MIT/GNU Scheme release notes

Stable release 9.0 (pending)

(Note that we're skipping the 8.x version numbers because long ago there was an 8.x series that we never released.)

In the past my (CPH) policy for a stable release was that the documentation had to be updated for the release before it went out. In practice, this has meant that there have been no stable releases in recent years. As of this release, we will no longer consider updated documentation a prerequisite for a stable release.

Major changes

• The compiler's C back end has been resurrected, allowing the system to be run on most computer architectures (under unix-like systems only).
• A new virtual machine has been designed and partially implemented. When finished, it will provide additional system portability.
• The system now runs on Intel Macs running Mac OS X with native-code compilation.
• The empty list and #f are now distinct objects.
• The garbage collector has been completely rewritten. The new design uses a single heap and a temporary memory region, which doubles the largest available heap space. The now-unavailable bchscheme was similar except that its temporary region was a file.

Incompatibilities with previous releases

• Support for SRFI 1 has forced a change in the behavior of the procedure reduce; code using the old reduce should adapt to the new behavior, or use reduce-left which implements the old behavior.
• The procedure record-type-default-inits now returns a list, not a vector.

System usage changes

• The compiler now generates type and range checks by default, in order to make compiled code more robust. The runtime system is now compiled this way as well. New declarations (no-type-checks) and (no-range-checks) allow these defaults to be overridden. This change will cause some performance degredation; we're interested in hearing about situations in which this is a significant problem.
• The compiler's verbosity has been significantly reduced.
• The system will now run on Windows XP SP2 when the no-execute permissions are enabled.
• Platform support for Cygwin has been added. This was a donation and hasn't been tested by us.
• The file specified by environment variable MITSCHEME_LOAD_OPTIONS is now considered optional rather than required.
• The --eval and --load command-line options have been changed so that their actions are queued to be evaluated by the REPL rather than being processed outside of the REPL context. This fixes various problems with the use of these options.
• Several problems have been fixed in the use of modifier keys under Windows and X11.

Changes to the runtime

• Defaulted optional arguments have a new value that is a self-evaluating constant. Previously such arguments were filled with a value that made them "unassigned".
• cond-expand now recognizes mit and mit/gnu as features, to assist porting programs.
• The URL support has been replaced by a new implementation of URIs.
• Basic support for mapping of pathnames to MIME types has been added.
• There is new syntax for expression comments: #;(+ 3 4).
• There is now support for access to the registry on Windows systems.
• The low-level Unicode support has been completely rewritten:
  • We now support UTF-8, UTF-16, and UTF-32.
  • There's a new character #\bom.
  • Parser buffers now deal with wide characters.
• The port abstraction has been completely rewritten to support character coding and a wider variety of line endings.
  • There's a new operation to unread a character.
  • Port encapsulation has been eliminated.
  • discard-char is now an alias for read-char.
  • open-tcp-stream-socket now takes only two arguments.
• Hash tables have been reimplemented for improved speed. In the process some less useful operations were removed. There are new procedures make-strong-eq-hash-table and make-strong-eqv-hash-table.
• The new procedure symbol provides an easy way to build new (interned) symbols.
• A new quoting syntax for symbols simplifies writing arbitrary symbols.
• The new procedures smallest-fixnum and largest-fixnum provide the limits on the fixnum representation.
• The new procedure channel-file-truncate can truncate an open file.
• Symbol names are now encoded in UTF-8. string->symbol accepts an ISO 8859-1 string and converts it, while symbol->string returns an ISO 8859-1 string (or signals an error if conversion impossible). New procedures utf8-string->symbol and symbol->utf8-string provide support for UTF-8 strings.
• string->number now accepts an optional argument; if given and true, and the input string isn't a number's representation, an error is signalled.
• The read procedure now accepts an optional second argument, an environment in which to look up control symbols such as *parser-radix*. This allows these variables to be scoped rather than dynamically bound, which in turn makes them much safer to use. Numerous callers of read have been changed to pass an appropriate environment here.
• New procedures count-matching-items, count-non-matching-items, reverse* and reverse*! have been implemented.
• The random-number generator has been changed to provide reasonable output for large moduli. The previous implementation limited the amount of randomness in that case.
• SRFIs 1, 2, 27, and 69 have been implemented.
  • The procedure random-source-pseudo-randomize! from SRFI 27 has not been implemented. While I agree that this could be useful, it effectively mandates a particular PRNG, and I don't want to be forced to use it.
  • The procedure hash from SRFI 69 has not been implemented, as it's a name conflict with a pre-existing procedure.
• The is now partial support for ISO 8601 date/time strings.
• There is now basic support for RDF and Turtle.
• There is now support for server-side programming using Apache and mod_lisp.

