display Property
This Info file contains edition 3.0 of the GNU Emacs Lisp Reference Manual, corresponding to GNU Emacs version 23.1.
This is edition 3.0 of the GNU Emacs Lisp Reference Manual,
corresponding to Emacs version 23.1.
Copyright © 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being “GNU General Public License,” with the Front-Cover texts being “A GNU Manual,” and with the Back-Cover Texts as in (a) below. A copy of the license is included in the section entitled “GNU Free Documentation License.”(a) The FSF's Back-Cover Text is: “You have the freedom to copy and modify this GNU manual. Buying copies from the FSF supports it in developing GNU and promoting software freedom.”
Appendices
--- The Detailed Node Listing --- ---------------------------------
Here are other nodes that are inferiors of those already listed, mentioned here so you can get to them in one step:
Introduction
Conventions
Format of Descriptions
Lisp Data Types
Programming Types
Character Type
Cons Cell and List Types
String Type
Editing Types
Numbers
Strings and Characters
Lists
Modifying Existing List Structure
Sequences, Arrays, and Vectors
Hash Tables
Symbols
Property Lists
Evaluation
Kinds of Forms
Control Structures
Nonlocal Exits
Errors
Variables
Scoping Rules for Variable Bindings
Buffer-Local Variables
Functions
Lambda Expressions
Macros
Common Problems Using Macros
Writing Customization Definitions
Customization Types
Loading
Byte Compilation
Advising Emacs Lisp Functions
Debugging Lisp Programs
The Lisp Debugger
Edebug
Breaks
The Outside Context
Edebug and Macros
Debugging Invalid Lisp Syntax
Reading and Printing Lisp Objects
Minibuffers
Completion
Command Loop
Defining Commands
Input Events
Reading Input
Keymaps
Menu Keymaps
Defining Menus
Major and Minor Modes
Hooks
Major Modes
Minor Modes
Mode Line Format
Font Lock Mode
Multiline Font Lock Constructs
Documentation
Files
Visiting Files
Information about Files
File Names
File Format Conversion
Backups and Auto-Saving
Backup Files
Buffers
Windows
Frames
Frame Parameters
Window Frame Parameters
Positions
Motion
Markers
Text
The Kill Ring
Indentation
Text Properties
Non-ASCII Characters
Coding Systems
Searching and Matching
Regular Expressions
Syntax of Regular Expressions
The Match Data
Syntax Tables
Syntax Descriptors
Parsing Expressions
Abbrevs and Abbrev Expansion
Processes
Receiving Output from Processes
Low-Level Network Access
Packing and Unpacking Byte Arrays
Emacs Display
The Echo Area
Reporting Warnings
Overlays
Faces
Fringes
The display Property
Images
Buttons
Abstract Display
Display Tables
Operating System Interface
Starting Up Emacs
Getting Out of Emacs
Terminal Input
Tips and Conventions
GNU Emacs Internals
Object Internals
Most of the GNU Emacs text editor is written in the programming language called Emacs Lisp. You can write new code in Emacs Lisp and install it as an extension to the editor. However, Emacs Lisp is more than a mere “extension language”; it is a full computer programming language in its own right. You can use it as you would any other programming language.
Because Emacs Lisp is designed for use in an editor, it has special features for scanning and parsing text as well as features for handling files, buffers, displays, subprocesses, and so on. Emacs Lisp is closely integrated with the editing facilities; thus, editing commands are functions that can also conveniently be called from Lisp programs, and parameters for customization are ordinary Lisp variables.
This manual attempts to be a full description of Emacs Lisp. For a beginner's introduction to Emacs Lisp, see An Introduction to Emacs Lisp Programming, by Bob Chassell, also published by the Free Software Foundation. This manual presumes considerable familiarity with the use of Emacs for editing; see The GNU Emacs Manual for this basic information.
Generally speaking, the earlier chapters describe features of Emacs Lisp that have counterparts in many programming languages, and later chapters describe features that are peculiar to Emacs Lisp or relate specifically to editing.
This is edition 3.0 of the GNU Emacs Lisp Reference Manual, corresponding to Emacs version 23.1.
This manual has gone through numerous drafts. It is nearly complete but not flawless. There are a few topics that are not covered, either because we consider them secondary (such as most of the individual modes) or because they are yet to be written. Because we are not able to deal with them completely, we have left out several parts intentionally.
The manual should be fully correct in what it does cover, and it is therefore open to criticism on anything it says—from specific examples and descriptive text, to the ordering of chapters and sections. If something is confusing, or you find that you have to look at the sources or experiment to learn something not covered in the manual, then perhaps the manual should be fixed. Please let us know.
As you use this manual, we ask that you send corrections as soon as you find them. If you think of a simple, real life example for a function or group of functions, please make an effort to write it up and send it in. Please reference any comments to the node name and function or variable name, as appropriate. Also state the number of the edition you are criticizing.
Please send comments and corrections using M-x report-emacs-bug.
Lisp (LISt Processing language) was first developed in the late 1950s at the Massachusetts Institute of Technology for research in artificial intelligence. The great power of the Lisp language makes it ideal for other purposes as well, such as writing editing commands.
Dozens of Lisp implementations have been built over the years, each with its own idiosyncrasies. Many of them were inspired by Maclisp, which was written in the 1960s at MIT's Project MAC. Eventually the implementors of the descendants of Maclisp came together and developed a standard for Lisp systems, called Common Lisp. In the meantime, Gerry Sussman and Guy Steele at MIT developed a simplified but very powerful dialect of Lisp, called Scheme.
GNU Emacs Lisp is largely inspired by Maclisp, and a little by Common Lisp. If you know Common Lisp, you will notice many similarities. However, many features of Common Lisp have been omitted or simplified in order to reduce the memory requirements of GNU Emacs. Sometimes the simplifications are so drastic that a Common Lisp user might be very confused. We will occasionally point out how GNU Emacs Lisp differs from Common Lisp. If you don't know Common Lisp, don't worry about it; this manual is self-contained.
A certain amount of Common Lisp emulation is available via the cl library. see Overview.
Emacs Lisp is not at all influenced by Scheme; but the GNU project has an implementation of Scheme, called Guile. We use Guile in all new GNU software that calls for extensibility.
This section explains the notational conventions that are used in this manual. You may want to skip this section and refer back to it later.
Throughout this manual, the phrases “the Lisp reader” and “the Lisp printer” refer to those routines in Lisp that convert textual representations of Lisp objects into actual Lisp objects, and vice versa. See Printed Representation, for more details. You, the person reading this manual, are thought of as “the programmer” and are addressed as “you.” “The user” is the person who uses Lisp programs, including those you write.
Examples of Lisp code are formatted like this: (list 1 2 3).
Names that represent metasyntactic variables, or arguments to a function
being described, are formatted like this: first-number.
nil and t
In Lisp, the symbol nil has three separate meanings: it
is a symbol with the name ‘nil’; it is the logical truth value
false; and it is the empty list—the list of zero elements.
When used as a variable, nil always has the value nil.
As far as the Lisp reader is concerned, ‘()’ and ‘nil’ are
identical: they stand for the same object, the symbol nil. The
different ways of writing the symbol are intended entirely for human
readers. After the Lisp reader has read either ‘()’ or ‘nil’,
there is no way to determine which representation was actually written
by the programmer.
In this manual, we write () when we wish to emphasize that it
means the empty list, and we write nil when we wish to emphasize
that it means the truth value false. That is a good convention to use
in Lisp programs also.
(cons 'foo ()) ; Emphasize the empty list (setq foo-flag nil) ; Emphasize the truth value false
In contexts where a truth value is expected, any non-nil value
is considered to be true. However, t is the preferred way
to represent the truth value true. When you need to choose a
value which represents true, and there is no other basis for
choosing, use t. The symbol t always has the value
t.
In Emacs Lisp, nil and t are special symbols that always
evaluate to themselves. This is so that you do not need to quote them
to use them as constants in a program. An attempt to change their
values results in a setting-constant error. See Constant Variables.
Return non-
nilif object is one of the two canonical boolean values:tornil.
A Lisp expression that you can evaluate is called a form. Evaluating a form always produces a result, which is a Lisp object. In the examples in this manual, this is indicated with ‘’:
(car '(1 2))
1
You can read this as “(car '(1 2)) evaluates to 1.”
When a form is a macro call, it expands into a new form for Lisp to evaluate. We show the result of the expansion with ‘==>’. We may or may not show the result of the evaluation of the expanded form.
(third '(a b c))
==> (car (cdr (cdr '(a b c))))
c
Sometimes to help describe one form we show another form that produces identical results. The exact equivalence of two forms is indicated with ‘==’.
(make-sparse-keymap) == (list 'keymap)
Many of the examples in this manual print text when they are
evaluated. If you execute example code in a Lisp Interaction buffer
(such as the buffer ‘*scratch*’), the printed text is inserted into
the buffer. If you execute the example by other means (such as by
evaluating the function eval-region), the printed text is
displayed in the echo area.
Examples in this manual indicate printed text with ‘-|’,
irrespective of where that text goes. The value returned by
evaluating the form (here bar) follows on a separate line with
‘’.
(progn (prin1 'foo) (princ "\n") (prin1 'bar))
-| foo
-| bar
bar
Some examples signal errors. This normally displays an error message in the echo area. We show the error message on a line starting with ‘error-->’. Note that ‘error-->’ itself does not appear in the echo area.
(+ 23 'x)
error--> Wrong type argument: number-or-marker-p, x
Some examples describe modifications to the contents of a buffer, by showing the “before” and “after” versions of the text. These examples show the contents of the buffer in question between two lines of dashes containing the buffer name. In addition, ‘-!-’ indicates the location of point. (The symbol for point, of course, is not part of the text in the buffer; it indicates the place between two characters where point is currently located.)
---------- Buffer: foo ----------
This is the -!-contents of foo.
---------- Buffer: foo ----------
(insert "changed ")
nil
---------- Buffer: foo ----------
This is the changed -!-contents of foo.
---------- Buffer: foo ----------
Functions, variables, macros, commands, user options, and special forms are described in this manual in a uniform format. The first line of a description contains the name of the item followed by its arguments, if any. The category—function, variable, or whatever—appears at the beginning of the line. The description follows on succeeding lines, sometimes with examples.
In a function description, the name of the function being described appears first. It is followed on the same line by a list of argument names. These names are also used in the body of the description, to stand for the values of the arguments.
The appearance of the keyword &optional in the argument list
indicates that the subsequent arguments may be omitted (omitted
arguments default to nil). Do not write &optional when
you call the function.
The keyword &rest (which must be followed by a single
argument name) indicates that any number of arguments can follow. The
single argument name following &rest will receive, as its
value, a list of all the remaining arguments passed to the function.
Do not write &rest when you call the function.
Here is a description of an imaginary function foo:
The function
foosubtracts integer1 from integer2, then adds all the rest of the arguments to the result. If integer2 is not supplied, then the number 19 is used by default.(foo 1 5 3 9) 16 (foo 5) 14More generally,
(foo w x y...) == (+ (- x w) y...)
Any argument whose name contains the name of a type (e.g., integer, integer1 or buffer) is expected to be of that type. A plural of a type (such as buffers) often means a list of objects of that type. Arguments named object may be of any type. (See Lisp Data Types, for a list of Emacs object types.) Arguments with other sorts of names (e.g., new-file) are discussed specifically in the description of the function. In some sections, features common to the arguments of several functions are described at the beginning.
See Lambda Expressions, for a more complete description of optional and rest arguments.
Command, macro, and special form descriptions have the same format, but the word `Function' is replaced by `Command', `Macro', or `Special Form', respectively. Commands are simply functions that may be called interactively; macros process their arguments differently from functions (the arguments are not evaluated), but are presented the same way.
Special form descriptions use a more complex notation to specify optional and repeated arguments because they can break the argument list down into separate arguments in more complicated ways. ‘[optional-arg]’ means that optional-arg is optional and ‘repeated-args...’ stands for zero or more arguments. Parentheses are used when several arguments are grouped into additional levels of list structure. Here is an example:
This imaginary special form implements a loop that executes the body forms and then increments the variable var on each iteration. On the first iteration, the variable has the value from; on subsequent iterations, it is incremented by one (or by inc if that is given). The loop exits before executing body if var equals to. Here is an example:
(count-loop (i 0 10) (prin1 i) (princ " ") (prin1 (aref vector i)) (terpri))If from and to are omitted, var is bound to
nilbefore the loop begins, and the loop exits if var is non-nilat the beginning of an iteration. Here is an example:(count-loop (done) (if (pending) (fixit) (setq done t)))In this special form, the arguments from and to are optional, but must both be present or both absent. If they are present, inc may optionally be specified as well. These arguments are grouped with the argument var into a list, to distinguish them from body, which includes all remaining elements of the form.
A variable is a name that can hold a value. Although nearly all variables can be set by the user, certain variables exist specifically so that users can change them; these are called user options. Ordinary variables and user options are described using a format like that for functions except that there are no arguments.
Here is a description of the imaginary electric-future-map
variable.
The value of this variable is a full keymap used by Electric Command Future mode. The functions in this map allow you to edit commands you have not yet thought about executing.
User option descriptions have the same format, but `Variable' is replaced by `User Option'.
These facilities provide information about which version of Emacs is in use.
This function returns a string describing the version of Emacs that is running. It is useful to include this string in bug reports.
(emacs-version) "GNU Emacs 23.1 (i686-pc-linux-gnu, GTK+ Version 2.14.4) of 2009-06-01 on cyd.mit.edu"If here is non-
nil, it inserts the text in the buffer before point, and returnsnil. When this function is called interactively, it prints the same information in the echo area, but giving a prefix argument makes here non-nil.
The value of this variable indicates the time at which Emacs was built at the local site. It is a list of three integers, like the value of
current-time(see Time of Day).emacs-build-time (18846 52016 156039)
The value of this variable is the version of Emacs being run. It is a string such as
"23.1.1". The last number in this string is not really part of the Emacs release version number; it is incremented each time you build Emacs in any given directory. A value with four numeric components, such as"22.0.91.1", indicates an unreleased test version.
The following two variables have existed since Emacs version 19.23:
The major version number of Emacs, as an integer. For Emacs version 23.1, the value is 23.
The minor version number of Emacs, as an integer. For Emacs version 23.1, the value is 1.
This manual was written by Robert Krawitz, Bil Lewis, Dan LaLiberte, Richard M. Stallman and Chris Welty, the volunteers of the GNU manual group, in an effort extending over several years. Robert J. Chassell helped to review and edit the manual, with the support of the Defense Advanced Research Projects Agency, ARPA Order 6082, arranged by Warren A. Hunt, Jr. of Computational Logic, Inc. Additional sections were written by Miles Bader, Lars Brinkhoff, Chong Yidong, Kenichi Handa, Lute Kamstra, Juri Linkov, Glenn Morris, Thien-Thi Nguyen, Dan Nicolaescu, Martin Rudalics, Kim F. Storm, Luc Teirlinck, and Eli Zaretskii.
Corrections were supplied by Drew Adams, Juanma Barranquero, Karl Berry, Jim Blandy, Bard Bloom, Stephane Boucher, David Boyes, Alan Carroll, Richard Davis, Lawrence R. Dodd, Peter Doornbosch, David A. Duff, Chris Eich, Beverly Erlebacher, David Eckelkamp, Ralf Fassel, Eirik Fuller, Stephen Gildea, Bob Glickstein, Eric Hanchrow, Jesper Harder, George Hartzell, Nathan Hess, Masayuki Ida, Dan Jacobson, Jak Kirman, Bob Knighten, Frederick M. Korz, Joe Lammens, Glenn M. Lewis, K. Richard Magill, Brian Marick, Roland McGrath, Stefan Monnier, Skip Montanaro, John Gardiner Myers, Thomas A. Peterson, Francesco Potorti, Friedrich Pukelsheim, Arnold D. Robbins, Raul Rockwell, Jason Rumney, Per Starbäck, Shinichirou Sugou, Kimmo Suominen, Edward Tharp, Bill Trost, Rickard Westman, Jean White, Eduard Wiebe, Matthew Wilding, Carl Witty, Dale Worley, Rusty Wright, and David D. Zuhn.
A Lisp object is a piece of data used and manipulated by Lisp programs. For our purposes, a type or data type is a set of possible objects.
Every object belongs to at least one type. Objects of the same type have similar structures and may usually be used in the same contexts. Types can overlap, and objects can belong to two or more types. Consequently, we can ask whether an object belongs to a particular type, but not for “the” type of an object.
A few fundamental object types are built into Emacs. These, from which all other types are constructed, are called primitive types. Each object belongs to one and only one primitive type. These types include integer, float, cons, symbol, string, vector, hash-table, subr, and byte-code function, plus several special types, such as buffer, that are related to editing. (See Editing Types.)
Each primitive type has a corresponding Lisp function that checks whether an object is a member of that type.
Lisp is unlike many other languages in that its objects are self-typing: the primitive type of each object is implicit in the object itself. For example, if an object is a vector, nothing can treat it as a number; Lisp knows it is a vector, not a number.
In most languages, the programmer must declare the data type of each variable, and the type is known by the compiler but not represented in the data. Such type declarations do not exist in Emacs Lisp. A Lisp variable can have any type of value, and it remembers whatever value you store in it, type and all. (Actually, a small number of Emacs Lisp variables can only take on values of a certain type. See Variables with Restricted Values.)
This chapter describes the purpose, printed representation, and read syntax of each of the standard types in GNU Emacs Lisp. Details on how to use these types can be found in later chapters.
The printed representation of an object is the format of the
output generated by the Lisp printer (the function prin1) for
that object. Every data type has a unique printed representation.
The read syntax of an object is the format of the input accepted
by the Lisp reader (the function read) for that object. This
is not necessarily unique; many kinds of object have more than one
syntax. See Read and Print.
In most cases, an object's printed representation is also a read syntax for the object. However, some types have no read syntax, since it does not make sense to enter objects of these types as constants in a Lisp program. These objects are printed in hash notation, which consists of the characters ‘#<’, a descriptive string (typically the type name followed by the name of the object), and a closing ‘>’. For example:
(current-buffer)
#<buffer objects.texi>
Hash notation cannot be read at all, so the Lisp reader signals the
error invalid-read-syntax whenever it encounters ‘#<’.
In other languages, an expression is text; it has no other form. In
Lisp, an expression is primarily a Lisp object and only secondarily the
text that is the object's read syntax. Often there is no need to
emphasize this distinction, but you must keep it in the back of your
mind, or you will occasionally be very confused.
When you evaluate an expression interactively, the Lisp interpreter
first reads the textual representation of it, producing a Lisp object,
and then evaluates that object (see Evaluation). However,
evaluation and reading are separate activities. Reading returns the
Lisp object represented by the text that is read; the object may or may
not be evaluated later. See Input Functions, for a description of
read, the basic function for reading objects.
A comment is text that is written in a program only for the sake of humans that read the program, and that has no effect on the meaning of the program. In Lisp, a semicolon (‘;’) starts a comment if it is not within a string or character constant. The comment continues to the end of line. The Lisp reader discards comments; they do not become part of the Lisp objects which represent the program within the Lisp system.
The ‘#@count’ construct, which skips the next count characters, is useful for program-generated comments containing binary data. The Emacs Lisp byte compiler uses this in its output files (see Byte Compilation). It isn't meant for source files, however.
See Comment Tips, for conventions for formatting comments.
There are two general categories of types in Emacs Lisp: those having to do with Lisp programming, and those having to do with editing. The former exist in many Lisp implementations, in one form or another. The latter are unique to Emacs Lisp.
The range of values for integers in Emacs Lisp is −268435456 to
268435455 (29 bits; i.e.,
-2**28
to
2**28 - 1)
on most machines. (Some machines may provide a wider range.) It is
important to note that the Emacs Lisp arithmetic functions do not check
for overflow. Thus (1+ 268435455) is −268435456 on most
machines.
The read syntax for integers is a sequence of (base ten) digits with an optional sign at the beginning and an optional period at the end. The printed representation produced by the Lisp interpreter never has a leading ‘+’ or a final ‘.’.
-1 ; The integer -1. 1 ; The integer 1. 1. ; Also the integer 1. +1 ; Also the integer 1. 536870913 ; Also the integer 1 on a 29-bit implementation.
See Numbers, for more information.
Floating point numbers are the computer equivalent of scientific
notation; you can think of a floating point number as a fraction
together with a power of ten. The precise number of significant
figures and the range of possible exponents is machine-specific; Emacs
uses the C data type double to store the value, and internally
this records a power of 2 rather than a power of 10.
The printed representation for floating point numbers requires either a decimal point (with at least one digit following), an exponent, or both. For example, ‘1500.0’, ‘15e2’, ‘15.0e2’, ‘1.5e3’, and ‘.15e4’ are five ways of writing a floating point number whose value is 1500. They are all equivalent.
See Numbers, for more information.
A character in Emacs Lisp is nothing more than an integer. In other words, characters are represented by their character codes. For example, the character A is represented as the integer 65.
Individual characters are used occasionally in programs, but it is more common to work with strings, which are sequences composed of characters. See String Type.
Characters in strings and buffers are currently limited to the range of 0 to 4194303—twenty two bits (see Character Codes). Codes 0 through 127 are ASCII codes; the rest are non-ASCII (see Non-ASCII Characters). Characters that represent keyboard input have a much wider range, to encode modifier keys such as Control, Meta and Shift.
There are special functions for producing a human-readable textual description of a character for the sake of messages. See Describing Characters.
Since characters are really integers, the printed representation of a character is a decimal number. This is also a possible read syntax for a character, but writing characters that way in Lisp programs is not clear programming. You should always use the special read syntax formats that Emacs Lisp provides for characters. These syntax formats start with a question mark.
The usual read syntax for alphanumeric characters is a question mark followed by the character; thus, ‘?A’ for the character A, ‘?B’ for the character B, and ‘?a’ for the character a.
For example:
?Q 81 ?q 113
You can use the same syntax for punctuation characters, but it is often a good idea to add a ‘\’ so that the Emacs commands for editing Lisp code don't get confused. For example, ‘?\(’ is the way to write the open-paren character. If the character is ‘\’, you must use a second ‘\’ to quote it: ‘?\\’.
You can express the characters control-g, backspace, tab, newline, vertical tab, formfeed, space, return, del, and escape as ‘?\a’, ‘?\b’, ‘?\t’, ‘?\n’, ‘?\v’, ‘?\f’, ‘?\s’, ‘?\r’, ‘?\d’, and ‘?\e’, respectively. (‘?\s’ followed by a dash has a different meaning—it applies the “super” modifier to the following character.) Thus,
?\a 7 ; control-g, C-g ?\b 8 ; backspace, <BS>, C-h ?\t 9 ; tab, <TAB>, C-i ?\n 10 ; newline, C-j ?\v 11 ; vertical tab, C-k ?\f 12 ; formfeed character, C-l ?\r 13 ; carriage return, <RET>, C-m ?\e 27 ; escape character, <ESC>, C-[ ?\s 32 ; space character, <SPC> ?\\ 92 ; backslash character, \ ?\d 127 ; delete character, <DEL>
These sequences which start with backslash are also known as escape sequences, because backslash plays the role of an “escape character”; this terminology has nothing to do with the character <ESC>. ‘\s’ is meant for use in character constants; in string constants, just write the space.
A backslash is allowed, and harmless, preceding any character without a special escape meaning; thus, ‘?\+’ is equivalent to ‘?+’. There is no reason to add a backslash before most characters. However, you should add a backslash before any of the characters ‘()\|;'`"#.,’ to avoid confusing the Emacs commands for editing Lisp code. You can also add a backslash before whitespace characters such as space, tab, newline and formfeed. However, it is cleaner to use one of the easily readable escape sequences, such as ‘\t’ or ‘\s’, instead of an actual whitespace character such as a tab or a space. (If you do write backslash followed by a space, you should write an extra space after the character constant to separate it from the following text.)
In addition to the specific escape sequences for special important control characters, Emacs provides several types of escape syntax that you can use to specify non-ASCII text characters.
You can specify characters by their Unicode values.
?\unnnn represents a character that maps to the Unicode
code point ‘U+nnnn’ (by convention, Unicode code points are
given in hexadecimal). There is a slightly different syntax for
specifying characters with code points higher than
U+ffff: \U00nnnnnn represents the character
whose code point is ‘U+nnnnnn’. The Unicode Standard only
defines code points up to ‘U+10ffff’, so if you specify a
code point higher than that, Emacs signals an error.
This peculiar and inconvenient syntax was adopted for compatibility with other programming languages. Unlike some other languages, Emacs Lisp supports this syntax only in character literals and strings.
The most general read syntax for a character represents the
character code in either octal or hex. To use octal, write a question
mark followed by a backslash and the octal character code (up to three
octal digits); thus, ‘?\101’ for the character A,
‘?\001’ for the character C-a, and ?\002 for the
character C-b. Although this syntax can represent any
ASCII character, it is preferred only when the precise octal
value is more important than the ASCII representation.
?\012 10 ?\n 10 ?\C-j 10
?\101 65 ?A 65
To use hex, write a question mark followed by a backslash, ‘x’,
and the hexadecimal character code. You can use any number of hex
digits, so you can represent any character code in this way.
Thus, ‘?\x41’ for the character A, ‘?\x1’ for the
character C-a, and ?\x8e0 for the Latin-1 character
‘a’ with grave accent.
Control characters can be represented using yet another read syntax. This consists of a question mark followed by a backslash, caret, and the corresponding non-control character, in either upper or lower case. For example, both ‘?\^I’ and ‘?\^i’ are valid read syntax for the character C-i, the character whose value is 9.
Instead of the ‘^’, you can use ‘C-’; thus, ‘?\C-i’ is equivalent to ‘?\^I’ and to ‘?\^i’:
?\^I 9 ?\C-I 9
In strings and buffers, the only control characters allowed are those that exist in ASCII; but for keyboard input purposes, you can turn any character into a control character with ‘C-’. The character codes for these non-ASCII control characters include the 2**26 bit as well as the code for the corresponding non-control character. Ordinary terminals have no way of generating non-ASCII control characters, but you can generate them straightforwardly using X and other window systems.
For historical reasons, Emacs treats the <DEL> character as the control equivalent of ?:
?\^? 127 ?\C-? 127
As a result, it is currently not possible to represent the character Control-?, which is a meaningful input character under X, using ‘\C-’. It is not easy to change this, as various Lisp files refer to <DEL> in this way.
For representing control characters to be found in files or strings, we recommend the ‘^’ syntax; for control characters in keyboard input, we prefer the ‘C-’ syntax. Which one you use does not affect the meaning of the program, but may guide the understanding of people who read it.
A meta character is a character typed with the <META> modifier key. The integer that represents such a character has the 2**27 bit set. We use high bits for this and other modifiers to make possible a wide range of basic character codes.
In a string, the 2**7 bit attached to an ASCII character indicates a meta character; thus, the meta characters that can fit in a string have codes in the range from 128 to 255, and are the meta versions of the ordinary ASCII characters. (In Emacs versions 18 and older, this convention was used for characters outside of strings as well.)
The read syntax for meta characters uses ‘\M-’. For example, ‘?\M-A’ stands for M-A. You can use ‘\M-’ together with octal character codes (see below), with ‘\C-’, or with any other syntax for a character. Thus, you can write M-A as ‘?\M-A’, or as ‘?\M-\101’. Likewise, you can write C-M-b as ‘?\M-\C-b’, ‘?\C-\M-b’, or ‘?\M-\002’.
The case of a graphic character is indicated by its character code; for example, ASCII distinguishes between the characters ‘a’ and ‘A’. But ASCII has no way to represent whether a control character is upper case or lower case. Emacs uses the 2**25 bit to indicate that the shift key was used in typing a control character. This distinction is possible only when you use X terminals or other special terminals; ordinary terminals do not report the distinction to the computer in any way. The Lisp syntax for the shift bit is ‘\S-’; thus, ‘?\C-\S-o’ or ‘?\C-\S-O’ represents the shifted-control-o character.
The X Window System defines three other modifier bits that can be set in a character: hyper, super and alt. The syntaxes for these bits are ‘\H-’, ‘\s-’ and ‘\A-’. (Case is significant in these prefixes.) Thus, ‘?\H-\M-\A-x’ represents Alt-Hyper-Meta-x. (Note that ‘\s’ with no following ‘-’ represents the space character.) Numerically, the bit values are 2**22 for alt, 2**23 for super and 2**24 for hyper.
A symbol in GNU Emacs Lisp is an object with a name. The symbol name serves as the printed representation of the symbol. In ordinary Lisp use, with one single obarray (see Creating Symbols), a symbol's name is unique—no two symbols have the same name.
A symbol can serve as a variable, as a function name, or to hold a property list. Or it may serve only to be distinct from all other Lisp objects, so that its presence in a data structure may be recognized reliably. In a given context, usually only one of these uses is intended. But you can use one symbol in all of these ways, independently.
A symbol whose name starts with a colon (‘:’) is called a keyword symbol. These symbols automatically act as constants, and are normally used only by comparing an unknown symbol with a few specific alternatives.
A symbol name can contain any characters whatever. Most symbol names are written with letters, digits, and the punctuation characters ‘-+=*/’. Such names require no special punctuation; the characters of the name suffice as long as the name does not look like a number. (If it does, write a ‘\’ at the beginning of the name to force interpretation as a symbol.) The characters ‘_~!@$%^&:<>{}?’ are less often used but also require no special punctuation. Any other characters may be included in a symbol's name by escaping them with a backslash. In contrast to its use in strings, however, a backslash in the name of a symbol simply quotes the single character that follows the backslash. For example, in a string, ‘\t’ represents a tab character; in the name of a symbol, however, ‘\t’ merely quotes the letter ‘t’. To have a symbol with a tab character in its name, you must actually use a tab (preceded with a backslash). But it's rare to do such a thing.
Common Lisp note: In Common Lisp, lower case letters are always “folded” to upper case, unless they are explicitly escaped. In Emacs Lisp, upper case and lower case letters are distinct.
Here are several examples of symbol names. Note that the ‘+’ in the fifth example is escaped to prevent it from being read as a number. This is not necessary in the fourth example because the rest of the name makes it invalid as a number.
foo ; A symbol named ‘foo’. FOO ; A symbol named ‘FOO’, different from ‘foo’. char-to-string ; A symbol named ‘char-to-string’. 1+ ; A symbol named ‘1+’ ; (not ‘+1’, which is an integer). \+1 ; A symbol named ‘+1’ ; (not a very readable name). \(*\ 1\ 2\) ; A symbol named ‘(* 1 2)’ (a worse name). +-*/_~!@$%^&=:<>{} ; A symbol named ‘+-*/_~!@$%^&=:<>{}’. ; These characters need not be escaped.
Normally the Lisp reader interns all symbols (see Creating Symbols). To prevent interning, you can write ‘#:’ before the name of the symbol.
A sequence is a Lisp object that represents an ordered set of elements. There are two kinds of sequence in Emacs Lisp, lists and arrays. Thus, an object of type list or of type array is also considered a sequence.
Arrays are further subdivided into strings, vectors, char-tables and
bool-vectors. Vectors can hold elements of any type, but string
elements must be characters, and bool-vector elements must be t
or nil. Char-tables are like vectors except that they are
indexed by any valid character code. The characters in a string can
have text properties like characters in a buffer (see Text Properties), but vectors do not support text properties, even when
their elements happen to be characters.
Lists, strings and the other array types are different, but they have
important similarities. For example, all have a length l, and all
have elements which can be indexed from zero to l minus one.
Several functions, called sequence functions, accept any kind of
sequence. For example, the function elt can be used to extract
an element of a sequence, given its index. See Sequences Arrays Vectors.
It is generally impossible to read the same sequence twice, since
sequences are always created anew upon reading. If you read the read
syntax for a sequence twice, you get two sequences with equal contents.
There is one exception: the empty list () always stands for the
same object, nil.
A cons cell is an object that consists of two slots, called the car slot and the cdr slot. Each slot can hold or refer to any Lisp object. We also say that “the car of this cons cell is” whatever object its car slot currently holds, and likewise for the cdr.
A note to C programmers: in Lisp, we do not distinguish between “holding” a value and “pointing to” the value, because pointers in Lisp are implicit.
A list is a series of cons cells, linked together so that the
cdr slot of each cons cell holds either the next cons cell or the
empty list. The empty list is actually the symbol nil.
See Lists, for functions that work on lists. Because most cons
cells are used as part of lists, the phrase list structure has
come to refer to any structure made out of cons cells.
Because cons cells are so central to Lisp, we also have a word for “an object which is not a cons cell.” These objects are called atoms.
The read syntax and printed representation for lists are identical, and consist of a left parenthesis, an arbitrary number of elements, and a right parenthesis. Here are examples of lists:
(A 2 "A") ; A list of three elements. () ; A list of no elements (the empty list). nil ; A list of no elements (the empty list). ("A ()") ; A list of one element: the string"A ()". (A ()) ; A list of two elements:Aand the empty list. (A nil) ; Equivalent to the previous. ((A B C)) ; A list of one element ; (which is a list of three elements).
Upon reading, each object inside the parentheses becomes an element
of the list. That is, a cons cell is made for each element. The
car slot of the cons cell holds the element, and its cdr
slot refers to the next cons cell of the list, which holds the next
element in the list. The cdr slot of the last cons cell is set to
hold nil.
The names car and cdr derive from the history of Lisp. The
original Lisp implementation ran on an IBM 704 computer which
divided words into two parts, called the “address” part and the
“decrement”; car was an instruction to extract the contents of
the address part of a register, and cdr an instruction to extract
the contents of the decrement. By contrast, “cons cells” are named
for the function cons that creates them, which in turn was named
for its purpose, the construction of cells.
A list can be illustrated by a diagram in which the cons cells are
shown as pairs of boxes, like dominoes. (The Lisp reader cannot read
such an illustration; unlike the textual notation, which can be
understood by both humans and computers, the box illustrations can be
understood only by humans.) This picture represents the three-element
list (rose violet buttercup):
--- --- --- --- --- ---
| | |--> | | |--> | | |--> nil
--- --- --- --- --- ---
| | |
| | |
--> rose --> violet --> buttercup
In this diagram, each box represents a slot that can hold or refer to any Lisp object. Each pair of boxes represents a cons cell. Each arrow represents a reference to a Lisp object, either an atom or another cons cell.
In this example, the first box, which holds the car of the first
cons cell, refers to or “holds” rose (a symbol). The second
box, holding the cdr of the first cons cell, refers to the next
pair of boxes, the second cons cell. The car of the second cons
cell is violet, and its cdr is the third cons cell. The
cdr of the third (and last) cons cell is nil.
Here is another diagram of the same list, (rose violet
buttercup), sketched in a different manner:
--------------- ---------------- -------------------
| car | cdr | | car | cdr | | car | cdr |
| rose | o-------->| violet | o-------->| buttercup | nil |
| | | | | | | | |
--------------- ---------------- -------------------
A list with no elements in it is the empty list; it is identical
to the symbol nil. In other words, nil is both a symbol
and a list.
Here is the list (A ()), or equivalently (A nil),
depicted with boxes and arrows:
--- --- --- ---
| | |--> | | |--> nil
--- --- --- ---
| |
| |
--> A --> nil
Here is a more complex illustration, showing the three-element list,
((pine needles) oak maple), the first element of which is a
two-element list:
--- --- --- --- --- ---
| | |--> | | |--> | | |--> nil
--- --- --- --- --- ---
| | |
| | |
| --> oak --> maple
|
| --- --- --- ---
--> | | |--> | | |--> nil
--- --- --- ---
| |
| |
--> pine --> needles
The same list represented in the second box notation looks like this:
-------------- -------------- --------------
| car | cdr | | car | cdr | | car | cdr |
| o | o------->| oak | o------->| maple | nil |
| | | | | | | | | |
-- | --------- -------------- --------------
|
|
| -------------- ----------------
| | car | cdr | | car | cdr |
------>| pine | o------->| needles | nil |
| | | | | |
-------------- ----------------
Dotted pair notation is a general syntax for cons cells that
represents the car and cdr explicitly. In this syntax,
(a . b) stands for a cons cell whose car is
the object a and whose cdr is the object b. Dotted
pair notation is more general than list syntax because the cdr
does not have to be a list. However, it is more cumbersome in cases
where list syntax would work. In dotted pair notation, the list
‘(1 2 3)’ is written as ‘(1 . (2 . (3 . nil)))’. For
nil-terminated lists, you can use either notation, but list
notation is usually clearer and more convenient. When printing a
list, the dotted pair notation is only used if the cdr of a cons
cell is not a list.
Here's an example using boxes to illustrate dotted pair notation.
This example shows the pair (rose . violet):
--- ---
| | |--> violet
--- ---
|
|
--> rose
You can combine dotted pair notation with list notation to represent
conveniently a chain of cons cells with a non-nil final cdr.
You write a dot after the last element of the list, followed by the
cdr of the final cons cell. For example, (rose violet
. buttercup) is equivalent to (rose . (violet . buttercup)).
The object looks like this:
--- --- --- ---
| | |--> | | |--> buttercup
--- --- --- ---
| |
| |
--> rose --> violet
The syntax (rose . violet . buttercup) is invalid because
there is nothing that it could mean. If anything, it would say to put
buttercup in the cdr of a cons cell whose cdr is already
used for violet.
The list (rose violet) is equivalent to (rose . (violet)),
and looks like this:
--- --- --- ---
| | |--> | | |--> nil
--- --- --- ---
| |
| |
--> rose --> violet
Similarly, the three-element list (rose violet buttercup)
is equivalent to (rose . (violet . (buttercup))).
It looks like this:
--- --- --- --- --- ---
| | |--> | | |--> | | |--> nil
--- --- --- --- --- ---
| | |
| | |
--> rose --> violet --> buttercup
An association list or alist is a specially-constructed list whose elements are cons cells. In each element, the car is considered a key, and the cdr is considered an associated value. (In some cases, the associated value is stored in the car of the cdr.) Association lists are often used as stacks, since it is easy to add or remove associations at the front of the list.
For example,
(setq alist-of-colors
'((rose . red) (lily . white) (buttercup . yellow)))
sets the variable alist-of-colors to an alist of three elements. In the
first element, rose is the key and red is the value.
See Association Lists, for a further explanation of alists and for functions that work on alists. See Hash Tables, for another kind of lookup table, which is much faster for handling a large number of keys.
An array is composed of an arbitrary number of slots for holding or referring to other Lisp objects, arranged in a contiguous block of memory. Accessing any element of an array takes approximately the same amount of time. In contrast, accessing an element of a list requires time proportional to the position of the element in the list. (Elements at the end of a list take longer to access than elements at the beginning of a list.)
Emacs defines four types of array: strings, vectors, bool-vectors, and char-tables.
A string is an array of characters and a vector is an array of
arbitrary objects. A bool-vector can hold only t or nil.
These kinds of array may have any length up to the largest integer.
Char-tables are sparse arrays indexed by any valid character code; they
can hold arbitrary objects.
The first element of an array has index zero, the second element has index 1, and so on. This is called zero-origin indexing. For example, an array of four elements has indices 0, 1, 2, and 3. The largest possible index value is one less than the length of the array. Once an array is created, its length is fixed.
All Emacs Lisp arrays are one-dimensional. (Most other programming languages support multidimensional arrays, but they are not essential; you can get the same effect with nested one-dimensional arrays.) Each type of array has its own read syntax; see the following sections for details.
The array type is a subset of the sequence type, and contains the string type, the vector type, the bool-vector type, and the char-table type.
A string is an array of characters. Strings are used for many purposes in Emacs, as can be expected in a text editor; for example, as the names of Lisp symbols, as messages for the user, and to represent text extracted from buffers. Strings in Lisp are constants: evaluation of a string returns the same string.
See Strings and Characters, for functions that operate on strings.
The read syntax for a string is a double-quote, an arbitrary number
of characters, and another double-quote, "like this". To
include a double-quote in a string, precede it with a backslash; thus,
"\"" is a string containing just a single double-quote
character. Likewise, you can include a backslash by preceding it with
another backslash, like this: "this \\ is a single embedded
backslash".
The newline character is not special in the read syntax for strings; if you write a new line between the double-quotes, it becomes a character in the string. But an escaped newline—one that is preceded by ‘\’—does not become part of the string; i.e., the Lisp reader ignores an escaped newline while reading a string. An escaped space ‘\ ’ is likewise ignored.
"It is useful to include newlines
in documentation strings,
but the newline is \
ignored if escaped."
"It is useful to include newlines
in documentation strings,
but the newline is ignored if escaped."
You can include a non-ASCII international character in a string constant by writing it literally. There are two text representations for non-ASCII characters in Emacs strings (and in buffers): unibyte and multibyte. If the string constant is read from a multibyte source, such as a multibyte buffer or string, or a file that would be visited as multibyte, then the character is read as a multibyte character, and that makes the string multibyte. If the string constant is read from a unibyte source, then the character is read as unibyte and that makes the string unibyte.
You can also represent a multibyte non-ASCII character with its character code: use a hex escape, ‘\xnnnnnnn’, with as many digits as necessary. (Multibyte non-ASCII character codes are all greater than 256.) Any character which is not a valid hex digit terminates this construct. If the next character in the string could be interpreted as a hex digit, write ‘\ ’ (backslash and space) to terminate the hex escape—for example, ‘\x8e0\ ’ represents one character, ‘a’ with grave accent. ‘\ ’ in a string constant is just like backslash-newline; it does not contribute any character to the string, but it does terminate the preceding hex escape.
You can represent a unibyte non-ASCII character with its character code, which must be in the range from 128 (0200 octal) to 255 (0377 octal). If you write all such character codes in octal and the string contains no other characters forcing it to be multibyte, this produces a unibyte string. However, using any hex escape in a string (even for an ASCII character) forces the string to be multibyte.
You can also specify characters in a string by their numeric values in Unicode, using ‘\u’ and ‘\U’ (see Character Type).
See Text Representations, for more information about the two text representations.
You can use the same backslash escape-sequences in a string constant
as in character literals (but do not use the question mark that begins a
character constant). For example, you can write a string containing the
nonprinting characters tab and C-a, with commas and spaces between
them, like this: "\t, \C-a". See Character Type, for a
description of the read syntax for characters.
However, not all of the characters you can write with backslash escape-sequences are valid in strings. The only control characters that a string can hold are the ASCII control characters. Strings do not distinguish case in ASCII control characters.
Properly speaking, strings cannot hold meta characters; but when a
string is to be used as a key sequence, there is a special convention
that provides a way to represent meta versions of ASCII
characters in a string. If you use the ‘\M-’ syntax to indicate
a meta character in a string constant, this sets the
2**7
bit of the character in the string. If the string is used in
define-key or lookup-key, this numeric code is translated
into the equivalent meta character. See Character Type.
Strings cannot hold characters that have the hyper, super, or alt modifiers.
A string can hold properties for the characters it contains, in addition to the characters themselves. This enables programs that copy text between strings and buffers to copy the text's properties with no special effort. See Text Properties, for an explanation of what text properties mean. Strings with text properties use a special read and print syntax:
#("characters" property-data...)
where property-data consists of zero or more elements, in groups of three as follows:
beg end plist
The elements beg and end are integers, and together specify a range of indices in the string; plist is the property list for that range. For example,
#("foo bar" 0 3 (face bold) 3 4 nil 4 7 (face italic))
represents a string whose textual contents are ‘foo bar’, in which
the first three characters have a face property with value
bold, and the last three have a face property with value
italic. (The fourth character has no text properties, so its
property list is nil. It is not actually necessary to mention
ranges with nil as the property list, since any characters not
mentioned in any range will default to having no properties.)
A vector is a one-dimensional array of elements of any type. It takes a constant amount of time to access any element of a vector. (In a list, the access time of an element is proportional to the distance of the element from the beginning of the list.)
The printed representation of a vector consists of a left square bracket, the elements, and a right square bracket. This is also the read syntax. Like numbers and strings, vectors are considered constants for evaluation.
[1 "two" (three)] ; A vector of three elements.
[1 "two" (three)]
See Vectors, for functions that work with vectors.
A char-table is a one-dimensional array of elements of any type, indexed by character codes. Char-tables have certain extra features to make them more useful for many jobs that involve assigning information to character codes—for example, a char-table can have a parent to inherit from, a default value, and a small number of extra slots to use for special purposes. A char-table can also specify a single value for a whole character set.
The printed representation of a char-table is like a vector except that there is an extra ‘#^’ at the beginning.
See Char-Tables, for special functions to operate on char-tables. Uses of char-tables include:
A bool-vector is a one-dimensional array of elements that
must be t or nil.
The printed representation of a bool-vector is like a string, except
that it begins with ‘#&’ followed by the length. The string
constant that follows actually specifies the contents of the bool-vector
as a bitmap—each “character” in the string contains 8 bits, which
specify the next 8 elements of the bool-vector (1 stands for t,
and 0 for nil). The least significant bits of the character
correspond to the lowest indices in the bool-vector.
(make-bool-vector 3 t)
#&3"^G"
(make-bool-vector 3 nil)
#&3"^@"
These results make sense, because the binary code for ‘C-g’ is 111 and ‘C-@’ is the character with code 0.
If the length is not a multiple of 8, the printed representation shows extra elements, but these extras really make no difference. For instance, in the next example, the two bool-vectors are equal, because only the first 3 bits are used:
(equal #&3"\377" #&3"\007")
t
A hash table is a very fast kind of lookup table, somewhat like an alist in that it maps keys to corresponding values, but much faster. Hash tables have no read syntax, and print using hash notation. See Hash Tables, for functions that operate on hash tables.
(make-hash-table)
#<hash-table 'eql nil 0/65 0x83af980>
Lisp functions are executable code, just like functions in other
programming languages. In Lisp, unlike most languages, functions are
also Lisp objects. A non-compiled function in Lisp is a lambda
expression: that is, a list whose first element is the symbol
lambda (see Lambda Expressions).
In most programming languages, it is impossible to have a function without a name. In Lisp, a function has no intrinsic name. A lambda expression can be called as a function even though it has no name; to emphasize this, we also call it an anonymous function (see Anonymous Functions). A named function in Lisp is just a symbol with a valid function in its function cell (see Defining Functions).
Most of the time, functions are called when their names are written in
Lisp expressions in Lisp programs. However, you can construct or obtain
a function object at run time and then call it with the primitive
functions funcall and apply. See Calling Functions.
A Lisp macro is a user-defined construct that extends the Lisp
language. It is represented as an object much like a function, but with
different argument-passing semantics. A Lisp macro has the form of a
list whose first element is the symbol macro and whose cdr
is a Lisp function object, including the lambda symbol.
Lisp macro objects are usually defined with the built-in
defmacro function, but any list that begins with macro is
a macro as far as Emacs is concerned. See Macros, for an explanation
of how to write a macro.
Warning: Lisp macros and keyboard macros (see Keyboard Macros) are entirely different things. When we use the word “macro” without qualification, we mean a Lisp macro, not a keyboard macro.
A primitive function is a function callable from Lisp but written in the C programming language. Primitive functions are also called subrs or built-in functions. (The word “subr” is derived from “subroutine.”) Most primitive functions evaluate all their arguments when they are called. A primitive function that does not evaluate all its arguments is called a special form (see Special Forms).
It does not matter to the caller of a function whether the function is primitive. However, this does matter if you try to redefine a primitive with a function written in Lisp. The reason is that the primitive function may be called directly from C code. Calls to the redefined function from Lisp will use the new definition, but calls from C code may still use the built-in definition. Therefore, we discourage redefinition of primitive functions.
The term function refers to all Emacs functions, whether written in Lisp or C. See Function Type, for information about the functions written in Lisp.
Primitive functions have no read syntax and print in hash notation with the name of the subroutine.
(symbol-function 'car) ; Access the function cell ; of the symbol. #<subr car> (subrp (symbol-function 'car)) ; Is this a primitive function? t ; Yes.
The byte compiler produces byte-code function objects. Internally, a byte-code function object is much like a vector; however, the evaluator handles this data type specially when it appears as a function to be called. See Byte Compilation, for information about the byte compiler.
The printed representation and read syntax for a byte-code function object is like that for a vector, with an additional ‘#’ before the opening ‘[’.
An autoload object is a list whose first element is the symbol
autoload. It is stored as the function definition of a symbol,
where it serves as a placeholder for the real definition. The autoload
object says that the real definition is found in a file of Lisp code
that should be loaded when necessary. It contains the name of the file,
plus some other information about the real definition.
After the file has been loaded, the symbol should have a new function definition that is not an autoload object. The new definition is then called as if it had been there to begin with. From the user's point of view, the function call works as expected, using the function definition in the loaded file.
An autoload object is usually created with the function
autoload, which stores the object in the function cell of a
symbol. See Autoload, for more details.
The types in the previous section are used for general programming purposes, and most of them are common to most Lisp dialects. Emacs Lisp provides several additional data types for purposes connected with editing.
A buffer is an object that holds text that can be edited (see Buffers). Most buffers hold the contents of a disk file (see Files) so they can be edited, but some are used for other purposes. Most buffers are also meant to be seen by the user, and therefore displayed, at some time, in a window (see Windows). But a buffer need not be displayed in any window. Each buffer has a designated position called point (see Positions); most editing commands act on the contents of the current buffer in the neighborhood of point. At any time, one buffer is the current buffer.
The contents of a buffer are much like a string, but buffers are not used like strings in Emacs Lisp, and the available operations are different. For example, you can insert text efficiently into an existing buffer, altering the buffer's contents, whereas “inserting” text into a string requires concatenating substrings, and the result is an entirely new string object.
Many of the standard Emacs functions manipulate or test the characters in the current buffer; a whole chapter in this manual is devoted to describing these functions (see Text).
Several other data structures are associated with each buffer:
The local keymap and variable list contain entries that individually override global bindings or values. These are used to customize the behavior of programs in different buffers, without actually changing the programs.
A buffer may be indirect, which means it shares the text of another buffer, but presents it differently. See Indirect Buffers.
Buffers have no read syntax. They print in hash notation, showing the buffer name.
(current-buffer)
#<buffer objects.texi>
A marker denotes a position in a specific buffer. Markers therefore have two components: one for the buffer, and one for the position. Changes in the buffer's text automatically relocate the position value as necessary to ensure that the marker always points between the same two characters in the buffer.
Markers have no read syntax. They print in hash notation, giving the current character position and the name of the buffer.
(point-marker)
#<marker at 10779 in objects.texi>
See Markers, for information on how to test, create, copy, and move markers.
A window describes the portion of the terminal screen that Emacs uses to display a buffer. Every window has one associated buffer, whose contents appear in the window. By contrast, a given buffer may appear in one window, no window, or several windows.
Though many windows may exist simultaneously, at any time one window is designated the selected window. This is the window where the cursor is (usually) displayed when Emacs is ready for a command. The selected window usually displays the current buffer, but this is not necessarily the case.
Windows are grouped on the screen into frames; each window belongs to one and only one frame. See Frame Type.
Windows have no read syntax. They print in hash notation, giving the window number and the name of the buffer being displayed. The window numbers exist to identify windows uniquely, since the buffer displayed in any given window can change frequently.
(selected-window)
#<window 1 on objects.texi>
See Windows, for a description of the functions that work on windows.
A frame is a screen area that contains one or more Emacs windows; we also use the term “frame” to refer to the Lisp object that Emacs uses to refer to the screen area.
Frames have no read syntax. They print in hash notation, giving the frame's title, plus its address in core (useful to identify the frame uniquely).
(selected-frame)
#<frame emacs@psilocin.gnu.org 0xdac80>
See Frames, for a description of the functions that work on frames.
A terminal is a device capable of displaying one or more Emacs frames (see Frame Type).
Terminals have no read syntax. They print in hash notation giving the terminal's ordinal number and its TTY device file name.
(get-device-terminal nil)
#<terminal 1 on /dev/tty>
A window configuration stores information about the positions, sizes, and contents of the windows in a frame, so you can recreate the same arrangement of windows later.
Window configurations do not have a read syntax; their print syntax looks like ‘#<window-configuration>’. See Window Configurations, for a description of several functions related to window configurations.
A frame configuration stores information about the positions,
sizes, and contents of the windows in all frames. It is not a
primitive type—it is actually a list whose car is
frame-configuration and whose cdr is an alist. Each alist
element describes one frame, which appears as the car of that
element.
See Frame Configurations, for a description of several functions related to frame configurations.
The word process usually means a running program. Emacs itself runs in a process of this sort. However, in Emacs Lisp, a process is a Lisp object that designates a subprocess created by the Emacs process. Programs such as shells, GDB, ftp, and compilers, running in subprocesses of Emacs, extend the capabilities of Emacs.
An Emacs subprocess takes textual input from Emacs and returns textual output to Emacs for further manipulation. Emacs can also send signals to the subprocess.
Process objects have no read syntax. They print in hash notation, giving the name of the process:
(process-list)
(#<process shell>)
See Processes, for information about functions that create, delete, return information about, send input or signals to, and receive output from processes.
A stream is an object that can be used as a source or sink for characters—either to supply characters for input or to accept them as output. Many different types can be used this way: markers, buffers, strings, and functions. Most often, input streams (character sources) obtain characters from the keyboard, a buffer, or a file, and output streams (character sinks) send characters to a buffer, such as a *Help* buffer, or to the echo area.
The object nil, in addition to its other meanings, may be used
as a stream. It stands for the value of the variable
standard-input or standard-output. Also, the object
t as a stream specifies input using the minibuffer
(see Minibuffers) or output in the echo area (see The Echo Area).
Streams have no special printed representation or read syntax, and print as whatever primitive type they are.
See Read and Print, for a description of functions related to streams, including parsing and printing functions.
A keymap maps keys typed by the user to commands. This mapping
controls how the user's command input is executed. A keymap is actually
a list whose car is the symbol keymap.
See Keymaps, for information about creating keymaps, handling prefix keys, local as well as global keymaps, and changing key bindings.
An overlay specifies properties that apply to a part of a buffer. Each overlay applies to a specified range of the buffer, and contains a property list (a list whose elements are alternating property names and values). Overlay properties are used to present parts of the buffer temporarily in a different display style. Overlays have no read syntax, and print in hash notation, giving the buffer name and range of positions.
See Overlays, for how to create and use overlays.
A font specifies how to display text on a graphical terminal. There are actually three separate font types—font objects, font specs, and font entities—each of which has slightly different properties. None of them have a read syntax; their print syntax looks like ‘#<font-object>’, ‘#<font-spec>’, and ‘#<font-entity>’ respectively. See Low-Level Font, for a description of these Lisp objects.
To represent shared or circular structures within a complex of Lisp objects, you can use the reader constructs ‘#n=’ and ‘#n#’.
Use #n= before an object to label it for later reference;
subsequently, you can use #n# to refer the same object in
another place. Here, n is some integer. For example, here is how
to make a list in which the first element recurs as the third element:
(#1=(a) b #1#)
This differs from ordinary syntax such as this
((a) b (a))
which would result in a list whose first and third elements look alike but are not the same Lisp object. This shows the difference:
(prog1 nil
(setq x '(#1=(a) b #1#)))
(eq (nth 0 x) (nth 2 x))
t
(setq x '((a) b (a)))
(eq (nth 0 x) (nth 2 x))
nil
You can also use the same syntax to make a circular structure, which appears as an “element” within itself. Here is an example:
#1=(a #1#)
This makes a list whose second element is the list itself. Here's how you can see that it really works:
(prog1 nil
(setq x '#1=(a #1#)))
(eq x (cadr x))
t
The Lisp printer can produce this syntax to record circular and shared
structure in a Lisp object, if you bind the variable print-circle
to a non-nil value. See Output Variables.
The Emacs Lisp interpreter itself does not perform type checking on the actual arguments passed to functions when they are called. It could not do so, since function arguments in Lisp do not have declared data types, as they do in other programming languages. It is therefore up to the individual function to test whether each actual argument belongs to a type that the function can use.
All built-in functions do check the types of their actual arguments
when appropriate, and signal a wrong-type-argument error if an
argument is of the wrong type. For example, here is what happens if you
pass an argument to + that it cannot handle:
(+ 2 'a)
error--> Wrong type argument: number-or-marker-p, a
If you want your program to handle different types differently, you must do explicit type checking. The most common way to check the type of an object is to call a type predicate function. Emacs has a type predicate for each type, as well as some predicates for combinations of types.
A type predicate function takes one argument; it returns t if
the argument belongs to the appropriate type, and nil otherwise.
Following a general Lisp convention for predicate functions, most type
predicates' names end with ‘p’.
Here is an example which uses the predicates listp to check for
a list and symbolp to check for a symbol.
(defun add-on (x)
(cond ((symbolp x)
;; If X is a symbol, put it on LIST.
(setq list (cons x list)))
((listp x)
;; If X is a list, add its elements to LIST.
(setq list (append x list)))
(t
;; We handle only symbols and lists.
(error "Invalid argument %s in add-on" x))))
Here is a table of predefined type predicates, in alphabetical order, with references to further information.
atomarraypbool-vector-pbufferpbyte-code-function-pcase-table-pchar-or-string-pchar-table-pcommandpconspdisplay-table-pfloatpfontpframe-configuration-pframe-live-pframepfunctionphash-table-pinteger-or-marker-pintegerpkeymappkeywordplistpmarkerpwholenumpnlistpnumberpnumber-or-marker-poverlaypprocesspsequencepstringpsubrpsymbolpsyntax-table-puser-variable-pvectorpwindow-configuration-pwindow-live-pwindowpbooleanpstring-or-null-pThe most general way to check the type of an object is to call the
function type-of. Recall that each object belongs to one and
only one primitive type; type-of tells you which one (see Lisp Data Types). But type-of knows nothing about non-primitive
types. In most cases, it is more convenient to use type predicates than
type-of.
This function returns a symbol naming the primitive type of object. The value is one of the symbols
bool-vector,buffer,char-table,compiled-function,cons,float,font-entity,font-object,font-spec,frame,hash-table,integer,marker,overlay,process,string,subr,symbol,vector,window, orwindow-configuration.(type-of 1) integer (type-of 'nil) symbol (type-of '()) ;()isnil. symbol (type-of '(x)) cons
Here we describe functions that test for equality between any two objects. Other functions test equality of contents between objects of specific types, e.g., strings. For these predicates, see the appropriate chapter describing the data type.
This function returns
tif object1 and object2 are the same object,nilotherwise.
eqreturnstif object1 and object2 are integers with the same value. Also, since symbol names are normally unique, if the arguments are symbols with the same name, they areeq. For other types (e.g., lists, vectors, strings), two arguments with the same contents or elements are not necessarilyeqto each other: they areeqonly if they are the same object, meaning that a change in the contents of one will be reflected by the same change in the contents of the other.(eq 'foo 'foo) t (eq 456 456) t (eq "asdf" "asdf") nil (eq "" "") t ;; This exception occurs because Emacs Lisp ;; makes just one multibyte empty string, to save space. (eq '(1 (2 (3))) '(1 (2 (3)))) nil (setq foo '(1 (2 (3)))) (1 (2 (3))) (eq foo foo) t (eq foo '(1 (2 (3)))) nil (eq [(1 2) 3] [(1 2) 3]) nil (eq (point-marker) (point-marker)) nilThe
make-symbolfunction returns an uninterned symbol, distinct from the symbol that is used if you write the name in a Lisp expression. Distinct symbols with the same name are noteq. See Creating Symbols.(eq (make-symbol "foo") 'foo) nil
This function returns
tif object1 and object2 have equal components,nilotherwise. Whereaseqtests if its arguments are the same object,equallooks inside nonidentical arguments to see if their elements or contents are the same. So, if two objects areeq, they areequal, but the converse is not always true.(equal 'foo 'foo) t (equal 456 456) t (equal "asdf" "asdf") t (eq "asdf" "asdf") nil (equal '(1 (2 (3))) '(1 (2 (3)))) t (eq '(1 (2 (3))) '(1 (2 (3)))) nil (equal [(1 2) 3] [(1 2) 3]) t (eq [(1 2) 3] [(1 2) 3]) nil (equal (point-marker) (point-marker)) t (eq (point-marker) (point-marker)) nilComparison of strings is case-sensitive, but does not take account of text properties—it compares only the characters in the strings. Use
equal-including-propertiesto also compare text properties. For technical reasons, a unibyte string and a multibyte string areequalif and only if they contain the same sequence of character codes and all these codes are either in the range 0 through 127 (ASCII) or 160 through 255 (eight-bit-graphic). (see Text Representations).(equal "asdf" "ASDF") nilHowever, two distinct buffers are never considered
equal, even if their textual contents are the same.
The test for equality is implemented recursively; for example, given
two cons cells x and y, (equal x y)
returns t if and only if both the expressions below return
t:
(equal (car x) (car y))
(equal (cdr x) (cdr y))
Because of this recursive method, circular lists may therefore cause infinite recursion (leading to an error).
This function behaves like
equalin all cases but also requires that for two strings to be equal, they have the same text properties.(equal "asdf" (propertize "asdf" '(asdf t))) t (equal-including-properties "asdf" (propertize "asdf" '(asdf t))) nil
GNU Emacs supports two numeric data types: integers and floating point numbers. Integers are whole numbers such as −3, 0, 7, 13, and 511. Their values are exact. Floating point numbers are numbers with fractional parts, such as −4.5, 0.0, or 2.71828. They can also be expressed in exponential notation: 1.5e2 equals 150; in this example, ‘e2’ stands for ten to the second power, and that is multiplied by 1.5. Floating point values are not exact; they have a fixed, limited amount of precision.
The range of values for an integer depends on the machine. The minimum range is −268435456 to 268435455 (29 bits; i.e., -2**28 to 2**28 - 1), but some machines may provide a wider range. Many examples in this chapter assume an integer has 29 bits. The Lisp reader reads an integer as a sequence of digits with optional initial sign and optional final period.
1 ; The integer 1. 1. ; The integer 1. +1 ; Also the integer 1. -1 ; The integer −1. 536870913 ; Also the integer 1, due to overflow. 0 ; The integer 0. -0 ; The integer 0.
The syntax for integers in bases other than 10 uses ‘#’ followed by a letter that specifies the radix: ‘b’ for binary, ‘o’ for octal, ‘x’ for hex, or ‘radixr’ to specify radix radix. Case is not significant for the letter that specifies the radix. Thus, ‘#binteger’ reads integer in binary, and ‘#radixrinteger’ reads integer in radix radix. Allowed values of radix run from 2 to 36. For example:
#b101100 44
#o54 44
#x2c 44
#24r1k 44
To understand how various functions work on integers, especially the bitwise operators (see Bitwise Operations), it is often helpful to view the numbers in their binary form.
In 29-bit binary, the decimal integer 5 looks like this:
0 0000 0000 0000 0000 0000 0000 0101
(We have inserted spaces between groups of 4 bits, and two spaces between groups of 8 bits, to make the binary integer easier to read.)
The integer −1 looks like this:
1 1111 1111 1111 1111 1111 1111 1111
−1 is represented as 29 ones. (This is called two's complement notation.)
The negative integer, −5, is creating by subtracting 4 from −1. In binary, the decimal integer 4 is 100. Consequently, −5 looks like this:
1 1111 1111 1111 1111 1111 1111 1011
In this implementation, the largest 29-bit binary integer value is 268,435,455 in decimal. In binary, it looks like this:
0 1111 1111 1111 1111 1111 1111 1111
Since the arithmetic functions do not check whether integers go outside their range, when you add 1 to 268,435,455, the value is the negative integer −268,435,456:
(+ 1 268435455)
-268435456
1 0000 0000 0000 0000 0000 0000 0000
Many of the functions described in this chapter accept markers for arguments in place of numbers. (See Markers.) Since the actual arguments to such functions may be either numbers or markers, we often give these arguments the name number-or-marker. When the argument value is a marker, its position value is used and its buffer is ignored.
The value of this variable is the largest integer that Emacs Lisp can handle.
The value of this variable is the smallest integer that Emacs Lisp can handle. It is negative.
See max-char, for the maximum value of a valid character codepoint.
Floating point numbers are useful for representing numbers that are
not integral. The precise range of floating point numbers is
machine-specific; it is the same as the range of the C data type
double on the machine you are using.
The read-syntax for floating point numbers requires either a decimal point (with at least one digit following), an exponent, or both. For example, ‘1500.0’, ‘15e2’, ‘15.0e2’, ‘1.5e3’, and ‘.15e4’ are five ways of writing a floating point number whose value is 1500. They are all equivalent. You can also use a minus sign to write negative floating point numbers, as in ‘-1.0’.
Most modern computers support the IEEE floating point standard,
which provides for positive infinity and negative infinity as floating point
values. It also provides for a class of values called NaN or
“not-a-number”; numerical functions return such values in cases where
there is no correct answer. For example, (/ 0.0 0.0) returns a
NaN. For practical purposes, there's no significant difference between
different NaN values in Emacs Lisp, and there's no rule for precisely
which NaN value should be used in a particular case, so Emacs Lisp
doesn't try to distinguish them (but it does report the sign, if you
print it). Here are the read syntaxes for these special floating
point values:
To test whether a floating point value is a NaN, compare it with
itself using =. That returns nil for a NaN, and
t for any other floating point value.
The value -0.0 is distinguishable from ordinary zero in
IEEE floating point, but Emacs Lisp equal and
= consider them equal values.
You can use logb to extract the binary exponent of a floating
point number (or estimate the logarithm of an integer):
This function returns the binary exponent of number. More precisely, the value is the logarithm of number base 2, rounded down to an integer.
(logb 10) 3 (logb 10.0e20) 69
The functions in this section test for numbers, or for a specific
type of number. The functions integerp and floatp can
take any type of Lisp object as argument (they would not be of much
use otherwise), but the zerop predicate requires a number as
its argument. See also integer-or-marker-p and
number-or-marker-p, in Predicates on Markers.
This predicate tests whether its argument is a floating point number and returns
tif so,nilotherwise.
floatpdoes not exist in Emacs versions 18 and earlier.
This predicate tests whether its argument is an integer, and returns
tif so,nilotherwise.
This predicate tests whether its argument is a number (either integer or floating point), and returns
tif so,nilotherwise.
The
wholenumppredicate (whose name comes from the phrase “whole-number-p”) tests to see whether its argument is a nonnegative integer, and returnstif so,nilotherwise. 0 is considered non-negative.
This predicate tests whether its argument is zero, and returns
tif so,nilotherwise. The argument must be a number.
(zerop x)is equivalent to(= x 0).
To test numbers for numerical equality, you should normally use
=, not eq. There can be many distinct floating point
number objects with the same numeric value. If you use eq to
compare them, then you test whether two values are the same
object. By contrast, = compares only the numeric values
of the objects.
At present, each integer value has a unique Lisp object in Emacs Lisp.
Therefore, eq is equivalent to = where integers are
concerned. It is sometimes convenient to use eq for comparing an
unknown value with an integer, because eq does not report an
error if the unknown value is not a number—it accepts arguments of any
type. By contrast, = signals an error if the arguments are not
numbers or markers. However, it is a good idea to use = if you
can, even for comparing integers, just in case we change the
representation of integers in a future Emacs version.
Sometimes it is useful to compare numbers with equal; it
treats two numbers as equal if they have the same data type (both
integers, or both floating point) and the same value. By contrast,
= can treat an integer and a floating point number as equal.
See Equality Predicates.
There is another wrinkle: because floating point arithmetic is not exact, it is often a bad idea to check for equality of two floating point values. Usually it is better to test for approximate equality. Here's a function to do this:
(defvar fuzz-factor 1.0e-6)
(defun approx-equal (x y)
(or (and (= x 0) (= y 0))
(< (/ (abs (- x y))
(max (abs x) (abs y)))
fuzz-factor)))
Common Lisp note: Comparing numbers in Common Lisp always requires
= because Common Lisp implements multi-word integers, and two
distinct integer objects can have the same numeric value. Emacs Lisp
can have just one integer object for any given value because it has a
limited range of integer values.
This function tests whether its arguments are numerically equal, and returns
tif so,nilotherwise.
This function acts like
eqexcept when both arguments are numbers. It compares numbers by type and numeric value, so that(eql 1.0 1)returnsnil, but(eql 1.0 1.0)and(eql 1 1)both returnt.
This function tests whether its arguments are numerically equal, and returns
tif they are not, andnilif they are.
This function tests whether its first argument is strictly less than its second argument. It returns
tif so,nilotherwise.
This function tests whether its first argument is less than or equal to its second argument. It returns
tif so,nilotherwise.
This function tests whether its first argument is strictly greater than its second argument. It returns
tif so,nilotherwise.
This function tests whether its first argument is greater than or equal to its second argument. It returns
tif so,nilotherwise.
This function returns the largest of its arguments. If any of the arguments is floating-point, the value is returned as floating point, even if it was given as an integer.
(max 20) 20 (max 1 2.5) 2.5 (max 1 3 2.5) 3.0
This function returns the smallest of its arguments. If any of the arguments is floating-point, the value is returned as floating point, even if it was given as an integer.
(min -4 1) -4
To convert an integer to floating point, use the function float.
This returns number converted to floating point. If number is already a floating point number,
floatreturns it unchanged.
There are four functions to convert floating point numbers to integers;
they differ in how they round. All accept an argument number
and an optional argument divisor. Both arguments may be
integers or floating point numbers. divisor may also be
nil. If divisor is nil or omitted, these
functions convert number to an integer, or return it unchanged
if it already is an integer. If divisor is non-nil, they
divide number by divisor and convert the result to an
integer. An arith-error results if divisor is 0.
This returns number, converted to an integer by rounding towards zero.
(truncate 1.2) 1 (truncate 1.7) 1 (truncate -1.2) -1 (truncate -1.7) -1
This returns number, converted to an integer by rounding downward (towards negative infinity).
If divisor is specified, this uses the kind of division operation that corresponds to
mod, rounding downward.(floor 1.2) 1 (floor 1.7) 1 (floor -1.2) -2 (floor -1.7) -2 (floor 5.99 3) 1
This returns number, converted to an integer by rounding upward (towards positive infinity).
(ceiling 1.2) 2 (ceiling 1.7) 2 (ceiling -1.2) -1 (ceiling -1.7) -1
This returns number, converted to an integer by rounding towards the nearest integer. Rounding a value equidistant between two integers may choose the integer closer to zero, or it may prefer an even integer, depending on your machine.
(round 1.2) 1 (round 1.7) 2 (round -1.2) -1 (round -1.7) -2
Emacs Lisp provides the traditional four arithmetic operations: addition, subtraction, multiplication, and division. Remainder and modulus functions supplement the division functions. The functions to add or subtract 1 are provided because they are traditional in Lisp and commonly used.
All of these functions except % return a floating point value
if any argument is floating.
It is important to note that in Emacs Lisp, arithmetic functions
do not check for overflow. Thus (1+ 268435455) may evaluate to
−268435456, depending on your hardware.
This function returns number-or-marker plus 1. For example,
(setq foo 4) 4 (1+ foo) 5This function is not analogous to the C operator
++—it does not increment a variable. It just computes a sum. Thus, if we continue,foo 4If you want to increment the variable, you must use
setq, like this:(setq foo (1+ foo)) 5
This function adds its arguments together. When given no arguments,
+returns 0.(+) 0 (+ 1) 1 (+ 1 2 3 4) 10
The
-function serves two purposes: negation and subtraction. When-has a single argument, the value is the negative of the argument. When there are multiple arguments,-subtracts each of the more-numbers-or-markers from number-or-marker, cumulatively. If there are no arguments, the result is 0.(- 10 1 2 3 4) 0 (- 10) -10 (-) 0
This function multiplies its arguments together, and returns the product. When given no arguments,
*returns 1.(*) 1 (* 1) 1 (* 1 2 3 4) 24
This function divides dividend by divisor and returns the quotient. If there are additional arguments divisors, then it divides dividend by each divisor in turn. Each argument may be a number or a marker.
If all the arguments are integers, then the result is an integer too. This means the result has to be rounded. On most machines, the result is rounded towards zero after each division, but some machines may round differently with negative arguments. This is because the Lisp function
/is implemented using the C division operator, which also permits machine-dependent rounding. As a practical matter, all known machines round in the standard fashion.If you divide an integer by 0, an
arith-errorerror is signaled. (See Errors.) Floating point division by zero returns either infinity or a NaN if your machine supports IEEE floating point; otherwise, it signals anarith-errorerror.(/ 6 2) 3 (/ 5 2) 2 (/ 5.0 2) 2.5 (/ 5 2.0) 2.5 (/ 5.0 2.0) 2.5 (/ 25 3 2) 4 (/ -17 6) -2 (could in theory be −3 on some machines)
This function returns the integer remainder after division of dividend by divisor. The arguments must be integers or markers.
For negative arguments, the remainder is in principle machine-dependent since the quotient is; but in practice, all known machines behave alike.
An
arith-errorresults if divisor is 0.(% 9 4) 1 (% -9 4) -1 (% 9 -4) 1 (% -9 -4) -1For any two integers dividend and divisor,
(+ (% dividend divisor) (* (/ dividend divisor) divisor))always equals dividend.
This function returns the value of dividend modulo divisor; in other words, the remainder after division of dividend by divisor, but with the same sign as divisor. The arguments must be numbers or markers.
Unlike
%,modreturns a well-defined result for negative arguments. It also permits floating point arguments; it rounds the quotient downward (towards minus infinity) to an integer, and uses that quotient to compute the remainder.An
arith-errorresults if divisor is 0.(mod 9 4) 1 (mod -9 4) 3 (mod 9 -4) -3 (mod -9 -4) -1 (mod 5.5 2.5) .5For any two numbers dividend and divisor,
(+ (mod dividend divisor) (* (floor dividend divisor) divisor))always equals dividend, subject to rounding error if either argument is floating point. For
floor, see Numeric Conversions.
The functions ffloor, fceiling, fround, and
ftruncate take a floating point argument and return a floating
point result whose value is a nearby integer. ffloor returns the
nearest integer below; fceiling, the nearest integer above;
ftruncate, the nearest integer in the direction towards zero;
fround, the nearest integer.
This function rounds float to the next lower integral value, and returns that value as a floating point number.
This function rounds float to the next higher integral value, and returns that value as a floating point number.
This function rounds float towards zero to an integral value, and returns that value as a floating point number.
This function rounds float to the nearest integral value, and returns that value as a floating point number.
In a computer, an integer is represented as a binary number, a sequence of bits (digits which are either zero or one). A bitwise operation acts on the individual bits of such a sequence. For example, shifting moves the whole sequence left or right one or more places, reproducing the same pattern “moved over.”
The bitwise operations in Emacs Lisp apply only to integers.
lsh, which is an abbreviation for logical shift, shifts the bits in integer1 to the left count places, or to the right if count is negative, bringing zeros into the vacated bits. If count is negative,lshshifts zeros into the leftmost (most-significant) bit, producing a positive result even if integer1 is negative. Contrast this withash, below.Here are two examples of
lsh, shifting a pattern of bits one place to the left. We show only the low-order eight bits of the binary pattern; the rest are all zero.(lsh 5 1) 10 ;; Decimal 5 becomes decimal 10. 00000101 00001010 (lsh 7 1) 14 ;; Decimal 7 becomes decimal 14. 00000111 00001110As the examples illustrate, shifting the pattern of bits one place to the left produces a number that is twice the value of the previous number.
Shifting a pattern of bits two places to the left produces results like this (with 8-bit binary numbers):
(lsh 3 2) 12 ;; Decimal 3 becomes decimal 12. 00000011 00001100On the other hand, shifting one place to the right looks like this:
(lsh 6 -1) 3 ;; Decimal 6 becomes decimal 3. 00000110 00000011 (lsh 5 -1) 2 ;; Decimal 5 becomes decimal 2. 00000101 00000010As the example illustrates, shifting one place to the right divides the value of a positive integer by two, rounding downward.
The function
lsh, like all Emacs Lisp arithmetic functions, does not check for overflow, so shifting left can discard significant bits and change the sign of the number. For example, left shifting 268,435,455 produces −2 on a 29-bit machine:(lsh 268435455 1) ; left shift -2In binary, in the 29-bit implementation, the argument looks like this:
;; Decimal 268,435,455 0 1111 1111 1111 1111 1111 1111 1111which becomes the following when left shifted:
;; Decimal −2 1 1111 1111 1111 1111 1111 1111 1110
ash(arithmetic shift) shifts the bits in integer1 to the left count places, or to the right if count is negative.
ashgives the same results aslshexcept when integer1 and count are both negative. In that case,ashputs ones in the empty bit positions on the left, whilelshputs zeros in those bit positions.Thus, with
ash, shifting the pattern of bits one place to the right looks like this:(ash -6 -1) -3 ;; Decimal −6 becomes decimal −3. 1 1111 1111 1111 1111 1111 1111 1010 1 1111 1111 1111 1111 1111 1111 1101In contrast, shifting the pattern of bits one place to the right with
lshlooks like this:(lsh -6 -1) 268435453 ;; Decimal −6 becomes decimal 268,435,453. 1 1111 1111 1111 1111 1111 1111 1010 0 1111 1111 1111 1111 1111 1111 1101Here are other examples:
; 29-bit binary values (lsh 5 2) ; 5 = 0 0000 0000 0000 0000 0000 0000 0101 20 ; = 0 0000 0000 0000 0000 0000 0001 0100 (ash 5 2) 20 (lsh -5 2) ; -5 = 1 1111 1111 1111 1111 1111 1111 1011 -20 ; = 1 1111 1111 1111 1111 1111 1110 1100 (ash -5 2) -20 (lsh 5 -2) ; 5 = 0 0000 0000 0000 0000 0000 0000 0101 1 ; = 0 0000 0000 0000 0000 0000 0000 0001 (ash 5 -2) 1 (lsh -5 -2) ; -5 = 1 1111 1111 1111 1111 1111 1111 1011 134217726 ; = 0 0111 1111 1111 1111 1111 1111 1110 (ash -5 -2) ; -5 = 1 1111 1111 1111 1111 1111 1111 1011 -2 ; = 1 1111 1111 1111 1111 1111 1111 1110
This function returns the “logical and” of the arguments: the nth bit is set in the result if, and only if, the nth bit is set in all the arguments. (“Set” means that the value of the bit is 1 rather than 0.)
For example, using 4-bit binary numbers, the “logical and” of 13 and 12 is 12: 1101 combined with 1100 produces 1100. In both the binary numbers, the leftmost two bits are set (i.e., they are 1's), so the leftmost two bits of the returned value are set. However, for the rightmost two bits, each is zero in at least one of the arguments, so the rightmost two bits of the returned value are 0's.
Therefore,
(logand 13 12) 12If
logandis not passed any argument, it returns a value of −1. This number is an identity element forlogandbecause its binary representation consists entirely of ones. Iflogandis passed just one argument, it returns that argument.; 29-bit binary values (logand 14 13) ; 14 = 0 0000 0000 0000 0000 0000 0000 1110 ; 13 = 0 0000 0000 0000 0000 0000 0000 1101 12 ; 12 = 0 0000 0000 0000 0000 0000 0000 1100 (logand 14 13 4) ; 14 = 0 0000 0000 0000 0000 0000 0000 1110 ; 13 = 0 0000 0000 0000 0000 0000 0000 1101 ; 4 = 0 0000 0000 0000 0000 0000 0000 0100 4 ; 4 = 0 0000 0000 0000 0000 0000 0000 0100 (logand) -1 ; -1 = 1 1111 1111 1111 1111 1111 1111 1111
This function returns the “inclusive or” of its arguments: the nth bit is set in the result if, and only if, the nth bit is set in at least one of the arguments. If there are no arguments, the result is zero, which is an identity element for this operation. If
logioris passed just one argument, it returns that argument.; 29-bit binary values (logior 12 5) ; 12 = 0 0000 0000 0000 0000 0000 0000 1100 ; 5 = 0 0000 0000 0000 0000 0000 0000 0101 13 ; 13 = 0 0000 0000 0000 0000 0000 0000 1101 (logior 12 5 7) ; 12 = 0 0000 0000 0000 0000 0000 0000 1100 ; 5 = 0 0000 0000 0000 0000 0000 0000 0101 ; 7 = 0 0000 0000 0000 0000 0000 0000 0111 15 ; 15 = 0 0000 0000 0000 0000 0000 0000 1111
This function returns the “exclusive or” of its arguments: the nth bit is set in the result if, and only if, the nth bit is set in an odd number of the arguments. If there are no arguments, the result is 0, which is an identity element for this operation. If
logxoris passed just one argument, it returns that argument.; 29-bit binary values (logxor 12 5) ; 12 = 0 0000 0000 0000 0000 0000 0000 1100 ; 5 = 0 0000 0000 0000 0000 0000 0000 0101 9 ; 9 = 0 0000 0000 0000 0000 0000 0000 1001 (logxor 12 5 7) ; 12 = 0 0000 0000 0000 0000 0000 0000 1100 ; 5 = 0 0000 0000 0000 0000 0000 0000 0101 ; 7 = 0 0000 0000 0000 0000 0000 0000 0111 14 ; 14 = 0 0000 0000 0000 0000 0000 0000 1110
This function returns the logical complement of its argument: the nth bit is one in the result if, and only if, the nth bit is zero in integer, and vice-versa.
(lognot 5) -6 ;; 5 = 0 0000 0000 0000 0000 0000 0000 0101 ;; becomes ;; -6 = 1 1111 1111 1111 1111 1111 1111 1010
These mathematical functions allow integers as well as floating point numbers as arguments.
These are the ordinary trigonometric functions, with argument measured in radians.
The value of
(asinarg)is a number between −pi/2 and pi/2 (inclusive) whose sine is arg; if, however, arg is out of range (outside [−1, 1]), it signals adomain-errorerror.
The value of
(acosarg)is a number between 0 and pi (inclusive) whose cosine is arg; if, however, arg is out of range (outside [−1, 1]), it signals adomain-errorerror.
The value of
(atany)is a number between −pi/2 and pi/2 (exclusive) whose tangent is y. If the optional second argument x is given, the value of(atan y x)is the angle in radians between the vector[x,y]and theXaxis.
This is the exponential function; it returns e to the power arg. e is a fundamental mathematical constant also called the base of natural logarithms.
This function returns the logarithm of arg, with base base. If you don't specify base, the base e is used. If arg is negative, it signals a
domain-errorerror.
This function returns the logarithm of arg, with base 10. If arg is negative, it signals a
domain-errorerror.(log10x)==(logx10), at least approximately.
This function returns x raised to power y. If both arguments are integers and y is positive, the result is an integer; in this case, overflow causes truncation, so watch out.
This returns the square root of arg. If arg is negative, it signals a
domain-errorerror.
A deterministic computer program cannot generate true random numbers. For most purposes, pseudo-random numbers suffice. A series of pseudo-random numbers is generated in a deterministic fashion. The numbers are not truly random, but they have certain properties that mimic a random series. For example, all possible values occur equally often in a pseudo-random series.
In Emacs, pseudo-random numbers are generated from a “seed” number.
Starting from any given seed, the random function always
generates the same sequence of numbers. Emacs always starts with the
same seed value, so the sequence of values of random is actually
the same in each Emacs run! For example, in one operating system, the
first call to (random) after you start Emacs always returns
−1457731, and the second one always returns −7692030. This
repeatability is helpful for debugging.
If you want random numbers that don't always come out the same, execute
(random t). This chooses a new seed based on the current time of
day and on Emacs's process ID number.
This function returns a pseudo-random integer. Repeated calls return a series of pseudo-random integers.
If limit is a positive integer, the value is chosen to be nonnegative and less than limit.
If limit is
t, it means to choose a new seed based on the current time of day and on Emacs's process ID number.On some machines, any integer representable in Lisp may be the result of
random. On other machines, the result can never be larger than a certain maximum or less than a certain (negative) minimum.
A string in Emacs Lisp is an array that contains an ordered sequence of characters. Strings are used as names of symbols, buffers, and files; to send messages to users; to hold text being copied between buffers; and for many other purposes. Because strings are so important, Emacs Lisp has many functions expressly for manipulating them. Emacs Lisp programs use strings more often than individual characters.
See Strings of Events, for special considerations for strings of keyboard character events.
Characters are represented in Emacs Lisp as integers; whether an integer is a character or not is determined only by how it is used. Thus, strings really contain integers. See Character Codes, for details about character representation in Emacs.
The length of a string (like any array) is fixed, and cannot be altered once the string exists. Strings in Lisp are not terminated by a distinguished character code. (By contrast, strings in C are terminated by a character with ASCII code 0.)
Since strings are arrays, and therefore sequences as well, you can
operate on them with the general array and sequence functions.
(See Sequences Arrays Vectors.) For example, you can access or
change individual characters in a string using the functions aref
and aset (see Array Functions).
There are two text representations for non-ASCII characters in Emacs strings (and in buffers): unibyte and multibyte (see Text Representations). For most Lisp programming, you don't need to be concerned with these two representations.
Sometimes key sequences are represented as unibyte strings. When a unibyte string is a key sequence, string elements in the range 128 to 255 represent meta characters (which are large integers) rather than character codes in the range 128 to 255. Strings cannot hold characters that have the hyper, super or alt modifiers; they can hold ASCII control characters, but no other control characters. They do not distinguish case in ASCII control characters. If you want to store such characters in a sequence, such as a key sequence, you must use a vector instead of a string. See Character Type, for more information about keyboard input characters.
Strings are useful for holding regular expressions. You can also
match regular expressions against strings with string-match
(see Regexp Search). The functions match-string
(see Simple Match Data) and replace-match (see Replacing Match) are useful for decomposing and modifying strings after
matching regular expressions against them.
Like a buffer, a string can contain text properties for the characters in it, as well as the characters themselves. See Text Properties. All the Lisp primitives that copy text from strings to buffers or other strings also copy the properties of the characters being copied.
See Text, for information about functions that display strings or copy them into buffers. See Character Type, and String Type, for information about the syntax of characters and strings. See Non-ASCII Characters, for functions to convert between text representations and to encode and decode character codes.
For more information about general sequence and array predicates, see Sequences Arrays Vectors, and Arrays.
This function returns
tif object is a string ornil. It returnsnilotherwise.
This function returns
tif object is a string or a character (i.e., an integer),nilotherwise.
The following functions create strings, either from scratch, or by putting strings together, or by taking them apart.
This function returns a string made up of count repetitions of character. If count is negative, an error is signaled.
(make-string 5 ?x) "xxxxx" (make-string 0 ?x) ""Other functions to compare with this one include
char-to-string(see String Conversion),make-vector(see Vectors), andmake-list(see Building Lists).
This returns a string containing the characters characters.
(string ?a ?b ?c) "abc"
This function returns a new string which consists of those characters from string in the range from (and including) the character at the index start up to (but excluding) the character at the index end. The first character is at index zero.
(substring "abcdefg" 0 3) "abc"In the above example, the index for ‘a’ is 0, the index for ‘b’ is 1, and the index for ‘c’ is 2. The index 3—which is the the fourth character in the string—marks the character position up to which the substring is copied. Thus, ‘abc’ is copied from the string
"abcdefg".A negative number counts from the end of the string, so that −1 signifies the index of the last character of the string. For example:
(substring "abcdefg" -3 -1) "ef"In this example, the index for ‘e’ is −3, the index for ‘f’ is −2, and the index for ‘g’ is −1. Therefore, ‘e’ and ‘f’ are included, and ‘g’ is excluded.
When
nilis used for end, it stands for the length of the string. Thus,(substring "abcdefg" -3 nil) "efg"Omitting the argument end is equivalent to specifying
nil. It follows that(substringstring0)returns a copy of all of string.(substring "abcdefg" 0) "abcdefg"But we recommend
copy-sequencefor this purpose (see Sequence Functions).If the characters copied from string have text properties, the properties are copied into the new string also. See Text Properties.
substringalso accepts a vector for the first argument. For example:(substring [a b (c) "d"] 1 3) [b (c)]A
wrong-type-argumenterror is signaled if start is not an integer or if end is neither an integer nornil. Anargs-out-of-rangeerror is signaled if start indicates a character following end, or if either integer is out of range for string.Contrast this function with
buffer-substring(see Buffer Contents), which returns a string containing a portion of the text in the current buffer. The beginning of a string is at index 0, but the beginning of a buffer is at index 1.
This works like
substringbut discards all text properties from the value. Also, start may be omitted ornil, which is equivalent to 0. Thus,(substring-no-propertiesstring)returns a copy of string, with all text properties removed.
This function returns a new string consisting of the characters in the arguments passed to it (along with their text properties, if any). The arguments may be strings, lists of numbers, or vectors of numbers; they are not themselves changed. If
concatreceives no arguments, it returns an empty string.(concat "abc" "-def") "abc-def" (concat "abc" (list 120 121) [122]) "abcxyz" ;;nilis an empty sequence. (concat "abc" nil "-def") "abc-def" (concat "The " "quick brown " "fox.") "The quick brown fox." (concat) ""This function always constructs a new string that is not
eqto any existing string, except when the result is the empty string (to save space, Emacs makes only one empty multibyte string).For information about other concatenation functions, see the description of
mapconcatin Mapping Functions,vconcatin Vector Functions, andappendin Building Lists. For concatenating individual command-line arguments into a string to be used as a shell command, see combine-and-quote-strings.
This function splits string into substrings based on the regular expression separators (see Regular Expressions). Each match for separators defines a splitting point; the substrings between splitting points are made into a list, which is returned.
If omit-nulls is
nil(or omitted), the result contains null strings whenever there are two consecutive matches for separators, or a match is adjacent to the beginning or end of string. If omit-nulls ist, these null strings are omitted from the result.If separators is
nil(or omitted), the default is the value ofsplit-string-default-separators.As a special case, when separators is
nil(or omitted), null strings are always omitted from the result. Thus:(split-string " two words ") ("two" "words")The result is not
("" "two" "words" ""), which would rarely be useful. If you need such a result, use an explicit value for separators:(split-string " two words " split-string-default-separators) ("" "two" "words" "")More examples:
(split-string "Soup is good food" "o") ("S" "up is g" "" "d f" "" "d") (split-string "Soup is good food" "o" t) ("S" "up is g" "d f" "d") (split-string "Soup is good food" "o+") ("S" "up is g" "d f" "d")Empty matches do count, except that
split-stringwill not look for a final empty match when it already reached the end of the string using a non-empty match or when string is empty:(split-string "aooob" "o*") ("" "a" "" "b" "") (split-string "ooaboo" "o*") ("" "" "a" "b" "") (split-string "" "") ("")However, when separators can match the empty string, omit-nulls is usually
t, so that the subtleties in the three previous examples are rarely relevant:(split-string "Soup is good food" "o*" t) ("S" "u" "p" " " "i" "s" " " "g" "d" " " "f" "d") (split-string "Nice doggy!" "" t) ("N" "i" "c" "e" " " "d" "o" "g" "g" "y" "!") (split-string "" "" t) nilSomewhat odd, but predictable, behavior can occur for certain “non-greedy” values of separators that can prefer empty matches over non-empty matches. Again, such values rarely occur in practice:
(split-string "ooo" "o*" t) nil (split-string "ooo" "\\|o+" t) ("o" "o" "o")If you need to split a string that is a shell command, where individual arguments could be quoted, see split-string-and-unquote.
The default value of separators for
split-string. Its usual value is"[ \f\t\n\r\v]+".
The most basic way to alter the contents of an existing string is with
aset (see Array Functions). (aset string
idx char) stores char into string at index
idx. Each character occupies one or more bytes, and if char
needs a different number of bytes from the character already present at
that index, aset signals an error.
A more powerful function is store-substring:
This function alters part of the contents of the string string, by storing obj starting at index idx. The argument obj may be either a character or a (smaller) string.
Since it is impossible to change the length of an existing string, it is an error if obj doesn't fit within string's actual length, or if any new character requires a different number of bytes from the character currently present at that point in string.
To clear out a string that contained a password, use
clear-string:
This makes string a unibyte string and clears its contents to zeros. It may also change string's length.
This function returns
tif the arguments represent the same character,nilotherwise. This function ignores differences in case ifcase-fold-searchis non-nil.(char-equal ?x ?x) t (let ((case-fold-search nil)) (char-equal ?x ?X)) nil
This function returns
tif the characters of the two strings match exactly. Symbols are also allowed as arguments, in which case their print names are used. Case is always significant, regardless ofcase-fold-search.(string= "abc" "abc") t (string= "abc" "ABC") nil (string= "ab" "ABC") nilThe function
string=ignores the text properties of the two strings. Whenequal(see Equality Predicates) compares two strings, it usesstring=.For technical reasons, a unibyte and a multibyte string are
equalif and only if they contain the same sequence of character codes and all these codes are either in the range 0 through 127 (ASCII) or 160 through 255 (eight-bit-graphic). However, when a unibyte string is converted to a multibyte string, all characters with codes in the range 160 through 255 are converted to characters with higher codes, whereas ASCII characters remain unchanged. Thus, a unibyte string and its conversion to multibyte are onlyequalif the string is all ASCII. Character codes 160 through 255 are not entirely proper in multibyte text, even though they can occur. As a consequence, the situation where a unibyte and a multibyte string areequalwithout both being all ASCII is a technical oddity that very few Emacs Lisp programmers ever get confronted with. See Text Representations.
This function compares two strings a character at a time. It scans both the strings at the same time to find the first pair of corresponding characters that do not match. If the lesser character of these two is the character from string1, then string1 is less, and this function returns
t. If the lesser character is the one from string2, then string1 is greater, and this function returnsnil. If the two strings match entirely, the value isnil.Pairs of characters are compared according to their character codes. Keep in mind that lower case letters have higher numeric values in the ASCII character set than their upper case counterparts; digits and many punctuation characters have a lower numeric value than upper case letters. An ASCII character is less than any non-ASCII character; a unibyte non-ASCII character is always less than any multibyte non-ASCII character (see Text Representations).
(string< "abc" "abd") t (string< "abd" "abc") nil (string< "123" "abc") tWhen the strings have different lengths, and they match up to the length of string1, then the result is
t. If they match up to the length of string2, the result isnil. A string of no characters is less than any other string.(string< "" "abc") t (string< "ab" "abc") t (string< "abc" "") nil (string< "abc" "ab") nil (string< "" "") nilSymbols are also allowed as arguments, in which case their print names are used.
This function compares the specified part of string1 with the specified part of string2. The specified part of string1 runs from index start1 up to index end1 (
nilmeans the end of the string). The specified part of string2 runs from index start2 up to index end2 (nilmeans the end of the string).The strings are both converted to multibyte for the comparison (see Text Representations) so that a unibyte string and its conversion to multibyte are always regarded as equal. If ignore-case is non-
nil, then case is ignored, so that upper case letters can be equal to lower case letters.If the specified portions of the two strings match, the value is
t. Otherwise, the value is an integer which indicates how many leading characters agree, and which string is less. Its absolute value is one plus the number of characters that agree at the beginning of the two strings. The sign is negative if string1 (or its specified portion) is less.
