MIT/GNU Scheme release notes

Testing release 7.7.91 (pending)

These are rough notes on the notable changes that have taken place since 7.7.90. Some of these changes will be available in the snapshots.

• The empty list and #f are now distinct objects.
• 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".
• Support for server-side programming using Apache and mod_lisp.
• Basic support for mapping of pathnames to MIME types.
• New syntax for expression comments: #;(+ 3 4).
• Support for access to the registry on Windows systems.
• Fix to allow no-execute permissions with Windows XP SP2.
• Improved support for XML:
  • rename xml-element-content to xml-element-contents
  • added flexibility for writing character data
  • support for UTF-8, UTF-16, UTF-32, and all ISO 8859 character sets
  • new support for XHTML 1.0 Strict and XHTML 1.1, including convenience procedures for building documents.
  • Support for 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.
  • Rewrite of XML naming support, to rationalize code and bring terminology into line with W3C.
• Complete rewrite of Unicode support:
  • support for UTF-8, UTF-16, UTF-32
  • new character #\bom
  • parser buffers now deal with wide characters
• Complete rewrite of the port abstraction (2004-02-16 00:38): support for character coding and a wider variety of line endings.
  • new operation to unread a character
  • port encapsulation eliminated
  • discard-char now returns information about its success
  • open-tcp-stream-socket now takes only two arguments
• Reimplementation of hash tables eliminates some operations.
• New procedures make-strong-eq-hash-table and make-strong-eqv-hash-table.
• New procedure symbol provides easy way to build new symbols.
• New quoting syntax for symbols simplifies writing arbitrary symbols.
• Basic support for RDF.
• New procedures smallest-fixnum and largest-fixnum.
• Support added for cygwin. This hasn't been tested by us.
• New procedure channel-file-truncate.
• 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.
• File specified by environment variable MITSCHEME_LOAD_OPTIONS is now considered optional rather than required.
• Change record-type-default-inits to return a list, not a vector.
• Change --eval and --load command-line options 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.
• Change read to accept an optional second argument, which is 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.
• Implement count-matching-items and count-non-matching-items.
• Fixed several problems related to the use of modifier keys under Windows and X11.
• Eliminated input-buffer/read-until-delimiter, input-buffer/discard-until-delimiter, and input-buffer/discard-char.
• New procedures reverse* and reverse*!.
• Random-number generator changed to provide reasonable output for large moduli. Previous implementation limited the amount of randomness in that case.
• Implement SRFI 27, except for random-source-pseudo-randomize!. 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.
• Implement SRFI 69, except that the procedure hash couldn't be implemented as it's a name conflict with a pre-existing procedure.
• Implement partial support for ISO 8601 date/time strings.
• New procedure decoded-time?.

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.