection
during the evaluation of THUNK, yielding an approximate idea of
what is causing allocation in your program.
Since GC does not occur very frequently, you may need to use the
LOOP parameter, to cause THUNK to be called LOOP times.
7.21 SXML
=========
SXML is a native representation of XML in terms of standard Scheme data
types: lists, symbols, and strings. For example, the simple XML
fragment:
Alfie text ")
⇒ (*TOP* (foo "text"))
(xml->sxml "text ")
⇒ (*TOP* (foo (@ (kind "bar")) "text"))
(xml->sxml "text ")
⇒ (*TOP* (http://example.org/ns1:foo "text"))
(xml->sxml "text "
#:namespaces '((ns1 . "http://example.org/ns1")))
⇒ (*TOP* (ns1:foo "text"))
(xml->sxml "\n Alfie the parrot! \n ")
⇒ (*TOP* (foo "\n" (bar " Alfie the parrot! ") "\n"))
(xml->sxml "\n Alfie the parrot! \n "
#:trim-whitespace? #t)
⇒ (*TOP* (foo (bar " Alfie the parrot! ")))
Parsed entities may be declared with the ‘#:entities’ keyword
argument, or handled with the ‘#:default-entity-handler’. By default,
only the standard ‘<’, ‘>’, ‘&’, ‘'’ and ‘"’
entities are defined, as well as the ‘&#N;’ and ‘&#xN;’ (decimal and
hexadecimal) numeric character entities.
(xml->sxml "& ")
⇒ (*TOP* (foo "&"))
(xml->sxml " ")
⇒ error: undefined entity: nbsp
(xml->sxml " ")
⇒ (*TOP* (foo "\xa0"))
(xml->sxml " "
#:entities '((nbsp . "\xa0")))
⇒ (*TOP* (foo "\xa0"))
(xml->sxml " &foo; "
#:default-entity-handler
(lambda (port name)
(case name
((nbsp) "\xa0")
(else
(format (current-warning-port)
"~a:~a:~a: undefined entitity: ~a\n"
(or (port-filename port) "")
(port-line port) (port-column port)
name)
(symbol->string name)))))
⊣ :0:17: undefined entitity: foo
⇒ (*TOP* (foo "\xa0 foo"))
By default, ‘xml->sxml’ skips over the ‘’ declaration, if
any. This behavior can be overridden with the ‘#:doctype-handler’
argument, which should be a procedure of three arguments: the “docname”
(a symbol), “systemid” (a string), and the internal doctype subset (as a
string or ‘#f’ if not present).
The handler should return keyword arguments as multiple values, as if
it were calling its continuation with keyword arguments. The
continuation accepts the ‘#:entities’ and ‘#:namespaces’ keyword
arguments, in the same format that ‘xml->sxml’ itself takes. These
entities and namespaces will be prepended to those given to the
‘xml->sxml’ invocation.
(define (handle-foo docname systemid internal-subset)
(case docname
((foo)
(values #:entities '((greets . "Hello, world!"))))
(else
(values))))
(xml->sxml "&greets;
"
#:doctype-handler handle-foo)
⇒ (*TOP* (p (i "Hello, world!")))
If the document has no doctype declaration, the DOCTYPE-HANDLER is
invoked with ‘#f’ for the three arguments.
In the future, the continuation may accept other keyword arguments,
for example to validate the parsed SXML against the doctype.
-- Scheme Procedure: sxml->xml tree [port]
Serialize the SXML tree TREE as XML. The output will be written to
the current output port, unless the optional argument PORT is
present.
-- Scheme Procedure: sxml->string sxml
Detag an sxml tree SXML into a string. Does not perform any
formatting.
7.21.3 SSAX: A Functional XML Parsing Toolkit
---------------------------------------------
Guile’s XML parser is based on Oleg Kiselyov’s powerful XML parsing
toolkit, SSAX.
7.21.3.1 History
................
Back in the 1990s, when the world was young again and XML was the
solution to all of its problems, there were basically two kinds of XML
parsers out there: DOM parsers and SAX parsers.
A DOM parser reads through an entire XML document, building up a tree
of “DOM objects” representing the document structure. They are very
easy to use, but sometimes you don’t actually want all of the
information in a document; building an object tree is not necessary if
all you want to do is to count word frequencies in a document, for
example.
SAX parsers were created to give the programmer more control on the
parsing process. A programmer gives the SAX parser a number of
“callbacks”: functions that will be called on various features of the
XML stream as they are encountered. SAX parsers are more efficient, but
much harder to use, as users typically have to manually maintain a stack
of open elements.
Kiselyov realized that the SAX programming model could be made much
simpler if the callbacks were formulated not as a linear fold across the
features of the XML stream, but as a _tree fold_ over the structure
implicit in the XML. In this way, the user has a very convenient,
functional-style interface that can still generate optimal parsers.
The ‘xml->sxml’ interface from the ‘(sxml simple)’ module is a
DOM-style parser built using SSAX, though it returns SXML instead of DOM
objects.
7.21.3.2 Implementation
.......................