Improved XML support

• xml-element-content has been renamed to xml-element-contents.
• Character data can now be provided in several different forms.
• We now support UTF-8, UTF-16, UTF-32, and all ISO 8859 character sets.
• We now support XHTML 1.0 Strict and XHTML 1.1, including convenience procedures for building documents.
• We now support XML-RPC.
• XML element attributes now have an opaque representation; previously they were pairs. Also, the attribute values are now guaranteed to be strings; unresolved entity references are no longer supported.
• The XML naming support has been rewritten, to rationalize the code and bring our terminology into line with W3C.

Changes to Edwin

• Edwin buffers are now allocated as external strings, which allows buffers to be as large as 32 MiB each.
• New parenthesis-editing minor mode M-x paredit-mode.
• Support for Lisppaste.

Changes to IMAIL

• IMAIL has improved sorting that works much better on large folders.
• IMAIL can now parse MIME in any folder, not just in IMAP folders.

Testing release 7.7.90

As of this release, MIT Scheme is a part of the GNU project and has been renamed MIT/GNU Scheme. The project is now hosted on Savannah. License text in the source files has been changed, and a license/warranty statement is now emitted during boot, to conform to the GNU coding standards.

This is the first testing release of MIT/GNU Scheme. I had originally planned to do a stable 7.8.0 release, but time pressures have made it difficult to bring the documentation up to date, so this release comes with out-of-date documentation. Additionally, there will be binaries only for GNU/Linux; users of other systems will have to wait for the stable release.

Incompatibilities with previous releases

• In releases 7.7.0 and 7.7.1, variable definitions (i.e. instances of the define special form) appearing inside let-syntax modified the environment outside of the let-syntax, while syntax definitions (instances of the define-syntax special form) modified the environment corresponding to the let-syntax form. However, according to R5RS this is incorrect: all definitions should modify the environment corresponding to the let-syntax form. The syntax has been changed to conform to R5RS.

• The record abstraction has received a major update. The primary purpose of this update has been to improve the performance of constructors, and to implement keyword constructors for records. As a consequence, the representation of record types has been changed. Because record types are constructed at load time, this has no effect on previously-compiled code.

  However, the define-structure macro was also changed to use these new facilities. The interface between define-structure and the record abstraction was changed to increase performance, and consequently previously-compiled instances of define-structure no longer work and must be recompiled.

  A further change to define-structure is that the initial-value expressions are interpreted in a different way. Previously, an undocumented feature of these expressions was that they could refer to other supplied record field names as free variables. This no longer works; instead these expressions are closed in the environment in which the define-structure macro appears.

  The default type-descriptor name for define-structure has changed. Previously, for a structure defined as

  (define-structure foo bar)
  

  the type descriptor was named foo. Now, the type descriptor is named rtd:foo. This change is useful primarily because it is common to name variables that hold objects of this type foo, and when the type descriptor has the same name, it causes confusion between references to the descriptor and unintended free references to an object. (After making this change, several such free references were found in the MIT/GNU Scheme code.)

  Finally, define-structure now defines a type descriptor for every structure definition, including structures without tags. Previously this was done only for tagged structures.

• The representation of character objects has been changed to provide direct support for Unicode. Previously, the representation had 16 bits of code and 5 bucky bits. The new representation has 21 bits of code and 4 bucky bits (the "top" bucky bit has been eliminated). This allows direct representation of the entire Unicode space.

  In addition, the syntax of characters has been extended to allow arbitrary Unicode characters to be represented. The new syntax is #\U+XXXX, where XXXX is a sequence of hexadecimal digits specifying a Unicode code point. This supersedes an undocumented syntax #\<codeXXXX>.

• The runtime library's support for Unicode has been completely rewritten, and now has support for UTF-8, UTF-16, and UTF-32 encodings, as well as support for wide strings. The UTF-8 codec has been fixed to signal errors for overlong coding sequences.

• The special form define-syntax has been changed so that the right-hand side may be a keyword. This can be used to make aliases for existing keywords, such as

  (define-syntax sequence begin)
  

• In pre-7.7 versions of MIT/GNU Scheme, the right-hand side of the special form define-syntax was a procedure, such as

  (define-syntax foo (lambda ...))
  

  This behavior was preserved in the 7.7 versions by a kludge that made the above equivalent to

  (define-syntax foo
    (non-hygienic-macro-transformer
     (lambda ...)))
  

  With this release, the old syntax has been eliminated. It is now necessary to use the non-hygienic-macro-transformer special form in these cases. (Note, however, that non-hygienic-macro-transformer is also a kludge and is not guaranteed to produce working macros. You should rewrite your macros in hygienic form to guarantee proper operation.)

• Command-line options now start with -- rather than -, again for compliance with GNU coding standards. The older - prefix still works but may eventually be dropped.

• The external representation of symbols has been extended to support the quoting mechanisms of Common Lisp. This means that there is a standard external representation for every interned symbol. For example, the notations |abcDEF|, foo|BAR|baz, and abc\ def respectively represent the symbols whose names are "abcDEF", "fooBARbaz", and "abc def".

  This change introduces an incompatibility in the way that symbols are printed. Previously, (write symbol) was equivalent to (write-string (symbol->string symbol)). Now, (write symbol) always writes the symbol out with appropriate quoting so that it will read back in as the same symbol.

Changes to the runtime system

• A new command-line option --batch-mode disables output of banners, prompts, and values. This is intended for use with shell scripts, where the Scheme program writes to standard output and the author doesn't want the output cluttered by the interactivity cues. Note that the effect of this option applies only to the top-level REPL; if an error occurs, all the interactivity cues are re-enabled in the error REPL.

• The following SRFIs are now supported: 0, 6, 8, 9, 23, and 30.

• The following newly-implemented procedures are notable:

  exact-positive-integer?
  host-big-endian?
  make-top-level-environment
  x-graphics/open-display?
  x-graphics/open-window?
  

• The tcp-server-connection-accept procedure now accepts an optional argument line-translation, which sets the line translation to be used for newly-accepted sockets. (Thanks to Arthur Gleckler)

• Output ports now track the current column. This is simple minded but should work for ASCII, at least.

• The URI support procedures, formerly a part of IMAIL, are now in the runtime library.

Changes to Edwin

• HTML mode is now used for ".xml" files, and PHP mode for ".inc" files.

• VC mode has a new editor variable vc-cvs-stay-local that implements a small subset of the corresponding functionality in GNU Emacs.

• The debug-on-*-error editor variables can now be set to 'ask, which causes the user to be prompted for the debugger when the corresponding error occurs. The default settings of these variables have been changed to be more appropriate for typical users.

Changes to XML support

• Support for XML namespaces has been implemented. One consequence of this is that the representation of XML names has been changed. It is no longer the case that XML names can be compared with eq?; instead one must use the new xml-name=?. Additionally, xml-intern now takes an optional second argument, which is the URI of the namespace. XML names that don't have an associated namespace URI are now ordinary interned symbols, which greatly simplifies reference to such names.

• Comments are preserved by the parser.

• The parser now distinguishes between <foo></foo> and <foo/> in its output. The former has a contents list of (""), while the latter has a contents list of ().

• Optional indentation is supported for DTD and attributes during output.

• The parser now supports handlers for processing instructions, which are invoked during parsing. A handler maps the text of a processing instruction to a list of XML items, which are inserted into the resulting XML structure in place of the processing instruction.

• The following new procedures are available to make XML input and output more convenient:

  read-xml
  read-xml-file
  write-xml-file
  string->xml
  substring->xml
  xml->string
  xml->wide-string
  

• All the remaining bugs identified by the XML conformance tests have been fixed, except support for UTF-16.

Stable release 7.7.1

Release 7.7.1 fixes several bugs in IMAIL; fixes a bug that prevented the use of server sockets on Windows systems; and fixes a bug that caused the debugger to generate errors in common circumstances.

Stable release 7.7.0

This release provides hygienic macro support, as defined in R4RS and R5RS. This is a complete rewrite of the syntax engine, so any program that uses macros should be rewritten to use the new engine. A subset of the old macro-definition syntax is still supported, but this will eventually be removed. Note that the new syntax engine has no effect on the compiled-code format; most binaries compiled by release 7.6.x should continue to work.

User-visible consequences to this change

• These syntactic keywords have been eliminated:

  define-macro
  in-package
  macro
  make-environment
  scode-quote
  sequence
  unassigned?
  using-syntax
  

• The syntactic keyword the-environment has been restricted to use in top-level environments. It is no longer allowed in the body of any binding form (e.g. lambda, let).

• Syntactic keywords are now stored in environments, rather than in a separate syntax-table structure. The environment abstraction has been enhanced to support this, as well as to make it more general. The changes are documented in the reference manual.

• The syntax-table abstraction has been eliminated, and most procedures and arguments involving syntax tables have been removed. One exception is the load procedure, which still accepts a syntax-table argument, but ignores it.

Other notable changes in this release

• Although the 7.6.1 release had a workaround for problems with certain AMD Athlon processors, the workaround was ineffective on machines running Windows operating systems (and possibly OS/2 systems as well). This version fixes that problem.

  The hash-table abstraction is now always loaded. It's not necessary to call load-option prior to use of hash tables. For upwards compatibility, calling (load-option 'hash-table) is still permitted but does nothing.