This function works like
assoc, except that key must be a string or symbol, and comparison is done usingcompare-strings. Symbols are converted to strings before testing. If case-fold is non-nil, it ignores case differences. Unlikeassoc, this function can also match elements of the alist that are strings or symbols rather than conses. In particular, alist can be a list of strings or symbols rather than an actual alist. See Association Lists.
See also the function compare-buffer-substrings in
Comparing Text, for a way to compare text in buffers. The
function string-match, which matches a regular expression
against a string, can be used for a kind of string comparison; see
Regexp Search.
This section describes functions for converting between characters,
strings and integers. format (see Formatting Strings) and
prin1-to-string (see Output Functions) can also convert
Lisp objects into strings. read-from-string (see Input Functions) can “convert” a string representation of a Lisp object
into an object. The functions string-make-multibyte and
string-make-unibyte convert the text representation of a string
(see Converting Representations).
See Documentation, for functions that produce textual descriptions
of text characters and general input events
(single-key-description and text-char-description). These
are used primarily for making help messages.
This function returns a new string containing one character, character. This function is semi-obsolete because the function
stringis more general. See Creating Strings.
This function returns the first character in string. If the string is empty, the function returns 0. The value is also 0 when the first character of string is the null character, ASCII code 0.
(string-to-char "ABC") 65 (string-to-char "xyz") 120 (string-to-char "") 0 (string-to-char "\000") 0This function may be eliminated in the future if it does not seem useful enough to retain.
This function returns a string consisting of the printed base-ten representation of number, which may be an integer or a floating point number. The returned value starts with a minus sign if the argument is negative.
(number-to-string 256) "256" (number-to-string -23) "-23" (number-to-string -23.5) "-23.5"
int-to-stringis a semi-obsolete alias for this function.See also the function
formatin Formatting Strings.
This function returns the numeric value of the characters in string. If base is non-
nil, it must be an integer between 2 and 16 (inclusive), and integers are converted in that base. If base isnil, then base ten is used. Floating point conversion only works in base ten; we have not implemented other radices for floating point numbers, because that would be much more work and does not seem useful. If string looks like an integer but its value is too large to fit into a Lisp integer,string-to-numberreturns a floating point result.The parsing skips spaces and tabs at the beginning of string, then reads as much of string as it can interpret as a number in the given base. (On some systems it ignores other whitespace at the beginning, not just spaces and tabs.) If the first character after the ignored whitespace is neither a digit in the given base, nor a plus or minus sign, nor the leading dot of a floating point number, this function returns 0.
(string-to-number "256") 256 (string-to-number "25 is a perfect square.") 25 (string-to-number "X256") 0 (string-to-number "-4.5") -4.5 (string-to-number "1e5") 100000.0
Here are some other functions that can convert to or from a string:
concatconcat can convert a vector or a list into a string.
See Creating Strings.
vconcatvconcat can convert a string into a vector. See Vector Functions.
appendappend can convert a string into a list. See Building Lists.
Formatting means constructing a string by substituting computed values at various places in a constant string. This constant string controls how the other values are printed, as well as where they appear; it is called a format string.
Formatting is often useful for computing messages to be displayed. In
fact, the functions message and error provide the same
formatting feature described here; they differ from format only
in how they use the result of formatting.
This function returns a new string that is made by copying string and then replacing any format specification in the copy with encodings of the corresponding objects. The arguments objects are the computed values to be formatted.
The characters in string, other than the format specifications, are copied directly into the output, including their text properties, if any.
A format specification is a sequence of characters beginning with a
‘%’. Thus, if there is a ‘%d’ in string, the
format function replaces it with the printed representation of
one of the values to be formatted (one of the arguments objects).
For example:
(format "The value of fill-column is %d." fill-column)
"The value of fill-column is 72."
Since format interprets ‘%’ characters as format
specifications, you should never pass an arbitrary string as
the first argument. This is particularly true when the string is
generated by some Lisp code. Unless the string is known to
never include any ‘%’ characters, pass "%s", described
below, as the first argument, and the string as the second, like this:
(format "%s" arbitrary-string)
If string contains more than one format specification, the format specifications correspond to successive values from objects. Thus, the first format specification in string uses the first such value, the second format specification uses the second such value, and so on. Any extra format specifications (those for which there are no corresponding values) cause an error. Any extra values to be formatted are ignored.
Certain format specifications require values of particular types. If you supply a value that doesn't fit the requirements, an error is signaled.
Here is a table of valid format specifications:
princ, not
prin1—see Output Functions). Thus, strings are represented
by their contents alone, with no ‘"’ characters, and symbols appear
without ‘\’ characters.
If the object is a string, its text properties are
copied into the output. The text properties of the ‘%s’ itself
are also copied, but those of the object take priority.
prin1—see Output Functions). Thus, strings are enclosed in ‘"’ characters, and
‘\’ characters appear where necessary before special characters.
(format "%% %d" 30) returns "% 30".
Any other format character results in an ‘Invalid format operation’ error.
Here are several examples:
(format "The name of this buffer is %s." (buffer-name))
"The name of this buffer is strings.texi."
(format "The buffer object prints as %s." (current-buffer))
"The buffer object prints as strings.texi."
(format "The octal value of %d is %o,
and the hex value is %x." 18 18 18)
"The octal value of 18 is 22,
and the hex value is 12."
A specification can have a width, which is a decimal number
between the ‘%’ and the specification character. If the printed
representation of the object contains fewer characters than this
width, format extends it with padding. The width specifier is
ignored for the ‘%%’ specification. Any padding introduced by
the width specifier normally consists of spaces inserted on the left:
(format "%5d is padded on the left with spaces" 123)
" 123 is padded on the left with spaces"
If the width is too small, format does not truncate the
object's printed representation. Thus, you can use a width to specify
a minimum spacing between columns with no risk of losing information.
In the following three examples, ‘%7s’ specifies a minimum width
of 7. In the first case, the string inserted in place of ‘%7s’
has only 3 letters, and needs 4 blank spaces as padding. In the
second case, the string "specification" is 13 letters wide but
is not truncated.
(format "The word `%7s' actually has %d letters in it."
"foo" (length "foo"))
"The word ` foo' actually has 3 letters in it."
(format "The word `%7s' actually has %d letters in it."
"specification" (length "specification"))
"The word `specification' actually has 13 letters in it."
Immediately after the ‘%’ and before the optional width specifier, you can also put certain flag characters.
The flag ‘+’ inserts a plus sign before a positive number, so that it always has a sign. A space character as flag inserts a space before a positive number. (Otherwise, positive numbers start with the first digit.) These flags are useful for ensuring that positive numbers and negative numbers use the same number of columns. They are ignored except for ‘%d’, ‘%e’, ‘%f’, ‘%g’, and if both flags are used, ‘+’ takes precedence.
The flag ‘#’ specifies an “alternate form” which depends on the format in use. For ‘%o’, it ensures that the result begins with a ‘0’. For ‘%x’ and ‘%X’, it prefixes the result with ‘0x’ or ‘0X’. For ‘%e’, ‘%f’, and ‘%g’, the ‘#’ flag means include a decimal point even if the precision is zero.
The flag ‘-’ causes the padding inserted by the width specifier, if any, to be inserted on the right rather than the left. The flag ‘0’ ensures that the padding consists of ‘0’ characters instead of spaces, inserted on the left. These flags are ignored for specification characters for which they do not make sense: ‘%s’, ‘%S’ and ‘%c’ accept the ‘0’ flag, but still pad with spaces on the left. If both ‘-’ and ‘0’ are present and valid, ‘-’ takes precedence.
(format "%06d is padded on the left with zeros" 123)
"000123 is padded on the left with zeros"
(format "%-6d is padded on the right" 123)
"123 is padded on the right"
(format "The word `%-7s' actually has %d letters in it."
"foo" (length "foo"))
"The word `foo ' actually has 3 letters in it."
All the specification characters allow an optional precision before the character (after the width, if present). The precision is a decimal-point ‘.’ followed by a digit-string. For the floating-point specifications (‘%e’, ‘%f’, ‘%g’), the precision specifies how many decimal places to show; if zero, the decimal-point itself is also omitted. For ‘%s’ and ‘%S’, the precision truncates the string to the given width, so ‘%.3s’ shows only the first three characters of the representation for object. Precision has no effect for other specification characters.
The character case functions change the case of single characters or of the contents of strings. The functions normally convert only alphabetic characters (the letters ‘A’ through ‘Z’ and ‘a’ through ‘z’, as well as non-ASCII letters); other characters are not altered. You can specify a different case conversion mapping by specifying a case table (see Case Tables).
These functions do not modify the strings that are passed to them as arguments.
The examples below use the characters ‘X’ and ‘x’ which have ASCII codes 88 and 120 respectively.
This function converts string-or-char, which should be either a character or a string, to lower case.
When string-or-char is a string, this function returns a new string in which each letter in the argument that is upper case is converted to lower case. When string-or-char is a character, this function returns the corresponding lower case character (an integer); if the original character is lower case, or is not a letter, the return value is equal to the original character.
(downcase "The cat in the hat") "the cat in the hat" (downcase ?X) 120
This function converts string-or-char, which should be either a character or a string, to upper case.
When string-or-char is a string, this function returns a new string in which each letter in the argument that is lower case is converted to upper case. When string-or-char is a character, this function returns the corresponding upper case character (an an integer); if the original character is upper case, or is not a letter, the return value is equal to the original character.
(upcase "The cat in the hat") "THE CAT IN THE HAT" (upcase ?x) 88
This function capitalizes strings or characters. If string-or-char is a string, the function returns a new string whose contents are a copy of string-or-char in which each word has been capitalized. This means that the first character of each word is converted to upper case, and the rest are converted to lower case.
The definition of a word is any sequence of consecutive characters that are assigned to the word constituent syntax class in the current syntax table (see Syntax Class Table).
When string-or-char is a character, this function does the same thing as
upcase.(capitalize "The cat in the hat") "The Cat In The Hat" (capitalize "THE 77TH-HATTED CAT") "The 77th-Hatted Cat" (capitalize ?x) 88
If string-or-char is a string, this function capitalizes the initials of the words in string-or-char, without altering any letters other than the initials. It returns a new string whose contents are a copy of string-or-char, in which each word has had its initial letter converted to upper case.
The definition of a word is any sequence of consecutive characters that are assigned to the word constituent syntax class in the current syntax table (see Syntax Class Table).
When the argument to
upcase-initialsis a character,upcase-initialshas the same result asupcase.(upcase-initials "The CAT in the hAt") "The CAT In The HAt"
See Text Comparison, for functions that compare strings; some of them ignore case differences, or can optionally ignore case differences.
You can customize case conversion by installing a special case table. A case table specifies the mapping between upper case and lower case letters. It affects both the case conversion functions for Lisp objects (see the previous section) and those that apply to text in the buffer (see Case Changes). Each buffer has a case table; there is also a standard case table which is used to initialize the case table of new buffers.
A case table is a char-table (see Char-Tables) whose subtype is
case-table. This char-table maps each character into the
corresponding lower case character. It has three extra slots, which
hold related tables:
In simple cases, all you need to specify is the mapping to lower-case; the three related tables will be calculated automatically from that one.
For some languages, upper and lower case letters are not in one-to-one correspondence. There may be two different lower case letters with the same upper case equivalent. In these cases, you need to specify the maps for both lower case and upper case.
The extra table canonicalize maps each character to a canonical equivalent; any two characters that are related by case-conversion have the same canonical equivalent character. For example, since ‘a’ and ‘A’ are related by case-conversion, they should have the same canonical equivalent character (which should be either ‘a’ for both of them, or ‘A’ for both of them).
The extra table equivalences is a map that cyclically permutes each equivalence class (of characters with the same canonical equivalent). (For ordinary ASCII, this would map ‘a’ into ‘A’ and ‘A’ into ‘a’, and likewise for each set of equivalent characters.)
When constructing a case table, you can provide nil for
canonicalize; then Emacs fills in this slot from the lower case
and upper case mappings. You can also provide nil for
equivalences; then Emacs fills in this slot from
canonicalize. In a case table that is actually in use, those
components are non-nil. Do not try to specify
equivalences without also specifying canonicalize.
Here are the functions for working with case tables:
This function makes table the standard case table, so that it will be used in any buffers created subsequently.
The
with-case-tablemacro saves the current case table, makes table the current case table, evaluates the body forms, and finally restores the case table. The return value is the value of the last form in body. The case table is restored even in case of an abnormal exit viathrowor error (see Nonlocal Exits).
Some language environments modify the case conversions of
ASCII characters; for example, in the Turkish language
environment, the ASCII character ‘I’ is downcased into
a Turkish “dotless i”. This can interfere with code that requires
ordinary ASCII case conversion, such as implementations of
ASCII-based network protocols. In that case, use the
with-case-table macro with the variable ascii-case-table,
which stores the unmodified case table for the ASCII
character set.
The case table for the ASCII character set. This should not be modified by any language environment settings.
The following three functions are convenient subroutines for packages that define non-ASCII character sets. They modify the specified case table case-table; they also modify the standard syntax table. See Syntax Tables. Normally you would use these functions to change the standard case table.
This function specifies a pair of corresponding letters, one upper case and one lower case.
This function makes characters l and r a matching pair of case-invariant delimiters.
This function makes char case-invariant, with syntax syntax.
This command displays a description of the contents of the current buffer's case table.
A list represents a sequence of zero or more elements (which may be any Lisp objects). The important difference between lists and vectors is that two or more lists can share part of their structure; in addition, you can insert or delete elements in a list without copying the whole list.
Lists in Lisp are not a primitive data type; they are built up from cons cells. A cons cell is a data object that represents an ordered pair. That is, it has two slots, and each slot holds, or refers to, some Lisp object. One slot is known as the car, and the other is known as the cdr. (These names are traditional; see Cons Cell Type.) cdr is pronounced “could-er.”
We say that “the car of this cons cell is” whatever object its car slot currently holds, and likewise for the cdr.
A list is a series of cons cells “chained together,” so that each
cell refers to the next one. There is one cons cell for each element of
the list. By convention, the cars of the cons cells hold the
elements of the list, and the cdrs are used to chain the list: the
cdr slot of each cons cell refers to the following cons cell. The
cdr of the last cons cell is nil. This asymmetry between
the car and the cdr is entirely a matter of convention; at the
level of cons cells, the car and cdr slots have the same
characteristics.
Since nil is the conventional value to put in the cdr of
the last cons cell in the list, we call that case a true list.
In Lisp, we consider the symbol nil a list as well as a
symbol; it is the list with no elements. For convenience, the symbol
nil is considered to have nil as its cdr (and also
as its car). Therefore, the cdr of a true list is always a
true list.
If the cdr of a list's last cons cell is some other value,
neither nil nor another cons cell, we call the structure a
dotted list, since its printed representation would use
‘.’. There is one other possibility: some cons cell's cdr
could point to one of the previous cons cells in the list. We call
that structure a circular list.
For some purposes, it does not matter whether a list is true, circular or dotted. If the program doesn't look far enough down the list to see the cdr of the final cons cell, it won't care. However, some functions that operate on lists demand true lists and signal errors if given a dotted list. Most functions that try to find the end of a list enter infinite loops if given a circular list.
Because most cons cells are used as part of lists, the phrase list structure has come to mean any structure made out of cons cells.
The cdr of any nonempty true list l is a list containing all the elements of l except the first.
See Cons Cell Type, for the read and print syntax of cons cells and lists, and for “box and arrow” illustrations of lists.
The following predicates test whether a Lisp object is an atom,
whether it is a cons cell or is a list, or whether it is the
distinguished object nil. (Many of these predicates can be
defined in terms of the others, but they are used so often that it is
worth having all of them.)
This function returns
tif object is a cons cell,nilotherwise.nilis not a cons cell, although it is a list.
This function returns
tif object is an atom,nilotherwise. All objects except cons cells are atoms. The symbolnilis an atom and is also a list; it is the only Lisp object that is both.(atom object) == (not (consp object))
This function returns
tif object is a cons cell ornil. Otherwise, it returnsnil.(listp '(1)) t (listp '()) t
This function is the opposite of
listp: it returnstif object is not a list. Otherwise, it returnsnil.(listp object) == (not (nlistp object))
This function returns
tif object isnil, and returnsnilotherwise. This function is identical tonot, but as a matter of clarity we usenullwhen object is considered a list andnotwhen it is considered a truth value (seenotin Combining Conditions).(null '(1)) nil (null '()) t
This function returns the value referred to by the first slot of the cons cell cons-cell. In other words, it returns the car of cons-cell.
As a special case, if cons-cell is
nil, this function returnsnil. Therefore, any list is a valid argument. An error is signaled if the argument is not a cons cell ornil.(car '(a b c)) a (car '()) nil
This function returns the value referred to by the second slot of the cons cell cons-cell. In other words, it returns the cdr of cons-cell.
As a special case, if cons-cell is
nil, this function returnsnil; therefore, any list is a valid argument. An error is signaled if the argument is not a cons cell ornil.(cdr '(a b c)) (b c) (cdr '()) nil
This function lets you take the car of a cons cell while avoiding errors for other data types. It returns the car of object if object is a cons cell,
nilotherwise. This is in contrast tocar, which signals an error if object is not a list.(car-safe object) == (let ((x object)) (if (consp x) (car x) nil))
This function lets you take the cdr of a cons cell while avoiding errors for other data types. It returns the cdr of object if object is a cons cell,
nilotherwise. This is in contrast tocdr, which signals an error if object is not a list.(cdr-safe object) == (let ((x object)) (if (consp x) (cdr x) nil))
This macro is a way of examining the car of a list, and taking it off the list, all at once.
It operates on the list which is stored in the symbol listname. It removes this element from the list by setting listname to the cdr of its old value—but it also returns the car of that list, which is the element being removed.
x (a b c) (pop x) a x (b c)
This function returns the nth element of list. Elements are numbered starting with zero, so the car of list is element number zero. If the length of list is n or less, the value is
nil.If n is negative,
nthreturns the first element of list.(nth 2 '(1 2 3 4)) 3 (nth 10 '(1 2 3 4)) nil (nth -3 '(1 2 3 4)) 1 (nth n x) == (car (nthcdr n x))The function
eltis similar, but applies to any kind of sequence. For historical reasons, it takes its arguments in the opposite order. See Sequence Functions.
This function returns the nth cdr of list. In other words, it skips past the first n links of list and returns what follows.
If n is zero or negative,
nthcdrreturns all of list. If the length of list is n or less,nthcdrreturnsnil.(nthcdr 1 '(1 2 3 4)) (2 3 4) (nthcdr 10 '(1 2 3 4)) nil (nthcdr -3 '(1 2 3 4)) (1 2 3 4)
This function returns the last link of list. The
carof this link is the list's last element. If list is null,nilis returned. If n is non-nil, the nth-to-last link is returned instead, or the whole of list if n is bigger than list's length.
This function returns the length of list, with no risk of either an error or an infinite loop. It generally returns the number of distinct cons cells in the list. However, for circular lists, the value is just an upper bound; it is often too large.
If list is not
nilor a cons cell,safe-lengthreturns 0.
The most common way to compute the length of a list, when you are not
worried that it may be circular, is with length. See Sequence Functions.
This function returns the list x with the last element, or the last n elements, removed. If n is greater than zero it makes a copy of the list so as not to damage the original list. In general,
(append (butlastx n) (lastx n))will return a list equal to x.
This is a version of
butlastthat works by destructively modifying thecdrof the appropriate element, rather than making a copy of the list.
Many functions build lists, as lists reside at the very heart of Lisp.
cons is the fundamental list-building function; however, it is
interesting to note that list is used more times in the source
code for Emacs than cons.
This function is the most basic function for building new list structure. It creates a new cons cell, making object1 the car, and object2 the cdr. It then returns the new cons cell. The arguments object1 and object2 may be any Lisp objects, but most often object2 is a list.
(cons 1 '(2)) (1 2) (cons 1 '()) (1) (cons 1 2) (1 . 2)
consis often used to add a single element to the front of a list. This is called consing the element onto the list. 1 For example:(setq list (cons newelt list))Note that there is no conflict between the variable named
listused in this example and the function namedlistdescribed below; any symbol can serve both purposes.
This function creates a list with objects as its elements. The resulting list is always
nil-terminated. If no objects are given, the empty list is returned.(list 1 2 3 4 5) (1 2 3 4 5) (list 1 2 '(3 4 5) 'foo) (1 2 (3 4 5) foo) (list) nil
This function creates a list of length elements, in which each element is object. Compare
make-listwithmake-string(see Creating Strings).(make-list 3 'pigs) (pigs pigs pigs) (make-list 0 'pigs) nil (setq l (make-list 3 '(a b)) ((a b) (a b) (a b)) (eq (car l) (cadr l)) t
This function returns a list containing all the elements of sequences. The sequences may be lists, vectors, bool-vectors, or strings, but the last one should usually be a list. All arguments except the last one are copied, so none of the arguments is altered. (See
nconcin Rearrangement, for a way to join lists with no copying.)More generally, the final argument to
appendmay be any Lisp object. The final argument is not copied or converted; it becomes the cdr of the last cons cell in the new list. If the final argument is itself a list, then its elements become in effect elements of the result list. If the final element is not a list, the result is a dotted list since its final cdr is notnilas required in a true list.
Here is an example of using append:
(setq trees '(pine oak))
(pine oak)
(setq more-trees (append '(maple birch) trees))
(maple birch pine oak)
trees
(pine oak)
more-trees
(maple birch pine oak)
(eq trees (cdr (cdr more-trees)))
t
You can see how append works by looking at a box diagram. The
variable trees is set to the list (pine oak) and then the
variable more-trees is set to the list (maple birch pine
oak). However, the variable trees continues to refer to the
original list:
more-trees trees
| |
| --- --- --- --- -> --- --- --- ---
--> | | |--> | | |--> | | |--> | | |--> nil
--- --- --- --- --- --- --- ---
| | | |
| | | |
--> maple -->birch --> pine --> oak
An empty sequence contributes nothing to the value returned by
append. As a consequence of this, a final nil argument
forces a copy of the previous argument:
trees
(pine oak)
(setq wood (append trees nil))
(pine oak)
wood
(pine oak)
(eq wood trees)
nil
This once was the usual way to copy a list, before the function
copy-sequence was invented. See Sequences Arrays Vectors.
Here we show the use of vectors and strings as arguments to append:
(append [a b] "cd" nil)
(a b 99 100)
With the help of apply (see Calling Functions), we can append
all the lists in a list of lists:
(apply 'append '((a b c) nil (x y z) nil))
(a b c x y z)
If no sequences are given, nil is returned:
(append)
nil
Here are some examples where the final argument is not a list:
(append '(x y) 'z)
(x y . z)
(append '(x y) [z])
(x y . [z])
The second example shows that when the final argument is a sequence but not a list, the sequence's elements do not become elements of the resulting list. Instead, the sequence becomes the final cdr, like any other non-list final argument.
This function creates a new list whose elements are the elements of list, but in reverse order. The original argument list is not altered.
(setq x '(1 2 3 4)) (1 2 3 4) (reverse x) (4 3 2 1) x (1 2 3 4)
This function returns a copy of the tree
tree. If tree is a cons cell, this makes a new cons cell with the same car and cdr, then recursively copies the car and cdr in the same way.Normally, when tree is anything other than a cons cell,
copy-treesimply returns tree. However, if vecp is non-nil, it copies vectors too (and operates recursively on their elements).
This returns a list of numbers starting with from and incrementing by separation, and ending at or just before to. separation can be positive or negative and defaults to 1. If to is
nilor numerically equal to from, the value is the one-element list(from). If to is less than from with a positive separation, or greater than from with a negative separation, the value isnilbecause those arguments specify an empty sequence.If separation is 0 and to is neither
nilnor numerically equal to from,number-sequencesignals an error, since those arguments specify an infinite sequence.All arguments can be integers or floating point numbers. However, floating point arguments can be tricky, because floating point arithmetic is inexact. For instance, depending on the machine, it may quite well happen that
(number-sequence 0.4 0.6 0.2)returns the one element list(0.4), whereas(number-sequence 0.4 0.8 0.2)returns a list with three elements. The nth element of the list is computed by the exact formula(+from(*n separation)). Thus, if one wants to make sure that to is included in the list, one can pass an expression of this exact type for to. Alternatively, one can replace to with a slightly larger value (or a slightly more negative value if separation is negative).Some examples:
(number-sequence 4 9) (4 5 6 7 8 9) (number-sequence 9 4 -1) (9 8 7 6 5 4) (number-sequence 9 4 -2) (9 7 5) (number-sequence 8) (8) (number-sequence 8 5) nil (number-sequence 5 8 -1) nil (number-sequence 1.5 6 2) (1.5 3.5 5.5)
These functions, and one macro, provide convenient ways to modify a list which is stored in a variable.
This macro provides an alternative way to write
(setqlistname(consnewelt listname)).(setq l '(a b)) (a b) (push 'c l) (c a b) l (c a b)
Two functions modify lists that are the values of variables.
This function sets the variable symbol by consing element onto the old value, if element is not already a member of that value. It returns the resulting list, whether updated or not. The value of symbol had better be a list already before the call.
add-to-listuses compare-fn to compare element against existing list members; if compare-fn isnil, it usesequal.Normally, if element is added, it is added to the front of symbol, but if the optional argument append is non-
nil, it is added at the end.The argument symbol is not implicitly quoted;
add-to-listis an ordinary function, likesetand unlikesetq. Quote the argument yourself if that is what you want.
Here's a scenario showing how to use add-to-list:
(setq foo '(a b))
(a b)
(add-to-list 'foo 'c) ;; Add c.
(c a b)
(add-to-list 'foo 'b) ;; No effect.
(c a b)
foo ;; foo was changed.
(c a b)
An equivalent expression for (add-to-list 'var
value) is this:
(or (member value var)
(setq var (cons value var)))
This function sets the variable symbol by inserting element into the old value, which must be a list, at the position specified by order. If element is already a member of the list, its position in the list is adjusted according to order. Membership is tested using
eq. This function returns the resulting list, whether updated or not.The order is typically a number (integer or float), and the elements of the list are sorted in non-decreasing numerical order.
order may also be omitted or
nil. Then the numeric order of element stays unchanged if it already has one; otherwise, element has no numeric order. Elements without a numeric list order are placed at the end of the list, in no particular order.Any other value for order removes the numeric order of element if it already has one; otherwise, it is equivalent to
nil.The argument symbol is not implicitly quoted;
add-to-ordered-listis an ordinary function, likesetand unlikesetq. Quote the argument yourself if that is what you want.The ordering information is stored in a hash table on symbol's
list-orderproperty.
Here's a scenario showing how to use add-to-ordered-list:
(setq foo '())
nil
(add-to-ordered-list 'foo 'a 1) ;; Add a.
(a)
(add-to-ordered-list 'foo 'c 3) ;; Add c.
(a c)
(add-to-ordered-list 'foo 'b 2) ;; Add b.
(a b c)
(add-to-ordered-list 'foo 'b 4) ;; Move b.
(a c b)
(add-to-ordered-list 'foo 'd) ;; Append d.
(a c b d)
(add-to-ordered-list 'foo 'e) ;; Add e.
(a c b e d)
foo ;; foo was changed.
(a c b e d)
You can modify the car and cdr contents of a cons cell with the
primitives setcar and setcdr. We call these “destructive”
operations because they change existing list structure.
Common Lisp note: Common Lisp uses functionsrplacaandrplacdto alter list structure; they change structure the same way assetcarandsetcdr, but the Common Lisp functions return the cons cell whilesetcarandsetcdrreturn the new car or cdr.
setcarChanging the car of a cons cell is done with setcar. When
used on a list, setcar replaces one element of a list with a
different element.
This function stores object as the new car of cons, replacing its previous car. In other words, it changes the car slot of cons to refer to object. It returns the value object. For example:
(setq x '(1 2)) (1 2) (setcar x 4) 4 x (4 2)
When a cons cell is part of the shared structure of several lists, storing a new car into the cons changes one element of each of these lists. Here is an example:
;; Create two lists that are partly shared. (setq x1 '(a b c)) (a b c) (setq x2 (cons 'z (cdr x1))) (z b c) ;; Replace the car of a shared link. (setcar (cdr x1) 'foo) foo x1 ; Both lists are changed. (a foo c) x2 (z foo c) ;; Replace the car of a link that is not shared. (setcar x1 'baz) baz x1 ; Only one list is changed. (baz foo c) x2 (z foo c)
Here is a graphical depiction of the shared structure of the two lists
in the variables x1 and x2, showing why replacing b
changes them both:
--- --- --- --- --- ---
x1---> | | |----> | | |--> | | |--> nil
--- --- --- --- --- ---
| --> | |
| | | |
--> a | --> b --> c
|
--- --- |
x2--> | | |--
--- ---
|
|
--> z
Here is an alternative form of box diagram, showing the same relationship:
x1:
-------------- -------------- --------------
| car | cdr | | car | cdr | | car | cdr |
| a | o------->| b | o------->| c | nil |
| | | -->| | | | | |
-------------- | -------------- --------------
|
x2: |
-------------- |
| car | cdr | |
| z | o----
| | |
--------------
The lowest-level primitive for modifying a cdr is setcdr:
This function stores object as the new cdr of cons, replacing its previous cdr. In other words, it changes the cdr slot of cons to refer to object. It returns the value object.
Here is an example of replacing the cdr of a list with a different list. All but the first element of the list are removed in favor of a different sequence of elements. The first element is unchanged, because it resides in the car of the list, and is not reached via the cdr.
(setq x '(1 2 3))
(1 2 3)
(setcdr x '(4))
(4)
x
(1 4)
You can delete elements from the middle of a list by altering the
cdrs of the cons cells in the list. For example, here we delete
the second element, b, from the list (a b c), by changing
the cdr of the first cons cell:
(setq x1 '(a b c))
(a b c)
(setcdr x1 (cdr (cdr x1)))
(c)
x1
(a c)
Here is the result in box notation:
--------------------
| |
-------------- | -------------- | --------------
| car | cdr | | | car | cdr | -->| car | cdr |
| a | o----- | b | o-------->| c | nil |
| | | | | | | | |
-------------- -------------- --------------
The second cons cell, which previously held the element b, still
exists and its car is still b, but it no longer forms part
of this list.
It is equally easy to insert a new element by changing cdrs:
(setq x1 '(a b c))
(a b c)
(setcdr x1 (cons 'd (cdr x1)))
(d b c)
x1
(a d b c)
Here is this result in box notation:
-------------- ------------- -------------
| car | cdr | | car | cdr | | car | cdr |
| a | o | -->| b | o------->| c | nil |
| | | | | | | | | | |
--------- | -- | ------------- -------------
| |
----- --------
| |
| --------------- |
| | car | cdr | |
-->| d | o------
| | |
---------------
Here are some functions that rearrange lists “destructively” by modifying the cdrs of their component cons cells. We call these functions “destructive” because they chew up the original lists passed to them as arguments, relinking their cons cells to form a new list that is the returned value.
See delq, in Sets And Lists, for another function
that modifies cons cells.
This function returns a list containing all the elements of lists. Unlike
append(see Building Lists), the lists are not copied. Instead, the last cdr of each of the lists is changed to refer to the following list. The last of the lists is not altered. For example:(setq x '(1 2 3)) (1 2 3) (nconc x '(4 5)) (1 2 3 4 5) x (1 2 3 4 5)Since the last argument of
nconcis not itself modified, it is reasonable to use a constant list, such as'(4 5), as in the above example. For the same reason, the last argument need not be a list:(setq x '(1 2 3)) (1 2 3) (nconc x 'z) (1 2 3 . z) x (1 2 3 . z)However, the other arguments (all but the last) must be lists.
A common pitfall is to use a quoted constant list as a non-last argument to
nconc. If you do this, your program will change each time you run it! Here is what happens:(defun add-foo (x) ; We want this function to add (nconc '(foo) x)) ;footo the front of its arg. (symbol-function 'add-foo) (lambda (x) (nconc (quote (foo)) x)) (setq xx (add-foo '(1 2))) ; It seems to work. (foo 1 2) (setq xy (add-foo '(3 4))) ; What happened? (foo 1 2 3 4) (eq xx xy) t (symbol-function 'add-foo) (lambda (x) (nconc (quote (foo 1 2 3 4) x)))
This function reverses the order of the elements of list. Unlike
reverse,nreversealters its argument by reversing the cdrs in the cons cells forming the list. The cons cell that used to be the last one in list becomes the first cons cell of the value.For example:
(setq x '(a b c)) (a b c) x (a b c) (nreverse x) (c b a) ;; The cons cell that was first is now last. x (a)To avoid confusion, we usually store the result of
nreverseback in the same variable which held the original list:(setq x (nreverse x))Here is the
nreverseof our favorite example,(a b c), presented graphically:Original list head: Reversed list: ------------- ------------- ------------ | car | cdr | | car | cdr | | car | cdr | | a | nil |<-- | b | o |<-- | c | o | | | | | | | | | | | | | | ------------- | --------- | - | -------- | - | | | | ------------- ------------
This function sorts list stably, though destructively, and returns the sorted list. It compares elements using predicate. A stable sort is one in which elements with equal sort keys maintain their relative order before and after the sort. Stability is important when successive sorts are used to order elements according to different criteria.
The argument predicate must be a function that accepts two arguments. It is called with two elements of list. To get an increasing order sort, the predicate should return non-
nilif the first element is “less than” the second, ornilif not.The comparison function predicate must give reliable results for any given pair of arguments, at least within a single call to
sort. It must be antisymmetric; that is, if a is less than b, b must not be less than a. It must be transitive—that is, if a is less than b, and b is less than c, then a must be less than c. If you use a comparison function which does not meet these requirements, the result ofsortis unpredictable.The destructive aspect of
sortis that it rearranges the cons cells forming list by changing cdrs. A nondestructive sort function would create new cons cells to store the elements in their sorted order. If you wish to make a sorted copy without destroying the original, copy it first withcopy-sequenceand then sort.Sorting does not change the cars of the cons cells in list; the cons cell that originally contained the element
ain list still hasain its car after sorting, but it now appears in a different position in the list due to the change of cdrs. For example:(setq nums '(1 3 2 6 5 4 0)) (1 3 2 6 5 4 0) (sort nums '<) (0 1 2 3 4 5 6) nums (1 2 3 4 5 6)Warning: Note that the list in
numsno longer contains 0; this is the same cons cell that it was before, but it is no longer the first one in the list. Don't assume a variable that formerly held the argument now holds the entire sorted list! Instead, save the result ofsortand use that. Most often we store the result back into the variable that held the original list:(setq nums (sort nums '<))See Sorting, for more functions that perform sorting. See
documentationin Accessing Documentation, for a useful example ofsort.
A list can represent an unordered mathematical set—simply consider a
value an element of a set if it appears in the list, and ignore the
order of the list. To form the union of two sets, use append (as
long as you don't mind having duplicate elements). You can remove
equal duplicates using delete-dups. Other useful
functions for sets include memq and delq, and their
equal versions, member and delete.
Common Lisp note: Common Lisp has functionsunion(which avoids duplicate elements) andintersectionfor set operations, but GNU Emacs Lisp does not have them. You can write them in Lisp if you wish.
This function tests to see whether object is a member of list. If it is,
memqreturns a list starting with the first occurrence of object. Otherwise, it returnsnil. The letter ‘q’ inmemqsays that it useseqto compare object against the elements of the list. For example:(memq 'b '(a b c b a)) (b c b a) (memq '(2) '((1) (2))) ;(2)and(2)are noteq. nil
This function destructively removes all elements
eqto object from list. The letter ‘q’ indelqsays that it useseqto compare object against the elements of the list, likememqandremq.
When delq deletes elements from the front of the list, it does so
simply by advancing down the list and returning a sublist that starts
after those elements:
(delq 'a '(a b c)) == (cdr '(a b c))
When an element to be deleted appears in the middle of the list, removing it involves changing the cdrs (see Setcdr).
(setq sample-list '(a b c (4)))
(a b c (4))
(delq 'a sample-list)
(b c (4))
sample-list
(a b c (4))
(delq 'c sample-list)
(a b (4))
sample-list
(a b (4))
Note that (delq 'c sample-list) modifies sample-list to
splice out the third element, but (delq 'a sample-list) does not
splice anything—it just returns a shorter list. Don't assume that a
variable which formerly held the argument list now has fewer
elements, or that it still holds the original list! Instead, save the
result of delq and use that. Most often we store the result back
into the variable that held the original list:
(setq flowers (delq 'rose flowers))
In the following example, the (4) that delq attempts to match
and the (4) in the sample-list are not eq:
(delq '(4) sample-list)
(a c (4))
If you want to delete elements that are equal to a given value,
use delete (see below).
This function returns a copy of list, with all elements removed which are
eqto object. The letter ‘q’ inremqsays that it useseqto compare object against the elements oflist.(setq sample-list '(a b c a b c)) (a b c a b c) (remq 'a sample-list) (b c b c) sample-list (a b c a b c)
The function
memqltests to see whether object is a member of list, comparing members with object usingeql, so floating point elements are compared by value. If object is a member,memqlreturns a list starting with its first occurrence in list. Otherwise, it returnsnil.Compare this with
memq:(memql 1.2 '(1.1 1.2 1.3)) ;1.2and1.2areeql. (1.2 1.3) (memq 1.2 '(1.1 1.2 1.3)) ;1.2and1.2are noteq. nil
The following three functions are like memq, delq and
remq, but use equal rather than eq to compare
elements. See Equality Predicates.
The function
membertests to see whether object is a member of list, comparing members with object usingequal. If object is a member,memberreturns a list starting with its first occurrence in list. Otherwise, it returnsnil.Compare this with
memq:(member '(2) '((1) (2))) ;(2)and(2)areequal. ((2)) (memq '(2) '((1) (2))) ;(2)and(2)are noteq. nil ;; Two strings with the same contents areequal. (member "foo" '("foo" "bar")) ("foo" "bar")
If
sequenceis a list, this function destructively removes all elementsequalto object from sequence. For lists,deleteis todelqasmemberis tomemq: it usesequalto compare elements with object, likemember; when it finds an element that matches, it cuts the element out just asdelqwould.If
sequenceis a vector or string,deletereturns a copy ofsequencewith all elementsequaltoobjectremoved.For example:
(setq l '((2) (1) (2))) (delete '(2) l) ((1)) l ((2) (1)) ;; If you want to changelreliably, ;; write(setq l (delete elt l)). (setq l '((2) (1) (2))) (delete '(1) l) ((2) (2)) l ((2) (2)) ;; In this case, it makes no difference whether you setl, ;; but you should do so for the sake of the other case. (delete '(2) [(2) (1) (2)]) [(1)]
This function is the non-destructive counterpart of
delete. It returns a copy ofsequence, a list, vector, or string, with elementsequaltoobjectremoved. For example:(remove '(2) '((2) (1) (2))) ((1)) (remove '(2) [(2) (1) (2)]) [(1)]
Common Lisp note: The functionsmember,deleteandremovein GNU Emacs Lisp are derived from Maclisp, not Common Lisp. The Common Lisp versions do not useequalto compare elements.
This function is like
member, except that object should be a string and that it ignores differences in letter-case and text representation: upper-case and lower-case letters are treated as equal, and unibyte strings are converted to multibyte prior to comparison.
This function destructively removes all
equalduplicates from list, stores the result in list and returns it. Of severalequaloccurrences of an element in list,delete-dupskeeps the first one.
See also the function add-to-list, in List Variables,
for a way to add an element to a list stored in a variable and used as a
set.
An association list, or alist for short, records a mapping from keys to values. It is a list of cons cells called associations: the car of each cons cell is the key, and the cdr is the associated value.2
Here is an example of an alist. The key pine is associated with
the value cones; the key oak is associated with
acorns; and the key maple is associated with seeds.
((pine . cones)
(oak . acorns)
(maple . seeds))
Both the values and the keys in an alist may be any Lisp objects.
For example, in the following alist, the symbol a is
associated with the number 1, and the string "b" is
associated with the list (2 3), which is the cdr of
the alist element:
((a . 1) ("b" 2 3))
Sometimes it is better to design an alist to store the associated value in the car of the cdr of the element. Here is an example of such an alist:
((rose red) (lily white) (buttercup yellow))
Here we regard red as the value associated with rose. One
advantage of this kind of alist is that you can store other related
information—even a list of other items—in the cdr of the
cdr. One disadvantage is that you cannot use rassq (see
below) to find the element containing a given value. When neither of
these considerations is important, the choice is a matter of taste, as
long as you are consistent about it for any given alist.
The same alist shown above could be regarded as having the
associated value in the cdr of the element; the value associated
with rose would be the list (red).
Association lists are often used to record information that you might otherwise keep on a stack, since new associations may be added easily to the front of the list. When searching an association list for an association with a given key, the first one found is returned, if there is more than one.
In Emacs Lisp, it is not an error if an element of an association list is not a cons cell. The alist search functions simply ignore such elements. Many other versions of Lisp signal errors in such cases.
Note that property lists are similar to association lists in several respects. A property list behaves like an association list in which each key can occur only once. See Property Lists, for a comparison of property lists and association lists.
This function returns the first association for key in alist, comparing key against the alist elements using
equal(see Equality Predicates). It returnsnilif no association in alist has a carequalto key. For example:(setq trees '((pine . cones) (oak . acorns) (maple . seeds))) ((pine . cones) (oak . acorns) (maple . seeds)) (assoc 'oak trees) (oak . acorns) (cdr (assoc 'oak trees)) acorns (assoc 'birch trees) nilHere is another example, in which the keys and values are not symbols:
(setq needles-per-cluster '((2 "Austrian Pine" "Red Pine") (3 "Pitch Pine") (5 "White Pine"))) (cdr (assoc 3 needles-per-cluster)) ("Pitch Pine") (cdr (assoc 2 needles-per-cluster)) ("Austrian Pine" "Red Pine")
The function assoc-string is much like assoc except
that it ignores certain differences between strings. See Text Comparison.
This function returns the first association with value value in alist. It returns
nilif no association in alist has a cdrequalto value.
rassocis likeassocexcept that it compares the cdr of each alist association instead of the car. You can think of this as “reverseassoc,” finding the key for a given value.
This function is like
associn that it returns the first association for key in alist, but it makes the comparison usingeqinstead ofequal.assqreturnsnilif no association in alist has a careqto key. This function is used more often thanassoc, sinceeqis faster thanequaland most alists use symbols as keys. See Equality Predicates.(setq trees '((pine . cones) (oak . acorns) (maple . seeds))) ((pine . cones) (oak . acorns) (maple . seeds)) (assq 'pine trees) (pine . cones)On the other hand,
assqis not usually useful in alists where the keys may not be symbols:(setq leaves '(("simple leaves" . oak) ("compound leaves" . horsechestnut))) (assq "simple leaves" leaves) nil (assoc "simple leaves" leaves) ("simple leaves" . oak)
This function returns the first association with value value in alist. It returns
nilif no association in alist has a cdreqto value.
rassqis likeassqexcept that it compares the cdr of each alist association instead of the car. You can think of this as “reverseassq,” finding the key for a given value.For example:
(setq trees '((pine . cones) (oak . acorns) (maple . seeds))) (rassq 'acorns trees) (oak . acorns) (rassq 'spores trees) nil
rassqcannot search for a value stored in the car of the cdr of an element:(setq colors '((rose red) (lily white) (buttercup yellow))) (rassq 'white colors) nilIn this case, the cdr of the association
(lily white)is not the symbolwhite, but rather the list(white). This becomes clearer if the association is written in dotted pair notation:(lily white) == (lily . (white))
This function searches alist for a match for key. For each element of alist, it compares the element (if it is an atom) or the element's car (if it is a cons) against key, by calling test with two arguments: the element or its car, and key. The arguments are passed in that order so that you can get useful results using
string-matchwith an alist that contains regular expressions (see Regexp Search). If test is omitted ornil,equalis used for comparison.If an alist element matches key by this criterion, then
assoc-defaultreturns a value based on this element. If the element is a cons, then the value is the element's cdr. Otherwise, the return value is default.If no alist element matches key,
assoc-defaultreturnsnil.
This function returns a two-level deep copy of alist: it creates a new copy of each association, so that you can alter the associations of the new alist without changing the old one.
(setq needles-per-cluster '((2 . ("Austrian Pine" "Red Pine")) (3 . ("Pitch Pine")) (5 . ("White Pine")))) ((2 "Austrian Pine" "Red Pine") (3 "Pitch Pine") (5 "White Pine")) (setq copy (copy-alist needles-per-cluster)) ((2 "Austrian Pine" "Red Pine") (3 "Pitch Pine") (5 "White Pine")) (eq needles-per-cluster copy) nil (equal needles-per-cluster copy) t (eq (car needles-per-cluster) (car copy)) nil (cdr (car (cdr needles-per-cluster))) ("Pitch Pine") (eq (cdr (car (cdr needles-per-cluster))) (cdr (car (cdr copy)))) tThis example shows how
copy-alistmakes it possible to change the associations of one copy without affecting the other:(setcdr (assq 3 copy) '("Martian Vacuum Pine")) (cdr (assq 3 needles-per-cluster)) ("Pitch Pine")
This function deletes from alist all the elements whose car is
eqto key, much as if you useddelqto delete each such element one by one. It returns the shortened alist, and often modifies the original list structure of alist. For correct results, use the return value ofassq-delete-allrather than looking at the saved value of alist.(setq alist '((foo 1) (bar 2) (foo 3) (lose 4))) ((foo 1) (bar 2) (foo 3) (lose 4)) (assq-delete-all 'foo alist) ((bar 2) (lose 4)) alist ((foo 1) (bar 2) (lose 4))
This function deletes from alist all the elements whose cdr is
eqto value. It returns the shortened alist, and often modifies the original list structure of alist.rassq-delete-allis likeassq-delete-allexcept that it compares the cdr of each alist association instead of the car.
This section describes functions for operating on rings. A ring is a fixed-size data structure that supports insertion, deletion, rotation, and modulo-indexed reference and traversal.
This returns a new ring capable of holding size objects. size should be an integer.
This returns the number of objects that ring currently contains. The value will never exceed that returned by
ring-size.
This returns a new ring which is a copy of ring. The new ring contains the same (
eq) objects as ring.
The newest element in the ring always has index 0. Higher indices correspond to older elements. Indices are computed modulo the ring length. Index −1 corresponds to the oldest element, −2 to the next-oldest, and so forth.
This returns the object in ring found at index index. index may be negative or greater than the ring length. If ring is empty,
ring-refsignals an error.
This inserts object into ring, making it the newest element, and returns object.
If the ring is full, insertion removes the oldest element to make room for the new element.
Remove an object from ring, and return that object. The argument index specifies which item to remove; if it is
nil, that means to remove the oldest item. If ring is empty,ring-removesignals an error.
This inserts object into ring, treating it as the oldest element. The return value is not significant.
If the ring is full, this function removes the newest element to make room for the inserted element.
If you are careful not to exceed the ring size, you can use the ring as a first-in-first-out queue. For example:
(let ((fifo (make-ring 5)))
(mapc (lambda (obj) (ring-insert fifo obj))
'(0 one "two"))
(list (ring-remove fifo) t
(ring-remove fifo) t
(ring-remove fifo)))
(0 t one t "two")
Recall that the sequence type is the union of two other Lisp types: lists and arrays. In other words, any list is a sequence, and any array is a sequence. The common property that all sequences have is that each is an ordered collection of elements.
An array is a fixed-length object with a slot for each of its elements. All the elements are accessible in constant time. The four types of arrays are strings, vectors, char-tables and bool-vectors.
A list is a sequence of elements, but it is not a single primitive object; it is made of cons cells, one cell per element. Finding the nth element requires looking through n cons cells, so elements farther from the beginning of the list take longer to access. But it is possible to add elements to the list, or remove elements.
The following diagram shows the relationship between these types:
_____________________________________________
| |
| Sequence |
| ______ ________________________________ |
| | | | | |
| | List | | Array | |
| | | | ________ ________ | |
| |______| | | | | | | |
| | | Vector | | String | | |
| | |________| |________| | |
| | ____________ _____________ | |
| | | | | | | |
| | | Char-table | | Bool-vector | | |
| | |____________| |_____________| | |
| |________________________________| |
|_____________________________________________|
In Emacs Lisp, a sequence is either a list or an array. The common property of all sequences is that they are ordered collections of elements. This section describes functions that accept any kind of sequence.
Returns
tif object is a list, vector, string, bool-vector, or char-table,nilotherwise.
This function returns the number of elements in sequence. If sequence is a dotted list, a
wrong-type-argumenterror is signaled. Circular lists may cause an infinite loop. For a char-table, the value returned is always one more than the maximum Emacs character code.See Definition of safe-length, for the related function
safe-length.(length '(1 2 3)) 3 (length ()) 0 (length "foobar") 6 (length [1 2 3]) 3 (length (make-bool-vector 5 nil)) 5
See also string-bytes, in Text Representations.
This function returns the element of sequence indexed by index. Legitimate values of index are integers ranging from 0 up to one less than the length of sequence. If sequence is a list, out-of-range values behave as for
nth. See Definition of nth. Otherwise, out-of-range values trigger anargs-out-of-rangeerror.(elt [1 2 3 4] 2) 3 (elt '(1 2 3 4) 2) 3 ;; We usestringto show clearly which charactereltreturns. (string (elt "1234" 2)) "3" (elt [1 2 3 4] 4) error--> Args out of range: [1 2 3 4], 4 (elt [1 2 3 4] -1) error--> Args out of range: [1 2 3 4], -1This function generalizes
aref(see Array Functions) andnth(see Definition of nth).
Returns a copy of sequence. The copy is the same type of object as the original sequence, and it has the same elements in the same order.
Storing a new element into the copy does not affect the original sequence, and vice versa. However, the elements of the new sequence are not copies; they are identical (
eq) to the elements of the original. Therefore, changes made within these elements, as found via the copied sequence, are also visible in the original sequence.If the sequence is a string with text properties, the property list in the copy is itself a copy, not shared with the original's property list. However, the actual values of the properties are shared. See Text Properties.
This function does not work for dotted lists. Trying to copy a circular list may cause an infinite loop.
See also
appendin Building Lists,concatin Creating Strings, andvconcatin Vector Functions, for other ways to copy sequences.(setq bar '(1 2)) (1 2) (setq x (vector 'foo bar)) [foo (1 2)] (setq y (copy-sequence x)) [foo (1 2)] (eq x y) nil (equal x y) t (eq (elt x 1) (elt y 1)) t ;; Replacing an element of one sequence. (aset x 0 'quux) x [quux (1 2)] y [foo (1 2)] ;; Modifying the inside of a shared element. (setcar (aref x 1) 69) x [quux (69 2)] y [foo (69 2)]
An array object has slots that hold a number of other Lisp objects, called the elements of the array. Any element of an array may be accessed in constant time. In contrast, the time to access an element of a list is proportional to the position of that element in the list.
Emacs defines four types of array, all one-dimensional:
strings (see String Type), vectors (see Vector Type), bool-vectors (see Bool-Vector Type), and
char-tables (see Char-Table Type). Vectors and char-tables
can hold elements of any type, but strings can only hold characters,
and bool-vectors can only hold t and nil.
All four kinds of array share these characteristics:
aref and aset, respectively (see Array Functions).
When you create an array, other than a char-table, you must specify its length. You cannot specify the length of a char-table, because that is determined by the range of character codes.
In principle, if you want an array of text characters, you could use either a string or a vector. In practice, we always choose strings for such applications, for four reasons:
By contrast, for an array of keyboard input characters (such as a key sequence), a vector may be necessary, because many keyboard input characters are outside the range that will fit in a string. See Key Sequence Input.
In this section, we describe the functions that accept all types of arrays.
This function returns
tif object is an array (i.e., a vector, a string, a bool-vector or a char-table).(arrayp [a]) t (arrayp "asdf") t (arrayp (syntax-table)) ;; A char-table. t
This function returns the indexth element of array. The first element is at index zero.
(setq primes [2 3 5 7 11 13]) [2 3 5 7 11 13] (aref primes 4) 11 (aref "abcdefg" 1) 98 ; ‘b’ is ASCII code 98.See also the function
elt, in Sequence Functions.
This function sets the indexth element of array to be object. It returns object.
(setq w [foo bar baz]) [foo bar baz] (aset w 0 'fu) fu w [fu bar baz] (setq x "asdfasfd") "asdfasfd" (aset x 3 ?Z) 90 x "asdZasfd"If array is a string and object is not a character, a
wrong-type-argumenterror results. The function converts a unibyte string to multibyte if necessary to insert a character.
This function fills the array array with object, so that each element of array is object. It returns array.
(setq a [a b c d e f g]) [a b c d e f g] (fillarray a 0) [0 0 0 0 0 0 0] a [0 0 0 0 0 0 0] (setq s "When in the course") "When in the course" (fillarray s ?-) "------------------"If array is a string and object is not a character, a
wrong-type-argumenterror results.
The general sequence functions copy-sequence and length
are often useful for objects known to be arrays. See Sequence Functions.
A vector is a general-purpose array whose elements can be any Lisp objects. (By contrast, the elements of a string can only be characters. See Strings and Characters.) Vectors are used in Emacs for many purposes: as key sequences (see Key Sequences), as symbol-lookup tables (see Creating Symbols), as part of the representation of a byte-compiled function (see Byte Compilation), and more.
In Emacs Lisp, the indices of the elements of a vector start from zero and count up from there.
Vectors are printed with square brackets surrounding the elements.
Thus, a vector whose elements are the symbols a, b and
a is printed as [a b a]. You can write vectors in the
same way in Lisp input.
A vector, like a string or a number, is considered a constant for evaluation: the result of evaluating it is the same vector. This does not evaluate or even examine the elements of the vector. See Self-Evaluating Forms.
Here are examples illustrating these principles:
(setq avector [1 two '(three) "four" [five]])
[1 two (quote (three)) "four" [five]]
(eval avector)
[1 two (quote (three)) "four" [five]]
(eq avector (eval avector))
t
Here are some functions that relate to vectors:
This function returns
tif object is a vector.(vectorp [a]) t (vectorp "asdf") nil
This function creates and returns a vector whose elements are the arguments, objects.
(vector 'foo 23 [bar baz] "rats") [foo 23 [bar baz] "rats"] (vector) []
This function returns a new vector consisting of length elements, each initialized to object.
(setq sleepy (make-vector 9 'Z)) [Z Z Z Z Z Z Z Z Z]
This function returns a new vector containing all the elements of sequences. The arguments sequences may be true lists, vectors, strings or bool-vectors. If no sequences are given, an empty vector is returned.
The value is a newly constructed vector that is not
eqto any existing vector.(setq a (vconcat '(A B C) '(D E F))) [A B C D E F] (eq a (vconcat a)) nil (vconcat) [] (vconcat [A B C] "aa" '(foo (6 7))) [A B C 97 97 foo (6 7)]The
vconcatfunction also allows byte-code function objects as arguments. This is a special feature to make it easy to access the entire contents of a byte-code function object. See Byte-Code Objects.For other concatenation functions, see
mapconcatin Mapping Functions,concatin Creating Strings, andappendin Building Lists.
The append function also provides a way to convert a vector into a
list with the same elements:
(setq avector [1 two (quote (three)) "four" [five]])
[1 two (quote (three)) "four" [five]]
(append avector nil)
(1 two (quote (three)) "four" [five])
A char-table is much like a vector, except that it is indexed by
character codes. Any valid character code, without modifiers, can be
used as an index in a char-table. You can access a char-table's
elements with aref and aset, as with any array. In
addition, a char-table can have extra slots to hold additional
data not associated with particular character codes. Like vectors,
char-tables are constants when evaluated, and can hold elements of any
type.
Each char-table has a subtype, a symbol, which serves two purposes:
display-table
as the subtype, and syntax tables are char-tables with
syntax-table as the subtype. The subtype can be queried using
the function char-table-subtype, described below.
char-table-extra-slots symbol property, which should be an
integer between 0 and 10. If the subtype has no such symbol property,
the char-table has no extra slots. See Property Lists, for
information about symbol properties.
A char-table can have a parent, which is another char-table. If
it does, then whenever the char-table specifies nil for a
particular character c, it inherits the value specified in the
parent. In other words, (aref char-table c) returns
the value from the parent of char-table if char-table itself
specifies nil.
A char-table can also have a default value. If so, then
(aref char-table c) returns the default value
whenever the char-table does not specify any other non-nil value.
Return a newly-created char-table, with subtype subtype (a symbol). Each element is initialized to init, which defaults to
nil. You cannot alter the subtype of a char-table after the char-table is created.There is no argument to specify the length of the char-table, because all char-tables have room for any valid character code as an index.
If subtype has the
char-table-extra-slotssymbol property, that specifies the number of extra slots in the char-table. This should be an integer between 0 and 10; otherwise,make-char-tableraises an error. If subtype has nochar-table-extra-slotssymbol property (see Property Lists), the char-table has no extra slots.
This function returns
tif object is a char-table, andnilotherwise.
There is no special function to access default values in a char-table.
To do that, use char-table-range (see below).
This function returns the parent of char-table. The parent is always either
nilor another char-table.
This function sets the parent of char-table to new-parent.
This function returns the contents of extra slot n of char-table. The number of extra slots in a char-table is determined by its subtype.
This function stores value in extra slot n of char-table.
A char-table can specify an element value for a single character code; it can also specify a value for an entire character set.
This returns the value specified in char-table for a range of characters range. Here are the possibilities for range:
nil- Refers to the default value.
- char
- Refers to the element for character char (supposing char is a valid character code).
(from.to)- A cons cell refers to all the characters in the inclusive range ‘[from..to]’.
This function sets the value in char-table for a range of characters range. Here are the possibilities for range:
nil- Refers to the default value.
t- Refers to the whole range of character codes.
- char
- Refers to the element for character char (supposing char is a valid character code).
(from.to)- A cons cell refers to all the characters in the inclusive range ‘[from..to]’.
This function calls its argument function for each element of char-table that has a non-
nilvalue. The call to function is with two arguments, a key and a value. The key is a possible range argument forchar-table-range—either a valid character or a cons cell(from.to), specifying a range of characters that share the same value. The value is what(char-table-rangechar-table key)returns.Overall, the key-value pairs passed to function describe all the values stored in char-table.
The return value is always
nil; to make calls tomap-char-tableuseful, function should have side effects. For example, here is how to examine the elements of the syntax table:(let (accumulator) (map-char-table #'(lambda (key value) (setq accumulator (cons (list (if (consp key) (list (car key) (cdr key)) key) value) accumulator))) (syntax-table)) accumulator) (((2597602 4194303) (2)) ((2597523 2597601) (3)) ... (65379 (5 . 65378)) (65378 (4 . 65379)) (65377 (1)) ... (12 (0)) (11 (3)) (10 (12)) (9 (0)) ((0 8) (3)))
A bool-vector is much like a vector, except that it stores only the
values t and nil. If you try to store any non-nil
value into an element of the bool-vector, the effect is to store
t there. As with all arrays, bool-vector indices start from 0,
and the length cannot be changed once the bool-vector is created.
Bool-vectors are constants when evaluated.
There are two special functions for working with bool-vectors; aside from that, you manipulate them with same functions used for other kinds of arrays.
Return a new bool-vector of length elements, each one initialized to initial.
Here is an example of creating, examining, and updating a bool-vector. Note that the printed form represents up to 8 boolean values as a single character.
(setq bv (make-bool-vector 5 t))
#&5"^_"
(aref bv 1)
t
(aset bv 3 nil)
nil
bv
#&5"^W"
These results make sense because the binary codes for control-_ and control-W are 11111 and 10111, respectively.
A hash table is a very fast kind of lookup table, somewhat like an alist (see Association Lists) in that it maps keys to corresponding values. It differs from an alist in these ways:
Emacs Lisp provides a general-purpose hash table data type, along with a series of functions for operating on them. Hash tables have no read syntax, and print in hash notation, like this:
(make-hash-table)
#<hash-table 'eql nil 0/65 0x83af980>
(The term “hash notation” refers to the initial ‘#’ character—see Printed Representation—and has nothing to do with the term “hash table.”)
Obarrays are also a kind of hash table, but they are a different type of object and are used only for recording interned symbols (see Creating Symbols).
The principal function for creating a hash table is
make-hash-table.
This function creates a new hash table according to the specified arguments. The arguments should consist of alternating keywords (particular symbols recognized specially) and values corresponding to them.
Several keywords make sense in
make-hash-table, but the only two that you really need to know about are:testand:weakness.
:testtest- This specifies the method of key lookup for this hash table. The default is
eql;eqandequalare other alternatives:
eql- Keys which are numbers are “the same” if they are
equal, that is, if they are equal in value and either both are integers or both are floating point numbers; otherwise, two distinct objects are never “the same.”eq- Any two distinct Lisp objects are “different” as keys.
equal- Two Lisp objects are “the same,” as keys, if they are equal according to
equal.You can use
define-hash-table-test(see Defining Hash) to define additional possibilities for test.:weaknessweak- The weakness of a hash table specifies whether the presence of a key or value in the hash table preserves it from garbage collection.
The value, weak, must be one of
nil,key,value,key-or-value,key-and-value, ortwhich is an alias forkey-and-value. If weak iskeythen the hash table does not prevent its keys from being collected as garbage (if they are not referenced anywhere else); if a particular key does get collected, the corresponding association is removed from the hash table.If weak is
value, then the hash table does not prevent values from being collected as garbage (if they are not referenced anywhere else); if a particular value does get collected, the corresponding association is removed from the hash table.If weak is
key-and-valueort, both the key and the value must be live in order to preserve the association. Thus, the hash table does not protect either keys or values from garbage collection; if either one is collected as garbage, that removes the association.If weak is
key-or-value, either the key or the value can preserve the association. Thus, associations are removed from the hash table when both their key and value would be collected as garbage (if not for references from weak hash tables).The default for weak is
nil, so that all keys and values referenced in the hash table are preserved from garbage collection.:sizesize- This specifies a hint for how many associations you plan to store in the hash table. If you know the approximate number, you can make things a little more efficient by specifying it this way. If you specify too small a size, the hash table will grow automatically when necessary, but doing that takes some extra time.
The default size is 65.
:rehash-sizerehash-size- When you add an association to a hash table and the table is “full,” it grows automatically. This value specifies how to make the hash table larger, at that time.
If rehash-size is an integer, it should be positive, and the hash table grows by adding that much to the nominal size. If rehash-size is a floating point number, it had better be greater than 1, and the hash table grows by multiplying the old size by that number.
The default value is 1.5.
:rehash-thresholdthreshold- This specifies the criterion for when the hash table is “full” (so it should be made larger). The value, threshold, should be a positive floating point number, no greater than 1. The hash table is “full” whenever the actual number of entries exceeds this fraction of the nominal size. The default for threshold is 0.8.
This is equivalent to
make-hash-table, but with a different style argument list. The argument test specifies the method of key lookup.This function is obsolete. Use
make-hash-tableinstead.
This section describes the functions for accessing and storing associations in a hash table. In general, any Lisp object can be used as a hash key, unless the comparison method imposes limits. Any Lisp object can also be used as the value.
This function looks up key in table, and returns its associated value—or default, if key has no association in table.
This function enters an association for key in table, with value value. If key already has an association in table, value replaces the old associated value.
This function removes the association for key from table, if there is one. If key has no association,
remhashdoes nothing.Common Lisp note: In Common Lisp,
remhashreturns non-nilif it actually removed an association andnilotherwise. In Emacs Lisp,remhashalways returnsnil.
This function removes all the associations from hash table table, so that it becomes empty. This is also called clearing the hash table.
Common Lisp note: In Common Lisp,
clrhashreturns the empty table. In Emacs Lisp, it returnsnil.
This function calls function once for each of the associations in table. The function function should accept two arguments—a key listed in table, and its associated value.
maphashreturnsnil.
You can define new methods of key lookup by means of
define-hash-table-test. In order to use this feature, you need
to understand how hash tables work, and what a hash code means.
You can think of a hash table conceptually as a large array of many
slots, each capable of holding one association. To look up a key,
gethash first computes an integer, the hash code, from the key.
It reduces this integer modulo the length of the array, to produce an
index in the array. Then it looks in that slot, and if necessary in
other nearby slots, to see if it has found the key being sought.
Thus, to define a new method of key lookup, you need to specify both a function to compute the hash code from a key, and a function to compare two keys directly.
This function defines a new hash table test, named name.
After defining name in this way, you can use it as the test argument in
make-hash-table. When you do that, the hash table will use test-fn to compare key values, and hash-fn to compute a “hash code” from a key value.The function test-fn should accept two arguments, two keys, and return non-
nilif they are considered “the same.”The function hash-fn should accept one argument, a key, and return an integer that is the “hash code” of that key. For good results, the function should use the whole range of integer values for hash codes, including negative integers.
The specified functions are stored in the property list of name under the property
hash-table-test; the property value's form is(test-fn hash-fn).
This function returns a hash code for Lisp object obj. This is an integer which reflects the contents of obj and the other Lisp objects it points to.
If two objects obj1 and obj2 are equal, then
(sxhashobj1)and(sxhashobj2)are the same integer.If the two objects are not equal, the values returned by
sxhashare usually different, but not always; once in a rare while, by luck, you will encounter two distinct-looking objects that give the same result fromsxhash.
This example creates a hash table whose keys are strings that are compared case-insensitively.
(defun case-fold-string= (a b)
(compare-strings a nil nil b nil nil t))
(defun case-fold-string-hash (a)
(sxhash (upcase a)))
(define-hash-table-test 'case-fold
'case-fold-string= 'case-fold-string-hash)
(make-hash-table :test 'case-fold)
Here is how you could define a hash table test equivalent to the
predefined test value equal. The keys can be any Lisp object,
and equal-looking objects are considered the same key.
(define-hash-table-test 'contents-hash 'equal 'sxhash)
(make-hash-table :test 'contents-hash)
Here are some other functions for working with hash tables.
This function creates and returns a copy of table. Only the table itself is copied—the keys and values are shared.
This returns the test value that was given when table was created, to specify how to hash and compare keys. See
make-hash-table(see Creating Hash).
This function returns the weak value that was specified for hash table table.
A symbol is an object with a unique name. This chapter describes symbols, their components, their property lists, and how they are created and interned. Separate chapters describe the use of symbols as variables and as function names; see Variables, and Functions. For the precise read syntax for symbols, see Symbol Type.
You can test whether an arbitrary Lisp object is a symbol
with symbolp:
Each symbol has four components (or “cells”), each of which references another object:
symbol-name in Creating Symbols.
symbol-value in
Accessing Variables.
symbol-function in Function Cells.
symbol-plist in Property Lists.
The print name cell always holds a string, and cannot be changed. The other three cells can be set individually to any specified Lisp object.
The print name cell holds the string that is the name of the symbol. Since symbols are represented textually by their names, it is important not to have two symbols with the same name. The Lisp reader ensures this: every time it reads a symbol, it looks for an existing symbol with the specified name before it creates a new one. (In GNU Emacs Lisp, this lookup uses a hashing algorithm and an obarray; see Creating Symbols.)
The value cell holds the symbol's value as a variable
(see Variables). That is what you get if you evaluate the symbol as
a Lisp expression (see Evaluation). Any Lisp object is a legitimate
value. Certain symbols have values that cannot be changed; these
include nil and t, and any symbol whose name starts with
‘:’ (those are called keywords). See Constant Variables.
We often refer to “the function foo” when we really mean
the function stored in the function cell of the symbol foo. We
make the distinction explicit only when necessary. In normal
usage, the function cell usually contains a function
(see Functions) or a macro (see Macros), as that is what the
Lisp interpreter expects to see there (see Evaluation). Keyboard
macros (see Keyboard Macros), keymaps (see Keymaps) and
autoload objects (see Autoloading) are also sometimes stored in
the function cells of symbols.
The property list cell normally should hold a correctly formatted property list (see Property Lists), as a number of functions expect to see a property list there.
The function cell or the value cell may be void, which means
that the cell does not reference any object. (This is not the same
thing as holding the symbol void, nor the same as holding the
symbol nil.) Examining a function or value cell that is void
results in an error, such as ‘Symbol's value as variable is void’.
The four functions symbol-name, symbol-value,
symbol-plist, and symbol-function return the contents of
the four cells of a symbol. Here as an example we show the contents of
the four cells of the symbol buffer-file-name:
(symbol-name 'buffer-file-name)
"buffer-file-name"
(symbol-value 'buffer-file-name)
"/gnu/elisp/symbols.texi"
(symbol-function 'buffer-file-name)
#<subr buffer-file-name>
(symbol-plist 'buffer-file-name)
(variable-documentation 29529)
Because this symbol is the variable which holds the name of the file
being visited in the current buffer, the value cell contents we see are
the name of the source file of this chapter of the Emacs Lisp Manual.
The property list cell contains the list (variable-documentation
29529) which tells the documentation functions where to find the
documentation string for the variable buffer-file-name in the
DOC-version file. (29529 is the offset from the beginning
of the DOC-version file to where that documentation string
begins—see Documentation Basics.) The function cell contains
the function for returning the name of the file.
buffer-file-name names a primitive function, which has no read
syntax and prints in hash notation (see Primitive Function Type). A
symbol naming a function written in Lisp would have a lambda expression
(or a byte-code object) in this cell.
A definition in Lisp is a special form that announces your intention to use a certain symbol in a particular way. In Emacs Lisp, you can define a symbol as a variable, or define it as a function (or macro), or both independently.
A definition construct typically specifies a value or meaning for the symbol for one kind of use, plus documentation for its meaning when used in this way. Thus, when you define a symbol as a variable, you can supply an initial value for the variable, plus documentation for the variable.
defvar and defconst are special forms that define a
symbol as a global variable. They are documented in detail in
Defining Variables. For defining user option variables that can
be customized, use defcustom (see Customization).
defun defines a symbol as a function, creating a lambda
expression and storing it in the function cell of the symbol. This
lambda expression thus becomes the function definition of the symbol.
(The term “function definition,” meaning the contents of the function
cell, is derived from the idea that defun gives the symbol its
definition as a function.) defsubst and defalias are two
other ways of defining a function. See Functions.
defmacro defines a symbol as a macro. It creates a macro
object and stores it in the function cell of the symbol. Note that a
given symbol can be a macro or a function, but not both at once, because
both macro and function definitions are kept in the function cell, and
that cell can hold only one Lisp object at any given time.
See Macros.
In Emacs Lisp, a definition is not required in order to use a symbol
as a variable or function. Thus, you can make a symbol a global
variable with setq, whether you define it first or not. The real
purpose of definitions is to guide programmers and programming tools.
They inform programmers who read the code that certain symbols are
intended to be used as variables, or as functions. In addition,
utilities such as etags and make-docfile recognize
definitions, and add appropriate information to tag tables and the
DOC-version file. See Accessing Documentation.
To understand how symbols are created in GNU Emacs Lisp, you must know how Lisp reads them. Lisp must ensure that it finds the same symbol every time it reads the same set of characters. Failure to do so would cause complete confusion.
When the Lisp reader encounters a symbol, it reads all the characters of the name. Then it “hashes” those characters to find an index in a table called an obarray. Hashing is an efficient method of looking something up. For example, instead of searching a telephone book cover to cover when looking up Jan Jones, you start with the J's and go from there. That is a simple version of hashing. Each element of the obarray is a bucket which holds all the symbols with a given hash code; to look for a given name, it is sufficient to look through all the symbols in the bucket for that name's hash code. (The same idea is used for general Emacs hash tables, but they are a different data type; see Hash Tables.)
If a symbol with the desired name is found, the reader uses that symbol. If the obarray does not contain a symbol with that name, the reader makes a new symbol and adds it to the obarray. Finding or adding a symbol with a certain name is called interning it, and the symbol is then called an interned symbol.
Interning ensures that each obarray has just one symbol with any particular name. Other like-named symbols may exist, but not in the same obarray. Thus, the reader gets the same symbols for the same names, as long as you keep reading with the same obarray.
Interning usually happens automatically in the reader, but sometimes other programs need to do it. For example, after the M-x command obtains the command name as a string using the minibuffer, it then interns the string, to get the interned symbol with that name.
No obarray contains all symbols; in fact, some symbols are not in any obarray. They are called uninterned symbols. An uninterned symbol has the same four cells as other symbols; however, the only way to gain access to it is by finding it in some other object or as the value of a variable.
Creating an uninterned symbol is useful in generating Lisp code, because an uninterned symbol used as a variable in the code you generate cannot clash with any variables used in other Lisp programs.
In Emacs Lisp, an obarray is actually a vector. Each element of the
vector is a bucket; its value is either an interned symbol whose name
hashes to that bucket, or 0 if the bucket is empty. Each interned
symbol has an internal link (invisible to the user) to the next symbol
in the bucket. Because these links are invisible, there is no way to
find all the symbols in an obarray except using mapatoms (below).
The order of symbols in a bucket is not significant.
In an empty obarray, every element is 0, so you can create an obarray
with (make-vector length 0). This is the only
valid way to create an obarray. Prime numbers as lengths tend
to result in good hashing; lengths one less than a power of two are also
good.
Do not try to put symbols in an obarray yourself. This does
not work—only intern can enter a symbol in an obarray properly.
Common Lisp note: In Common Lisp, a single symbol may be interned in several obarrays.
Most of the functions below take a name and sometimes an obarray as
arguments. A wrong-type-argument error is signaled if the name
is not a string, or if the obarray is not a vector.
This function returns the string that is symbol's name. For example:
(symbol-name 'foo) "foo"Warning: Changing the string by substituting characters does change the name of the symbol, but fails to update the obarray, so don't do it!
This function returns a newly-allocated, uninterned symbol whose name is name (which must be a string). Its value and function definition are void, and its property list is
nil. In the example below, the value ofsymis noteqtofoobecause it is a distinct uninterned symbol whose name is also ‘foo’.(setq sym (make-symbol "foo")) foo (eq sym 'foo) nil
This function returns the interned symbol whose name is name. If there is no such symbol in the obarray obarray,
interncreates a new one, adds it to the obarray, and returns it. If obarray is omitted, the value of the global variableobarrayis used.(setq sym (intern "foo")) foo (eq sym 'foo) t (setq sym1 (intern "foo" other-obarray)) foo (eq sym1 'foo) nil
Common Lisp note: In Common Lisp, you can intern an existing symbol
in an obarray. In Emacs Lisp, you cannot do this, because the argument
to intern must be a string, not a symbol.
This function returns the symbol in obarray whose name is name, or
nilif obarray has no symbol with that name. Therefore, you can useintern-softto test whether a symbol with a given name is already interned. If obarray is omitted, the value of the global variableobarrayis used.The argument name may also be a symbol; in that case, the function returns name if name is interned in the specified obarray, and otherwise
nil.(intern-soft "frazzle") ; No such symbol exists. nil (make-symbol "frazzle") ; Create an uninterned one. frazzle (intern-soft "frazzle") ; That one cannot be found. nil (setq sym (intern "frazzle")) ; Create an interned one. frazzle (intern-soft "frazzle") ; That one can be found! frazzle (eq sym 'frazzle) ; And it is the same one. t
This function calls function once with each symbol in the obarray obarray. Then it returns
nil. If obarray is omitted, it defaults to the value ofobarray, the standard obarray for ordinary symbols.(setq count 0) 0 (defun count-syms (s) (setq count (1+ count))) count-syms (mapatoms 'count-syms) nil count 1871See
documentationin Accessing Documentation, for another example usingmapatoms.
This function deletes symbol from the obarray obarray. If
symbolis not actually in the obarray,uninterndoes nothing. If obarray isnil, the current obarray is used.If you provide a string instead of a symbol as symbol, it stands for a symbol name. Then
uninterndeletes the symbol (if any) in the obarray which has that name. If there is no such symbol,uninterndoes nothing.If
uninterndoes delete a symbol, it returnst. Otherwise it returnsnil.
A property list (plist for short) is a list of paired elements. Each of the pairs associates a property name (usually a symbol) with a property or value.
Every symbol has a cell that stores a property list (see Symbol Components). This property list is used to record information about the symbol, such as its variable documentation and the name of the file where it was defined.
Property lists can also be used in other contexts. For instance, you can assign property lists to character positions in a string or buffer. See Text Properties.
The property names and values in a property list can be any Lisp
objects, but the names are usually symbols. Property list functions
compare the property names using eq. Here is an example of a
property list, found on the symbol progn when the compiler is
loaded:
(lisp-indent-function 0 byte-compile byte-compile-progn)
Here lisp-indent-function and byte-compile are property
names, and the other two elements are the corresponding values.
Association lists (see Association Lists) are very similar to property lists. In contrast to association lists, the order of the pairs in the property list is not significant since the property names must be distinct.
Property lists are better than association lists for attaching
information to various Lisp function names or variables. If your
program keeps all of its associations in one association list, it will
typically need to search that entire list each time it checks for an
association. This could be slow. By contrast, if you keep the same
information in the property lists of the function names or variables
themselves, each search will scan only the length of one property list,
which is usually short. This is why the documentation for a variable is
recorded in a property named variable-documentation. The byte
compiler likewise uses properties to record those functions needing
special treatment.
However, association lists have their own advantages. Depending on your application, it may be faster to add an association to the front of an association list than to update a property. All properties for a symbol are stored in the same property list, so there is a possibility of a conflict between different uses of a property name. (For this reason, it is a good idea to choose property names that are probably unique, such as by beginning the property name with the program's usual name-prefix for variables and functions.) An association list may be used like a stack where associations are pushed on the front of the list and later discarded; this is not possible with a property list.
This function sets symbol's property list to plist. Normally, plist should be a well-formed property list, but this is not enforced. The return value is plist.
(setplist 'foo '(a 1 b (2 3) c nil)) (a 1 b (2 3) c nil) (symbol-plist 'foo) (a 1 b (2 3) c nil)For symbols in special obarrays, which are not used for ordinary purposes, it may make sense to use the property list cell in a nonstandard fashion; in fact, the abbrev mechanism does so (see Abbrevs).
This function finds the value of the property named property in symbol's property list. If there is no such property,
nilis returned. Thus, there is no distinction between a value ofniland the absence of the property.The name property is compared with the existing property names using
eq, so any object is a legitimate property.See
putfor an example.
This function puts value onto symbol's property list under the property name property, replacing any previous property value. The
putfunction returns value.(put 'fly 'verb 'transitive) 'transitive (put 'fly 'noun '(a buzzing little bug)) (a buzzing little bug) (get 'fly 'verb) transitive (symbol-plist 'fly) (verb transitive noun (a buzzing little bug))
These functions are useful for manipulating property lists that are stored in places other than symbols:
This returns the value of the property property stored in the property list plist. It accepts a malformed plist argument. If property is not found in the plist, it returns
nil. For example,(plist-get '(foo 4) 'foo) 4 (plist-get '(foo 4 bad) 'foo) 4 (plist-get '(foo 4 bad) 'bad)nil(plist-get '(foo 4 bad) 'bar) nil
This stores value as the value of the property property in the property list plist. It may modify plist destructively, or it may construct a new list structure without altering the old. The function returns the modified property list, so you can store that back in the place where you got plist. For example,
(setq my-plist '(bar t foo 4)) (bar t foo 4) (setq my-plist (plist-put my-plist 'foo 69)) (bar t foo 69) (setq my-plist (plist-put my-plist 'quux '(a))) (bar t foo 69 quux (a))
You could define put in terms of plist-put as follows:
(defun put (symbol prop value)
(setplist symbol
(plist-put (symbol-plist symbol) prop value)))
Like
plist-getexcept that it compares properties usingequalinstead ofeq.
Like
plist-putexcept that it compares properties usingequalinstead ofeq.
This returns non-
nilif plist contains the given property. Unlikeplist-get, this allows you to distinguish between a missing property and a property with the valuenil. The value is actually the tail of plist whosecaris property.
The evaluation of expressions in Emacs Lisp is performed by the
Lisp interpreter—a program that receives a Lisp object as input
and computes its value as an expression. How it does this depends
on the data type of the object, according to rules described in this
chapter. The interpreter runs automatically to evaluate portions of
your program, but can also be called explicitly via the Lisp primitive
function eval.
The Lisp interpreter, or evaluator, is the part of Emacs that computes the value of an expression that is given to it. When a function written in Lisp is called, the evaluator computes the value of the function by evaluating the expressions in the function body. Thus, running any Lisp program really means running the Lisp interpreter.
A Lisp object that is intended for evaluation is called an expression or a form. The fact that forms are data objects and not merely text is one of the fundamental differences between Lisp-like languages and typical programming languages. Any object can be evaluated, but in practice only numbers, symbols, lists and strings are evaluated very often.
In subsequent sections, we will describe the details of what evaluation means for each kind of form.
It is very common to read a Lisp form and then evaluate the form,
but reading and evaluation are separate activities, and either can be
performed alone. Reading per se does not evaluate anything; it
converts the printed representation of a Lisp object to the object
itself. It is up to the caller of read to specify whether this
object is a form to be evaluated, or serves some entirely different
purpose. See Input Functions.
Evaluation is a recursive process, and evaluating a form often
involves evaluating parts within that form. For instance, when you
evaluate a function call form such as (car x), Emacs
first evaluates the argument (the subform x). After evaluating
the argument, Emacs executes the function (car), and if
the function is written in Lisp, execution works by evaluating the
body of the function. (In this example, however, car is
not a Lisp function; it is a primitive function implemented in C.)
See Functions, for more information about functions and function
calls.
Evaluation takes place in a context called the environment, which consists of the current values and bindings of all Lisp variables (see Variables).3 Whenever a form refers to a variable without creating a new binding for it, the variable evaluates to the value given by the current environment. Evaluating a form may create a new environment for recursive evaluation, by binding variables (see Local Variables). Such environments are temporary, and vanish when the evaluation of the form is complete.
Evaluating a form may also make changes that persist; these changes
are called side effects. An example of a form that produces a
side effect is (setq foo 1).
Do not confuse evaluation with command key interpretation. The
editor command loop translates keyboard input into a command (an
interactively callable function) using the active keymaps, and then
uses call-interactively to execute that command. Executing the
command usually involves evaluation, if the command is written in
Lisp; however, this step is not considered a part of command key
interpretation. See Command Loop.
A Lisp object that is intended to be evaluated is called a form. How Emacs evaluates a form depends on its data type. Emacs has three different kinds of form that are evaluated differently: symbols, lists, and “all other types.” This section describes all three kinds, one by one, starting with the “all other types” which are self-evaluating forms.
A self-evaluating form is any form that is not a list or
symbol. Self-evaluating forms evaluate to themselves: the result of
evaluation is the same object that was evaluated. Thus, the number 25
evaluates to 25, and the string "foo" evaluates to the string
"foo". Likewise, evaluating a vector does not cause evaluation
of the elements of the vector—it returns the same vector with its
contents unchanged.
'123 ; A number, shown without evaluation. 123 123 ; Evaluated as usual---result is the same. 123 (eval '123) ; Evaluated ``by hand''---result is the same. 123 (eval (eval '123)) ; Evaluating twice changes nothing. 123
It is common to write numbers, characters, strings, and even vectors in Lisp code, taking advantage of the fact that they self-evaluate. However, it is quite unusual to do this for types that lack a read syntax, because there's no way to write them textually. It is possible to construct Lisp expressions containing these types by means of a Lisp program. Here is an example:
;; Build an expression containing a buffer object. (setq print-exp (list 'print (current-buffer))) (print #<buffer eval.texi>) ;; Evaluate it. (eval print-exp) -| #<buffer eval.texi> #<buffer eval.texi>
When a symbol is evaluated, it is treated as a variable. The result is the variable's value, if it has one. If it has none (if its value cell is void), an error is signaled. For more information on the use of variables, see Variables.
In the following example, we set the value of a symbol with
setq. Then we evaluate the symbol, and get back the value that
setq stored.
(setq a 123)
123
(eval 'a)
123
a
123
The symbols nil and t are treated specially, so that the
value of nil is always nil, and the value of t is
always t; you cannot set or bind them to any other values. Thus,
these two symbols act like self-evaluating forms, even though
eval treats them like any other symbol. A symbol whose name
starts with ‘:’ also self-evaluates in the same way; likewise,
its value ordinarily cannot be changed. See Constant Variables.
A form that is a nonempty list is either a function call, a macro call, or a special form, according to its first element. These three kinds of forms are evaluated in different ways, described below. The remaining list elements constitute the arguments for the function, macro, or special form.
The first step in evaluating a nonempty list is to examine its first element. This element alone determines what kind of form the list is and how the rest of the list is to be processed. The first element is not evaluated, as it would be in some Lisp dialects such as Scheme.
If the first element of the list is a symbol then evaluation examines the symbol's function cell, and uses its contents instead of the original symbol. If the contents are another symbol, this process, called symbol function indirection, is repeated until it obtains a non-symbol. See Function Names, for more information about symbol function indirection.
One possible consequence of this process is an infinite loop, in the
event that a symbol's function cell refers to the same symbol. Or a
symbol may have a void function cell, in which case the subroutine
symbol-function signals a void-function error. But if
neither of these things happens, we eventually obtain a non-symbol,
which ought to be a function or other suitable object.
More precisely, we should now have a Lisp function (a lambda
expression), a byte-code function, a primitive function, a Lisp macro,
a special form, or an autoload object. Each of these types is a case
described in one of the following sections. If the object is not one
of these types, Emacs signals an invalid-function error.
The following example illustrates the symbol indirection process. We
use fset to set the function cell of a symbol and
symbol-function to get the function cell contents
(see Function Cells). Specifically, we store the symbol car
into the function cell of first, and the symbol first into
the function cell of erste.
;; Build this function cell linkage:
;; ------------- ----- ------- -------
;; | #<subr car> | <-- | car | <-- | first | <-- | erste |
;; ------------- ----- ------- -------
(symbol-function 'car)
#<subr car>
(fset 'first 'car)
car
(fset 'erste 'first)
first
(erste '(1 2 3)) ; Call the function referenced by erste.
1
By contrast, the following example calls a function without any symbol function indirection, because the first element is an anonymous Lisp function, not a symbol.
((lambda (arg) (erste arg))
'(1 2 3))
1
Executing the function itself evaluates its body; this does involve
symbol function indirection when calling erste.
The built-in function indirect-function provides an easy way to
perform symbol function indirection explicitly.
This function returns the meaning of function as a function. If function is a symbol, then it finds function's function definition and starts over with that value. If function is not a symbol, then it returns function itself.
This function signals a
void-functionerror if the final symbol is unbound and optional argument noerror isnilor omitted. Otherwise, if noerror is non-nil, it returnsnilif the final symbol is unbound.It signals a
cyclic-function-indirectionerror if there is a loop in the chain of symbols.Here is how you could define
indirect-functionin Lisp:(defun indirect-function (function) (if (symbolp function) (indirect-function (symbol-function function)) function))
If the first element of a list being evaluated is a Lisp function
object, byte-code object or primitive function object, then that list is
a function call. For example, here is a call to the function
+:
(+ 1 x)
The first step in evaluating a function call is to evaluate the
remaining elements of the list from left to right. The results are the
actual argument values, one value for each list element. The next step
is to call the function with this list of arguments, effectively using
the function apply (see Calling Functions). If the function
is written in Lisp, the arguments are used to bind the argument
variables of the function (see Lambda Expressions); then the forms
in the function body are evaluated in order, and the value of the last
body form becomes the value of the function call.
If the first element of a list being evaluated is a macro object, then the list is a macro call. When a macro call is evaluated, the elements of the rest of the list are not initially evaluated. Instead, these elements themselves are used as the arguments of the macro. The macro definition computes a replacement form, called the expansion of the macro, to be evaluated in place of the original form. The expansion may be any sort of form: a self-evaluating constant, a symbol, or a list. If the expansion is itself a macro call, this process of expansion repeats until some other sort of form results.
Ordinary evaluation of a macro call finishes by evaluating the expansion. However, the macro expansion is not necessarily evaluated right away, or at all, because other programs also expand macro calls, and they may or may not evaluate the expansions.
Normally, the argument expressions are not evaluated as part of computing the macro expansion, but instead appear as part of the expansion, so they are computed when the expansion is evaluated.
For example, given a macro defined as follows:
(defmacro cadr (x)
(list 'car (list 'cdr x)))
an expression such as (cadr (assq 'handler list)) is a macro
call, and its expansion is:
(car (cdr (assq 'handler list)))
Note that the argument (assq 'handler list) appears in the
expansion.
See Macros, for a complete description of Emacs Lisp macros.
A special form is a primitive function specially marked so that its arguments are not all evaluated. Most special forms define control structures or perform variable bindings—things which functions cannot do.
Each special form has its own rules for which arguments are evaluated and which are used without evaluation. Whether a particular argument is evaluated may depend on the results of evaluating other arguments.
Here is a list, in alphabetical order, of all of the special forms in Emacs Lisp with a reference to where each is described.
andcatchcondcondition-casedefconstdefmacrodefundefvarfunctionifinteractiveletlet*orprog1prog2prognquotesave-current-buffersave-excursionsave-restrictionsave-window-excursionsetqsetq-defaulttrack-mouseunwind-protectwhilewith-output-to-temp-bufferCommon Lisp note: Here are some comparisons of special forms in GNU Emacs Lisp and Common Lisp.setq,if, andcatchare special forms in both Emacs Lisp and Common Lisp.defunis a special form in Emacs Lisp, but a macro in Common Lisp.save-excursionis a special form in Emacs Lisp, but doesn't exist in Common Lisp.throwis a special form in Common Lisp (because it must be able to throw multiple values), but it is a function in Emacs Lisp (which doesn't have multiple values).
The autoload feature allows you to call a function or macro whose function definition has not yet been loaded into Emacs. It specifies which file contains the definition. When an autoload object appears as a symbol's function definition, calling that symbol as a function automatically loads the specified file; then it calls the real definition loaded from that file. See Autoload.
The special form quote returns its single argument, as written,
without evaluating it. This provides a way to include constant symbols
and lists, which are not self-evaluating objects, in a program. (It is
not necessary to quote self-evaluating objects such as numbers, strings,
and vectors.)
Because quote is used so often in programs, Lisp provides a
convenient read syntax for it. An apostrophe character (‘'’)
followed by a Lisp object (in read syntax) expands to a list whose first
element is quote, and whose second element is the object. Thus,
the read syntax 'x is an abbreviation for (quote x).
Here are some examples of expressions that use quote:
(quote (+ 1 2))
(+ 1 2)
(quote foo)
foo
'foo
foo
''foo
(quote foo)
'(quote foo)
(quote foo)
['foo]
[(quote foo)]
Other quoting constructs include function (see Anonymous Functions), which causes an anonymous lambda expression written in Lisp
to be compiled, and ‘`’ (see Backquote), which is used to quote
only part of a list, while computing and substituting other parts.
Most often, forms are evaluated automatically, by virtue of their
occurrence in a program being run. On rare occasions, you may need to
write code that evaluates a form that is computed at run time, such as
after reading a form from text being edited or getting one from a
property list. On these occasions, use the eval function.
The functions and variables described in this section evaluate forms, specify limits to the evaluation process, or record recently returned values. Loading a file also does evaluation (see Loading).
It is generally cleaner and more flexible to store a function in a
data structure, and call it with funcall or apply, than
to store an expression in the data structure and evaluate it. Using
functions provides the ability to pass information to them as
arguments.
This is the basic function evaluating an expression. It evaluates form in the current environment and returns the result. How the evaluation proceeds depends on the type of the object (see Forms).
Since
evalis a function, the argument expression that appears in a call toevalis evaluated twice: once as preparation beforeevalis called, and again by theevalfunction itself. Here is an example:(setq foo 'bar) bar (setq bar 'baz) baz ;; Hereevalreceives argumentfoo(eval 'foo) bar ;; Hereevalreceives argumentbar, which is the value offoo(eval foo) bazThe number of currently active calls to
evalis limited tomax-lisp-eval-depth(see below).
This function evaluates the forms in the current buffer in the region defined by the positions start and end. It reads forms from the region and calls
evalon them until the end of the region is reached, or until an error is signaled and not handled.By default,
eval-regiondoes not produce any output. However, if stream is non-nil, any output produced by output functions (see Output Functions), as well as the values that result from evaluating the expressions in the region are printed using stream. See Output Streams.If read-function is non-
nil, it should be a function, which is used instead ofreadto read expressions one by one. This function is called with one argument, the stream for reading input. You can also use the variableload-read-function(see How Programs Do Loading) to specify this function, but it is more robust to use the read-function argument.
eval-regiondoes not move point. It always returnsnil.
This is similar to
eval-region, but the arguments provide different optional features.eval-bufferoperates on the entire accessible portion of buffer buffer-or-name. buffer-or-name can be a buffer, a buffer name (a string), ornil(or omitted), which means to use the current buffer. stream is used as ineval-region, unless stream isniland print non-nil. In that case, values that result from evaluating the expressions are still discarded, but the output of the output functions is printed in the echo area. filename is the file name to use forload-history(see Unloading), and defaults tobuffer-file-name(see Buffer File Name). If unibyte is non-nil,readconverts strings to unibyte whenever possible.
This variable defines the maximum depth allowed in calls to
eval,apply, andfuncallbefore an error is signaled (with error message"Lisp nesting exceeds max-lisp-eval-depth").This limit, with the associated error when it is exceeded, is one way Emacs Lisp avoids infinite recursion on an ill-defined function. If you increase the value of
max-lisp-eval-depthtoo much, such code can cause stack overflow instead. The depth limit counts internal uses ofeval,apply, andfuncall, such as for calling the functions mentioned in Lisp expressions, and recursive evaluation of function call arguments and function body forms, as well as explicit calls in Lisp code.The default value of this variable is 400. If you set it to a value less than 100, Lisp will reset it to 100 if the given value is reached. Entry to the Lisp debugger increases the value, if there is little room left, to make sure the debugger itself has room to execute.
max-specpdl-sizeprovides another limit on nesting. See Local Variables.
The value of this variable is a list of the values returned by all the expressions that were read, evaluated, and printed from buffers (including the minibuffer) by the standard Emacs commands which do this. (Note that this does not include evaluation in ‘*ielm*’ buffers, nor evaluation using C-j in
lisp-interaction-mode.) The elements are ordered most recent first.(setq x 1) 1 (list 'A (1+ 2) auto-save-default) (A 3 t) values ((A 3 t) 1 ...)This variable is useful for referring back to values of forms recently evaluated. It is generally a bad idea to print the value of
valuesitself, since this may be very long. Instead, examine particular elements, like this:;; Refer to the most recent evaluation result. (nth 0 values) (A 3 t) ;; That put a new element on, ;; so all elements move back one. (nth 1 values) (A 3 t) ;; This gets the element that was next-to-most-recent ;; before this example. (nth 3 values) 1
A Lisp program consists of expressions or forms (see Forms). We control the order of execution of these forms by enclosing them in control structures. Control structures are special forms which control when, whether, or how many times to execute the forms they contain.
The simplest order of execution is sequential execution: first form a, then form b, and so on. This is what happens when you write several forms in succession in the body of a function, or at top level in a file of Lisp code—the forms are executed in the order written. We call this textual order. For example, if a function body consists of two forms a and b, evaluation of the function evaluates first a and then b. The result of evaluating b becomes the value of the function.
Explicit control structures make possible an order of execution other than sequential.
Emacs Lisp provides several kinds of control structure, including other varieties of sequencing, conditionals, iteration, and (controlled) jumps—all discussed below. The built-in control structures are special forms since their subforms are not necessarily evaluated or not evaluated sequentially. You can use macros to define your own control structure constructs (see Macros).
Evaluating forms in the order they appear is the most common way
control passes from one form to another. In some contexts, such as in a
function body, this happens automatically. Elsewhere you must use a
control structure construct to do this: progn, the simplest
control construct of Lisp.
A progn special form looks like this:
(progn a b c ...)
and it says to execute the forms a, b, c, and so on, in
that order. These forms are called the body of the progn form.
The value of the last form in the body becomes the value of the entire
progn. (progn) returns nil.
In the early days of Lisp, progn was the only way to execute
two or more forms in succession and use the value of the last of them.
But programmers found they often needed to use a progn in the
body of a function, where (at that time) only one form was allowed. So
the body of a function was made into an “implicit progn”:
several forms are allowed just as in the body of an actual progn.
Many other control structures likewise contain an implicit progn.
As a result, progn is not used as much as it was many years ago.
It is needed now most often inside an unwind-protect, and,
or, or in the then-part of an if.
This special form evaluates all of the forms, in textual order, returning the result of the final form.
(progn (print "The first form") (print "The second form") (print "The third form")) -| "The first form" -| "The second form" -| "The third form" "The third form"
Two other control constructs likewise evaluate a series of forms but return a different value:
This special form evaluates form1 and all of the forms, in textual order, returning the result of form1.
(prog1 (print "The first form") (print "The second form") (print "The third form")) -| "The first form" -| "The second form" -| "The third form" "The first form"Here is a way to remove the first element from a list in the variable
x, then return the value of that former element:(prog1 (car x) (setq x (cdr x)))
This special form evaluates form1, form2, and all of the following forms, in textual order, returning the result of form2.
(prog2 (print "The first form") (print "The second form") (print "The third form")) -| "The first form" -| "The second form" -| "The third form" "The second form"
Conditional control structures choose among alternatives. Emacs Lisp
has four conditional forms: if, which is much the same as in
other languages; when and unless, which are variants of
if; and cond, which is a generalized case statement.
ifchooses between the then-form and the else-forms based on the value of condition. If the evaluated condition is non-nil, then-form is evaluated and the result returned. Otherwise, the else-forms are evaluated in textual order, and the value of the last one is returned. (The else part ofifis an example of an implicitprogn. See Sequencing.)If condition has the value
nil, and no else-forms are given,ifreturnsnil.
ifis a special form because the branch that is not selected is never evaluated—it is ignored. Thus, in the example below,trueis not printed because(if nil (print 'true) 'very-false) very-false
This is a variant of
ifwhere there are no else-forms, and possibly several then-forms. In particular,(when condition a b c)is entirely equivalent to
(if condition (progn a b c) nil)
This is a variant of
ifwhere there is no then-form:(unless condition a b c)is entirely equivalent to
(if condition nil a b c)
condchooses among an arbitrary number of alternatives. Each clause in thecondmust be a list. The car of this list is the condition; the remaining elements, if any, the body-forms. Thus, a clause looks like this:(condition body-forms...)
condtries the clauses in textual order, by evaluating the condition of each clause. If the value of condition is non-nil, the clause “succeeds”; thencondevaluates its body-forms, and the value of the last of body-forms becomes the value of thecond. The remaining clauses are ignored.If the value of condition is
nil, the clause “fails,” so thecondmoves on to the following clause, trying its condition.If every condition evaluates to
nil, so that every clause fails,condreturnsnil.A clause may also look like this:
(condition)Then, if condition is non-
nilwhen tested, the value of condition becomes the value of thecondform.The following example has four clauses, which test for the cases where the value of
xis a number, string, buffer and symbol, respectively:(cond ((numberp x) x) ((stringp x) x) ((bufferp x) (setq temporary-hack x) ; multiple body-forms (buffer-name x)) ; in one clause ((symbolp x) (symbol-value x)))Often we want to execute the last clause whenever none of the previous clauses was successful. To do this, we use
tas the condition of the last clause, like this:(tbody-forms). The formtevaluates tot, which is nevernil, so this clause never fails, provided thecondgets to it at all.For example,
(setq a 5) (cond ((eq a 'hack) 'foo) (t "default")) "default"This
condexpression returnsfooif the value ofaishack, and returns the string"default"otherwise.
Any conditional construct can be expressed with cond or with
if. Therefore, the choice between them is a matter of style.
For example:
(if a b c)
==
(cond (a b) (t c))
This section describes three constructs that are often used together
with if and cond to express complicated conditions. The
constructs and and or can also be used individually as
kinds of multiple conditional constructs.
This function tests for the falsehood of condition. It returns
tif condition isnil, andnilotherwise. The functionnotis identical tonull, and we recommend using the namenullif you are testing for an empty list.
The
andspecial form tests whether all the conditions are true. It works by evaluating the conditions one by one in the order written.If any of the conditions evaluates to
nil, then the result of theandmust benilregardless of the remaining conditions; soandreturnsnilright away, ignoring the remaining conditions.If all the conditions turn out non-
nil, then the value of the last of them becomes the value of theandform. Just(and), with no conditions, returnst, appropriate because all the conditions turned out non-nil. (Think about it; which one did not?)Here is an example. The first condition returns the integer 1, which is not
nil. Similarly, the second condition returns the integer 2, which is notnil. The third condition isnil, so the remaining condition is never evaluated.(and (print 1) (print 2) nil (print 3)) -| 1 -| 2 nilHere is a more realistic example of using
and:(if (and (consp foo) (eq (car foo) 'x)) (message "foo is a list starting with x"))Note that
(car foo)is not executed if(consp foo)returnsnil, thus avoiding an error.
andexpressions can also be written using eitheriforcond. Here's how:(and arg1 arg2 arg3) == (if arg1 (if arg2 arg3)) == (cond (arg1 (cond (arg2 arg3))))
The
orspecial form tests whether at least one of the conditions is true. It works by evaluating all the conditions one by one in the order written.If any of the conditions evaluates to a non-
nilvalue, then the result of theormust be non-nil; soorreturns right away, ignoring the remaining conditions. The value it returns is the non-nilvalue of the condition just evaluated.If all the conditions turn out
nil, then theorexpression returnsnil. Just(or), with no conditions, returnsnil, appropriate because all the conditions turned outnil. (Think about it; which one did not?)For example, this expression tests whether
xis eithernilor the integer zero:(or (eq x nil) (eq x 0))Like the
andconstruct,orcan be written in terms ofcond. For example:(or arg1 arg2 arg3) == (cond (arg1) (arg2) (arg3))You could almost write
orin terms ofif, but not quite:(if arg1 arg1 (if arg2 arg2 arg3))This is not completely equivalent because it can evaluate arg1 or arg2 twice. By contrast,
(orarg1 arg2 arg3)never evaluates any argument more than once.
Iteration means executing part of a program repetitively. For
example, you might want to repeat some computation once for each element
of a list, or once for each integer from 0 to n. You can do this
in Emacs Lisp with the special form while:
whilefirst evaluates condition. If the result is non-nil, it evaluates forms in textual order. Then it reevaluates condition, and if the result is non-nil, it evaluates forms again. This process repeats until condition evaluates tonil.There is no limit on the number of iterations that may occur. The loop will continue until either condition evaluates to
nilor until an error orthrowjumps out of it (see Nonlocal Exits).The value of a
whileform is alwaysnil.(setq num 0) 0 (while (< num 4) (princ (format "Iteration %d." num)) (setq num (1+ num))) -| Iteration 0. -| Iteration 1. -| Iteration 2. -| Iteration 3. nilTo write a “repeat...until” loop, which will execute something on each iteration and then do the end-test, put the body followed by the end-test in a
prognas the first argument ofwhile, as shown here:(while (progn (forward-line 1) (not (looking-at "^$"))))This moves forward one line and continues moving by lines until it reaches an empty line. It is peculiar in that the
whilehas no body, just the end test (which also does the real work of moving point).
The dolist and dotimes macros provide convenient ways to
write two common kinds of loops.
This construct executes body once for each element of list, binding the variable var locally to hold the current element. Then it returns the value of evaluating result, or
nilif result is omitted. For example, here is how you could usedolistto define thereversefunction:(defun reverse (list) (let (value) (dolist (elt list value) (setq value (cons elt value)))))
This construct executes body once for each integer from 0 (inclusive) to count (exclusive), binding the variable var to the integer for the current iteration. Then it returns the value of evaluating result, or
nilif result is omitted. Here is an example of usingdotimesto do something 100 times:(dotimes (i 100) (insert "I will not obey absurd orders\n"))
A nonlocal exit is a transfer of control from one point in a program to another remote point. Nonlocal exits can occur in Emacs Lisp as a result of errors; you can also use them under explicit control. Nonlocal exits unbind all variable bindings made by the constructs being exited.
catch and throwMost control constructs affect only the flow of control within the
construct itself. The function throw is the exception to this
rule of normal program execution: it performs a nonlocal exit on
request. (There are other exceptions, but they are for error handling
only.) throw is used inside a catch, and jumps back to
that catch. For example:
(defun foo-outer ()
(catch 'foo
(foo-inner)))
(defun foo-inner ()
...
(if x
(throw 'foo t))
...)
The throw form, if executed, transfers control straight back to
the corresponding catch, which returns immediately. The code
following the throw is not executed. The second argument of
throw is used as the return value of the catch.
The function throw finds the matching catch based on the
first argument: it searches for a catch whose first argument is
eq to the one specified in the throw. If there is more
than one applicable catch, the innermost one takes precedence.
Thus, in the above example, the throw specifies foo, and
the catch in foo-outer specifies the same symbol, so that
catch is the applicable one (assuming there is no other matching
catch in between).
Executing throw exits all Lisp constructs up to the matching
catch, including function calls. When binding constructs such as
let or function calls are exited in this way, the bindings are
unbound, just as they are when these constructs exit normally
(see Local Variables). Likewise, throw restores the buffer
and position saved by save-excursion (see Excursions), and
the narrowing status saved by save-restriction and the window
selection saved by save-window-excursion (see Window Configurations). It also runs any cleanups established with the
unwind-protect special form when it exits that form
(see Cleanups).
The throw need not appear lexically within the catch
that it jumps to. It can equally well be called from another function
called within the catch. As long as the throw takes place
chronologically after entry to the catch, and chronologically
before exit from it, it has access to that catch. This is why
throw can be used in commands such as exit-recursive-edit
that throw back to the editor command loop (see Recursive Editing).
Common Lisp note: Most other versions of Lisp, including Common Lisp, have several ways of transferring control nonsequentially:return,return-from, andgo, for example. Emacs Lisp has onlythrow.
catchestablishes a return point for thethrowfunction. The return point is distinguished from other such return points by tag, which may be any Lisp object exceptnil. The argument tag is evaluated normally before the return point is established.With the return point in effect,
catchevaluates the forms of the body in textual order. If the forms execute normally (without error or nonlocal exit) the value of the last body form is returned from thecatch.If a
throwis executed during the execution of body, specifying the same value tag, thecatchform exits immediately; the value it returns is whatever was specified as the second argument ofthrow.
The purpose of
throwis to return from a return point previously established withcatch. The argument tag is used to choose among the various existing return points; it must beeqto the value specified in thecatch. If multiple return points match tag, the innermost one is used.The argument value is used as the value to return from that
catch.If no return point is in effect with tag tag, then a
no-catcherror is signaled with data(tag value).
catch and throwOne way to use catch and throw is to exit from a doubly
nested loop. (In most languages, this would be done with a “go to.”)
Here we compute (foo i j) for i and j
varying from 0 to 9:
(defun search-foo ()
(catch 'loop
(let ((i 0))
(while (< i 10)
(let ((j 0))
(while (< j 10)
(if (foo i j)
(throw 'loop (list i j)))
(setq j (1+ j))))
(setq i (1+ i))))))
If foo ever returns non-nil, we stop immediately and return a
list of i and j. If foo always returns nil, the
catch returns normally, and the value is nil, since that
is the result of the while.
Here are two tricky examples, slightly different, showing two
return points at once. First, two return points with the same tag,
hack:
(defun catch2 (tag)
(catch tag
(throw 'hack 'yes)))
catch2
(catch 'hack
(print (catch2 'hack))
'no)
-| yes
no
Since both return points have tags that match the throw, it goes to
the inner one, the one established in catch2. Therefore,
catch2 returns normally with value yes, and this value is
printed. Finally the second body form in the outer catch, which is
'no, is evaluated and returned from the outer catch.
Now let's change the argument given to catch2:
(catch 'hack
(print (catch2 'quux))
'no)
yes
We still have two return points, but this time only the outer one has
the tag hack; the inner one has the tag quux instead.
Therefore, throw makes the outer catch return the value
yes. The function print is never called, and the
body-form 'no is never evaluated.
When Emacs Lisp attempts to evaluate a form that, for some reason, cannot be evaluated, it signals an error.
When an error is signaled, Emacs's default reaction is to print an error message and terminate execution of the current command. This is the right thing to do in most cases, such as if you type C-f at the end of the buffer.
In complicated programs, simple termination may not be what you want.
For example, the program may have made temporary changes in data
structures, or created temporary buffers that should be deleted before
the program is finished. In such cases, you would use
unwind-protect to establish cleanup expressions to be
evaluated in case of error. (See Cleanups.) Occasionally, you may
wish the program to continue execution despite an error in a subroutine.
In these cases, you would use condition-case to establish
error handlers to recover control in case of error.
Resist the temptation to use error handling to transfer control from
one part of the program to another; use catch and throw
instead. See Catch and Throw.
Signaling an error means beginning error processing. Error processing normally aborts all or part of the running program and returns to a point that is set up to handle the error (see Processing of Errors). Here we describe how to signal an error.
Most errors are signaled “automatically” within Lisp primitives
which you call for other purposes, such as if you try to take the
car of an integer or move forward a character at the end of the
buffer. You can also signal errors explicitly with the functions
error and signal.
Quitting, which happens when the user types C-g, is not considered an error, but it is handled almost like an error. See Quitting.
Every error specifies an error message, one way or another. The message should state what is wrong (“File does not exist”), not how things ought to be (“File must exist”). The convention in Emacs Lisp is that error messages should start with a capital letter, but should not end with any sort of punctuation.
This function signals an error with an error message constructed by applying
format(see Formatting Strings) to format-string and args.These examples show typical uses of
error:(error "That is an error -- try something else") error--> That is an error -- try something else (error "You have committed %d errors" 10) error--> You have committed 10 errors
errorworks by callingsignalwith two arguments: the error symbolerror, and a list containing the string returned byformat.Warning: If you want to use your own string as an error message verbatim, don't just write
(errorstring). If string contains ‘%’, it will be interpreted as a format specifier, with undesirable results. Instead, use(error "%s"string).
This function signals an error named by error-symbol. The argument data is a list of additional Lisp objects relevant to the circumstances of the error.
The argument error-symbol must be an error symbol—a symbol bearing a property
error-conditionswhose value is a list of condition names. This is how Emacs Lisp classifies different sorts of errors. See Error Symbols, for a description of error symbols, error conditions and condition names.If the error is not handled, the two arguments are used in printing the error message. Normally, this error message is provided by the
error-messageproperty of error-symbol. If data is non-nil, this is followed by a colon and a comma separated list of the unevaluated elements of data. Forerror, the error message is the car of data (that must be a string). Subcategories offile-errorare handled specially.The number and significance of the objects in data depends on error-symbol. For example, with a
wrong-type-argumenterror, there should be two objects in the list: a predicate that describes the type that was expected, and the object that failed to fit that type.Both error-symbol and data are available to any error handlers that handle the error:
condition-casebinds a local variable to a list of the form(error-symbol.data)(see Handling Errors).The function
signalnever returns (though in older Emacs versions it could sometimes return).(signal 'wrong-number-of-arguments '(x y)) error--> Wrong number of arguments: x, y (signal 'no-such-error '("My unknown error condition")) error--> peculiar error: "My unknown error condition"
Common Lisp note: Emacs Lisp has nothing like the Common Lisp concept of continuable errors.
When an error is signaled, signal searches for an active
handler for the error. A handler is a sequence of Lisp
expressions designated to be executed if an error happens in part of the
Lisp program. If the error has an applicable handler, the handler is
executed, and control resumes following the handler. The handler
executes in the environment of the condition-case that
established it; all functions called within that condition-case
have already been exited, and the handler cannot return to them.
If there is no applicable handler for the error, it terminates the
current command and returns control to the editor command loop. (The
command loop has an implicit handler for all kinds of errors.) The
command loop's handler uses the error symbol and associated data to
print an error message. You can use the variable
command-error-function to control how this is done:
This variable, if non-
nil, specifies a function to use to handle errors that return control to the Emacs command loop. The function should take three arguments: data, a list of the same form thatcondition-casewould bind to its variable; context, a string describing the situation in which the error occurred, or (more often)nil; and caller, the Lisp function which called the primitive that signaled the error.
An error that has no explicit handler may call the Lisp debugger. The
debugger is enabled if the variable debug-on-error (see Error Debugging) is non-nil. Unlike error handlers, the debugger runs
in the environment of the error, so that you can examine values of
variables precisely as they were at the time of the error.
The usual effect of signaling an error is to terminate the command
that is running and return immediately to the Emacs editor command loop.
You can arrange to trap errors occurring in a part of your program by
establishing an error handler, with the special form
condition-case. A simple example looks like this:
(condition-case nil
(delete-file filename)
(error nil))
This deletes the file named filename, catching any error and
returning nil if an error occurs4.
The condition-case construct is often used to trap errors that
are predictable, such as failure to open a file in a call to
insert-file-contents. It is also used to trap errors that are
totally unpredictable, such as when the program evaluates an expression
read from the user.
The second argument of condition-case is called the
protected form. (In the example above, the protected form is a
call to delete-file.) The error handlers go into effect when
this form begins execution and are deactivated when this form returns.
They remain in effect for all the intervening time. In particular, they
are in effect during the execution of functions called by this form, in
their subroutines, and so on. This is a good thing, since, strictly
speaking, errors can be signaled only by Lisp primitives (including
signal and error) called by the protected form, not by the
protected form itself.
The arguments after the protected form are handlers. Each handler
lists one or more condition names (which are symbols) to specify
which errors it will handle. The error symbol specified when an error
is signaled also defines a list of condition names. A handler applies
to an error if they have any condition names in common. In the example
above, there is one handler, and it specifies one condition name,
error, which covers all errors.
The search for an applicable handler checks all the established handlers
starting with the most recently established one. Thus, if two nested
condition-case forms offer to handle the same error, the inner of
the two gets to handle it.
If an error is handled by some condition-case form, this
ordinarily prevents the debugger from being run, even if
debug-on-error says this error should invoke the debugger.
If you want to be able to debug errors that are caught by a
condition-case, set the variable debug-on-signal to a
non-nil value. You can also specify that a particular handler
should let the debugger run first, by writing debug among the
conditions, like this:
(condition-case nil
(delete-file filename)
((debug error) nil))
The effect of debug here is only to prevent
condition-case from suppressing the call to the debugger. Any
given error will invoke the debugger only if debug-on-error and
the other usual filtering mechanisms say it should. See Error Debugging.
Once Emacs decides that a certain handler handles the error, it
returns control to that handler. To do so, Emacs unbinds all variable
bindings made by binding constructs that are being exited, and
executes the cleanups of all unwind-protect forms that are
being exited. Once control arrives at the handler, the body of the
handler executes normally.
After execution of the handler body, execution returns from the
condition-case form. Because the protected form is exited
completely before execution of the handler, the handler cannot resume
execution at the point of the error, nor can it examine variable
bindings that were made within the protected form. All it can do is
clean up and proceed.
Error signaling and handling have some resemblance to throw and
catch (see Catch and Throw), but they are entirely separate
facilities. An error cannot be caught by a catch, and a
throw cannot be handled by an error handler (though using
throw when there is no suitable catch signals an error
that can be handled).
This special form establishes the error handlers handlers around the execution of protected-form. If protected-form executes without error, the value it returns becomes the value of the
condition-caseform; in this case, thecondition-casehas no effect. Thecondition-caseform makes a difference when an error occurs during protected-form.Each of the handlers is a list of the form
(conditions body...). Here conditions is an error condition name to be handled, or a list of condition names (which can includedebugto allow the debugger to run before the handler); body is one or more Lisp expressions to be executed when this handler handles an error. Here are examples of handlers:(error nil) (arith-error (message "Division by zero")) ((arith-error file-error) (message "Either division by zero or failure to open a file"))Each error that occurs has an error symbol that describes what kind of error it is. The
error-conditionsproperty of this symbol is a list of condition names (see Error Symbols). Emacs searches all the activecondition-caseforms for a handler that specifies one or more of these condition names; the innermost matchingcondition-casehandles the error. Within thiscondition-case, the first applicable handler handles the error.After executing the body of the handler, the
condition-casereturns normally, using the value of the last form in the handler body as the overall value.The argument var is a variable.
condition-casedoes not bind this variable when executing the protected-form, only when it handles an error. At that time, it binds var locally to an error description, which is a list giving the particulars of the error. The error description has the form(error-symbol.data). The handler can refer to this list to decide what to do. For example, if the error is for failure opening a file, the file name is the second element of data—the third element of the error description.If var is
nil, that means no variable is bound. Then the error symbol and associated data are not available to the handler.
This function returns the error message string for a given error descriptor. It is useful if you want to handle an error by printing the usual error message for that error. See Definition of signal.
Here is an example of using condition-case to handle the error
that results from dividing by zero. The handler displays the error
message (but without a beep), then returns a very large number.
(defun safe-divide (dividend divisor)
(condition-case err
;; Protected form.
(/ dividend divisor)
;; The handler.
(arith-error ; Condition.
;; Display the usual message for this error.
(message "%s" (error-message-string err))
1000000)))
safe-divide
(safe-divide 5 0)
-| Arithmetic error: (arith-error)
1000000
The handler specifies condition name arith-error so that it will handle only division-by-zero errors. Other kinds of errors will not be handled, at least not by this condition-case. Thus,
(safe-divide nil 3)
error--> Wrong type argument: number-or-marker-p, nil
Here is a condition-case that catches all kinds of errors,
including those signaled with error:
(setq baz 34)
34
(condition-case err
(if (eq baz 35)
t
;; This is a call to the function error.
(error "Rats! The variable %s was %s, not 35" 'baz baz))
;; This is the handler; it is not a form.
(error (princ (format "The error was: %s" err))
2))
-| The error was: (error "Rats! The variable baz was 34, not 35")
2
This construct executes body, ignoring any errors that occur during its execution. If the execution is without error,
ignore-errorsreturns the value of the last form in body; otherwise, it returnsnil.Here's the example at the beginning of this subsection rewritten using
ignore-errors:(ignore-errors (delete-file filename))
When you signal an error, you specify an error symbol to specify the kind of error you have in mind. Each error has one and only one error symbol to categorize it. This is the finest classification of errors defined by the Emacs Lisp language.
These narrow classifications are grouped into a hierarchy of wider
classes called error conditions, identified by condition
names. The narrowest such classes belong to the error symbols
themselves: each error symbol is also a condition name. There are also
condition names for more extensive classes, up to the condition name
error which takes in all kinds of errors (but not quit).
Thus, each error has one or more condition names: error, the
error symbol if that is distinct from error, and perhaps some
intermediate classifications.
In order for a symbol to be an error symbol, it must have an
error-conditions property which gives a list of condition names.
This list defines the conditions that this kind of error belongs to.
(The error symbol itself, and the symbol error, should always be
members of this list.) Thus, the hierarchy of condition names is
defined by the error-conditions properties of the error symbols.
Because quitting is not considered an error, the value of the
error-conditions property of quit is just (quit).
In addition to the error-conditions list, the error symbol
should have an error-message property whose value is a string to
be printed when that error is signaled but not handled. If the
error symbol has no error-message property or if the
error-message property exists, but is not a string, the error
message ‘peculiar error’ is used. See Definition of signal.
Here is how we define a new error symbol, new-error:
(put 'new-error
'error-conditions
'(error my-own-errors new-error))
(error my-own-errors new-error)
(put 'new-error 'error-message "A new error")
"A new error"
This error has three condition names: new-error, the narrowest
classification; my-own-errors, which we imagine is a wider
classification; and error, which is the widest of all.
The error string should start with a capital letter but it should not end with a period. This is for consistency with the rest of Emacs.
Naturally, Emacs will never signal new-error on its own; only
an explicit call to signal (see Definition of signal) in
your code can do this:
(signal 'new-error '(x y))
error--> A new error: x, y
This error can be handled through any of the three condition names.
This example handles new-error and any other errors in the class
my-own-errors:
(condition-case foo
(bar nil t)
(my-own-errors nil))
The significant way that errors are classified is by their condition
names—the names used to match errors with handlers. An error symbol
serves only as a convenient way to specify the intended error message
and list of condition names. It would be cumbersome to give
signal a list of condition names rather than one error symbol.
By contrast, using only error symbols without condition names would
seriously decrease the power of condition-case. Condition names
make it possible to categorize errors at various levels of generality
when you write an error handler. Using error symbols alone would
eliminate all but the narrowest level of classification.
See Standard Errors, for a list of all the standard error symbols and their conditions.
The unwind-protect construct is essential whenever you
temporarily put a data structure in an inconsistent state; it permits
you to make the data consistent again in the event of an error or
throw. (Another more specific cleanup construct that is used only for
changes in buffer contents is the atomic change group; Atomic Changes.)
unwind-protectexecutes body-form with a guarantee that the cleanup-forms will be evaluated if control leaves body-form, no matter how that happens. body-form may complete normally, or execute athrowout of theunwind-protect, or cause an error; in all cases, the cleanup-forms will be evaluated.If body-form finishes normally,
unwind-protectreturns the value of body-form, after it evaluates the cleanup-forms. If body-form does not finish,unwind-protectdoes not return any value in the normal sense.Only body-form is protected by the
unwind-protect. If any of the cleanup-forms themselves exits nonlocally (via athrowor an error),unwind-protectis not guaranteed to evaluate the rest of them. If the failure of one of the cleanup-forms has the potential to cause trouble, then protect it with anotherunwind-protectaround that form.The number of currently active
unwind-protectforms counts, together with the number of local variable bindings, against the limitmax-specpdl-size(see Local Variables).
For example, here we make an invisible buffer for temporary use, and make sure to kill it before finishing:
(save-excursion
(let ((buffer (get-buffer-create " *temp*")))
(set-buffer buffer)
(unwind-protect
body-form
(kill-buffer buffer))))
You might think that we could just as well write (kill-buffer
(current-buffer)) and dispense with the variable buffer.
However, the way shown above is safer, if body-form happens to
get an error after switching to a different buffer! (Alternatively,
you could write another save-excursion around body-form,
to ensure that the temporary buffer becomes current again in time to
kill it.)
Emacs includes a standard macro called with-temp-buffer which
expands into more or less the code shown above (see Current Buffer). Several of the macros defined in
this manual use unwind-protect in this way.
Here is an actual example derived from an FTP package. It creates a
process (see Processes) to try to establish a connection to a remote
machine. As the function ftp-login is highly susceptible to
numerous problems that the writer of the function cannot anticipate, it
is protected with a form that guarantees deletion of the process in the
event of failure. Otherwise, Emacs might fill up with useless
subprocesses.
(let ((win nil))
(unwind-protect
(progn
(setq process (ftp-setup-buffer host file))
(if (setq win (ftp-login process host user password))
(message "Logged in")
(error "Ftp login failed")))
(or win (and process (delete-process process)))))
This example has a small bug: if the user types C-g to
quit, and the quit happens immediately after the function
ftp-setup-buffer returns but before the variable process is
set, the process will not be killed. There is no easy way to fix this bug,
but at least it is very unlikely.
A variable is a name used in a program to stand for a value. Nearly all programming languages have variables of some sort. In the text of a Lisp program, variables are written using the syntax for symbols.
In Lisp, unlike most programming languages, programs are represented primarily as Lisp objects and only secondarily as text. The Lisp objects used for variables are symbols: the symbol name is the variable name, and the variable's value is stored in the value cell of the symbol. The use of a symbol as a variable is independent of its use as a function name. See Symbol Components.
The textual form of a Lisp program is given by the read syntax of the Lisp objects that constitute the program. Hence, a variable in a textual Lisp program is written using the read syntax for the symbol representing the variable.
The simplest way to use a variable is globally. This means that the variable has just one value at a time, and this value is in effect (at least for the moment) throughout the Lisp system. The value remains in effect until you specify a new one. When a new value replaces the old one, no trace of the old value remains in the variable.
You specify a value for a symbol with setq. For example,
(setq x '(a b))
gives the variable x the value (a b). Note that
setq is a special form (see Special Forms); it does not
evaluate its first argument, the name of the variable, but it does
evaluate the second argument, the new value.
Once the variable has a value, you can refer to it by using the symbol itself as an expression. Thus,
x (a b)
assuming the setq form shown above has already been executed.
If you do set the same variable again, the new value replaces the old one:
x
(a b)
(setq x 4)
4
x
4
In Emacs Lisp, certain symbols normally evaluate to themselves. These
include nil and t, as well as any symbol whose name starts
with ‘:’ (these are called keywords). These symbols cannot
be rebound, nor can their values be changed. Any attempt to set or bind
nil or t signals a setting-constant error. The
same is true for a keyword (a symbol whose name starts with ‘:’),
if it is interned in the standard obarray, except that setting such a
symbol to itself is not an error.
nil == 'nil
nil
(setq nil 500)
error--> Attempt to set constant symbol: nil
function returns
tif object is a symbol whose name starts with ‘:’, interned in the standard obarray, and returnsnilotherwise.
Global variables have values that last until explicitly superseded with new values. Sometimes it is useful to create variable values that exist temporarily—only until a certain part of the program finishes. These values are called local, and the variables so used are called local variables.
For example, when a function is called, its argument variables receive
new local values that last until the function exits. The let
special form explicitly establishes new local values for specified
variables; these last until exit from the let form.
Establishing a local value saves away the variable's previous value (or lack of one). We say that the previous value is shadowed and not visible. Both global and local values may be shadowed (see Scope). After the life span of the local value is over, the previous value (or lack of one) is restored.
If you set a variable (such as with setq) while it is local,
this replaces the local value; it does not alter the global value, or
previous local values, that are shadowed. To model this behavior, we
speak of a local binding of the variable as well as a local value.
The local binding is a conceptual place that holds a local value.
Entering a function, or a special form such as let, creates the
local binding; exiting the function or the let removes the
local binding. While the local binding lasts, the variable's value is
stored within it. Using setq or set while there is a
local binding stores a different value into the local binding; it does
not create a new binding.
We also speak of the global binding, which is where (conceptually) the global value is kept.
A variable can have more than one local binding at a time (for
example, if there are nested let forms that bind it). In such a
case, the most recently created local binding that still exists is the
current binding of the variable. (This rule is called
dynamic scoping; see Variable Scoping.) If there are no
local bindings, the variable's global binding is its current binding.
We sometimes call the current binding the most-local existing
binding, for emphasis. Ordinary evaluation of a symbol always returns
the value of its current binding.
The special forms let and let* exist to create
local bindings.
This special form binds variables according to bindings and then evaluates all of the forms in textual order. The
let-form returns the value of the last form in forms.Each of the bindings is either (i) a symbol, in which case that symbol is bound to
nil; or (ii) a list of the form(symbol value-form), in which case symbol is bound to the result of evaluating value-form. If value-form is omitted,nilis used.All of the value-forms in bindings are evaluated in the order they appear and before binding any of the symbols to them. Here is an example of this:
zis bound to the old value ofy, which is 2, not the new value ofy, which is 1.(setq y 2) 2 (let ((y 1) (z y)) (list y z)) (1 2)
This special form is like
let, but it binds each variable right after computing its local value, before computing the local value for the next variable. Therefore, an expression in bindings can reasonably refer to the preceding symbols bound in thislet*form. Compare the following example with the example above forlet.(setq y 2) 2 (let* ((y 1) (z y)) ; Use the just-established value ofy. (list y z)) (1 1)
Here is a complete list of the other facilities that create local bindings:
Variables can also have buffer-local bindings (see Buffer-Local Variables); a few variables have terminal-local bindings (see Multiple Terminals). These kinds of bindings work somewhat like ordinary local bindings, but they are localized depending on “where” you are in Emacs, rather than localized in time.
This variable defines the limit on the total number of local variable bindings and
unwind-protectcleanups (see Cleaning Up from Nonlocal Exits) that are allowed before Emacs signals an error (with data"Variable binding depth exceeds max-specpdl-size").This limit, with the associated error when it is exceeded, is one way that Lisp avoids infinite recursion on an ill-defined function.
max-lisp-eval-depthprovides another limit on depth of nesting. See Eval.The default value is 1000. Entry to the Lisp debugger increases the value, if there is little room left, to make sure the debugger itself has room to execute.
If you have never given a symbol any value as a global variable, we
say that that symbol's global value is void. In other words, the
symbol's value cell does not have any Lisp object in it. If you try to
evaluate the symbol, you get a void-variable error rather than
a value.
Note that a value of nil is not the same as void. The symbol
nil is a Lisp object and can be the value of a variable just as any
other object can be; but it is a value. A void variable does not
have any value.
After you have given a variable a value, you can make it void once more
using makunbound.
This function makes the current variable binding of symbol void. Subsequent attempts to use this symbol's value as a variable will signal the error
void-variable, unless and until you set it again.
makunboundreturns symbol.(makunbound 'x) ; Make the global value ofxvoid. x x error--> Symbol's value as variable is void: xIf symbol is locally bound,
makunboundaffects the most local existing binding. This is the only way a symbol can have a void local binding, since all the constructs that create local bindings create them with values. In this case, the voidness lasts at most as long as the binding does; when the binding is removed due to exit from the construct that made it, the previous local or global binding is reexposed as usual, and the variable is no longer void unless the newly reexposed binding was void all along.(setq x 1) ; Put a value in the global binding. 1 (let ((x 2)) ; Locally bind it. (makunbound 'x) ; Void the local binding. x) error--> Symbol's value as variable is void: x x ; The global binding is unchanged. 1 (let ((x 2)) ; Locally bind it. (let ((x 3)) ; And again. (makunbound 'x) ; Void the innermost-local binding. x)) ; And refer: it's void. error--> Symbol's value as variable is void: x (let ((x 2)) (let ((x 3)) (makunbound 'x)) ; Void inner binding, then remove it. x) ; Now outerletbinding is visible. 2
A variable that has been made void with makunbound is
indistinguishable from one that has never received a value and has
always been void.
You can use the function boundp to test whether a variable is
currently void.
boundpreturnstif variable (a symbol) is not void; more precisely, if its current binding is not void. It returnsnilotherwise.(boundp 'abracadabra) ; Starts out void. nil (let ((abracadabra 5)) ; Locally bind it. (boundp 'abracadabra)) t (boundp 'abracadabra) ; Still globally void. nil (setq abracadabra 5) ; Make it globally nonvoid. 5 (boundp 'abracadabra) t
You may announce your intention to use a symbol as a global variable
with a variable definition: a special form, either defconst
or defvar.
In Emacs Lisp, definitions serve three purposes. First, they inform
people who read the code that certain symbols are intended to be
used a certain way (as variables). Second, they inform the Lisp system
of these things, supplying a value and documentation. Third, they
provide information to utilities such as etags and
make-docfile, which create data bases of the functions and
variables in a program.
The difference between defconst and defvar is primarily
a matter of intent, serving to inform human readers of whether the value
should ever change. Emacs Lisp does not restrict the ways in which a
variable can be used based on defconst or defvar
declarations. However, it does make a difference for initialization:
defconst unconditionally initializes the variable, while
defvar initializes it only if it is void.
This special form defines symbol as a variable and can also initialize and document it. The definition informs a person reading your code that symbol is used as a variable that might be set or changed. Note that symbol is not evaluated; the symbol to be defined must appear explicitly in the
defvar.If symbol is void and value is specified,
defvarevaluates it and sets symbol to the result. But if symbol already has a value (i.e., it is not void), value is not even evaluated, and symbol's value remains unchanged. If value is omitted, the value of symbol is not changed in any case.If symbol has a buffer-local binding in the current buffer,
defvaroperates on the default value, which is buffer-independent, not the current (buffer-local) binding. It sets the default value if the default value is void. See Buffer-Local Variables.When you evaluate a top-level
defvarform with C-M-x in Emacs Lisp mode (eval-defun), a special feature ofeval-defunarranges to set the variable unconditionally, without testing whether its value is void.If the doc-string argument appears, it specifies the documentation for the variable. (This opportunity to specify documentation is one of the main benefits of defining the variable.) The documentation is stored in the symbol's
variable-documentationproperty. The Emacs help functions (see Documentation) look for this property.If the documentation string begins with the character ‘*’, Emacs allows users to set it interactively using the
set-variablecommand. However, you should nearly always usedefcustominstead ofdefvarto define such variables, so that users can use M-x customize and related commands to set them. In that case, it is not necessary to begin the documentation string with ‘*’. See Customization.Here are some examples. This form defines
foobut does not initialize it:(defvar foo) fooThis example initializes the value of
barto23, and gives it a documentation string:(defvar bar 23 "The normal weight of a bar.") barThe following form changes the documentation string for
bar, making it a user option, but does not change the value, sincebaralready has a value. (The addition(1+ nil)would get an error if it were evaluated, but since it is not evaluated, there is no error.)(defvar bar (1+ nil) "*The normal weight of a bar.") bar bar 23Here is an equivalent expression for the
defvarspecial form:(defvar symbol value doc-string) == (progn (if (not (boundp 'symbol)) (setq symbol value)) (if 'doc-string (put 'symbol 'variable-documentation 'doc-string)) 'symbol)The
defvarform returns symbol, but it is normally used at top level in a file where its value does not matter.
This special form defines symbol as a value and initializes it. It informs a person reading your code that symbol has a standard global value, established here, that should not be changed by the user or by other programs. Note that symbol is not evaluated; the symbol to be defined must appear explicitly in the
defconst.
defconstalways evaluates value, and sets the value of symbol to the result. If symbol does have a buffer-local binding in the current buffer,defconstsets the default value, not the buffer-local value. (But you should not be making buffer-local bindings for a symbol that is defined withdefconst.)Here,
piis a constant that presumably ought not to be changed by anyone (attempts by the Indiana State Legislature notwithstanding). As the second form illustrates, however, this is only advisory.(defconst pi 3.1415 "Pi to five places.") pi (setq pi 3) pi pi 3
This function returns
tif variable is a user option—a variable intended to be set by the user for customization—andnilotherwise. (Variables other than user options exist for the internal purposes of Lisp programs, and users need not know about them.)User option variables are distinguished from other variables either though being declared using
defcustom5 or by the first character of theirvariable-documentationproperty. If the property exists and is a string, and its first character is ‘*’, then the variable is a user option. Aliases of user options are also user options.
If a user option variable has a variable-interactive property,
the set-variable command uses that value to control reading the
new value for the variable. The property's value is used as if it were
specified in interactive (see Using Interactive). However,
this feature is largely obsoleted by defcustom
(see Customization).
Warning: If the defconst and defvar special
forms are used while the variable has a local binding (made with
let, or a function argument), they set the local-binding's
value; the top-level binding is not changed. This is not what you
usually want. To prevent it, use these special forms at top level in
a file, where normally no local binding is in effect, and make sure to
load the file before making a local binding for the variable.
When you define a variable whose value is a function, or a list of functions, use a name that ends in ‘-function’ or ‘-functions’, respectively.
There are several other variable name conventions; here is a complete list:
nil for “good” arguments and nil for “bad”
arguments.
nil or not.
Since such variables often end up acquiring more values over time,
this convention is not strongly recommended.
When you define a variable, always consider whether you should mark it as “safe” or “risky”; see File Local Variables.
When defining and initializing a variable that holds a complicated
value (such as a keymap with bindings in it), it's best to put the
entire computation of the value into the defvar, like this:
(defvar my-mode-map
(let ((map (make-sparse-keymap)))
(define-key map "\C-c\C-a" 'my-command)
...
map)
docstring)
This method has several benefits. First, if the user quits while
loading the file, the variable is either still uninitialized or
initialized properly, never in-between. If it is still uninitialized,
reloading the file will initialize it properly. Second, reloading the
file once the variable is initialized will not alter it; that is
important if the user has run hooks to alter part of the contents (such
as, to rebind keys). Third, evaluating the defvar form with
C-M-x will reinitialize the map completely.
Putting so much code in the defvar form has one disadvantage:
it puts the documentation string far away from the line which names the
variable. Here's a safe way to avoid that:
(defvar my-mode-map nil
docstring)
(unless my-mode-map
(let ((map (make-sparse-keymap)))
(define-key map "\C-c\C-a" 'my-command)
...
(setq my-mode-map map)))
This has all the same advantages as putting the initialization inside
the defvar, except that you must type C-M-x twice, once on
each form, if you do want to reinitialize the variable.
But be careful not to write the code like this:
(defvar my-mode-map nil
docstring)
(unless my-mode-map
(setq my-mode-map (make-sparse-keymap))
(define-key my-mode-map "\C-c\C-a" 'my-command)
...)
This code sets the variable, then alters it, but it does so in more than
one step. If the user quits just after the setq, that leaves the
variable neither correctly initialized nor void nor nil. Once
that happens, reloading the file will not initialize the variable; it
will remain incomplete.
The usual way to reference a variable is to write the symbol which
names it (see Symbol Forms). This requires you to specify the
variable name when you write the program. Usually that is exactly what
you want to do. Occasionally you need to choose at run time which
variable to reference; then you can use symbol-value.
This function returns the value of symbol. This is the value in the innermost local binding of the symbol, or its global value if it has no local bindings.
(setq abracadabra 5) 5 (setq foo 9) 9 ;; Here the symbolabracadabra;; is the symbol whose value is examined. (let ((abracadabra 'foo)) (symbol-value 'abracadabra)) foo ;; Here, the value ofabracadabra, ;; which isfoo, ;; is the symbol whose value is examined. (let ((abracadabra 'foo)) (symbol-value abracadabra)) 9 (symbol-value 'abracadabra) 5A
void-variableerror is signaled if the current binding of symbol is void.
The usual way to change the value of a variable is with the special
form setq. When you need to compute the choice of variable at
run time, use the function set.
This special form is the most common method of changing a variable's value. Each symbol is given a new value, which is the result of evaluating the corresponding form. The most-local existing binding of the symbol is changed.
setqdoes not evaluate symbol; it sets the symbol that you write. We say that this argument is automatically quoted. The ‘q’ insetqstands for “quoted.”The value of the
setqform is the value of the last form.(setq x (1+ 2)) 3 x ;xnow has a global value. 3 (let ((x 5)) (setq x 6) ; The local binding ofxis set. x) 6 x ; The global value is unchanged. 3Note that the first form is evaluated, then the first symbol is set, then the second form is evaluated, then the second symbol is set, and so on:
(setq x 10 ; Notice thatxis set before y (1+ x)) ; the value ofyis computed. 11
This function sets symbol's value to value, then returns value. Since
setis a function, the expression written for symbol is evaluated to obtain the symbol to set.The most-local existing binding of the variable is the binding that is set; shadowed bindings are not affected.
(set one 1) error--> Symbol's value as variable is void: one (set 'one 1) 1 (set 'two 'one) one (set two 2) ;twoevaluates to symbolone. 2 one ; So it isonethat was set. 2 (let ((one 1)) ; This binding ofoneis set, (set 'one 3) ; not the global value. one) 3 one 2If symbol is not actually a symbol, a
wrong-type-argumenterror is signaled.(set '(x y) 'z) error--> Wrong type argument: symbolp, (x y)Logically speaking,
setis a more fundamental primitive thansetq. Any use ofsetqcan be trivially rewritten to useset;setqcould even be defined as a macro, given the availability ofset. However,setitself is rarely used; beginners hardly need to know about it. It is useful only for choosing at run time which variable to set. For example, the commandset-variable, which reads a variable name from the user and then sets the variable, needs to useset.Common Lisp note: In Common Lisp,setalways changes the symbol's “special” or dynamic value, ignoring any lexical bindings. In Emacs Lisp, all variables and all bindings are dynamic, sosetalways affects the most local existing binding.
A given symbol foo can have several local variable bindings,
established at different places in the Lisp program, as well as a global
binding. The most recently established binding takes precedence over
the others.
Local bindings in Emacs Lisp have indefinite scope and dynamic extent. Scope refers to where textually in the source code the binding can be accessed. “Indefinite scope” means that any part of the program can potentially access the variable binding. Extent refers to when, as the program is executing, the binding exists. “Dynamic extent” means that the binding lasts as long as the activation of the construct that established it.
The combination of dynamic extent and indefinite scope is called dynamic scoping. By contrast, most programming languages use lexical scoping, in which references to a local variable must be located textually within the function or block that binds the variable.
Common Lisp note: Variables declared “special” in Common Lisp are dynamically scoped, like all variables in Emacs Lisp.
Emacs Lisp uses indefinite scope for local variable bindings. This means that any function anywhere in the program text might access a given binding of a variable. Consider the following function definitions:
(defun binder (x) ;xis bound inbinder. (foo 5)) ;foois some other function. (defun user () ;xis used ``free'' inuser. (list x))
In a lexically scoped language, the binding of x in
binder would never be accessible in user, because
user is not textually contained within the function
binder. However, in dynamically-scoped Emacs Lisp, user
may or may not refer to the binding of x established in
binder, depending on the circumstances:
user directly without calling binder at all,
then whatever binding of x is found, it cannot come from
binder.
foo as follows and then call binder, then the
binding made in binder will be seen in user:
(defun foo (lose)
(user))
foo as follows and then call binder,
then the binding made in binder will not be seen in
user:
(defun foo (x)
(user))
Here, when foo is called by binder, it binds x.
(The binding in foo is said to shadow the one made in
binder.) Therefore, user will access the x bound
by foo instead of the one bound by binder.
Emacs Lisp uses dynamic scoping because simple implementations of lexical scoping are slow. In addition, every Lisp system needs to offer dynamic scoping at least as an option; if lexical scoping is the norm, there must be a way to specify dynamic scoping instead for a particular variable. It might not be a bad thing for Emacs to offer both, but implementing it with dynamic scoping only was much easier.
Extent refers to the time during program execution that a variable name is valid. In Emacs Lisp, a variable is valid only while the form that bound it is executing. This is called dynamic extent. “Local” or “automatic” variables in most languages, including C and Pascal, have dynamic extent.
One alternative to dynamic extent is indefinite extent. This means that a variable binding can live on past the exit from the form that made the binding. Common Lisp and Scheme, for example, support this, but Emacs Lisp does not.
To illustrate this, the function below, make-add, returns a
function that purports to add n to its own argument m. This
would work in Common Lisp, but it does not do the job in Emacs Lisp,
because after the call to make-add exits, the variable n
is no longer bound to the actual argument 2.
(defun make-add (n)
(function (lambda (m) (+ n m)))) ; Return a function.
make-add
(fset 'add2 (make-add 2)) ; Define function add2
; with (make-add 2).
(lambda (m) (+ n m))
(add2 4) ; Try to add 2 to 4.
error--> Symbol's value as variable is void: n
Some Lisp dialects have “closures,” objects that are like functions but record additional variable bindings. Emacs Lisp does not have closures.
A simple sample implementation (which is not how Emacs Lisp actually works) may help you understand dynamic binding. This technique is called deep binding and was used in early Lisp systems.
Suppose there is a stack of bindings, which are variable-value pairs.
At entry to a function or to a let form, we can push bindings
onto the stack for the arguments or local variables created there. We
can pop those bindings from the stack at exit from the binding
construct.
We can find the value of a variable by searching the stack from top to bottom for a binding for that variable; the value from that binding is the value of the variable. To set the variable, we search for the current binding, then store the new value into that binding.
As you can see, a function's bindings remain in effect as long as it continues execution, even during its calls to other functions. That is why we say the extent of the binding is dynamic. And any other function can refer to the bindings, if it uses the same variables while the bindings are in effect. That is why we say the scope is indefinite.
The actual implementation of variable scoping in GNU Emacs Lisp uses a technique called shallow binding. Each variable has a standard place in which its current value is always found—the value cell of the symbol.
In shallow binding, setting the variable works by storing a value in the value cell. Creating a new binding works by pushing the old value (belonging to a previous binding) onto a stack, and storing the new local value in the value cell. Eliminating a binding works by popping the old value off the stack, into the value cell.
We use shallow binding because it has the same results as deep binding, but runs faster, since there is never a need to search for a binding.
Binding a variable in one function and using it in another is a powerful technique, but if used without restraint, it can make programs hard to understand. There are two clean ways to use this technique:
You should write comments to inform other programmers that they can see all uses of the variable before them, and to advise them not to add uses elsewhere.
case-fold-search is defined as “non-nil means ignore case
when searching”; various search and replace functions refer to it
directly or through their subroutines, but do not bind or set it.
Then you can bind the variable in other programs, knowing reliably what the effect will be.
In either case, you should define the variable with defvar.
This helps other people understand your program by telling them to look
for inter-function usage. It also avoids a warning from the byte
compiler. Choose the variable's name to avoid name conflicts—don't
use short names like x.
Global and local variable bindings are found in most programming languages in one form or another. Emacs, however, also supports additional, unusual kinds of variable binding, such as buffer-local bindings, which apply only in one buffer. Having different values for a variable in different buffers is an important customization method. (Variables can also have bindings that are local to each terminal, or to each frame. See Multiple Terminals, and See Frame-Local Variables.)
A buffer-local variable has a buffer-local binding associated with a particular buffer. The binding is in effect when that buffer is current; otherwise, it is not in effect. If you set the variable while a buffer-local binding is in effect, the new value goes in that binding, so its other bindings are unchanged. This means that the change is visible only in the buffer where you made it.
The variable's ordinary binding, which is not associated with any specific buffer, is called the default binding. In most cases, this is the global binding.
A variable can have buffer-local bindings in some buffers but not in other buffers. The default binding is shared by all the buffers that don't have their own bindings for the variable. (This includes all newly-created buffers.) If you set the variable in a buffer that does not have a buffer-local binding for it, this sets the default binding, so the new value is visible in all the buffers that see the default binding.
The most common use of buffer-local bindings is for major modes to change
variables that control the behavior of commands. For example, C mode and
Lisp mode both set the variable paragraph-start to specify that only
blank lines separate paragraphs. They do this by making the variable
buffer-local in the buffer that is being put into C mode or Lisp mode, and
then setting it to the new value for that mode. See Major Modes.
The usual way to make a buffer-local binding is with
make-local-variable, which is what major mode commands typically
use. This affects just the current buffer; all other buffers (including
those yet to be created) will continue to share the default value unless
they are explicitly given their own buffer-local bindings.
A more powerful operation is to mark the variable as
automatically buffer-local by calling
make-variable-buffer-local. You can think of this as making the
variable local in all buffers, even those yet to be created. More
precisely, the effect is that setting the variable automatically makes
the variable local to the current buffer if it is not already so. All
buffers start out by sharing the default value of the variable as usual,
but setting the variable creates a buffer-local binding for the current
buffer. The new value is stored in the buffer-local binding, leaving
the default binding untouched. This means that the default value cannot
be changed with setq in any buffer; the only way to change it is
with setq-default.
Warning: When a variable has buffer-local
bindings in one or more buffers, let rebinds the binding that's
currently in effect. For instance, if the current buffer has a
buffer-local value, let temporarily rebinds that. If no
buffer-local bindings are in effect, let rebinds
the default value. If inside the let you then change to a
different current buffer in which a different binding is in effect,
you won't see the let binding any more. And if you exit the
let while still in the other buffer, you won't see the
unbinding occur (though it will occur properly). Here is an example
to illustrate:
(setq foo 'g)
(set-buffer "a")
(make-local-variable 'foo)
(setq foo 'a)
(let ((foo 'temp))
;; foo 'temp ; let binding in buffer ‘a’
(set-buffer "b")
;; foo 'g ; the global value since foo is not local in ‘b’
body...)
foo 'g ; exiting restored the local value in buffer ‘a’,
; but we don't see that in buffer ‘b’
(set-buffer "a") ; verify the local value was restored
foo 'a
Note that references to foo in body access the
buffer-local binding of buffer ‘b’.
When a file specifies local variable values, these become buffer-local values when you visit the file. See File Variables.
A buffer-local variable cannot be made frame-local (see Frame-Local Variables) or terminal-local (see Multiple Terminals).
This function creates a buffer-local binding in the current buffer for variable (a symbol). Other buffers are not affected. The value returned is variable.
The buffer-local value of variable starts out as the same value variable previously had. If variable was void, it remains void.
;; In buffer ‘b1’: (setq foo 5) ; Affects all buffers. 5 (make-local-variable 'foo) ; Now it is local in ‘b1’. foo foo ; That did not change 5 ; the value. (setq foo 6) ; Change the value 6 ; in ‘b1’. foo 6 ;; In buffer ‘b2’, the value hasn't changed. (save-excursion (set-buffer "b2") foo) 5Making a variable buffer-local within a
let-binding for that variable does not work reliably, unless the buffer in which you do this is not current either on entry to or exit from thelet. This is becauseletdoes not distinguish between different kinds of bindings; it knows only which variable the binding was made for.If the variable is terminal-local (see Multiple Terminals), or frame-local (see Frame-Local Variables), this function signals an error. Such variables cannot have buffer-local bindings as well.
Warning: do not use
make-local-variablefor a hook variable. The hook variables are automatically made buffer-local as needed if you use the local argument toadd-hookorremove-hook.
This function marks variable (a symbol) automatically buffer-local, so that any subsequent attempt to set it will make it local to the current buffer at the time.
A peculiar wrinkle of this feature is that binding the variable (with
letor other binding constructs) does not create a buffer-local binding for it. Only setting the variable (withsetorsetq), while the variable does not have alet-style binding that was made in the current buffer, does so.If variable does not have a default value, then calling this command will give it a default value of
nil. If variable already has a default value, that value remains unchanged. Subsequently callingmakunboundon variable will result in a void buffer-local value and leave the default value unaffected.The value returned is variable.
Warning: Don't assume that you should use
make-variable-buffer-localfor user-option variables, simply because users might want to customize them differently in different buffers. Users can make any variable local, when they wish to. It is better to leave the choice to them.The time to use
make-variable-buffer-localis when it is crucial that no two buffers ever share the same binding. For example, when a variable is used for internal purposes in a Lisp program which depends on having separate values in separate buffers, then usingmake-variable-buffer-localcan be the best solution.
This returns
tif variable is buffer-local in buffer buffer (which defaults to the current buffer); otherwise,nil.
This returns
tif variable will become buffer-local in buffer buffer (which defaults to the current buffer) if it is set there.
This function returns the buffer-local binding of variable (a symbol) in buffer buffer. If variable does not have a buffer-local binding in buffer buffer, it returns the default value (see Default Value) of variable instead.
This function returns a list describing the buffer-local variables in buffer buffer. (If buffer is omitted, the current buffer is used.) It returns an association list (see Association Lists) in which each element contains one buffer-local variable and its value. However, when a variable's buffer-local binding in buffer is void, then the variable appears directly in the resulting list.
(make-local-variable 'foobar) (makunbound 'foobar) (make-local-variable 'bind-me) (setq bind-me 69) (setq lcl (buffer-local-variables)) ;; First, built-in variables local in all buffers: ((mark-active . nil) (buffer-undo-list . nil) (mode-name . "Fundamental") ... ;; Next, non-built-in buffer-local variables. ;; This one is buffer-local and void: foobar ;; This one is buffer-local and nonvoid: (bind-me . 69))Note that storing new values into the cdrs of cons cells in this list does not change the buffer-local values of the variables.
This function deletes the buffer-local binding (if any) for variable (a symbol) in the current buffer. As a result, the default binding of variable becomes visible in this buffer. This typically results in a change in the value of variable, since the default value is usually different from the buffer-local value just eliminated.
If you kill the buffer-local binding of a variable that automatically becomes buffer-local when set, this makes the default value visible in the current buffer. However, if you set the variable again, that will once again create a buffer-local binding for it.
kill-local-variablereturns variable.This function is a command because it is sometimes useful to kill one buffer-local variable interactively, just as it is useful to create buffer-local variables interactively.
This function eliminates all the buffer-local variable bindings of the current buffer except for variables marked as “permanent” and local hook functions that have a non-
nilpermanent-local-hookproperty (see Setting Hooks). As a result, the buffer will see the default values of most variables.This function also resets certain other information pertaining to the buffer: it sets the local keymap to
nil, the syntax table to the value of(standard-syntax-table), the case table to(standard-case-table), and the abbrev table to the value offundamental-mode-abbrev-table.The very first thing this function does is run the normal hook
change-major-mode-hook(see below).Every major mode command begins by calling this function, which has the effect of switching to Fundamental mode and erasing most of the effects of the previous major mode. To ensure that this does its job, the variables that major modes set should not be marked permanent.
kill-all-local-variablesreturnsnil.
The function
kill-all-local-variablesruns this normal hook before it does anything else. This gives major modes a way to arrange for something special to be done if the user switches to a different major mode. It is also useful for buffer-specific minor modes that should be forgotten if the user changes the major mode.For best results, make this variable buffer-local, so that it will disappear after doing its job and will not interfere with the subsequent major mode. See Hooks.
A buffer-local variable is permanent if the variable name (a
symbol) has a permanent-local property that is non-nil.
Permanent locals are appropriate for data pertaining to where the file
came from or how to save it, rather than with how to edit the contents.
The global value of a variable with buffer-local bindings is also called the default value, because it is the value that is in effect whenever neither the current buffer nor the selected frame has its own binding for the variable.
The functions default-value and setq-default access and
change a variable's default value regardless of whether the current
buffer has a buffer-local binding. For example, you could use
setq-default to change the default setting of
paragraph-start for most buffers; and this would work even when
you are in a C or Lisp mode buffer that has a buffer-local value for
this variable.
The special forms defvar and defconst also set the
default value (if they set the variable at all), rather than any
buffer-local value.
This function returns symbol's default value. This is the value that is seen in buffers and frames that do not have their own values for this variable. If symbol is not buffer-local, this is equivalent to
symbol-value(see Accessing Variables).
The function
default-boundptells you whether symbol's default value is nonvoid. If(default-boundp 'foo)returnsnil, then(default-value 'foo)would get an error.
default-boundpis todefault-valueasboundpis tosymbol-value.
This special form gives each symbol a new default value, which is the result of evaluating the corresponding form. It does not evaluate symbol, but does evaluate form. The value of the
setq-defaultform is the value of the last form.If a symbol is not buffer-local for the current buffer, and is not marked automatically buffer-local,
setq-defaulthas the same effect assetq. If symbol is buffer-local for the current buffer, then this changes the value that other buffers will see (as long as they don't have a buffer-local value), but not the value that the current buffer sees.;; In buffer ‘foo’: (make-local-variable 'buffer-local) buffer-local (setq buffer-local 'value-in-foo) value-in-foo (setq-default buffer-local 'new-default) new-default buffer-local value-in-foo (default-value 'buffer-local) new-default ;; In (the new) buffer ‘bar’: buffer-local new-default (default-value 'buffer-local) new-default (setq buffer-local 'another-default) another-default (default-value 'buffer-local) another-default ;; Back in buffer ‘foo’: buffer-local value-in-foo (default-value 'buffer-local) another-default
This function is like
setq-default, except that symbol is an ordinary evaluated argument.(set-default (car '(a b c)) 23) 23 (default-value 'a) 23
A file can specify local variable values; Emacs uses these to create buffer-local bindings for those variables in the buffer visiting that file. See Local Variables in Files, for basic information about file-local variables. This section describes the functions and variables that affect how file-local variables are processed.
If a file-local variable could specify an arbitrary function or Lisp expression that would be called later, visiting a file could take over your Emacs. Emacs protects against this by automatically setting only those file-local variables whose specified values are known to be safe. Other file-local variables are set only if the user agrees.
For additional safety, read-circle is temporarily bound to
nil when Emacs reads file-local variables (see Input Functions). This prevents the Lisp reader from recognizing circular
and shared Lisp structures (see Circular Objects).
This variable controls whether to process file-local variables. The possible values are:
t(the default)- Set the safe variables, and query (once) about any unsafe variables.
:safe- Set only the safe variables and do not query.
:all- Set all the variables and do not query.
nil- Don't set any variables.
- anything else
- Query (once) about all the variables.
This function parses, and binds or evaluates as appropriate, any local variables specified by the contents of the current buffer. The variable
enable-local-variableshas its effect here. However, this function does not look for the ‘mode:’ local variable in the ‘-*-’ line.set-auto-modedoes that, also takingenable-local-variablesinto account (see Auto Major Mode).This function works by walking the alist stored in
file-local-variables-alistand applying each local variable in turn. It callsbefore-hack-local-variables-hookandhack-local-variables-hookbefore and after applying the variables, respectively.If the optional argument mode-only is non-
nil, then all this function does is returntif the ‘-*-’ line or the local variables list specifies a mode andnilotherwise. It does not set the mode nor any other file-local variable.
This buffer-local variable holds the alist of file-local variable settings. Each element of the alist is of the form
(var.value), where var is a symbol of the local variable and value is its value. When Emacs visits a file, it first collects all the file-local variables into this alist, and then thehack-local-variablesfunction applies them one by one.
Emacs calls this hook immediately before applying file-local variables stored in
file-local-variables-alist.
Emacs calls this hook immediately after it finishes applying file-local variables stored in
file-local-variables-alist.
You can specify safe values for a variable with a
safe-local-variable property. The property has to be a
function of one argument; any value is safe if the function returns
non-nil given that value. Many commonly-encountered file
variables have safe-local-variable properties; these include
fill-column, fill-prefix, and indent-tabs-mode.
For boolean-valued variables that are safe, use booleanp as the
property value. Lambda expressions should be quoted so that
describe-variable can display the predicate.
This variable provides another way to mark some variable values as safe. It is a list of cons cells
(var.val), where var is a variable name and val is a value which is safe for that variable.When Emacs asks the user whether or not to obey a set of file-local variable specifications, the user can choose to mark them as safe. Doing so adds those variable/value pairs to
safe-local-variable-values, and saves it to the user's custom file.
This function returns non-
nilif it is safe to give sym the value val, based on the above criteria.
Some variables are considered risky. A variable whose name
ends in any of ‘-command’, ‘-frame-alist’, ‘-function’,
‘-functions’, ‘-hook’, ‘-hooks’, ‘-form’,
‘-forms’, ‘-map’, ‘-map-alist’, ‘-mode-alist’,
‘-program’, or ‘-predicate’ is considered risky. The
variables ‘font-lock-keywords’, ‘font-lock-keywords’
followed by a digit, and ‘font-lock-syntactic-keywords’ are also
considered risky. Finally, any variable whose name has a
non-nil risky-local-variable property is considered
risky.
This function returns non-
nilif sym is a risky variable, based on the above criteria.
If a variable is risky, it will not be entered automatically into
safe-local-variable-values as described above. Therefore,
Emacs will always query before setting a risky variable, unless the
user explicitly allows the setting by customizing
safe-local-variable-values directly.
This variable holds a list of variables that should not be given local values by files. Any value specified for one of these variables is completely ignored.
The ‘Eval:’ “variable” is also a potential loophole, so Emacs normally asks for confirmation before handling it.
This variable controls processing of ‘Eval:’ in ‘-*-’ lines or local variables lists in files being visited. A value of
tmeans process them unconditionally;nilmeans ignore them; anything else means ask the user what to do for each file. The default value ismaybe.
This variable holds a list of expressions that are safe to evaluate when found in the ‘Eval:’ “variable” in a file local variables list.
If the expression is a function call and the function has a
safe-local-eval-function property, the property value
determines whether the expression is safe to evaluate. The property
value can be a predicate to call to test the expression, a list of
such predicates (it's safe if any predicate succeeds), or t
(always safe provided the arguments are constant).
Text properties are also potential loopholes, since their values could include functions to call. So Emacs discards all text properties from string values specified for file-local variables.
A directory can specify local variable values common to all files in that directory; Emacs uses these to create buffer-local bindings for those variables in buffers visiting any file in that directory. This is useful when the files in the directory belong to some project and therefore share the same local variables.
There are two different methods for specifying directory local variables: by putting them in a special file, or by defining a project class for that directory.
This constant is the name of the file where Emacs expects to find the directory-local variables. The name of the file is .dir-locals.el6. A file by that name in a directory causes Emacs to apply its settings to any file in that directory or any of its subdirectories. If some of the subdirectories have their own .dir-locals.el files, Emacs uses the settings from the deepest file it finds starting from the file's directory and moving up the directory tree. The file specifies local variables as a specially formatted list; see Per-directory Local Variables, for more details.
This function reads the
.dir-locals.elfile and stores the directory-local variables infile-local-variables-alistthat is local to the buffer visiting any file in the directory, without applying them. It also stores the directory-local settings indir-locals-class-alist, where it defines a special class for the directory in which .dir-locals.el file was found. This function works by callingdir-locals-set-class-variablesanddir-locals-set-directory-class, described below.
This function defines a set of variable settings for the named class, which is a symbol. You can later assign the class to one or more directories, and Emacs will apply those variable settings to all files in those directories. The list in variables can be of one of the two forms:
(major-mode.alist)or(directory.list). With the first form, if the file's buffer turns on a mode that is derived from major-mode, then the all the variables in the associated alist are applied; alist should be of the form(name.value). A special valuenilfor major-mode means the settings are applicable to any mode.With the second form of variables, if directory is the initial substring of the file's directory, then list is applied recursively by following the above rules; list should be of one of the two forms accepted by this function in variables.
This function assigns class to all the files in
directoryand its subdirectories. Thereafter, all the variable settings specified for class will be applied to any visited file in directory and its children. class must have been already defined bydir-locals-set-class-variables
This alist holds the class symbols and the associated variable settings. It is updated by
dir-locals-set-class-variables.
This alist holds directory names, their assigned class names, and modification times of the associated directory local variables file. It is updated by
dir-locals-set-directory-class.
In addition to buffer-local variable bindings (see Buffer-Local Variables), Emacs supports frame-local bindings. A frame-local binding for a variable is in effect in a frame for which it was defined.
In practice, frame-local variables have not proven very useful.
Ordinary frame parameters are generally used instead (see Frame Parameters). The function make-variable-frame-local, which
was used to define frame-local variables, has been deprecated since
Emacs 22.2. However, you can still define a frame-specific binding
for a variable var in frame frame, by setting the
var frame parameter for that frame:
(modify-frame-parameters frame '((var . value)))
This causes the variable var to be bound to the specified
value in the named frame. To check the frame-specific
values of such variables, use frame-parameter. See Parameter Access.
Note that you cannot have a frame-local binding for a variable that has a buffer-local binding.
It is sometimes useful to make two variables synonyms, so that both
variables always have the same value, and changing either one also
changes the other. Whenever you change the name of a
variable—either because you realize its old name was not well
chosen, or because its meaning has partly changed—it can be useful
to keep the old name as an alias of the new one for
compatibility. You can do this with defvaralias.
This function defines the symbol new-alias as a variable alias for symbol base-variable. This means that retrieving the value of new-alias returns the value of base-variable, and changing the value of new-alias changes the value of base-variable. The two aliased variable names always share the same value and the same bindings.
If the docstring argument is non-
nil, it specifies the documentation for new-alias; otherwise, the alias gets the same documentation as base-variable has, if any, unless base-variable is itself an alias, in which case new-alias gets the documentation of the variable at the end of the chain of aliases.This function returns base-variable.
Variable aliases are convenient for replacing an old name for a
variable with a new name. make-obsolete-variable declares that
the old name is obsolete and therefore that it may be removed at some
stage in the future.
This function makes the byte compiler warn that the variable obsolete-name is obsolete. If current-name is a symbol, it is the variable's new name; then the warning message says to use current-name instead of obsolete-name. If current-name is a string, this is the message and there is no replacement variable.
If provided, when should be a string indicating when the variable was first made obsolete—for example, a date or a release number.
You can make two variables synonyms and declare one obsolete at the
same time using the macro define-obsolete-variable-alias.
This macro marks the variable obsolete-name as obsolete and also makes it an alias for the variable current-name. It is equivalent to the following:
(defvaralias obsolete-name current-name docstring) (make-obsolete-variable obsolete-name current-name when)
This function returns the variable at the end of the chain of aliases of variable. If variable is not a symbol, or if variable is not defined as an alias, the function returns variable.
This function signals a
cyclic-variable-indirectionerror if there is a loop in the chain of symbols.
(defvaralias 'foo 'bar)
(indirect-variable 'foo)
bar
(indirect-variable 'bar)
bar
(setq bar 2)
bar
2
foo
2
(setq foo 0)
bar
0
foo
0
Ordinary Lisp variables can be assigned any value that is a valid
Lisp object. However, certain Lisp variables are not defined in Lisp,
but in C. Most of these variables are defined in the C code using
DEFVAR_LISP. Like variables defined in Lisp, these can take on
any value. However, some variables are defined using
DEFVAR_INT or DEFVAR_BOOL. See Writing Emacs Primitives, in particular the
description of functions of the type syms_of_filename,
for a brief discussion of the C implementation.
Variables of type DEFVAR_BOOL can only take on the values
nil or t. Attempting to assign them any other value
will set them to t:
(let ((display-hourglass 5))
display-hourglass)
t
Variables of type DEFVAR_INT can only take on integer values.
Attempting to assign them any other value will result in an error:
(setq window-min-height 5.0)
error--> Wrong type argument: integerp, 5.0
A Lisp program is composed mainly of Lisp functions. This chapter explains what functions are, how they accept arguments, and how to define them.
In a general sense, a function is a rule for carrying on a computation given several values called arguments. The result of the computation is called the value of the function. The computation can also have side effects: lasting changes in the values of variables or the contents of data structures.
Here are important terms for functions in Emacs Lisp and for other function-like objects.
car or append. These functions are also called
built-in functions, or subrs. (Special forms are also
considered primitives.)
Usually the reason we implement a function as a primitive is either
because it is fundamental, because it provides a low-level interface
to operating system services, or because it needs to run fast.
Primitives can be modified or added only by changing the C sources and
recompiling the editor. See Writing Emacs Primitives.
command-execute can invoke; it
is a possible definition for a key sequence. Some functions are
commands; a function written in Lisp is a command if it contains an
interactive declaration (see Defining Commands). Such a function
can be called from Lisp expressions like other functions; in this case,
the fact that the function is a command makes no difference.
Keyboard macros (strings and vectors) are commands also, even though
they are not functions. A symbol is a command if its function
definition is a command; such symbols can be invoked with M-x.
The symbol is a function as well if the definition is a function.
See Interactive Call.
This function returns
tif object is any kind of function, i.e. can be passed tofuncall. Note thatfunctionpreturnsnilfor special forms (see Special Forms).
Unlike functionp, the next three functions do not
treat a symbol as its function definition.
This function returns
tif object is a built-in function (i.e., a Lisp primitive).(subrp 'message) ;messageis a symbol, nil ; not a subr object. (subrp (symbol-function 'message)) t
This function returns
tif object is a byte-code function. For example:(byte-code-function-p (symbol-function 'next-line)) t
This function provides information about the argument list of a primitive, subr. The returned value is a pair
(min.max). min is the minimum number of args. max is the maximum number or the symbolmany, for a function with&restarguments, or the symbolunevalledif subr is a special form.
A function written in Lisp is a list that looks like this:
(lambda (arg-variables...)
[documentation-string]
[interactive-declaration]
body-forms...)
Such a list is called a lambda expression. In Emacs Lisp, it actually is valid as an expression—it evaluates to itself. In some other Lisp dialects, a lambda expression is not a valid expression at all. In either case, its main use is not to be evaluated as an expression, but to be called as a function.
A function written in Lisp (a “lambda expression”) is a list that looks like this:
(lambda (arg-variables...)
[documentation-string]
[interactive-declaration]
body-forms...)
The first element of a lambda expression is always the symbol
lambda. This indicates that the list represents a function. The
reason functions are defined to start with lambda is so that
other lists, intended for other uses, will not accidentally be valid as
functions.
The second element is a list of symbols—the argument variable names. This is called the lambda list. When a Lisp function is called, the argument values are matched up against the variables in the lambda list, which are given local bindings with the values provided. See Local Variables.
The documentation string is a Lisp string object placed within the function definition to describe the function for the Emacs help facilities. See Function Documentation.
The interactive declaration is a list of the form (interactive
code-string). This declares how to provide arguments if the
function is used interactively. Functions with this declaration are called
commands; they can be called using M-x or bound to a key.
Functions not intended to be called in this way should not have interactive
declarations. See Defining Commands, for how to write an interactive
declaration.
The rest of the elements are the body of the function: the Lisp code to do the work of the function (or, as a Lisp programmer would say, “a list of Lisp forms to evaluate”). The value returned by the function is the value returned by the last element of the body.
Consider for example the following function:
(lambda (a b c) (+ a b c))
We can call this function by writing it as the car of an expression, like this:
((lambda (a b c) (+ a b c))
1 2 3)
This call evaluates the body of the lambda expression with the variable
a bound to 1, b bound to 2, and c bound to 3.
Evaluation of the body adds these three numbers, producing the result 6;
therefore, this call to the function returns the value 6.
Note that the arguments can be the results of other function calls, as in this example:
((lambda (a b c) (+ a b c))
1 (* 2 3) (- 5 4))
This evaluates the arguments 1, (* 2 3), and (- 5
4) from left to right. Then it applies the lambda expression to the
argument values 1, 6 and 1 to produce the value 8.
It is not often useful to write a lambda expression as the car of
a form in this way. You can get the same result, of making local
variables and giving them values, using the special form let
(see Local Variables). And let is clearer and easier to use.
In practice, lambda expressions are either stored as the function
definitions of symbols, to produce named functions, or passed as
arguments to other functions (see Anonymous Functions).
However, calls to explicit lambda expressions were very useful in the
old days of Lisp, before the special form let was invented. At
that time, they were the only way to bind and initialize local
variables.
Our simple sample function, (lambda (a b c) (+ a b c)),
specifies three argument variables, so it must be called with three
arguments: if you try to call it with only two arguments or four
arguments, you get a wrong-number-of-arguments error.
It is often convenient to write a function that allows certain
arguments to be omitted. For example, the function substring
accepts three arguments—a string, the start index and the end
index—but the third argument defaults to the length of the
string if you omit it. It is also convenient for certain functions to
accept an indefinite number of arguments, as the functions list
and + do.
To specify optional arguments that may be omitted when a function
is called, simply include the keyword &optional before the optional
arguments. To specify a list of zero or more extra arguments, include the
keyword &rest before one final argument.
Thus, the complete syntax for an argument list is as follows:
(required-vars...
[&optional optional-vars...]
[&rest rest-var])
The square brackets indicate that the &optional and &rest
clauses, and the variables that follow them, are optional.
A call to the function requires one actual argument for each of the
required-vars. There may be actual arguments for zero or more of
the optional-vars, and there cannot be any actual arguments beyond
that unless the lambda list uses &rest. In that case, there may
be any number of extra actual arguments.
If actual arguments for the optional and rest variables are omitted,
then they always default to nil. There is no way for the
function to distinguish between an explicit argument of nil and
an omitted argument. However, the body of the function is free to
consider nil an abbreviation for some other meaningful value.
This is what substring does; nil as the third argument to
substring means to use the length of the string supplied.
Common Lisp note: Common Lisp allows the function to specify what
default value to use when an optional argument is omitted; Emacs Lisp
always uses nil. Emacs Lisp does not support “supplied-p”
variables that tell you whether an argument was explicitly passed.
For example, an argument list that looks like this:
(a b &optional c d &rest e)
binds a and b to the first two actual arguments, which are
required. If one or two more arguments are provided, c and
d are bound to them respectively; any arguments after the first
four are collected into a list and e is bound to that list. If
there are only two arguments, c is nil; if two or three
arguments, d is nil; if four arguments or fewer, e
is nil.
There is no way to have required arguments following optional
ones—it would not make sense. To see why this must be so, suppose
that c in the example were optional and d were required.
Suppose three actual arguments are given; which variable would the
third argument be for? Would it be used for the c, or for
d? One can argue for both possibilities. Similarly, it makes
no sense to have any more arguments (either required or optional)
after a &rest argument.
Here are some examples of argument lists and proper calls:
((lambda (n) (1+ n)) ; One required: 1) ; requires exactly one argument. 2 ((lambda (n &optional n1) ; One required and one optional: (if n1 (+ n n1) (1+ n))) ; 1 or 2 arguments. 1 2) 3 ((lambda (n &rest ns) ; One required and one rest: (+ n (apply '+ ns))) ; 1 or more arguments. 1 2 3 4 5) 15
A lambda expression may optionally have a documentation string just after the lambda list. This string does not affect execution of the function; it is a kind of comment, but a systematized comment which actually appears inside the Lisp world and can be used by the Emacs help facilities. See Documentation, for how the documentation-string is accessed.
It is a good idea to provide documentation strings for all the functions in your program, even those that are called only from within your program. Documentation strings are like comments, except that they are easier to access.
The first line of the documentation string should stand on its own,
because apropos displays just this first line. It should consist
of one or two complete sentences that summarize the function's purpose.
The start of the documentation string is usually indented in the source file, but since these spaces come before the starting double-quote, they are not part of the string. Some people make a practice of indenting any additional lines of the string so that the text lines up in the program source. That is a mistake. The indentation of the following lines is inside the string; what looks nice in the source code will look ugly when displayed by the help commands.
You may wonder how the documentation string could be optional, since there are required components of the function that follow it (the body). Since evaluation of a string returns that string, without any side effects, it has no effect if it is not the last form in the body. Thus, in practice, there is no confusion between the first form of the body and the documentation string; if the only body form is a string then it serves both as the return value and as the documentation.
The last line of the documentation string can specify calling conventions different from the actual function arguments. Write text like this:
\(fn arglist)
following a blank line, at the beginning of the line, with no newline following it inside the documentation string. (The ‘\’ is used to avoid confusing the Emacs motion commands.) The calling convention specified in this way appears in help messages in place of the one derived from the actual arguments of the function.
This feature is particularly useful for macro definitions, since the arguments written in a macro definition often do not correspond to the way users think of the parts of the macro call.
In most computer languages, every function has a name; the idea of a
function without a name is nonsensical. In Lisp, a function in the
strictest sense has no name. It is simply a list whose first element is
lambda, a byte-code function object, or a primitive subr-object.
However, a symbol can serve as the name of a function. This happens when you put the function in the symbol's function cell (see Symbol Components). Then the symbol itself becomes a valid, callable function, equivalent to the list or subr-object that its function cell refers to. The contents of the function cell are also called the symbol's function definition. The procedure of using a symbol's function definition in place of the symbol is called symbol function indirection; see Function Indirection.
In practice, nearly all functions are given names in this way and
referred to through their names. For example, the symbol car works
as a function and does what it does because the primitive subr-object
#<subr car> is stored in its function cell.
We give functions names because it is convenient to refer to them by
their names in Lisp expressions. For primitive subr-objects such as
#<subr car>, names are the only way you can refer to them: there
is no read syntax for such objects. For functions written in Lisp, the
name is more convenient to use in a call than an explicit lambda
expression. Also, a function with a name can refer to itself—it can
be recursive. Writing the function's name in its own definition is much
more convenient than making the function definition point to itself
(something that is not impossible but that has various disadvantages in
practice).
We often identify functions with the symbols used to name them. For
example, we often speak of “the function car,” not
distinguishing between the symbol car and the primitive
subr-object that is its function definition. For most purposes, the
distinction is not important.
Even so, keep in mind that a function need not have a unique name. While
a given function object usually appears in the function cell of only
one symbol, this is just a matter of convenience. It is easy to store
it in several symbols using fset; then each of the symbols is
equally well a name for the same function.
A symbol used as a function name may also be used as a variable; these two uses of a symbol are independent and do not conflict. (Some Lisp dialects, such as Scheme, do not distinguish between a symbol's value and its function definition; a symbol's value as a variable is also its function definition.) If you have not given a symbol a function definition, you cannot use it as a function; whether the symbol has a value as a variable makes no difference to this.
We usually give a name to a function when it is first created. This
is called defining a function, and it is done with the
defun special form.
defunis the usual way to define new Lisp functions. It defines the symbol name as a function that looks like this:(lambda argument-list . body-forms)
defunstores this lambda expression in the function cell of name. It returns the value name, but usually we ignore this value.As described previously, argument-list is a list of argument names and may include the keywords
&optionaland&rest(see Lambda Expressions). Also, the first two of the body-forms may be a documentation string and an interactive declaration.There is no conflict if the same symbol name is also used as a variable, since the symbol's value cell is independent of the function cell. See Symbol Components.
Here are some examples:
(defun foo () 5) foo (foo) 5 (defun bar (a &optional b &rest c) (list a b c)) bar (bar 1 2 3 4 5) (1 2 (3 4 5)) (bar 1) (1 nil nil) (bar) error--> Wrong number of arguments. (defun capitalize-backwards () "Upcase the last letter of a word." (interactive) (backward-word 1) (forward-word 1) (backward-char 1) (capitalize-word 1)) capitalize-backwardsBe careful not to redefine existing functions unintentionally.
defunredefines even primitive functions such ascarwithout any hesitation or notification. Redefining a function already defined is often done deliberately, and there is no way to distinguish deliberate redefinition from unintentional redefinition.
This special form defines the symbol name as a function, with definition definition (which can be any valid Lisp function). It returns definition.
If docstring is non-
nil, it becomes the function documentation of name. Otherwise, any documentation provided by definition is used.The proper place to use
defaliasis where a specific function name is being defined—especially where that name appears explicitly in the source file being loaded. This is becausedefaliasrecords which file defined the function, just likedefun(see Unloading).By contrast, in programs that manipulate function definitions for other purposes, it is better to use
fset, which does not keep such records. See Function Cells.
You cannot create a new primitive function with defun or
defalias, but you can use them to change the function definition of
any symbol, even one such as car or x-popup-menu whose
normal definition is a primitive. However, this is risky: for
instance, it is next to impossible to redefine car without
breaking Lisp completely. Redefining an obscure function such as
x-popup-menu is less dangerous, but it still may not work as
you expect. If there are calls to the primitive from C code, they
call the primitive's C definition directly, so changing the symbol's
definition will have no effect on them.
See also defsubst, which defines a function like defun
and tells the Lisp compiler to open-code it. See Inline Functions.
Defining functions is only half the battle. Functions don't do anything until you call them, i.e., tell them to run. Calling a function is also known as invocation.
The most common way of invoking a function is by evaluating a list.
For example, evaluating the list (concat "a" "b") calls the
function concat with arguments "a" and "b".
See Evaluation, for a description of evaluation.
When you write a list as an expression in your program, you specify
which function to call, and how many arguments to give it, in the text
of the program. Usually that's just what you want. Occasionally you
need to compute at run time which function to call. To do that, use
the function funcall. When you also need to determine at run
time how many arguments to pass, use apply.
funcallcalls function with arguments, and returns whatever function returns.Since
funcallis a function, all of its arguments, including function, are evaluated beforefuncallis called. This means that you can use any expression to obtain the function to be called. It also means thatfuncalldoes not see the expressions you write for the arguments, only their values. These values are not evaluated a second time in the act of calling function; the operation offuncallis like the normal procedure for calling a function, once its arguments have already been evaluated.The argument function must be either a Lisp function or a primitive function. Special forms and macros are not allowed, because they make sense only when given the “unevaluated” argument expressions.
funcallcannot provide these because, as we saw above, it never knows them in the first place.(setq f 'list) list (funcall f 'x 'y 'z) (x y z) (funcall f 'x 'y '(z)) (x y (z)) (funcall 'and t nil) error--> Invalid function: #<subr and>Compare these examples with the examples of
apply.
applycalls function with arguments, just likefuncallbut with one difference: the last of arguments is a list of objects, which are passed to function as separate arguments, rather than a single list. We say thatapplyspreads this list so that each individual element becomes an argument.
applyreturns the result of calling function. As withfuncall, function must either be a Lisp function or a primitive function; special forms and macros do not make sense inapply.(setq f 'list) list (apply f 'x 'y 'z) error--> Wrong type argument: listp, z (apply '+ 1 2 '(3 4)) 10 (apply '+ '(1 2 3 4)) 10 (apply 'append '((a b c) nil (x y z) nil)) (a b c x y z)For an interesting example of using
apply, see Definition of mapcar.
Sometimes it is useful to fix some of the function's arguments at certain values, and leave the rest of arguments for when the function is actually called. The act of fixing some of the function's arguments is called partial application of the function7. The result is a new function that accepts the rest of arguments and calls the original function with all the arguments combined.
Here's how to do partial application in Emacs Lisp:
This function returns a new function which, when called, will call func with the list of arguments composed from args and additional arguments specified at the time of the call. If func accepts n arguments, then a call to
apply-partiallywith m<n arguments will produce a new function of n-m arguments.Here's how we could define the built-in function
1+, if it didn't exist, usingapply-partiallyand+, another built-in function:(defalias '1+ (apply-partially '+ 1) "Increment argument by one.") (1+ 10) 11
It is common for Lisp functions to accept functions as arguments or
find them in data structures (especially in hook variables and property
lists) and call them using funcall or apply. Functions
that accept function arguments are often called functionals.
Sometimes, when you call a functional, it is useful to supply a no-op function as the argument. Here are two different kinds of no-op function:
A mapping function applies a given function (not a
special form or macro) to each element of a list or other collection.
Emacs Lisp has several such functions; mapcar and
mapconcat, which scan a list, are described here.
See Definition of mapatoms, for the function mapatoms which
maps over the symbols in an obarray. See Definition of maphash,
for the function maphash which maps over key/value associations
in a hash table.
These mapping functions do not allow char-tables because a char-table
is a sparse array whose nominal range of indices is very large. To map
over a char-table in a way that deals properly with its sparse nature,
use the function map-char-table (see Char-Tables).
mapcarapplies function to each element of sequence in turn, and returns a list of the results.The argument sequence can be any kind of sequence except a char-table; that is, a list, a vector, a bool-vector, or a string. The result is always a list. The length of the result is the same as the length of sequence. For example:
(mapcar 'car '((a b) (c d) (e f))) (a c e) (mapcar '1+ [1 2 3]) (2 3 4) (mapcar 'char-to-string "abc") ("a" "b" "c") ;; Call each function inmy-hooks. (mapcar 'funcall my-hooks) (defun mapcar* (function &rest args) "Apply FUNCTION to successive cars of all ARGS. Return the list of results." ;; If no list is exhausted, (if (not (memq nil args)) ;; apply function to cars. (cons (apply function (mapcar 'car args)) (apply 'mapcar* function ;; Recurse for rest of elements. (mapcar 'cdr args))))) (mapcar* 'cons '(a b c) '(1 2 3 4)) ((a . 1) (b . 2) (c . 3))
mapcis likemapcarexcept that function is used for side-effects only—the values it returns are ignored, not collected into a list.mapcalways returns sequence.
mapconcatapplies function to each element of sequence: the results, which must be strings, are concatenated. Between each pair of result strings,mapconcatinserts the string separator. Usually separator contains a space or comma or other suitable punctuation.The argument function must be a function that can take one argument and return a string. The argument sequence can be any kind of sequence except a char-table; that is, a list, a vector, a bool-vector, or a string.
(mapconcat 'symbol-name '(The cat in the hat) " ") "The cat in the hat" (mapconcat (function (lambda (x) (format "%c" (1+ x)))) "HAL-8000" "") "IBM.9111"
In Lisp, a function is a list that starts with lambda, a
byte-code function compiled from such a list, or alternatively a
primitive subr-object; names are “extra.” Although usually functions
are defined with defun and given names at the same time, it is
occasionally more concise to use an explicit lambda expression—an
anonymous function. Such a list is valid wherever a function name is.
Any method of creating such a list makes a valid function. Even this:
(setq silly (append '(lambda (x)) (list (list '+ (* 3 4) 'x))))
(lambda (x) (+ 12 x))
This computes a list that looks like (lambda (x) (+ 12 x)) and
makes it the value (not the function definition!) of
silly.
Here is how we might call this function:
(funcall silly 1)
13
(It does not work to write (silly 1), because this function
is not the function definition of silly. We have not given
silly any function definition, just a value as a variable.)
Most of the time, anonymous functions are constants that appear in
your program. For example, you might want to pass one as an argument to
the function mapcar, which applies any given function to each
element of a list.
Here we define a function change-property which
uses a function as its third argument:
(defun change-property (symbol prop function)
(let ((value (get symbol prop)))
(put symbol prop (funcall function value))))
Here we define a function that uses change-property,
passing it a function to double a number:
(defun double-property (symbol prop)
(change-property symbol prop '(lambda (x) (* 2 x))))
In such cases, we usually use the special form function instead
of simple quotation to quote the anonymous function, like this:
(defun double-property (symbol prop)
(change-property symbol prop
(function (lambda (x) (* 2 x)))))
Using function instead of quote makes a difference if you
compile the function double-property. For example, if you
compile the second definition of double-property, the anonymous
function is compiled as well. By contrast, if you compile the first
definition which uses ordinary quote, the argument passed to
change-property is the precise list shown:
(lambda (x) (* x 2))
The Lisp compiler cannot assume this list is a function, even though it
looks like one, since it does not know what change-property will
do with the list. Perhaps it will check whether the car of the third
element is the symbol *! Using function tells the
compiler it is safe to go ahead and compile the constant function.
Nowadays it is possible to omit function entirely, like this:
(defun double-property (symbol prop)
(change-property symbol prop (lambda (x) (* 2 x))))
This is because lambda itself implies function.
We sometimes write function instead of quote when
quoting the name of a function, but this usage is just a sort of
comment:
(function symbol) == (quote symbol) == 'symbol
The read syntax #' is a short-hand for using function.
For example,
#'(lambda (x) (* x x))
is equivalent to
(function (lambda (x) (* x x)))
This special form returns function-object without evaluating it. In this, it is equivalent to
quote. However, it serves as a note to the Emacs Lisp compiler that function-object is intended to be used only as a function, and therefore can safely be compiled. Contrast this withquote, in Quoting.
See describe-symbols example, for a realistic example using
function and an anonymous function.
The function definition of a symbol is the object stored in the function cell of the symbol. The functions described here access, test, and set the function cell of symbols.
See also the function indirect-function. See Definition of indirect-function.
This returns the object in the function cell of symbol. If the symbol's function cell is void, a
void-functionerror is signaled.This function does not check that the returned object is a legitimate function.
(defun bar (n) (+ n 2)) bar (symbol-function 'bar) (lambda (n) (+ n 2)) (fset 'baz 'bar) bar (symbol-function 'baz) bar
If you have never given a symbol any function definition, we say that
that symbol's function cell is void. In other words, the function
cell does not have any Lisp object in it. If you try to call such a symbol
as a function, it signals a void-function error.
Note that void is not the same as nil or the symbol
void. The symbols nil and void are Lisp objects,
and can be stored into a function cell just as any other object can be
(and they can be valid functions if you define them in turn with
defun). A void function cell contains no object whatsoever.
You can test the voidness of a symbol's function definition with
fboundp. After you have given a symbol a function definition, you
can make it void once more using fmakunbound.
This function returns
tif the symbol has an object in its function cell,nilotherwise. It does not check that the object is a legitimate function.
This function makes symbol's function cell void, so that a subsequent attempt to access this cell will cause a
void-functionerror. It returns symbol. (See alsomakunbound, in Void Variables.)(defun foo (x) x) foo (foo 1) 1 (fmakunbound 'foo) foo (foo 1) error--> Symbol's function definition is void: foo
This function stores definition in the function cell of symbol. The result is definition. Normally definition should be a function or the name of a function, but this is not checked. The argument symbol is an ordinary evaluated argument.
There are three normal uses of this function:
- Copying one symbol's function definition to another—in other words, making an alternate name for a function. (If you think of this as the definition of the new name, you should use
defaliasinstead offset; see Definition of defalias.)- Giving a symbol a function definition that is not a list and therefore cannot be made with
defun. For example, you can usefsetto give a symbols1a function definition which is another symbols2; thens1serves as an alias for whatever definitions2presently has. (Once again usedefaliasinstead offsetif you think of this as the definition ofs1.)- In constructs for defining or altering functions. If
defunwere not a primitive, it could be written in Lisp (as a macro) usingfset.Here are examples of these uses:
;; Savefoo's definition inold-foo. (fset 'old-foo (symbol-function 'foo)) ;; Make the symbolcarthe function definition ofxfirst. ;; (Most likely,defaliaswould be better thanfsethere.) (fset 'xfirst 'car) car (xfirst '(1 2 3)) 1 (symbol-function 'xfirst) car (symbol-function (symbol-function 'xfirst)) #<subr car> ;; Define a named keyboard macro. (fset 'kill-two-lines "\^u2\^k") "\^u2\^k" ;; Here is a function that alters other functions. (defun copy-function-definition (new old) "Define NEW with the same function definition as OLD." (fset new (symbol-function old)))
fset is sometimes used to save the old definition of a
function before redefining it. That permits the new definition to
invoke the old definition. But it is unmodular and unclean for a Lisp
file to redefine a function defined elsewhere. If you want to modify
a function defined by another package, it is cleaner to use
defadvice (see Advising Functions).
You can use make-obsolete to declare a function obsolete. This
indicates that the function may be removed at some stage in the future.
This function makes the byte compiler warn that the function obsolete-name is obsolete. If current-name is a symbol, the warning message says to use current-name instead of obsolete-name. current-name does not need to be an alias for obsolete-name; it can be a different function with similar functionality. If current-name is a string, it is the warning message.
If provided, when should be a string indicating when the function was first made obsolete—for example, a date or a release number.
You can define a function as an alias and declare it obsolete at the
same time using the macro define-obsolete-function-alias.
This macro marks the function obsolete-name obsolete and also defines it as an alias for the function current-name. It is equivalent to the following:
(defalias obsolete-name current-name docstring) (make-obsolete obsolete-name current-name when)
You can define an inline function by using defsubst instead
of defun. An inline function works just like an ordinary
function except for one thing: when you compile a call to the function,
the function's definition is open-coded into the caller.
Making a function inline makes explicit calls run faster. But it also has disadvantages. For one thing, it reduces flexibility; if you change the definition of the function, calls already inlined still use the old definition until you recompile them.
Another disadvantage is that making a large function inline can increase the size of compiled code both in files and in memory. Since the speed advantage of inline functions is greatest for small functions, you generally should not make large functions inline.
Also, inline functions do not behave well with respect to debugging,
tracing, and advising (see Advising Functions). Since ease of
debugging and the flexibility of redefining functions are important
features of Emacs, you should not make a function inline, even if it's
small, unless its speed is really crucial, and you've timed the code
to verify that using defun actually has performance problems.
It's possible to define a macro to expand into the same code that an
inline function would execute. (See Macros.) But the macro would be
limited to direct use in expressions—a macro cannot be called with
apply, mapcar and so on. Also, it takes some work to
convert an ordinary function into a macro. To convert it into an inline
function is very easy; simply replace defun with defsubst.
Since each argument of an inline function is evaluated exactly once, you
needn't worry about how many times the body uses the arguments, as you
do for macros. (See Argument Evaluation.)
Inline functions can be used and open-coded later on in the same file, following the definition, just like macros.
Byte-compiling a file often produces warnings about functions that the compiler doesn't know about (see Compiler Errors). Sometimes this indicates a real problem, but usually the functions in question are defined in other files which would be loaded if that code is run. For example, byte-compiling fortran.el used to warn:
In end of data:
fortran.el:2152:1:Warning: the function `gud-find-c-expr' is not known
to be defined.
In fact, gud-find-c-expr is only used in the function that
Fortran mode uses for the local value of
gud-find-expr-function, which is a callback from GUD; if it is
called, the GUD functions will be loaded. When you know that such a
warning does not indicate a real problem, it is good to suppress the
warning. That makes new warnings which might mean real problems more
visible. You do that with declare-function.
All you need to do is add a declare-function statement before the
first use of the function in question:
(declare-function gud-find-c-expr "gud.el" nil)
This says that gud-find-c-expr is defined in gud.el (the
‘.el’ can be omitted). The compiler takes for granted that that file
really defines the function, and does not check.
The optional third argument specifies the argument list of
gud-find-c-expr. In this case, it takes no arguments
(nil is different from not specifying a value). In other
cases, this might be something like (file &optional overwrite).
You don't have to specify the argument list, but if you do the
byte compiler can check that the calls match the declaration.
Tell the byte compiler to assume that function is defined, with arguments arglist, and that the definition should come from the file file. fileonly non-
nilmeans only check that file exists, not that it actually defines function.
To verify that these functions really are declared where
declare-function says they are, use check-declare-file
to check all declare-function calls in one source file, or use
check-declare-directory check all the files in and under a
certain directory.
These commands find the file that ought to contain a function's
definition using locate-library; if that finds no file, they
expand the definition file name relative to the directory of the file
that contains the declare-function call.
You can also say that a function is defined by C code by specifying
a file name ending in ‘.c’. check-declare-file looks for
these files in the C source code directory. This is useful only when
you call a function that is defined only on certain systems. Most
of the primitive functions of Emacs are always defined so they will
never give you a warning.
Sometimes a file will optionally use functions from an external package.
If you prefix the filename in the declare-function statement with
‘ext:’, then it will be checked if it is found, otherwise skipped
without error.
There are some function definitions that ‘check-declare’ does not
understand (e.g. defstruct and some other macros). In such cases,
you can pass a non-nil fileonly argument to
declare-function, meaning to only check that the file exists, not
that it actually defines the function. Note that to do this without
having to specify an argument list, you should set the arglist
argument to t (because nil means an empty argument list, as
opposed to an unspecified one).
Some major modes such as SES call functions that are stored in user files. (see Top, for more information on SES.) User files sometimes have poor pedigrees—you can get a spreadsheet from someone you've just met, or you can get one through email from someone you've never met. So it is risky to call a function whose source code is stored in a user file until you have determined that it is safe.
Returns
nilif form is a safe Lisp expression, or returns a list that describes why it might be unsafe. The argument unsafep-vars is a list of symbols known to have temporary bindings at this point; it is mainly used for internal recursive calls. The current buffer is an implicit argument, which provides a list of buffer-local bindings.
Being quick and simple, unsafep does a very light analysis and
rejects many Lisp expressions that are actually safe. There are no
known cases where unsafep returns nil for an unsafe
expression. However, a “safe” Lisp expression can return a string
with a display property, containing an associated Lisp
expression to be executed after the string is inserted into a buffer.
This associated expression can be a virus. In order to be safe, you
must delete properties from all strings calculated by user code before
inserting them into buffers.
Here is a table of several functions that do things related to function calling and function definitions. They are documented elsewhere, but we provide cross references here.
applyautoloadcall-interactivelycalled-interactively-pcommandpdocumentationevalfuncallfunctionignoreindirect-functioninteractiveinteractive-pmapatomsmapcarmap-char-tablemapconcatundefinedMacros enable you to define new control constructs and other language features. A macro is defined much like a function, but instead of telling how to compute a value, it tells how to compute another Lisp expression which will in turn compute the value. We call this expression the expansion of the macro.
Macros can do this because they operate on the unevaluated expressions for the arguments, not on the argument values as functions do. They can therefore construct an expansion containing these argument expressions or parts of them.
If you are using a macro to do something an ordinary function could do, just for the sake of speed, consider using an inline function instead. See Inline Functions.
Suppose we would like to define a Lisp construct to increment a
variable value, much like the ++ operator in C. We would like to
write (inc x) and have the effect of (setq x (1+ x)).
Here's a macro definition that does the job:
(defmacro inc (var)
(list 'setq var (list '1+ var)))
When this is called with (inc x), the argument var is the
symbol x—not the value of x, as it would
be in a function. The body of the macro uses this to construct the
expansion, which is (setq x (1+ x)). Once the macro definition
returns this expansion, Lisp proceeds to evaluate it, thus incrementing
x.
A macro call looks just like a function call in that it is a list which starts with the name of the macro. The rest of the elements of the list are the arguments of the macro.
Evaluation of the macro call begins like evaluation of a function call except for one crucial difference: the macro arguments are the actual expressions appearing in the macro call. They are not evaluated before they are given to the macro definition. By contrast, the arguments of a function are results of evaluating the elements of the function call list.
Having obtained the arguments, Lisp invokes the macro definition just
as a function is invoked. The argument variables of the macro are bound
to the argument values from the macro call, or to a list of them in the
case of a &rest argument. And the macro body executes and
returns its value just as a function body does.
The second crucial difference between macros and functions is that the value returned by the macro body is not the value of the macro call. Instead, it is an alternate expression for computing that value, also known as the expansion of the macro. The Lisp interpreter proceeds to evaluate the expansion as soon as it comes back from the macro.
Since the expansion is evaluated in the normal manner, it may contain calls to other macros. It may even be a call to the same macro, though this is unusual.
You can see the expansion of a given macro call by calling
macroexpand.
This function expands form, if it is a macro call. If the result is another macro call, it is expanded in turn, until something which is not a macro call results. That is the value returned by
macroexpand. If form is not a macro call to begin with, it is returned as given.Note that
macroexpanddoes not look at the subexpressions of form (although some macro definitions may do so). Even if they are macro calls themselves,macroexpanddoes not expand them.The function
macroexpanddoes not expand calls to inline functions. Normally there is no need for that, since a call to an inline function is no harder to understand than a call to an ordinary function.If environment is provided, it specifies an alist of macro definitions that shadow the currently defined macros. Byte compilation uses this feature.
(defmacro inc (var) (list 'setq var (list '1+ var))) inc (macroexpand '(inc r)) (setq r (1+ r)) (defmacro inc2 (var1 var2) (list 'progn (list 'inc var1) (list 'inc var2))) inc2 (macroexpand '(inc2 r s)) (progn (inc r) (inc s)) ;incnot expanded here.
macroexpand-allexpands macros likemacroexpand, but will look for and expand all macros in form, not just at the top-level. If no macros are expanded, the return value iseqto form.Repeating the example used for
macroexpandabove withmacroexpand-all, we see thatmacroexpand-alldoes expand the embedded calls toinc:(macroexpand-all '(inc2 r s)) (progn (setq r (1+ r)) (setq s (1+ s)))
You might ask why we take the trouble to compute an expansion for a macro and then evaluate the expansion. Why not have the macro body produce the desired results directly? The reason has to do with compilation.
When a macro call appears in a Lisp program being compiled, the Lisp compiler calls the macro definition just as the interpreter would, and receives an expansion. But instead of evaluating this expansion, it compiles the expansion as if it had appeared directly in the program. As a result, the compiled code produces the value and side effects intended for the macro, but executes at full compiled speed. This would not work if the macro body computed the value and side effects itself—they would be computed at compile time, which is not useful.
In order for compilation of macro calls to work, the macros must
already be defined in Lisp when the calls to them are compiled. The
compiler has a special feature to help you do this: if a file being
compiled contains a defmacro form, the macro is defined
temporarily for the rest of the compilation of that file.
Byte-compiling a file also executes any require calls at
top-level in the file, so you can ensure that necessary macro
definitions are available during compilation by requiring the files
that define them (see Named Features). To avoid loading the macro
definition files when someone runs the compiled program, write
eval-when-compile around the require calls (see Eval During Compile).
A Lisp macro is a list whose car is macro. Its cdr should
be a function; expansion of the macro works by applying the function
(with apply) to the list of unevaluated argument-expressions
from the macro call.
It is possible to use an anonymous Lisp macro just like an anonymous
function, but this is never done, because it does not make sense to pass
an anonymous macro to functionals such as mapcar. In practice,
all Lisp macros have names, and they are usually defined with the
special form defmacro.
defmacrodefines the symbol name as a macro that looks like this:(macro lambda argument-list . body-forms)(Note that the cdr of this list is a function—a lambda expression.) This macro object is stored in the function cell of name. The value returned by evaluating the
defmacroform is name, but usually we ignore this value.The shape and meaning of argument-list is the same as in a function, and the keywords
&restand&optionalmay be used (see Argument List). Macros may have a documentation string, but anyinteractivedeclaration is ignored since macros cannot be called interactively.
The body of the macro definition can include a declare form,
which can specify how <TAB> should indent macro calls, and how to
step through them for Edebug.
A
declareform is used in a macro definition to specify various additional information about it. Two kinds of specification are currently supported:
(debugedebug-form-spec)- Specify how to step through macro calls for Edebug. See Instrumenting Macro Calls.
(indentindent-spec)- Specify how to indent calls to this macro. See Indenting Macros, for more details.
A
declareform only has its special effect in the body of adefmacroform if it immediately follows the documentation string, if present, or the argument list otherwise. (Strictly speaking, severaldeclareforms can follow the documentation string or argument list, but since adeclareform can have several specs, they can always be combined into a single form.) When used at other places in adefmacroform, or outside adefmacroform,declarejust returnsnilwithout evaluating any specs.
No macro absolutely needs a declare form, because that form
has no effect on how the macro expands, on what the macro means in the
program. It only affects secondary features: indentation and Edebug.
Macros often need to construct large list structures from a mixture of constants and nonconstant parts. To make this easier, use the ‘`’ syntax (usually called backquote).
Backquote allows you to quote a list, but selectively evaluate
elements of that list. In the simplest case, it is identical to the
special form quote (see Quoting). For example, these
two forms yield identical results:
`(a list of (+ 2 3) elements)
(a list of (+ 2 3) elements)
'(a list of (+ 2 3) elements)
(a list of (+ 2 3) elements)
The special marker ‘,’ inside of the argument to backquote indicates a value that isn't constant. Backquote evaluates the argument of ‘,’ and puts the value in the list structure:
(list 'a 'list 'of (+ 2 3) 'elements)
(a list of 5 elements)
`(a list of ,(+ 2 3) elements)
(a list of 5 elements)
Substitution with ‘,’ is allowed at deeper levels of the list structure also. For example:
(defmacro t-becomes-nil (variable)
`(if (eq ,variable t)
(setq ,variable nil)))
(t-becomes-nil foo)
== (if (eq foo t) (setq foo nil))
You can also splice an evaluated value into the resulting list, using the special marker ‘,@’. The elements of the spliced list become elements at the same level as the other elements of the resulting list. The equivalent code without using ‘`’ is often unreadable. Here are some examples:
(setq some-list '(2 3))
(2 3)
(cons 1 (append some-list '(4) some-list))
(1 2 3 4 2 3)
`(1 ,@some-list 4 ,@some-list)
(1 2 3 4 2 3)
(setq list '(hack foo bar))
(hack foo bar)
(cons 'use
(cons 'the
(cons 'words (append (cdr list) '(as elements)))))
(use the words foo bar as elements)
`(use the words ,@(cdr list) as elements)
(use the words foo bar as elements)
The basic facts of macro expansion have counterintuitive consequences. This section describes some important consequences that can lead to trouble, and rules to follow to avoid trouble.
The most common problem in writing macros is doing some of the real work prematurely—while expanding the macro, rather than in the expansion itself. For instance, one real package had this macro definition:
(defmacro my-set-buffer-multibyte (arg)
(if (fboundp 'set-buffer-multibyte)
(set-buffer-multibyte arg)))
With this erroneous macro definition, the program worked fine when
interpreted but failed when compiled. This macro definition called
set-buffer-multibyte during compilation, which was wrong, and
then did nothing when the compiled package was run. The definition
that the programmer really wanted was this:
(defmacro my-set-buffer-multibyte (arg)
(if (fboundp 'set-buffer-multibyte)
`(set-buffer-multibyte ,arg)))
This macro expands, if appropriate, into a call to
set-buffer-multibyte that will be executed when the compiled
program is actually run.
When defining a macro you must pay attention to the number of times the arguments will be evaluated when the expansion is executed. The following macro (used to facilitate iteration) illustrates the problem. This macro allows us to write a simple “for” loop such as one might find in Pascal.
(defmacro for (var from init to final do &rest body)
"Execute a simple \"for\" loop.
For example, (for i from 1 to 10 do (print i))."
(list 'let (list (list var init))
(cons 'while (cons (list '<= var final)
(append body (list (list 'inc var)))))))
for
(for i from 1 to 3 do
(setq square (* i i))
(princ (format "\n%d %d" i square)))
==>
(let ((i 1))
(while (<= i 3)
(setq square (* i i))
(princ (format "\n%d %d" i square))
(inc i)))
-|1 1
-|2 4
-|3 9
nil
The arguments from, to, and do in this macro are
“syntactic sugar”; they are entirely ignored. The idea is that you
will write noise words (such as from, to, and do)
in those positions in the macro call.
Here's an equivalent definition simplified through use of backquote:
(defmacro for (var from init to final do &rest body)
"Execute a simple \"for\" loop.
For example, (for i from 1 to 10 do (print i))."
`(let ((,var ,init))
(while (<= ,var ,final)
,@body
(inc ,var))))
Both forms of this definition (with backquote and without) suffer from
the defect that final is evaluated on every iteration. If
final is a constant, this is not a problem. If it is a more
complex form, say (long-complex-calculation x), this can slow
down the execution significantly. If final has side effects,
executing it more than once is probably incorrect.
A well-designed macro definition takes steps to avoid this problem by
producing an expansion that evaluates the argument expressions exactly
once unless repeated evaluation is part of the intended purpose of the
macro. Here is a correct expansion for the for macro:
(let ((i 1)
(max 3))
(while (<= i max)
(setq square (* i i))
(princ (format "%d %d" i square))
(inc i)))
Here is a macro definition that creates this expansion:
(defmacro for (var from init to final do &rest body)
"Execute a simple for loop: (for i from 1 to 10 do (print i))."
`(let ((,var ,init)
(max ,final))
(while (<= ,var max)
,@body
(inc ,var))))
Unfortunately, this fix introduces another problem, described in the following section.
In the previous section, the definition of for was fixed as
follows to make the expansion evaluate the macro arguments the proper
number of times:
(defmacro for (var from init to final do &rest body)
"Execute a simple for loop: (for i from 1 to 10 do (print i))."
`(let ((,var ,init)
(max ,final))
(while (<= ,var max)
,@body
(inc ,var))))
The new definition of for has a new problem: it introduces a
local variable named max which the user does not expect. This
causes trouble in examples such as the following:
(let ((max 0))
(for x from 0 to 10 do
(let ((this (frob x)))
(if (< max this)
(setq max this)))))
The references to max inside the body of the for, which
are supposed to refer to the user's binding of max, really access
the binding made by for.
The way to correct this is to use an uninterned symbol instead of
max (see Creating Symbols). The uninterned symbol can be
bound and referred to just like any other symbol, but since it is
created by for, we know that it cannot already appear in the
user's program. Since it is not interned, there is no way the user can
put it into the program later. It will never appear anywhere except
where put by for. Here is a definition of for that works
this way:
(defmacro for (var from init to final do &rest body)
"Execute a simple for loop: (for i from 1 to 10 do (print i))."
(let ((tempvar (make-symbol "max")))
`(let ((,var ,init)
(,tempvar ,final))
(while (<= ,var ,tempvar)
,@body
(inc ,var)))))
This creates an uninterned symbol named max and puts it in the
expansion instead of the usual interned symbol max that appears
in expressions ordinarily.
Another problem can happen if the macro definition itself
evaluates any of the macro argument expressions, such as by calling
eval (see Eval). If the argument is supposed to refer to the
user's variables, you may have trouble if the user happens to use a
variable with the same name as one of the macro arguments. Inside the
macro body, the macro argument binding is the most local binding of this
variable, so any references inside the form being evaluated do refer to
it. Here is an example:
(defmacro foo (a)
(list 'setq (eval a) t))
foo
(setq x 'b)
(foo x) ==> (setq b t)
t ; and b has been set.
;; but
(setq a 'c)
(foo a) ==> (setq a t)
t ; but this set a, not c.
It makes a difference whether the user's variable is named a or
x, because a conflicts with the macro argument variable
a.
Another problem with calling eval in a macro definition is that
it probably won't do what you intend in a compiled program. The
byte compiler runs macro definitions while compiling the program, when
the program's own computations (which you might have wished to access
with eval) don't occur and its local variable bindings don't
exist.
To avoid these problems, don't evaluate an argument expression while computing the macro expansion. Instead, substitute the expression into the macro expansion, so that its value will be computed as part of executing the expansion. This is how the other examples in this chapter work.
Occasionally problems result from the fact that a macro call is expanded each time it is evaluated in an interpreted function, but is expanded only once (during compilation) for a compiled function. If the macro definition has side effects, they will work differently depending on how many times the macro is expanded.
Therefore, you should avoid side effects in computation of the macro expansion, unless you really know what you are doing.
One special kind of side effect can't be avoided: constructing Lisp objects. Almost all macro expansions include constructed lists; that is the whole point of most macros. This is usually safe; there is just one case where you must be careful: when the object you construct is part of a quoted constant in the macro expansion.
If the macro is expanded just once, in compilation, then the object is constructed just once, during compilation. But in interpreted execution, the macro is expanded each time the macro call runs, and this means a new object is constructed each time.
In most clean Lisp code, this difference won't matter. It can matter only if you perform side-effects on the objects constructed by the macro definition. Thus, to avoid trouble, avoid side effects on objects constructed by macro definitions. Here is an example of how such side effects can get you into trouble:
(defmacro empty-object ()
(list 'quote (cons nil nil)))
(defun initialize (condition)
(let ((object (empty-object)))
(if condition
(setcar object condition))
object))
If initialize is interpreted, a new list (nil) is
constructed each time initialize is called. Thus, no side effect
survives between calls. If initialize is compiled, then the
macro empty-object is expanded during compilation, producing a
single “constant” (nil) that is reused and altered each time
initialize is called.
One way to avoid pathological cases like this is to think of
empty-object as a funny kind of constant, not as a memory
allocation construct. You wouldn't use setcar on a constant such
as '(nil), so naturally you won't use it on (empty-object)
either.
You can use the declare form in the macro definition to
specify how to <TAB> should indent indent calls to the macro. You
write it like this:
(declare (indent indent-spec))
Here are the possibilities for indent-spec:
nildefunlisp-body-indent
more columns than the open-parenthesis starting the containing
expression. If the argument is distinguished and is either the first
or second argument, it is indented twice that many extra columns.
If the argument is distinguished and not the first or second argument,
the line uses the standard pattern.
parse-partial-sexp (a Lisp primitive for
indentation and nesting computation) when it parses up to the
beginning of this line.
This chapter describes how to declare user options for customization, and also customization groups for classifying them. We use the term customization item to include both kinds of customization definitions—as well as face definitions (see Defining Faces).
All kinds of customization declarations (for variables and groups, and for faces) accept keyword arguments for specifying various information. This section describes some keywords that apply to all kinds.
All of these keywords, except :tag, can be used more than once
in a given item. Each use of the keyword has an independent effect.
The keyword :tag is an exception because any given item can only
display one name.
:tag label:group group:group in a defgroup, it makes the new group a subgroup of
group.
If you use this keyword more than once, you can put a single item into
more than one group. Displaying any of those groups will show this
item. Please don't overdo this, since the result would be annoying.
:link link-dataThere are several alternatives you can use for link-data:
(custom-manual info-node)"(emacs)Top". The link appears as
‘[Manual]’ in the customization buffer and enters the built-in
Info reader on info-node.
(info-link info-node)custom-manual except that the link appears
in the customization buffer with the Info node name.
(url-link url)browse-url-browser-function.
(emacs-commentary-link library)(emacs-library-link library)(file-link file)find-file when the user invokes this link.
(function-link function)describe-function when the user invokes this link.
(variable-link variable)describe-variable when the user invokes this link.
(custom-group-link group)You can specify the text to use in the customization buffer by adding
:tag name after the first element of the link-data;
for example, (info-link :tag "foo" "(emacs)Top") makes a link to
the Emacs manual which appears in the buffer as ‘foo’.
An item can have more than one external link; however, most items have
none at all.
:load fileload-library, and only if the file is
not already loaded.
:require feature(require 'feature) when your saved customizations
set the value of this item. feature should be a symbol.
The most common reason to use :require is when a variable enables
a feature such as a minor mode, and just setting the variable won't have
any effect unless the code which implements the mode is loaded.
:version version:package-version '(package . version)This keyword takes priority over :version.
package should be the official name of the package, such as MH-E
or Gnus. If the package package is released as part of Emacs,
package and version should appear in the value of
customize-package-emacs-version-alist.
Packages distributed as part of Emacs that use the
:package-version keyword must also update the
customize-package-emacs-version-alist variable.
This alist provides a mapping for the versions of Emacs that are associated with versions of a package listed in the
:package-versionkeyword. Its elements look like this:(package (pversion . eversion)...)For each package, which is a symbol, there are one or more elements that contain a package version pversion with an associated Emacs version eversion. These versions are strings. For example, the MH-E package updates this alist with the following:
(add-to-list 'customize-package-emacs-version-alist '(MH-E ("6.0" . "22.1") ("6.1" . "22.1") ("7.0" . "22.1") ("7.1" . "22.1") ("7.2" . "22.1") ("7.3" . "22.1") ("7.4" . "22.1") ("8.0" . "22.1")))The value of package needs to be unique and it needs to match the package value appearing in the
:package-versionkeyword. Since the user might see the value in a error message, a good choice is the official name of the package, such as MH-E or Gnus.
Each Emacs Lisp package should have one main customization group which contains all the options, faces and other groups in the package. If the package has a small number of options and faces, use just one group and put everything in it. When there are more than twelve or so options and faces, then you should structure them into subgroups, and put the subgroups under the package's main customization group. It is OK to put some of the options and faces in the package's main group alongside the subgroups.
The package's main or only group should be a member of one or more of
the standard customization groups. (To display the full list of them,
use M-x customize.) Choose one or more of them (but not too
many), and add your group to each of them using the :group
keyword.
The way to declare new customization groups is with defgroup.
Declare group as a customization group containing members. Do not quote the symbol group. The argument doc specifies the documentation string for the group.
The argument members is a list specifying an initial set of customization items to be members of the group. However, most often members is
nil, and you specify the group's members by using the:groupkeyword when defining those members.If you want to specify group members through members, each element should have the form
(name widget). Here name is a symbol, and widget is a widget type for editing that symbol. Useful widgets arecustom-variablefor a variable,custom-facefor a face, andcustom-groupfor a group.When you introduce a new group into Emacs, use the
:versionkeyword in thedefgroup; then you need not use it for the individual members of the group.In addition to the common keywords (see Common Keywords), you can also use this keyword in
defgroup:
The prefix-discarding feature is currently turned off, which means
that :prefix currently has no effect. We did this because we
found that discarding the specified prefixes often led to confusing
names for options. This happened because the people who wrote the
defgroup definitions for various groups added :prefix
keywords whenever they make logical sense—that is, whenever the
variables in the library have a common prefix.
In order to obtain good results with :prefix, it would be
necessary to check the specific effects of discarding a particular
prefix, given the specific items in a group and their names and
documentation. If the resulting text is not clear, then :prefix
should not be used in that case.
It should be possible to recheck all the customization groups, delete
the :prefix specifications which give unclear results, and then
turn this feature back on, if someone would like to do the work.
Use defcustom to declare user-customizable variables.
This macro declares option as a customizable user option. You should not quote option.
This causes the function
user-variable-pto returntwhen given option as an argument. See Defining Variables. The argument doc specifies the documentation string for the variable. (Note that there is no need to start doc with a ‘*’.)The argument standard is an expression that specifies the standard value for option. Evaluating the
defcustomform evaluates standard, but does not necessarily install the standard value. If option already has a default value,defcustomdoes not change it. If the user has saved a customization for option,defcustominstalls the user's customized value as option's default value. If neither of those cases applies,defcustominstalls the result of evaluating standard as the default value.The expression standard can be evaluated at various other times, too—whenever the customization facility needs to know option's standard value. So be sure to use an expression which is harmless to evaluate at any time. We recommend avoiding backquotes in standard, because they are not expanded when editing the value, so list values will appear to have the wrong structure.
Every
defcustomshould specify:groupat least once.If you specify the
:setkeyword, to make the variable take other special actions when set through the customization buffer, the variable's documentation string should tell the user specifically how to do the same job in hand-written Lisp code.When you evaluate a
defcustomform with C-M-x in Emacs Lisp mode (eval-defun), a special feature ofeval-defunarranges to set the variable unconditionally, without testing whether its value is void. (The same feature applies todefvar.) See Defining Variables.If you put a
defcustomin a file that is preloaded at dump time (see Building Emacs), and the standard value installed for the variable at that time might not be correct, usecustom-reevaluate-setting, described below, to re-evaluate the standard value during or after Emacs startup.
defcustom accepts the following additional keywords:
:type type:options value-listThis is meaningful only for certain types, currently including
hook, plist and alist. See the definition of the
individual types for a description of how to use :options.
:set setfunctionset-default.
:get getfunctiondefault-value.
You have to really understand the workings of Custom to use
:get correctly. It is meant for values that are treated in
Custom as variables but are not actually stored in Lisp variables. It
is almost surely a mistake to specify getfunction for a value
that really is stored in a Lisp variable.
:initialize functiondefcustom is evaluated. It should take two arguments,
the option name (a symbol) and the value. Here are some predefined
functions meant for use in this way:
custom-initialize-set:set function to initialize the variable, but
do not reinitialize it if it is already non-void.
custom-initialize-defaultcustom-initialize-set, but use the function
set-default to set the variable, instead of the variable's
:set function. This is the usual choice for a variable whose
:set function enables or disables a minor mode; with this choice,
defining the variable will not call the minor mode function, but
customizing the variable will do so.
custom-initialize-reset:set function to initialize the variable. If
the variable is already non-void, reset it by calling the :set
function using the current value (returned by the :get method).
This is the default :initialize function.
custom-initialize-changed:set function to initialize the variable, if it is
already set or has been customized; otherwise, just use
set-default.
custom-initialize-safe-setcustom-initialize-safe-defaultcustom-initialize-set
(custom-initialize-default, respectively), but catch errors.
If an error occurs during initialization, they set the variable to
nil using set-default, and throw no error.
These two functions are only meant for options defined in pre-loaded files, where some variables or functions used to compute the option's value may not yet be defined. The option normally gets updated in startup.el, ignoring the previously computed value. Because of this typical usage, the value which these two functions compute normally only matters when, after startup, one unsets the option's value and then reevaluates the defcustom. By that time, the necessary variables and functions will be defined, so there will not be an error.
:risky valuerisky-local-variable property to
value (see File Local Variables).
:safe functionsafe-local-variable property to
function (see File Local Variables).
:set-after variables:set-after if setting this variable won't work properly unless
those other variables already have their intended values.
It is useful to specify the :require keyword for an option
that “turns on” a certain feature. This causes Emacs to load the
feature, if it is not already loaded, whenever the option is set.
See Common Keywords. Here is an example, from the library
saveplace.el:
(defcustom save-place nil
"Non-nil means automatically save place in each file..."
:type 'boolean
:require 'saveplace
:group 'save-place)
If a customization item has a type such as hook or
alist, which supports :options, you can add additional
values to the list from outside the defcustom declaration by
calling custom-add-frequent-value. For example, if you define a
function my-lisp-mode-initialization intended to be called from
emacs-lisp-mode-hook, you might want to add that to the list of
reasonable values for emacs-lisp-mode-hook, but not by editing
its definition. You can do it thus:
(custom-add-frequent-value 'emacs-lisp-mode-hook
'my-lisp-mode-initialization)
For the customization option symbol, add value to the list of reasonable values.
The precise effect of adding a value depends on the customization type of symbol.
Internally, defcustom uses the symbol property
standard-value to record the expression for the standard value,
saved-value to record the value saved by the user with the
customization buffer, and customized-value to record the value
set by the user with the customization buffer, but not saved.
See Property Lists. These properties are lists, the car of which
is an expression that evaluates to the value.
This function re-evaluates the standard value of symbol, which should be a user option declared via
defcustom. (If the variable was customized, this function re-evaluates the saved value instead.) This is useful for customizable options that are defined before their value could be computed correctly, such as variables defined in packages that are loaded at dump time, but depend on the run-time information. For example, the value could be a file whose precise name depends on the hierarchy of files when Emacs runs, or a name of a program that needs to be searched at run time.A good place to put calls to this function is in the function
command-linethat is run during startup (see Startup Summary) or in the various hooks it calls.
When you define a user option with defcustom, you must specify
its customization type. That is a Lisp object which describes (1)
which values are legitimate and (2) how to display the value in the
customization buffer for editing.
You specify the customization type in defcustom with the
:type keyword. The argument of :type is evaluated, but
only once when the defcustom is executed, so it isn't useful
for the value to vary. Normally we use a quoted constant. For
example:
(defcustom diff-command "diff"
"The command to use to run diff."
:type '(string)
:group 'diff)
In general, a customization type is a list whose first element is a symbol, one of the customization type names defined in the following sections. After this symbol come a number of arguments, depending on the symbol. Between the type symbol and its arguments, you can optionally write keyword-value pairs (see Type Keywords).
Some type symbols do not use any arguments; those are called
simple types. For a simple type, if you do not use any
keyword-value pairs, you can omit the parentheses around the type
symbol. For example just string as a customization type is
equivalent to (string).
All customization types are implemented as widgets; see Introduction, for details.
This section describes all the simple customization types.
sexpsexp as a fall-back for any option, if you don't want to
take the time to work out a more specific type to use.
integernumberfloatstringregexpstring except that the string must be a valid regular
expression.
characterfile(file :must-match t)directoryhook:options keyword in a hook variable's
defcustom to specify a list of functions recommended for use in
the hook; see Variable Definitions.
alistYou can specify the key and value types like this:
(alist :key-type key-type :value-type value-type)
where key-type and value-type are customization type
specifications. The default key type is sexp, and the default
value type is sexp.
The user can add any key matching the specified key type, but you can
give some keys a preferential treatment by specifying them with the
:options (see Variable Definitions). The specified keys
will always be shown in the customize buffer (together with a suitable
value), with a checkbox to include or exclude or disable the key/value
pair from the alist. The user will not be able to edit the keys
specified by the :options keyword argument.
The argument to the :options keywords should be a list of
specifications for reasonable keys in the alist. Ordinarily, they are
simply atoms, which stand for themselves as. For example:
:options '("foo" "bar" "baz")
specifies that there are three “known” keys, namely "foo",
"bar" and "baz", which will always be shown first.
You may want to restrict the value type for specific keys, for
example, the value associated with the "bar" key can only be an
integer. You can specify this by using a list instead of an atom in
the list. The first element will specify the key, like before, while
the second element will specify the value type. For example:
:options '("foo" ("bar" integer) "baz")
Finally, you may want to change how the key is presented. By default,
the key is simply shown as a const, since the user cannot change
the special keys specified with the :options keyword. However,
you may want to use a more specialized type for presenting the key, like
function-item if you know it is a symbol with a function binding.
This is done by using a customization type specification instead of a
symbol for the key.
:options '("foo" ((function-item some-function) integer)
"baz")
Many alists use lists with two elements, instead of cons cells. For example,
(defcustom list-alist '(("foo" 1) ("bar" 2) ("baz" 3))
"Each element is a list of the form (KEY VALUE).")
instead of
(defcustom cons-alist '(("foo" . 1) ("bar" . 2) ("baz" . 3))
"Each element is a cons-cell (KEY . VALUE).")
Because of the way lists are implemented on top of cons cells, you can
treat list-alist in the example above as a cons cell alist, where
the value type is a list with a single element containing the real
value.
(defcustom list-alist '(("foo" 1) ("bar" 2) ("baz" 3))
"Each element is a list of the form (KEY VALUE)."
:type '(alist :value-type (group integer)))
The group widget is used here instead of list only because
the formatting is better suited for the purpose.
Similarly, you can have alists with more values associated with each key, using variations of this trick:
(defcustom person-data '(("brian" 50 t)
("dorith" 55 nil)
("ken" 52 t))
"Alist of basic info about people.
Each element has the form (NAME AGE MALE-FLAG)."
:type '(alist :value-type (group integer boolean)))
(defcustom pets '(("brian")
("dorith" "dog" "guppy")
("ken" "cat"))
"Alist of people's pets.
In an element (KEY . VALUE), KEY is the person's name,
and the VALUE is a list of that person's pets."
:type '(alist :value-type (repeat string)))
plistplist custom type is similar to the alist (see above),
except that the information is stored as a property list, i.e. a list of
this form:
(key value key value key value ...)
The default :key-type for plist is symbol,
rather than sexp.
symbolfunctionvariablefacebooleannil or t. Note that by
using choice and const together (see the next section),
you can specify that the value must be nil or t, but also
specify the text to describe each value in a way that fits the specific
meaning of the alternative.
coding-systemcolorWhen none of the simple types is appropriate, you can use composite types, which build new types from other types or from specified data. The specified types or data are called the arguments of the composite type. The composite type normally looks like this:
(constructor arguments...)
but you can also add keyword-value pairs before the arguments, like this:
(constructor {keyword value}... arguments...)
Here is a table of constructors and how to use them to write composite types:
(cons car-type cdr-type)(cons string
symbol) is a customization type which matches values such as
("foo" . foo).
In the customization buffer, the car and the cdr are
displayed and edited separately, each according to the type
that you specify for it.
(list element-types...)For example, (list integer string function) describes a list of
three elements; the first element must be an integer, the second a
string, and the third a function.
In the customization buffer, each element is displayed and edited
separately, according to the type specified for it.
(group element-types...)list except for the formatting
of text in the Custom buffer. list labels each
element value with its tag; group does not.
(vector element-types...)list except that the value must be a vector instead of a
list. The elements work the same as in list.
(choice alternative-types...)(choice integer string) allows either an
integer or a string.
In the customization buffer, the user selects an alternative using a menu, and can then edit the value in the usual way for that alternative.
Normally the strings in this menu are determined automatically from the
choices; however, you can specify different strings for the menu by
including the :tag keyword in the alternatives. For example, if
an integer stands for a number of spaces, while a string is text to use
verbatim, you might write the customization type this way,
(choice (integer :tag "Number of spaces")
(string :tag "Literal text"))
so that the menu offers ‘Number of spaces’ and ‘Literal text’.
In any alternative for which nil is not a valid value, other than
a const, you should specify a valid default for that alternative
using the :value keyword. See Type Keywords.
If some values are covered by more than one of the alternatives, customize will choose the first alternative that the value fits. This means you should always list the most specific types first, and the most general last. Here's an example of proper usage:
(choice (const :tag "Off" nil)
symbol (sexp :tag "Other"))
This way, the special value nil is not treated like other
symbols, and symbols are not treated like other Lisp expressions.
(radio element-types...)choice, except that the choices are displayed
using `radio buttons' rather than a menu. This has the advantage of
displaying documentation for the choices when applicable and so is often
a good choice for a choice between constant functions
(function-item customization types).
(const value)The main use of const is inside of choice. For example,
(choice integer (const nil)) allows either an integer or
nil.
:tag is often used with const, inside of choice.
For example,
(choice (const :tag "Yes" t)
(const :tag "No" nil)
(const :tag "Ask" foo))
describes a variable for which t means yes, nil means no,
and foo means “ask.”
(other value)The main use of other is as the last element of choice.
For example,
(choice (const :tag "Yes" t)
(const :tag "No" nil)
(other :tag "Ask" foo))
describes a variable for which t means yes, nil means no,
and anything else means “ask.” If the user chooses ‘Ask’ from
the menu of alternatives, that specifies the value foo; but any
other value (not t, nil or foo) displays as
‘Ask’, just like foo.
(function-item function)const, but used for values which are functions. This
displays the documentation string as well as the function name.
The documentation string is either the one you specify with
:doc, or function's own documentation string.
(variable-item variable)const, but used for values which are variable names. This
displays the documentation string as well as the variable name. The
documentation string is either the one you specify with :doc, or
variable's own documentation string.
(set types...)This appears in the customization buffer as a checklist, so that each of
types may have either one corresponding element or none. It is
not possible to specify two different elements that match the same one
of types. For example, (set integer symbol) allows one
integer and/or one symbol in the list; it does not allow multiple
integers or multiple symbols. As a result, it is rare to use
nonspecific types such as integer in a set.
Most often, the types in a set are const types, as
shown here:
(set (const :bold) (const :italic))
Sometimes they describe possible elements in an alist:
(set (cons :tag "Height" (const height) integer)
(cons :tag "Width" (const width) integer))
That lets the user specify a height value optionally
and a width value optionally.
(repeat element-type)(restricted-sexp :match-alternatives criteria)nil or non-nil according to
the argument. Using a predicate in the list says that objects for which
the predicate returns non-nil are acceptable.
'object. This sort of element
in the list says that object itself is an acceptable value.
For example,
(restricted-sexp :match-alternatives
(integerp 't 'nil))
allows integers, t and nil as legitimate values.
The customization buffer shows all legitimate values using their read syntax, and the user edits them textually.
Here is a table of the keywords you can use in keyword-value pairs in a composite type:
:tag tagchoice.
:match-alternatives criteriarestricted-sexp.
:args argument-list(const :args (foo)) is equivalent to
(const foo). You rarely need to write :args explicitly,
because normally the arguments are recognized automatically as
whatever follows the last keyword-value pair.
The :inline feature lets you splice a variable number of
elements into the middle of a list or vector. You use it in a
set, choice or repeat type which appears among the
element-types of a list or vector.
Normally, each of the element-types in a list or vector
describes one and only one element of the list or vector. Thus, if an
element-type is a repeat, that specifies a list of unspecified
length which appears as one element.
But when the element-type uses :inline, the value it matches is
merged directly into the containing sequence. For example, if it
matches a list with three elements, those become three elements of the
overall sequence. This is analogous to using ‘,@’ in the backquote
construct.
For example, to specify a list whose first element must be baz
and whose remaining arguments should be zero or more of foo and
bar, use this customization type:
(list (const baz) (set :inline t (const foo) (const bar)))
This matches values such as (baz), (baz foo), (baz bar)
and (baz foo bar).
When the element-type is a choice, you use :inline not
in the choice itself, but in (some of) the alternatives of the
choice. For example, to match a list which must start with a
file name, followed either by the symbol t or two strings, use
this customization type:
(list file
(choice (const t)
(list :inline t string string)))
If the user chooses the first alternative in the choice, then the
overall list has two elements and the second element is t. If
the user chooses the second alternative, then the overall list has three
elements and the second and third must be strings.
You can specify keyword-argument pairs in a customization type after the type name symbol. Here are the keywords you can use, and their meanings:
:value defaultchoice; it specifies the default value to use, at first, if and
when the user selects this alternative with the menu in the
customization buffer.
Of course, if the actual value of the option fits this alternative, it will appear showing the actual value, not default.
If nil is not a valid value for the alternative, then it is
essential to specify a valid default with :value.
:format format-string:action
attribute specifies what the button will do if the user invokes it;
its value is a function which takes two arguments—the widget which
the button appears in, and the event.
There is no way to specify two different buttons with different
actions.
:sample-face.
:tag
keyword.
:action action:button-face face:button-prefix prefix:button-suffix suffixnil:tag tag:doc doc:format, and use ‘%d’ or ‘%h’
in that value.
The usual reason to specify a documentation string for a type is to
provide more information about the meanings of alternatives inside a
:choice type or the parts of some other composite type.
:help-echo motion-docwidget-forward or
widget-backward, it will display the string motion-doc in
the echo area. In addition, motion-doc is used as the mouse
help-echo string and may actually be a function or form evaluated
to yield a help string. If it is a function, it is called with one
argument, the widget.
:match functionnil if
the value is acceptable.
:validate functionnil if the widget's
current value is valid for the widget. Otherwise, it should return
the widget containing the invalid data, and set that widget's
:error property to a string explaining the error.
In the previous sections we have described how to construct elaborate
type specifications for defcustom. In some cases you may want
to give such a type specification a name. The obvious case is when
you are using the same type for many user options: rather than repeat
the specification for each option, you can give the type specification
a name, and use that name each defcustom. The other case is
when a user option's value is a recursive data structure. To make it
possible for a datatype to refer to itself, it needs to have a name.
Since custom types are implemented as widgets, the way to define a new customize type is to define a new widget. We are not going to describe the widget interface here in details, see Introduction, for that. Instead we are going to demonstrate the minimal functionality needed for defining new customize types by a simple example.
(define-widget 'binary-tree-of-string 'lazy
"A binary tree made of cons-cells and strings."
:offset 4
:tag "Node"
:type '(choice (string :tag "Leaf" :value "")
(cons :tag "Interior"
:value ("" . "")
binary-tree-of-string
binary-tree-of-string)))
(defcustom foo-bar ""
"Sample variable holding a binary tree of strings."
:type 'binary-tree-of-string)
The function to define a new widget is called define-widget. The
first argument is the symbol we want to make a new widget type. The
second argument is a symbol representing an existing widget, the new
widget is going to be defined in terms of difference from the existing
widget. For the purpose of defining new customization types, the
lazy widget is perfect, because it accepts a :type keyword
argument with the same syntax as the keyword argument to
defcustom with the same name. The third argument is a
documentation string for the new widget. You will be able to see that
string with the M-x widget-browse <RET> binary-tree-of-string
<RET> command.
After these mandatory arguments follow the keyword arguments. The most
important is :type, which describes the data type we want to match
with this widget. Here a binary-tree-of-string is described as
being either a string, or a cons-cell whose car and cdr are themselves
both binary-tree-of-string. Note the reference to the widget
type we are currently in the process of defining. The :tag
attribute is a string to name the widget in the user interface, and the
:offset argument is there to ensure that child nodes are
indented four spaces relative to the parent node, making the tree
structure apparent in the customization buffer.
The defcustom shows how the new widget can be used as an ordinary
customization type.
The reason for the name lazy is that the other composite
widgets convert their inferior widgets to internal form when the
widget is instantiated in a buffer. This conversion is recursive, so
the inferior widgets will convert their inferior widgets. If
the data structure is itself recursive, this conversion is an infinite
recursion. The lazy widget prevents the recursion: it convert
its :type argument only when needed.
Loading a file of Lisp code means bringing its contents into the Lisp environment in the form of Lisp objects. Emacs finds and opens the file, reads the text, evaluates each form, and then closes the file.
The load functions evaluate all the expressions in a file just
as the eval-buffer function evaluates all the
expressions in a buffer. The difference is that the load functions
read and evaluate the text in the file as found on disk, not the text
in an Emacs buffer.
The loaded file must contain Lisp expressions, either as source code or as byte-compiled code. Each form in the file is called a top-level form. There is no special format for the forms in a loadable file; any form in a file may equally well be typed directly into a buffer and evaluated there. (Indeed, most code is tested this way.) Most often, the forms are function definitions and variable definitions.
A file containing Lisp code is often called a library. Thus, the “Rmail library” is a file containing code for Rmail mode. Similarly, a “Lisp library directory” is a directory of files containing Lisp code.
Emacs Lisp has several interfaces for loading. For example,
autoload creates a placeholder object for a function defined in a
file; trying to call the autoloading function loads the file to get the
function's real definition (see Autoload). require loads a
file if it isn't already loaded (see Named Features). Ultimately,
all these facilities call the load function to do the work.
This function finds and opens a file of Lisp code, evaluates all the forms in it, and closes the file.
To find the file,
loadfirst looks for a file named filename.elc, that is, for a file whose name is filename with the extension ‘.elc’ appended. If such a file exists, it is loaded. If there is no file by that name, thenloadlooks for a file named filename.el. If that file exists, it is loaded. Finally, if neither of those names is found,loadlooks for a file named filename with nothing appended, and loads it if it exists. (Theloadfunction is not clever about looking at filename. In the perverse case of a file named foo.el.el, evaluation of(load "foo.el")will indeed find it.)If Auto Compression mode is enabled, as it is by default, then if
loadcan not find a file, it searches for a compressed version of the file before trying other file names. It decompresses and loads it if it exists. It looks for compressed versions by appending each of the suffixes injka-compr-load-suffixesto the file name. The value of this variable must be a list of strings. Its standard value is(".gz").If the optional argument nosuffix is non-
nil, thenloaddoes not try the suffixes ‘.elc’ and ‘.el’. In this case, you must specify the precise file name you want, except that, if Auto Compression mode is enabled,loadwill still usejka-compr-load-suffixesto find compressed versions. By specifying the precise file name and usingtfor nosuffix, you can prevent perverse file names such as foo.el.el from being tried.If the optional argument must-suffix is non-
nil, thenloadinsists that the file name used must end in either ‘.el’ or ‘.elc’ (possibly extended with a compression suffix), unless it contains an explicit directory name.If filename is a relative file name, such as foo or baz/foo.bar,
loadsearches for the file using the variableload-path. It appends filename to each of the directories listed inload-path, and loads the first file it finds whose name matches. The current default directory is tried only if it is specified inload-path, wherenilstands for the default directory.loadtries all three possible suffixes in the first directory inload-path, then all three suffixes in the second directory, and so on. See Library Search.If you get a warning that foo.elc is older than foo.el, it means you should consider recompiling foo.el. See Byte Compilation.
When loading a source file (not compiled),
loadperforms character set translation just as Emacs would do when visiting the file. See Coding Systems.Messages like ‘Loading foo...’ and ‘Loading foo...done’ appear in the echo area during loading unless nomessage is non-
nil.Any unhandled errors while loading a file terminate loading. If the load was done for the sake of
autoload, any function definitions made during the loading are undone.If
loadcan't find the file to load, then normally it signals the errorfile-error(with ‘Cannot open load file filename’). But if missing-ok is non-nil, thenloadjust returnsnil.You can use the variable
load-read-functionto specify a function forloadto use instead ofreadfor reading expressions. See below.
loadreturnstif the file loads successfully.
This command loads the file filename. If filename is a relative file name, then the current default directory is assumed. This command does not use
load-path, and does not append suffixes. However, it does look for compressed versions (if Auto Compression Mode is enabled). Use this command if you wish to specify precisely the file name to load.
This command loads the library named library. It is equivalent to
load, except in how it reads its argument interactively.
This variable is non-
nilif Emacs is in the process of loading a file, and it isnilotherwise.
This variable specifies an alternate expression-reading function for
loadandeval-regionto use instead ofread. The function should accept one argument, just asreaddoes.Normally, the variable's value is
nil, which means those functions should useread.Instead of using this variable, it is cleaner to use another, newer feature: to pass the function as the read-function argument to
eval-region. See Eval.
For information about how load is used in building Emacs, see
Building Emacs.
We now describe some technical details about the exact suffixes that
load tries.
This is a list of suffixes indicating (compiled or source) Emacs Lisp files. It should not include the empty string.
loaduses these suffixes in order when it appends Lisp suffixes to the specified file name. The standard value is(".elc" ".el")which produces the behavior described in the previous section.
This is a list of suffixes that indicate representations of the same file. This list should normally start with the empty string. When
loadsearches for a file it appends the suffixes in this list, in order, to the file name, before searching for another file.Enabling Auto Compression mode appends the suffixes in
jka-compr-load-suffixesto this list and disabling Auto Compression mode removes them again. The standard value ofload-file-rep-suffixesif Auto Compression mode is disabled is(""). Given that the standard value ofjka-compr-load-suffixesis(".gz"), the standard value ofload-file-rep-suffixesif Auto Compression mode is enabled is("" ".gz").
This function returns the list of all suffixes that
loadshould try, in order, when its must-suffix argument is non-nil. This takes bothload-suffixesandload-file-rep-suffixesinto account. Ifload-suffixes,jka-compr-load-suffixesandload-file-rep-suffixesall have their standard values, this function returns(".elc" ".elc.gz" ".el" ".el.gz")if Auto Compression mode is enabled and(".elc" ".el")if Auto Compression mode is disabled.
To summarize, load normally first tries the suffixes in the
value of (get-load-suffixes) and then those in
load-file-rep-suffixes. If nosuffix is non-nil,
it skips the former group, and if must-suffix is non-nil,
it skips the latter group.
When Emacs loads a Lisp library, it searches for the library
in a list of directories specified by the variable load-path.
The value of this variable is a list of directories to search when loading files with
load. Each element is a string (which must be a directory name) ornil(which stands for the current working directory).
The value of load-path is initialized from the environment
variable EMACSLOADPATH, if that exists; otherwise its default
value is specified in emacs/src/epaths.h when Emacs is built.
Then the list is expanded by adding subdirectories of the directories
in the list.
The syntax of EMACSLOADPATH is the same as used for PATH;
‘:’ (or ‘;’, according to the operating system) separates
directory names, and ‘.’ is used for the current default directory.
Here is an example of how to set your EMACSLOADPATH variable from
a csh .login file:
setenv EMACSLOADPATH .:/user/bil/emacs:/usr/local/share/emacs/20.3/lisp
Here is how to set it using sh:
export EMACSLOADPATH
EMACSLOADPATH=.:/user/bil/emacs:/usr/local/share/emacs/20.3/lisp
Here is an example of code you can place in your init file (see Init File) to add several directories to the front of your default
load-path:
(setq load-path
(append (list nil "/user/bil/emacs"
"/usr/local/lisplib"
"~/emacs")
load-path))
In this example, the path searches the current working directory first, followed then by the /user/bil/emacs directory, the /usr/local/lisplib directory, and the ~/emacs directory, which are then followed by the standard directories for Lisp code.
Dumping Emacs uses a special value of load-path. If the value of
load-path at the end of dumping is unchanged (that is, still the
same special value), the dumped Emacs switches to the ordinary
load-path value when it starts up, as described above. But if
load-path has any other value at the end of dumping, that value
is used for execution of the dumped Emacs also.
Therefore, if you want to change load-path temporarily for
loading a few libraries in site-init.el or site-load.el,
you should bind load-path locally with let around the
calls to load.
The default value of load-path, when running an Emacs which has
been installed on the system, includes two special directories (and
their subdirectories as well):
"/usr/local/share/emacs/version/site-lisp"
and
"/usr/local/share/emacs/site-lisp"
The first one is for locally installed packages for a particular Emacs version; the second is for locally installed packages meant for use with all installed Emacs versions.
There are several reasons why a Lisp package that works well in one Emacs version can cause trouble in another. Sometimes packages need updating for incompatible changes in Emacs; sometimes they depend on undocumented internal Emacs data that can change without notice; sometimes a newer Emacs version incorporates a version of the package, and should be used only with that version.
Emacs finds these directories' subdirectories and adds them to
load-path when it starts up. Both immediate subdirectories and
subdirectories multiple levels down are added to load-path.
Not all subdirectories are included, though. Subdirectories whose names do not start with a letter or digit are excluded. Subdirectories named RCS or CVS are excluded. Also, a subdirectory which contains a file named .nosearch is excluded. You can use these methods to prevent certain subdirectories of the site-lisp directories from being searched.
If you run Emacs from the directory where it was built—that is, an
executable that has not been formally installed—then load-path
normally contains two additional directories. These are the lisp
and site-lisp subdirectories of the main build directory. (Both
are represented as absolute file names.)
This command finds the precise file name for library library. It searches for the library in the same way
loaddoes, and the argument nosuffix has the same meaning as inload: don't add suffixes ‘.elc’ or ‘.el’ to the specified name library.If the path is non-
nil, that list of directories is used instead ofload-path.When
locate-libraryis called from a program, it returns the file name as a string. When the user runslocate-libraryinteractively, the argument interactive-call ist, and this tellslocate-libraryto display the file name in the echo area.
When Emacs Lisp programs contain string constants with non-ASCII characters, these can be represented within Emacs either as unibyte strings or as multibyte strings (see Text Representations). Which representation is used depends on how the file is read into Emacs. If it is read with decoding into multibyte representation, the text of the Lisp program will be multibyte text, and its string constants will be multibyte strings. If a file containing Latin-1 characters (for example) is read without decoding, the text of the program will be unibyte text, and its string constants will be unibyte strings. See Coding Systems.
To make the results more predictable, Emacs always performs decoding into the multibyte representation when loading Lisp files, even if it was started with the ‘--unibyte’ option. This means that string constants with non-ASCII characters translate into multibyte strings. The only exception is when a particular file specifies no decoding.
The reason Emacs is designed this way is so that Lisp programs give
predictable results, regardless of how Emacs was started. In addition,
this enables programs that depend on using multibyte text to work even
in a unibyte Emacs. Of course, such programs should be designed to
notice whether the user prefers unibyte or multibyte text, by checking
default-enable-multibyte-characters, and convert representations
appropriately.
In most Emacs Lisp programs, the fact that non-ASCII strings are
multibyte strings should not be noticeable, since inserting them in
unibyte buffers converts them to unibyte automatically. However, if
this does make a difference, you can force a particular Lisp file to be
interpreted as unibyte by writing ‘-*-unibyte: t;-*-’ in a
comment on the file's first line. With that designator, the file will
unconditionally be interpreted as unibyte, even in an ordinary
multibyte Emacs session. This can matter when making keybindings to
non-ASCII characters written as ?vliteral.
The autoload facility allows you to make a function or macro known in Lisp, but put off loading the file that defines it. The first call to the function automatically reads the proper file to install the real definition and other associated code, then runs the real definition as if it had been loaded all along.
There are two ways to set up an autoloaded function: by calling
autoload, and by writing a special “magic” comment in the
source before the real definition. autoload is the low-level
primitive for autoloading; any Lisp program can call autoload at
any time. Magic comments are the most convenient way to make a function
autoload, for packages installed along with Emacs. These comments do
nothing on their own, but they serve as a guide for the command
update-file-autoloads, which constructs calls to autoload
and arranges to execute them when Emacs is built.
This function defines the function (or macro) named function so as to load automatically from filename. The string filename specifies the file to load to get the real definition of function.
If filename does not contain either a directory name, or the suffix
.elor.elc, thenautoloadinsists on adding one of these suffixes, and it will not load from a file whose name is just filename with no added suffix. (The variableload-suffixesspecifies the exact required suffixes.)The argument docstring is the documentation string for the function. Specifying the documentation string in the call to
autoloadmakes it possible to look at the documentation without loading the function's real definition. Normally, this should be identical to the documentation string in the function definition itself. If it isn't, the function definition's documentation string takes effect when it is loaded.If interactive is non-
nil, that says function can be called interactively. This lets completion in M-x work without loading function's real definition. The complete interactive specification is not given here; it's not needed unless the user actually calls function, and when that happens, it's time to load the real definition.You can autoload macros and keymaps as well as ordinary functions. Specify type as
macroif function is really a macro. Specify type askeymapif function is really a keymap. Various parts of Emacs need to know this information without loading the real definition.An autoloaded keymap loads automatically during key lookup when a prefix key's binding is the symbol function. Autoloading does not occur for other kinds of access to the keymap. In particular, it does not happen when a Lisp program gets the keymap from the value of a variable and calls
define-key; not even if the variable name is the same symbol function.If function already has a non-void function definition that is not an autoload object,
autoloaddoes nothing and returnsnil. If the function cell of function is void, or is already an autoload object, then it is defined as an autoload object like this:(autoload filename docstring interactive type)For example,
(symbol-function 'run-prolog) (autoload "prolog" 169681 t nil)In this case,
"prolog"is the name of the file to load, 169681 refers to the documentation string in the emacs/etc/DOC-version file (see Documentation Basics),tmeans the function is interactive, andnilthat it is not a macro or a keymap.
The autoloaded file usually contains other definitions and may require
or provide one or more features. If the file is not completely loaded
(due to an error in the evaluation of its contents), any function
definitions or provide calls that occurred during the load are
undone. This is to ensure that the next attempt to call any function
autoloading from this file will try again to load the file. If not for
this, then some of the functions in the file might be defined by the
aborted load, but fail to work properly for the lack of certain
subroutines not loaded successfully because they come later in the file.
If the autoloaded file fails to define the desired Lisp function or
macro, then an error is signaled with data "Autoloading failed to
define function function-name".
A magic autoload comment (often called an autoload cookie)
consists of ‘;;;###autoload’, on a line by itself,
just before the real definition of the function in its
autoloadable source file. The command M-x update-file-autoloads
writes a corresponding autoload call into loaddefs.el.
(The string that serves as the autoload cookie and the name of the
file generated by update-file-autoloads can be changed from the
above defaults, see below.)
Building Emacs loads loaddefs.el and thus calls autoload.
M-x update-directory-autoloads is even more powerful; it updates
autoloads for all files in the current directory.
The same magic comment can copy any kind of form into
loaddefs.el. If the form following the magic comment is not a
function-defining form or a defcustom form, it is copied
verbatim. “Function-defining forms” include define-skeleton,
define-derived-mode, define-generic-mode and
define-minor-mode as well as defun and
defmacro. To save space, a defcustom form is converted to
a defvar in loaddefs.el, with some additional information
if it uses :require.
You can also use a magic comment to execute a form at build time without executing it when the file itself is loaded. To do this, write the form on the same line as the magic comment. Since it is in a comment, it does nothing when you load the source file; but M-x update-file-autoloads copies it to loaddefs.el, where it is executed while building Emacs.
The following example shows how doctor is prepared for
autoloading with a magic comment:
;;;###autoload
(defun doctor ()
"Switch to *doctor* buffer and start giving psychotherapy."
(interactive)
(switch-to-buffer "*doctor*")
(doctor-mode))
Here's what that produces in loaddefs.el:
(autoload (quote doctor) "doctor" "\
Switch to *doctor* buffer and start giving psychotherapy.
\(fn)" t nil)
The backslash and newline immediately following the double-quote are a
convention used only in the preloaded uncompiled Lisp files such as
loaddefs.el; they tell make-docfile to put the
documentation string in the etc/DOC file. See Building Emacs.
See also the commentary in lib-src/make-docfile.c. ‘(fn)’
in the usage part of the documentation string is replaced with the
function's name when the various help functions (see Help Functions) display it.
If you write a function definition with an unusual macro that is not
one of the known and recognized function definition methods, use of an
ordinary magic autoload comment would copy the whole definition into
loaddefs.el. That is not desirable. You can put the desired
autoload call into loaddefs.el instead by writing this:
;;;###autoload (autoload 'foo "myfile")
(mydefunmacro foo
...)
You can use a non-default string as the autoload cookie and have the corresponding autoload calls written into a file whose name is different from the default loaddefs.el. Emacs provides two variables to control this:
The value of this variable should be a string whose syntax is a Lisp comment. M-x update-file-autoloads copies the Lisp form that follows the cookie into the autoload file it generates. The default value of this variable is
";;;###autoload".
The value of this variable names an Emacs Lisp file where the autoload calls should go. The default value is loaddefs.el, but you can override that, e.g., in the “Local Variables” section of a .el file (see File Local Variables). The autoload file is assumed to contain a trailer starting with a formfeed character.
You can load a given file more than once in an Emacs session. For example, after you have rewritten and reinstalled a function definition by editing it in a buffer, you may wish to return to the original version; you can do this by reloading the file it came from.
When you load or reload files, bear in mind that the load and
load-library functions automatically load a byte-compiled file
rather than a non-compiled file of similar name. If you rewrite a file
that you intend to save and reinstall, you need to byte-compile the new
version; otherwise Emacs will load the older, byte-compiled file instead
of your newer, non-compiled file! If that happens, the message
displayed when loading the file includes, ‘(compiled; note, source is
newer)’, to remind you to recompile it.
When writing the forms in a Lisp library file, keep in mind that the
file might be loaded more than once. For example, think about whether
each variable should be reinitialized when you reload the library;
defvar does not change the value if the variable is already
initialized. (See Defining Variables.)
The simplest way to add an element to an alist is like this:
(push '(leif-mode " Leif") minor-mode-alist)
But this would add multiple elements if the library is reloaded. To
avoid the problem, use add-to-list (see List Variables):
(add-to-list 'minor-mode-alist '(leif-mode " Leif"))
Occasionally you will want to test explicitly whether a library has
already been loaded. If the library uses provide to provide a
named feature, you can use featurep earlier in the file to test
whether the provide call has been executed before (see Named Features). Alternatively, you could use something like this:
(defvar foo-was-loaded nil)
(unless foo-was-loaded
execute-first-time-only
(setq foo-was-loaded t))
provide and require are an alternative to
autoload for loading files automatically. They work in terms of
named features. Autoloading is triggered by calling a specific
function, but a feature is loaded the first time another program asks
for it by name.
A feature name is a symbol that stands for a collection of functions, variables, etc. The file that defines them should provide the feature. Another program that uses them may ensure they are defined by requiring the feature. This loads the file of definitions if it hasn't been loaded already.
To require the presence of a feature, call require with the
feature name as argument. require looks in the global variable
features to see whether the desired feature has been provided
already. If not, it loads the feature from the appropriate file. This
file should call provide at the top level to add the feature to
features; if it fails to do so, require signals an error.
For example, in emacs/lisp/prolog.el,
the definition for run-prolog includes the following code:
(defun run-prolog ()
"Run an inferior Prolog process, with I/O via buffer *prolog*."
(interactive)
(require 'comint)
(switch-to-buffer (make-comint "prolog" prolog-program-name))
(inferior-prolog-mode))
The expression (require 'comint) loads the file comint.el
if it has not yet been loaded. This ensures that make-comint is
defined. Features are normally named after the files that provide them,
so that require need not be given the file name.
The comint.el file contains the following top-level expression:
(provide 'comint)
This adds comint to the global features list, so that
(require 'comint) will henceforth know that nothing needs to be
done.
When require is used at top level in a file, it takes effect
when you byte-compile that file (see Byte Compilation) as well as
when you load it. This is in case the required package contains macros
that the byte compiler must know about. It also avoids byte compiler
warnings for functions and variables defined in the file loaded with
require.
Although top-level calls to require are evaluated during
byte compilation, provide calls are not. Therefore, you can
ensure that a file of definitions is loaded before it is byte-compiled
by including a provide followed by a require for the same
feature, as in the following example.
(provide 'my-feature) ; Ignored by byte compiler,
; evaluated by load.
(require 'my-feature) ; Evaluated by byte compiler.
The compiler ignores the provide, then processes the
require by loading the file in question. Loading the file does
execute the provide call, so the subsequent require call
does nothing when the file is loaded.
This function announces that feature is now loaded, or being loaded, into the current Emacs session. This means that the facilities associated with feature are or will be available for other Lisp programs.
The direct effect of calling
provideis if not already in features then to add feature to the front of that list and call anyeval-after-loadcode waiting for it (see Hooks for Loading). The argument feature must be a symbol.providereturns feature.If provided, subfeatures should be a list of symbols indicating a set of specific subfeatures provided by this version of feature. You can test the presence of a subfeature using
featurep. The idea of subfeatures is that you use them when a package (which is one feature) is complex enough to make it useful to give names to various parts or functionalities of the package, which might or might not be loaded, or might or might not be present in a given version. See Network Feature Testing, for an example.features (bar bish) (provide 'foo) foo features (foo bar bish)When a file is loaded to satisfy an autoload, and it stops due to an error in the evaluation of its contents, any function definitions or
providecalls that occurred during the load are undone. See Autoload.
This function checks whether feature is present in the current Emacs session (using
(featurepfeature); see below). The argument feature must be a symbol.If the feature is not present, then
requireloads filename withload. If filename is not supplied, then the name of the symbol feature is used as the base file name to load. However, in this case,requireinsists on finding feature with an added ‘.el’ or ‘.elc’ suffix (possibly extended with a compression suffix); a file whose name is just feature won't be used. (The variableload-suffixesspecifies the exact required Lisp suffixes.)If noerror is non-
nil, that suppresses errors from actual loading of the file. In that case,requirereturnsnilif loading the file fails. Normally,requirereturns feature.If loading the file succeeds but does not provide feature,
requiresignals an error, ‘Required feature feature was not provided’.
This function returns
tif feature has been provided in the current Emacs session (i.e., if feature is a member offeatures.) If subfeature is non-nil, then the function returnstonly if that subfeature is provided as well (i.e. if subfeature is a member of thesubfeatureproperty of the feature symbol.)
The value of this variable is a list of symbols that are the features loaded in the current Emacs session. Each symbol was put in this list with a call to
provide. The order of the elements in thefeatureslist is not significant.
This function returns the name of the file that defined symbol. If type is
nil, then any kind of definition is acceptable. If type isdefun,defvar, ordefface, that specifies function definition, variable definition, or face definition only.The value is normally an absolute file name. It can also be
nil, if the definition is not associated with any file. If symbol specifies an autoloaded function, the value can be a relative file name without extension.
The basis for symbol-file is the data in the variable
load-history.
This variable's value is an alist connecting library file names with the names of functions and variables they define, the features they provide, and the features they require.
Each element is a list and describes one library. The car of the list is the absolute file name of the library, as a string. The rest of the list elements have these forms:
- var
- The symbol var was defined as a variable.
(defun .fun)- The function fun was defined.
(t .fun)- The function fun was previously an autoload before this library redefined it as a function. The following element is always
(defun .fun), which represents defining fun as a function.(autoload .fun)- The function fun was defined as an autoload.
(defface .face)- The face face was defined.
(require .feature)- The feature feature was required.
(provide .feature)- The feature feature was provided.
The value of
load-historymay have one element whose car isnil. This element describes definitions made witheval-bufferon a buffer that is not visiting a file.
The command eval-region updates load-history, but does so
by adding the symbols defined to the element for the file being visited,
rather than replacing that element. See Eval.
You can discard the functions and variables loaded by a library to
reclaim memory for other Lisp objects. To do this, use the function
unload-feature:
This command unloads the library that provided feature feature. It undefines all functions, macros, and variables defined in that library with
defun,defalias,defsubst,defmacro,defconst,defvar, anddefcustom. It then restores any autoloads formerly associated with those symbols. (Loading saves these in theautoloadproperty of the symbol.)Before restoring the previous definitions,
unload-featurerunsremove-hookto remove functions in the library from certain hooks. These hooks include variables whose names end in ‘hook’ or ‘-hooks’, plus those listed inunload-feature-special-hooks, as well asauto-mode-alist. This is to prevent Emacs from ceasing to function because important hooks refer to functions that are no longer defined.Standard unloading activities also undoes ELP profiling of functions in that library, unprovides any features provided by the library, and cancels timers held in variables defined by the library.
If these measures are not sufficient to prevent malfunction, a library can define an explicit unloader named feature
-unload-function. If that symbol is defined as a function,unload-featurecalls it with no arguments before doing anything else. It can do whatever is appropriate to unload the library. If it returnsnil,unload-featureproceeds to take the normal unload actions. Otherwise it considers the job to be done.Ordinarily,
unload-featurerefuses to unload a library on which other loaded libraries depend. (A library a depends on library b if a contains arequirefor b.) If the optional argument force is non-nil, dependencies are ignored and you can unload any library.
The unload-feature function is written in Lisp; its actions are
based on the variable load-history.
This variable holds a list of hooks to be scanned before unloading a library, to remove functions defined in the library.
You can ask for code to be executed if and when a particular library is
loaded, by calling eval-after-load.
This function arranges to evaluate form at the end of loading the file library, each time library is loaded. If library is already loaded, it evaluates form right away. Don't forget to quote form!
You don't need to give a directory or extension in the file name library—normally you just give a bare file name, like this:
(eval-after-load "edebug" '(def-edebug-spec c-point t))To restrict which files can trigger the evaluation, include a directory or an extension or both in library. Only a file whose absolute true name (i.e., the name with all symbolic links chased out) matches all the given name components will match. In the following example, my_inst.elc or my_inst.elc.gz in some directory
..../foo/barwill trigger the evaluation, but not my_inst.el:(eval-after-load "foo/bar/my_inst.elc" ...)library can also be a feature (i.e. a symbol), in which case form is evaluated when
(providelibrary)is called.An error in form does not undo the load, but does prevent execution of the rest of form.
In general, well-designed Lisp programs should not use this feature.
The clean and modular ways to interact with a Lisp library are (1)
examine and set the library's variables (those which are meant for
outside use), and (2) call the library's functions. If you wish to
do (1), you can do it immediately—there is no need to wait for when
the library is loaded. To do (2), you must load the library (preferably
with require).
But it is OK to use eval-after-load in your personal
customizations if you don't feel they must meet the design standards for
programs meant for wider use.
This variable, an alist built by
eval-after-load, holds the expressions to evaluate when particular libraries are loaded. Each element looks like this:(regexp-or-feature forms...)The key regexp-or-feature is either a regular expression or a symbol, and the value is a list of forms. The forms are evaluated when the key matches the absolute true name of the file being
loaded or the symbol beingprovided.
Emacs Lisp has a compiler that translates functions written in Lisp into a special representation called byte-code that can be executed more efficiently. The compiler replaces Lisp function definitions with byte-code. When a byte-code function is called, its definition is evaluated by the byte-code interpreter.
Because the byte-compiled code is evaluated by the byte-code interpreter, instead of being executed directly by the machine's hardware (as true compiled code is), byte-code is completely transportable from machine to machine without recompilation. It is not, however, as fast as true compiled code.
Compiling a Lisp file with the Emacs byte compiler always reads the file as multibyte text, even if Emacs was started with ‘--unibyte’, unless the file specifies otherwise. This is so that compilation gives results compatible with running the same file without compilation. See Loading Non-ASCII.
In general, any version of Emacs can run byte-compiled code produced by recent earlier versions of Emacs, but the reverse is not true.
If you do not want a Lisp file to be compiled, ever, put a file-local
variable binding for no-byte-compile into it, like this:
;; -*-no-byte-compile: t; -*-
See Compilation Errors, for how to investigate errors occurring in byte compilation.
A byte-compiled function is not as efficient as a primitive function written in C, but runs much faster than the version written in Lisp. Here is an example:
(defun silly-loop (n)
"Return time before and after N iterations of a loop."
(let ((t1 (current-time-string)))
(while (> (setq n (1- n))
0))
(list t1 (current-time-string))))
silly-loop
(silly-loop 50000000)
("Wed Mar 11 21:10:19 2009"
"Wed Mar 11 21:10:41 2009") ; 22 seconds
(byte-compile 'silly-loop)
[Compiled code not shown]
(silly-loop 50000000)
("Wed Mar 11 21:12:26 2009"
"Wed Mar 11 21:12:32 2009") ; 6 seconds
In this example, the interpreted code required 22 seconds to run, whereas the byte-compiled code required 6 seconds. These results are representative, but actual results will vary greatly.
You can byte-compile an individual function or macro definition with
the byte-compile function. You can compile a whole file with
byte-compile-file, or several files with
byte-recompile-directory or batch-byte-compile.
The byte compiler produces error messages and warnings about each file in a buffer called ‘*Compile-Log*’. These report things in your program that suggest a problem but are not necessarily erroneous.
Be careful when writing macro calls in files that you may someday
byte-compile. Macro calls are expanded when they are compiled, so the
macros must already be defined for proper compilation. For more
details, see Compiling Macros. If a program does not work the
same way when compiled as it does when interpreted, erroneous macro
definitions are one likely cause (see Problems with Macros).
Inline (defsubst) functions are less troublesome; if you
compile a call to such a function before its definition is known, the
call will still work right, it will just run slower.
Normally, compiling a file does not evaluate the file's contents or
load the file. But it does execute any require calls at top
level in the file. One way to ensure that necessary macro definitions
are available during compilation is to require the file that defines
them (see Named Features). To avoid loading the macro definition files
when someone runs the compiled program, write
eval-when-compile around the require calls (see Eval During Compile).
This function byte-compiles the function definition of symbol, replacing the previous definition with the compiled one. The function definition of symbol must be the actual code for the function; i.e., the compiler does not follow indirection to another symbol.
byte-compilereturns the new, compiled definition of symbol.If symbol's definition is a byte-code function object,
byte-compiledoes nothing and returnsnil. Lisp records only one function definition for any symbol, and if that is already compiled, non-compiled code is not available anywhere. So there is no way to “compile the same definition again.”(defun factorial (integer) "Compute factorial of INTEGER." (if (= 1 integer) 1 (* integer (factorial (1- integer))))) factorial (byte-compile 'factorial) #[(integer) "^H\301U\203^H^@\301\207\302^H\303^HS!\"\207" [integer 1 * factorial] 4 "Compute factorial of INTEGER."]The result is a byte-code function object. The string it contains is the actual byte-code; each character in it is an instruction or an operand of an instruction. The vector contains all the constants, variable names and function names used by the function, except for certain primitives that are coded as special instructions.
If the argument to
byte-compileis alambdaexpression, it returns the corresponding compiled code, but does not store it anywhere.
This command reads the defun containing point, compiles it, and evaluates the result. If you use this on a defun that is actually a function definition, the effect is to install a compiled version of that function.
compile-defunnormally displays the result of evaluation in the echo area, but if arg is non-nil, it inserts the result in the current buffer after the form it compiled.
This function compiles a file of Lisp code named filename into a file of byte-code. The output file's name is made by changing the ‘.el’ suffix into ‘.elc’; if filename does not end in ‘.el’, it adds ‘.elc’ to the end of filename.
Compilation works by reading the input file one form at a time. If it is a definition of a function or macro, the compiled function or macro definition is written out. Other forms are batched together, then each batch is compiled, and written so that its compiled code will be executed when the file is read. All comments are discarded when the input file is read.
This command returns
tif there were no errors andnilotherwise. When called interactively, it prompts for the file name.If load is non-
nil, this command loads the compiled file after compiling it. Interactively, load is the prefix argument.% ls -l push* -rw-r--r-- 1 lewis 791 Oct 5 20:31 push.el (byte-compile-file "~/emacs/push.el") t % ls -l push* -rw-r--r-- 1 lewis 791 Oct 5 20:31 push.el -rw-rw-rw- 1 lewis 638 Oct 8 20:25 push.elc
This command recompiles every ‘.el’ file in directory (or its subdirectories) that needs recompilation. A file needs recompilation if a ‘.elc’ file exists but is older than the ‘.el’ file.
When a ‘.el’ file has no corresponding ‘.elc’ file, flag says what to do. If it is
nil, this command ignores these files. If flag is 0, it compiles them. If it is neithernilnor 0, it asks the user whether to compile each such file, and asks about each subdirectory as well.Interactively,
byte-recompile-directoryprompts for directory and flag is the prefix argument.If force is non-
nil, this command recompiles every ‘.el’ file that has a ‘.elc’ file.The returned value is unpredictable.
This function runs
byte-compile-fileon files specified on the command line. This function must be used only in a batch execution of Emacs, as it kills Emacs on completion. An error in one file does not prevent processing of subsequent files, but no output file will be generated for it, and the Emacs process will terminate with a nonzero status code.If noforce is non-
nil, this function does not recompile files that have an up-to-date ‘.elc’ file.% emacs -batch -f batch-byte-compile *.el
This function actually interprets byte-code. A byte-compiled function is actually defined with a body that calls
byte-code. Don't call this function yourself—only the byte compiler knows how to generate valid calls to this function.In Emacs version 18, byte-code was always executed by way of a call to the function
byte-code. Nowadays, byte-code is usually executed as part of a byte-code function object, and only rarely through an explicit call tobyte-code.
Functions and variables loaded from a byte-compiled file access their documentation strings dynamically from the file whenever needed. This saves space within Emacs, and makes loading faster because the documentation strings themselves need not be processed while loading the file. Actual access to the documentation strings becomes slower as a result, but this normally is not enough to bother users.
Dynamic access to documentation strings does have drawbacks:
If your site installs Emacs following the usual procedures, these problems will never normally occur. Installing a new version uses a new directory with a different name; as long as the old version remains installed, its files will remain unmodified in the places where they are expected to be.
However, if you have built Emacs yourself and use it from the directory where you built it, you will experience this problem occasionally if you edit and recompile Lisp files. When it happens, you can cure the problem by reloading the file after recompiling it.
You can turn off this feature at compile time by setting
byte-compile-dynamic-docstrings to nil; this is useful
mainly if you expect to change the file, and you want Emacs processes
that have already loaded it to keep working when the file changes.
You can do this globally, or for one source file by specifying a
file-local binding for the variable. One way to do that is by adding
this string to the file's first line:
-*-byte-compile-dynamic-docstrings: nil;-*-
If this is non-
nil, the byte compiler generates compiled files that are set up for dynamic loading of documentation strings.
The dynamic documentation string feature writes compiled files that use a special Lisp reader construct, ‘#@count’. This construct skips the next count characters. It also uses the ‘#$’ construct, which stands for “the name of this file, as a string.” It is usually best not to use these constructs in Lisp source files, since they are not designed to be clear to humans reading the file.
When you compile a file, you can optionally enable the dynamic function loading feature (also known as lazy loading). With dynamic function loading, loading the file doesn't fully read the function definitions in the file. Instead, each function definition contains a place-holder which refers to the file. The first time each function is called, it reads the full definition from the file, to replace the place-holder.
The advantage of dynamic function loading is that loading the file becomes much faster. This is a good thing for a file which contains many separate user-callable functions, if using one of them does not imply you will probably also use the rest. A specialized mode which provides many keyboard commands often has that usage pattern: a user may invoke the mode, but use only a few of the commands it provides.
The dynamic loading feature has certain disadvantages:
These problems will never happen in normal circumstances with installed Emacs files. But they are quite likely to happen with Lisp files that you are changing. The easiest way to prevent these problems is to reload the new compiled file immediately after each recompilation.
The byte compiler uses the dynamic function loading feature if the
variable byte-compile-dynamic is non-nil at compilation
time. Do not set this variable globally, since dynamic loading is
desirable only for certain files. Instead, enable the feature for
specific source files with file-local variable bindings. For example,
you could do it by writing this text in the source file's first line:
-*-byte-compile-dynamic: t;-*-
If this is non-
nil, the byte compiler generates compiled files that are set up for dynamic function loading.
If function is a byte-code function object, this immediately finishes loading the byte code of function from its byte-compiled file, if it is not fully loaded already. Otherwise, it does nothing. It always returns function.
These features permit you to write code to be evaluated during compilation of a program.
This form marks body to be evaluated both when you compile the containing code and when you run it (whether compiled or not).
You can get a similar result by putting body in a separate file and referring to that file with
require. That method is preferable when body is large. Effectivelyrequireis automaticallyeval-and-compile, the package is loaded both when compiling and executing.
autoloadis also effectivelyeval-and-compiletoo. It's recognized when compiling, so uses of such a function don't produce “not known to be defined” warnings.Most uses of
eval-and-compileare fairly sophisticated.If a macro has a helper function to build its result, and that macro is used both locally and outside the package, then
eval-and-compileshould be used to get the helper both when compiling and then later when running.If functions are defined programmatically (with
fsetsay), theneval-and-compilecan be used to have that done at compile-time as well as run-time, so calls to those functions are checked (and warnings about “not known to be defined” suppressed).
This form marks body to be evaluated at compile time but not when the compiled program is loaded. The result of evaluation by the compiler becomes a constant which appears in the compiled program. If you load the source file, rather than compiling it, body is evaluated normally.
If you have a constant that needs some calculation to produce,
eval-when-compilecan do that at compile-time. For example,(defvar my-regexp (eval-when-compile (regexp-opt '("aaa" "aba" "abb"))))If you're using another package, but only need macros from it (the byte compiler will expand those), then
eval-when-compilecan be used to load it for compiling, but not executing. For example,(eval-when-compile (require 'my-macro-package)) ;; only macros needed from thisThe same sort of thing goes for macros and
defsubstfunctions defined locally and only for use within the file. They are needed for compiling the file, but in most cases they are not needed for execution of the compiled file. For example,(eval-when-compile (unless (fboundp 'some-new-thing) (defmacro 'some-new-thing () (compatibility code))))This is often good for code that's only a fallback for compatibility with other versions of Emacs.
Common Lisp Note: At top level,
eval-when-compileis analogous to the Common Lisp idiom(eval-when (compile eval) ...). Elsewhere, the Common Lisp ‘#.’ reader macro (but not when interpreting) is closer to whateval-when-compiledoes.
Byte compilation outputs all errors and warnings into the buffer ‘*Compile-Log*’. The messages include file names and line numbers that identify the location of the problem. The usual Emacs commands for operating on compiler diagnostics work properly on these messages.
However, the warnings about functions that were used but not defined are always “located” at the end of the file, so these commands won't find the places they are really used. To do that, you must search for the function names.
You can suppress the compiler warning for calling an undefined
function func by conditionalizing the function call on an
fboundp test, like this:
(if (fboundp 'func) ...(func ...)...)
The call to func must be in the then-form of the
if, and func must appear quoted in the call to
fboundp. (This feature operates for cond as well.)
You can tell the compiler that a function is defined using
declare-function (see Declaring Functions). Likewise, you
can tell the compiler that a variable is defined using defvar
with no initial value.
You can suppress the compiler warning for a specific use of an
undefined variable variable by conditionalizing its use on a
boundp test, like this:
(if (boundp 'variable) ...variable...)
The reference to variable must be in the then-form of the
if, and variable must appear quoted in the call to
boundp.
You can suppress any and all compiler warnings within a certain
expression using the construct with-no-warnings:
In execution, this is equivalent to
(prognbody...), but the compiler does not issue warnings for anything that occurs inside body.We recommend that you use this construct around the smallest possible piece of code, to avoid missing possible warnings other than one one you intend to suppress.
More precise control of warnings is possible by setting the variable
byte-compile-warnings.
Byte-compiled functions have a special data type: they are byte-code function objects.
Internally, a byte-code function object is much like a vector; however, the evaluator handles this data type specially when it appears as a function to be called. The printed representation for a byte-code function object is like that for a vector, with an additional ‘#’ before the opening ‘[’.
A byte-code function object must have at least four elements; there is no maximum number, but only the first six elements have any normal use. They are:
nil. The value may
be a number or a list, in case the documentation string is stored in a
file. Use the function documentation to get the real
documentation string (see Accessing Documentation).
nil for a function that isn't interactive.
Here's an example of a byte-code function object, in printed
representation. It is the definition of the command
backward-sexp.
#[(&optional arg)
"^H\204^F^@\301^P\302^H[!\207"
[arg 1 forward-sexp]
2
254435
"p"]
The primitive way to create a byte-code object is with
make-byte-code:
This function constructs and returns a byte-code function object with elements as its elements.
You should not try to come up with the elements for a byte-code function yourself, because if they are inconsistent, Emacs may crash when you call the function. Always leave it to the byte compiler to create these objects; it makes the elements consistent (we hope).
You can access the elements of a byte-code object using aref;
you can also use vconcat to create a vector with the same
elements.
People do not write byte-code; that job is left to the byte compiler. But we provide a disassembler to satisfy a cat-like curiosity. The disassembler converts the byte-compiled code into human-readable form.
The byte-code interpreter is implemented as a simple stack machine. It pushes values onto a stack of its own, then pops them off to use them in calculations whose results are themselves pushed back on the stack. When a byte-code function returns, it pops a value off the stack and returns it as the value of the function.
In addition to the stack, byte-code functions can use, bind, and set ordinary Lisp variables, by transferring values between variables and the stack.
This command displays the disassembled code for object. In interactive use, or if buffer-or-name is
nilor omitted, the output goes in a buffer named ‘*Disassemble*’. If buffer-or-name is non-nil, it must be a buffer or the name of an existing buffer. Then the output goes there, at point, and point is left before the output.The argument object can be a function name, a lambda expression or a byte-code object. If it is a lambda expression,
disassemblecompiles it and disassembles the resulting compiled code.
Here are two examples of using the disassemble function. We
have added explanatory comments to help you relate the byte-code to the
Lisp source; these do not appear in the output of disassemble.
(defun factorial (integer)
"Compute factorial of an integer."
(if (= 1 integer) 1
(* integer (factorial (1- integer)))))
factorial
(factorial 4)
24
(disassemble 'factorial)
-| byte-code for factorial:
doc: Compute factorial of an integer.
args: (integer)
0 varref integer ; Get the value of integer
; and push it onto the stack.
1 constant 1 ; Push 1 onto stack.
2 eqlsign ; Pop top two values off stack, compare
; them, and push result onto stack.
3 goto-if-nil 1 ; Pop and test top of stack;
; if nil, go to 1,
; else continue.
6 constant 1 ; Push 1 onto top of stack.
7 return ; Return the top element
; of the stack.
8:1 varref integer ; Push value of integer onto stack.
9 constant factorial ; Push factorial onto stack.
10 varref integer ; Push value of integer onto stack.
11 sub1 ; Pop integer, decrement value,
; push new value onto stack.
12 call 1 ; Call function factorial using
; the first (i.e., the top) element
; of the stack as the argument;
; push returned value onto stack.
13 mult ; Pop top two values off stack, multiply
; them, and push result onto stack.
14 return ; Return the top element of stack.
The silly-loop function is somewhat more complex:
(defun silly-loop (n)
"Return time before and after N iterations of a loop."
(let ((t1 (current-time-string)))
(while (> (setq n (1- n))
0))
(list t1 (current-time-string))))
silly-loop
(disassemble 'silly-loop)
-| byte-code for silly-loop:
doc: Return time before and after N iterations of a loop.
args: (n)
0 constant current-time-string ; Push
; current-time-string
; onto top of stack.
1 call 0 ; Call current-time-string
; with no argument,
; pushing result onto stack.
2 varbind t1 ; Pop stack and bind t1
; to popped value.
3:1 varref n ; Get value of n from
; the environment and push
; the value onto the stack.
4 sub1 ; Subtract 1 from top of stack.
5 dup ; Duplicate the top of the stack;
; i.e., copy the top of
; the stack and push the
; copy onto the stack.
6 varset n ; Pop the top of the stack,
; and bind n to the value.
; In effect, the sequence dup varset
; copies the top of the stack
; into the value of n
; without popping it.
7 constant 0 ; Push 0 onto stack.
8 gtr ; Pop top two values off stack,
; test if n is greater than 0
; and push result onto stack.
9 goto-if-not-nil 1 ; Goto 1 if n > 0
; (this continues the while loop)
; else continue.
12 varref t1 ; Push value of t1 onto stack.
13 constant current-time-string ; Push current-time-string
; onto top of stack.
14 call 0 ; Call current-time-string again.
15 unbind 1 ; Unbind t1 in local environment.
16 list2 ; Pop top two elements off stack,
; create a list of them,
; and push list onto stack.
17 return ; Return value of the top of stack.
The advice feature lets you add to the existing definition of a function, by advising the function. This is a cleaner method for a library to customize functions defined within Emacs—cleaner than redefining the whole function.
Each function can have multiple pieces of advice, separately defined. Each defined piece of advice can be enabled or disabled explicitly. All the enabled pieces of advice for any given function actually take effect when you activate advice for that function, or when you define or redefine the function. Note that enabling a piece of advice and activating advice for a function are not the same thing.
Usage Note: Advice is useful for altering the behavior of existing calls to an existing function. If you want the new behavior for new calls, or for key bindings, you should define a new function (or a new command) which uses the existing function.
Usage note: Advising a function can cause confusion in debugging, since people who debug calls to the original function may not notice that it has been modified with advice. Therefore, if you have the possibility to change the code of that function (or ask someone to do so) to run a hook, please solve the problem that way. Advice should be reserved for the cases where you cannot get the function changed.
In particular, this means that a file in Emacs should not put advice on a function in Emacs. There are currently a few exceptions to this convention, but we aim to correct them.
The command next-line moves point down vertically one or more
lines; it is the standard binding of C-n. When used on the last
line of the buffer, this command inserts a newline to create a line to
move to if next-line-add-newlines is non-nil (its default
is nil.)
Suppose you wanted to add a similar feature to previous-line,
which would insert a new line at the beginning of the buffer for the
command to move to (when next-line-add-newlines is
non-nil). How could you do this?
You could do it by redefining the whole function, but that is not modular. The advice feature provides a cleaner alternative: you can effectively add your code to the existing function definition, without actually changing or even seeing that definition. Here is how to do this:
(defadvice previous-line (before next-line-at-end
(&optional arg try-vscroll))
"Insert an empty line when moving up from the top line."
(if (and next-line-add-newlines (= arg 1)
(save-excursion (beginning-of-line) (bobp)))
(progn
(beginning-of-line)
(newline))))
This expression defines a piece of advice for the function
previous-line. This piece of advice is named
next-line-at-end, and the symbol before says that it is
before-advice which should run before the regular definition of
previous-line. (&optional arg try-vscroll) specifies
how the advice code can refer to the function's arguments.
When this piece of advice runs, it creates an additional line, in the situation where that is appropriate, but does not move point to that line. This is the correct way to write the advice, because the normal definition will run afterward and will move back to the newly inserted line.
Defining the advice doesn't immediately change the function
previous-line. That happens when you activate the advice,
like this:
(ad-activate 'previous-line)
This is what actually begins to use the advice that has been defined so
far for the function previous-line. Henceforth, whenever that
function is run, whether invoked by the user with C-p or
M-x, or called from Lisp, it runs the advice first, and its
regular definition second.
This example illustrates before-advice, which is one class of advice: it runs before the function's base definition. There are two other advice classes: after-advice, which runs after the base definition, and around-advice, which lets you specify an expression to wrap around the invocation of the base definition.
To define a piece of advice, use the macro defadvice. A call
to defadvice has the following syntax, which is based on the
syntax of defun and defmacro, but adds more:
(defadvice function (class name
[position] [arglist]
flags...)
[documentation-string]
[interactive-form]
body-forms...)
Here, function is the name of the function (or macro or special form) to be advised. From now on, we will write just “function” when describing the entity being advised, but this always includes macros and special forms.
In place of the argument list in an ordinary definition, an advice definition calls for several different pieces of information.
class specifies the class of the advice—one of before,
after, or around. Before-advice runs before the function
itself; after-advice runs after the function itself; around-advice is
wrapped around the execution of the function itself. After-advice and
around-advice can override the return value by setting
ad-return-value.
While advice is executing, after the function's original definition has been executed, this variable holds its return value, which will ultimately be returned to the caller after finishing all the advice. After-advice and around-advice can arrange to return some other value by storing it in this variable.
The argument name is the name of the advice, a non-nil
symbol. The advice name uniquely identifies one piece of advice, within all
the pieces of advice in a particular class for a particular
function. The name allows you to refer to the piece of
advice—to redefine it, or to enable or disable it.
The optional position specifies where, in the current list of
advice of the specified class, this new advice should be placed.
It should be either first, last or a number that specifies
a zero-based position (first is equivalent to 0). If no position
is specified, the default is first. Position values outside the
range of existing positions in this class are mapped to the beginning or
the end of the range, whichever is closer. The position value is
ignored when redefining an existing piece of advice.
The optional arglist can be used to define the argument list for the sake of advice. This becomes the argument list of the combined definition that is generated in order to run the advice (see Combined Definition). Therefore, the advice expressions can use the argument variables in this list to access argument values.
The argument list used in advice need not be the same as the argument list used in the original function, but must be compatible with it, so that it can handle the ways the function is actually called. If two pieces of advice for a function both specify an argument list, they must specify the same argument list.
See Argument Access in Advice, for more information about argument lists and advice, and a more flexible way for advice to access the arguments.
The remaining elements, flags, are symbols that specify further information about how to use this piece of advice. Here are the valid symbols and their meanings:
activateThis flag has no immediate effect if function itself is not defined yet (a
situation known as forward advice), because it is impossible to
activate an undefined function's advice. However, defining
function will automatically activate its advice.
protectunwind-protect form, so that it will execute even if the
previous code gets an error or uses throw. See Cleanups.
compileactivate is also specified.
See Combined Definition.
disablepreactivatedefadvice is
compiled or macroexpanded. This generates a compiled advised definition
according to the current advice state, which will be used during
activation if appropriate. See Preactivation.
This is useful only if this defadvice is byte-compiled.
The optional documentation-string serves to document this piece of
advice. When advice is active for function, the documentation for
function (as returned by documentation) combines the
documentation strings of all the advice for function with the
documentation string of its original function definition.
The optional interactive-form form can be supplied to change the interactive behavior of the original function. If more than one piece of advice has an interactive-form, then the first one (the one with the smallest position) found among all the advice takes precedence.
The possibly empty list of body-forms specifies the body of the advice. The body of an advice can access or change the arguments, the return value, the binding environment, and perform any other kind of side effect.
Warning: When you advise a macro, keep in mind that macros are expanded when a program is compiled, not when a compiled program is run. All subroutines used by the advice need to be available when the byte compiler expands the macro.
Around-advice lets you “wrap” a Lisp expression “around” the
original function definition. You specify where the original function
definition should go by means of the special symbol ad-do-it.
Where this symbol occurs inside the around-advice body, it is replaced
with a progn containing the forms of the surrounded code