‘(sxml ssax)’ is a package of low-to-high level lexing and parsing
procedures that can be combined to yield a SAX, a DOM, a validating
parser, or a parser intended for a particular document type. The
procedures in the package can be used separately to tokenize or parse
various pieces of XML documents. The package supports XML Namespaces,
internal and external parsed entities, user-controlled handling of
whitespace, and validation. This module therefore is intended to be a
framework, a set of “Lego blocks” you can use to build a parser
following any discipline and performing validation to any degree. As an
example of the parser construction, the source file includes a
semi-validating SXML parser.
SSAX has a “sequential” feel of SAX yet a “functional style” of DOM.
Like a SAX parser, the framework scans the document only once and
permits incremental processing. An application that handles document
elements in order can run as efficiently as possible. _Unlike_ a SAX
parser, the framework does not require an application register stateful
callbacks and surrender control to the parser. Rather, it is the
application that can drive the framework – calling its functions to get
the current lexical or syntax element. These functions do not maintain
or mutate any state save the input port. Therefore, the framework
permits parsing of XML in a pure functional style, with the input port
being a monad (or a linear, read-once parameter).
Besides the PORT, there is another monad – SEED. Most of the middle-
and high-level parsers are single-threaded through the SEED. The
functions of this framework do not process or affect the SEED in any
way: they simply pass it around as an instance of an opaque datatype.
User functions, on the other hand, can use the seed to maintain user’s
state, to accumulate parsing results, etc. A user can freely mix their
own functions with those of the framework. On the other hand, the user
may wish to instantiate a high-level parser: ‘SSAX:make-elem-parser’ or
‘SSAX:make-parser’. In the latter case, the user must provide functions
of specific signatures, which are called at predictable moments during
the parsing: to handle character data, element data, or processing
instructions (PI). The functions are always given the SEED, among other
parameters, and must return the new SEED.
From a functional point of view, XML parsing is a combined
pre-post-order traversal of a “tree” that is the XML document itself.
This down-and-up traversal tells the user about an element when its
start tag is encountered. The user is notified about the element once
more, after all element’s children have been handled. The process of
XML parsing therefore is a fold over the raw XML document. Unlike a
fold over trees defined in [1], the parser is necessarily
single-threaded – obviously as elements in a text XML document are laid
down sequentially. The parser therefore is a tree fold that has been
transformed to accept an accumulating parameter [1,2].
Formally, the denotational semantics of the parser can be expressed
as
parser:: (Start-tag -> Seed -> Seed) ->
(Start-tag -> Seed -> Seed -> Seed) ->
(Char-Data -> Seed -> Seed) ->
XML-text-fragment -> Seed -> Seed
parser fdown fup fchar " content " seed
= fup "" seed
(parser fdown fup fchar "content" (fdown "" seed))
parser fdown fup fchar "char-data content" seed
= parser fdown fup fchar "content" (fchar "char-data" seed)
parser fdown fup fchar "elem-content content" seed
= parser fdown fup fchar "content" (
parser fdown fup fchar "elem-content" seed)
Compare the last two equations with the left fold
fold-left kons elem:list seed = fold-left kons list (kons elem seed)
The real parser created by ‘SSAX:make-parser’ is slightly more
complicated, to account for processing instructions, entity references,
namespaces, processing of document type declaration, etc.
The XML standard document referred to in this module is
The present file also defines a procedure that parses the text of an
XML document or of a separate element into SXML, an S-expression-based
model of an XML Information Set. SXML is also an Abstract Syntax Tree
of an XML document. SXML is similar but not identical to DOM; SXML is
particularly suitable for Scheme-based XML/HTML authoring, SXPath
queries, and tree transformations. See SXML.html for more details.
SXML is a term implementation of evaluation of the XML document [3].
The other implementation is context-passing.
The present frameworks fully supports the XML Namespaces
Recommendation: x ->
where
::= ( ...)
::= ( *preorder* . ) |
( *macro* . ) |
( . ) |
( . )
::= XMLname | *text* | *default*
:: x [] ->
The ‘pre-post-order’ function, in the ‘(sxml transform)’ module,
visits the nodes and nodelists pre-post-order (depth-first). For each
‘’ of the form ‘(NAME ...)’, it looks up an association
with the given NAME among its . If failed, ‘pre-post-order’
tries to locate a ‘*default*’ binding. It’s an error if the latter
attempt fails as well. Having found a binding, the ‘pre-post-order’
function first checks to see if the binding is of the form
( *preorder* . )
If it is, the handler is ’applied’ to the current node. Otherwise,
the pre-post-order function first calls itself recursively for each
child of the current node, with prepended to the
in effect. The result of these calls is passed to the
(along with the head of the current ). To be more
precise, the handler is _applied_ to the head of the current node and
its processed children. The result of the handler, which should also be
a ‘’, replaces the current . If the current is a
text string or other atom, a special binding with a symbol ‘*text*’ is
looked up.
A binding can also be of a form
( *macro* . )
This is equivalent to ‘*preorder*’ described above. However, the
result is re-processed again, with the current stylesheet.
7.21.4.2 Usage
..............
-- Scheme Procedure: SRV:send-reply . fragments
Output the FRAGMENTS to the current output port.
The fragments are a list of strings, characters, numbers, thunks,
‘#f’, ‘#t’ – and other fragments. The function traverses the tree
depth-first, writes out strings and characters, executes thunks,
and ignores ‘#f’ and ‘'()’. The function returns ‘#t’ if anything
was written at all; otherwise the result is ‘#f’ If ‘#t’ occurs
among the fragments, it is not written out but causes the result of
‘SRV:send-reply’ to be ‘#t’.
-- Scheme Procedure: foldts fdown fup fhere seed tree
-- Scheme Procedure: post-order tree bindings
-- Scheme Procedure: pre-post-order tree bindings
-- Scheme Procedure: replace-range beg-pred end-pred forest
7.21.5 SXML Tree Fold
---------------------
7.21.5.1 Overview
.................
‘(sxml fold)’ defines a number of variants of the “fold” algorithm for
use in transforming SXML trees. Additionally it defines the layout
operator, ‘fold-layout’, which might be described as a context-passing
variant of SSAX’s ‘pre-post-order’.
7.21.5.2 Usage
..............
-- Scheme Procedure: foldt fup fhere tree
The standard multithreaded tree fold.
FUP is of type [a] -> a. FHERE is of type object -> a.
-- Scheme Procedure: foldts fdown fup fhere seed tree
The single-threaded tree fold originally defined in SSAX. *Note
SSAX::, for more information.
-- Scheme Procedure: foldts* fdown fup fhere seed tree
A variant of ‘foldts’ that allows pre-order tree rewrites.
Originally defined in Andy Wingo’s 2007 paper, _Applications of
fold to XML transformation_.
-- Scheme Procedure: fold-values proc list . seeds
A variant of ‘fold’ that allows multi-valued seeds. Note that the
order of the arguments differs from that of ‘fold’. *Note SRFI-1
Fold and Map::.
-- Scheme Procedure: foldts*-values fdown fup fhere tree . seeds
A variant of ‘foldts*’ that allows multi-valued seeds. Originally
defined in Andy Wingo’s 2007 paper, _Applications of fold to XML
transformation_.
-- Scheme Procedure: fold-layout tree bindings params layout stylesheet
A traversal combinator in the spirit of ‘pre-post-order’. *Note
Transforming SXML::.
‘fold-layout’ was originally presented in Andy Wingo’s 2007 paper,
_Applications of fold to XML transformation_.
bindings := (...)
binding := ( ...)
| (*default* . )
| (*text* . )
tag :=
handler-pair := (pre-layout . )
| (post . )
| (bindings . )
| (pre . )
| (macro . )
PRE-LAYOUT-HANDLER
A function of three arguments:
KIDS
the kids of the current node, before traversal
PARAMS
the params of the current node
LAYOUT
the layout coming into this node
PRE-LAYOUT-HANDLER is expected to use this information to
return a layout to pass to the kids. The default
implementation returns the layout given in the arguments.
POST-HANDLER
A function of five arguments:
TAG
the current tag being processed
PARAMS
the params of the current node
LAYOUT
the layout coming into the current node, before any kids
were processed
KLAYOUT
the layout after processing all of the children
KIDS
the already-processed child nodes
POST-HANDLER should return two values, the layout to pass to
the next node and the final tree.
TEXT-HANDLER
TEXT-HANDLER is a function of three arguments:
TEXT
the string
PARAMS
the current params
LAYOUT
the current layout
TEXT-HANDLER should return two values, the layout to pass to
the next node and the value to which the string should
transform.
7.21.6 SXPath
-------------
7.21.6.1 Overview
.................
SXPath: SXML Query Language
===========================
SXPath is a query language for SXML, an instance of XML Information set
(Infoset) in the form of s-expressions. See ‘(sxml ssax)’ for the
definition of SXML and more details. SXPath is also a translation into
Scheme of an XML Path Language, XPath (http://www.w3.org/TR/xpath).
XPath and SXPath describe means of selecting a set of Infoset’s items or
their properties.
To facilitate queries, XPath maps the XML Infoset into an explicit
tree, and introduces important notions of a location path and a current,
context node. A location path denotes a selection of a set of nodes
relative to a context node. Any XPath tree has a distinguished, root
node – which serves as the context node for absolute location paths.
Location path is recursively defined as a location step joined with a
location path. A location step is a simple query of the database
relative to a context node. A step may include expressions that further
filter the selected set. Each node in the resulting set is used as a
context node for the adjoining location path. The result of the step is
a union of the sets returned by the latter location paths.
The SXML representation of the XML Infoset (see SSAX.scm) is rather
suitable for querying as it is. Bowing to the XPath specification, we
will refer to SXML information items as ’Nodes’:
::= | |
| "text string" |
This production can also be described as
::= (name . ) | "text string"
An (ordered) set of nodes is just a list of the constituent nodes: