Chez Scheme Version 8.0 Release Notes
Copyright © 2010 Cadence Research Systems
All Rights Reserved
March 2010

1. Overview
2. Functionality Changes
2.1. Experimental library-group form (8.0)
2.2. import-notify parameter (8.0)
2.3. Library inspection procedures (8.0)
2.4. compile-program return value (8.0)
2.5. apropos-list, apropos, and library exports (8.0)
2.6. Automatic library compilation (8.0/7.9.4)
2.7. FreeBSD x86_64 support (8.0)
2.8. OpenSolaris x86/x86_64 support (8.0)
2.9. New and improved pathname operations (8.0/7.9.4)
2.10. Unicode path names in Windows (8.0/7.9.4)
2.11. Library directories and extensions (7.9.4/7.9.3)
2.12. New top-level begin semantics (7.9.4)
2.13. New foreign-data manipulation routines (7.9.4)
2.14. Extended foreign-procedure types (7.9.4)
2.15. UTF-16LE, UTF-16BE codecs and changes to utf-16-codec (7.9.4)
2.16. Support for non-Unicode character sets (7.9.4)
2.17. New current-transcoder parameter (7.9.4)
2.18. New transcoder? predicate (7.9.4)
2.19. New port and file operations (7.9.4)
2.20. Console error port (7.9.4)
2.21. Debug changes (7.9.4)
2.22. More programmer control over collection (7.9.4)
2.23. New system querying procedures (7.9.4/7.5)
2.24. New date/time conversion procedures (7.9.4)
2.25. New sleep procedure (7.9.4)
2.26. Profile palette change (7.9.4)
2.27. New gensym printing option (7.9.4)
2.28. Compile-time Values and Properties (7.9.4)
2.29. New define-values syntax (7.9.4)
2.30. create-exception-state procedure (7.9.4)
2.31. New environment-mutable? predicate (7.9.4)
2.32. with-interrupts-disabled (7.9.4)
2.33. custom-port-buffer-size parameter (7.9.4)
2.34. make-boot-file (7.9.4)
2.35. Expression editor enabled for scripts (7.9.4)
2.36. Windows heap search path (7.9.4)
2.37. New (chezscheme) and (chezscheme csv7) libraries (7.9.4/7.9.3)
2.38. Procedures for manipulating top-level keyword bindings (7.9.4/7.9.3)
2.39. Compiling and loading programs and libraries (7.9.4/7.9.3/7.5)
2.40. #!fold-case and #!no-fold-case (7.9.3)
2.41. New --optimize-level command-line option (7.9.3)
2.42. file-buffer-size parameter (7.9.3)
2.43. current-exception-state parameter (7.9.3)
2.44. fork-thread and multiple values (7.9.3)
2.45. system return value (7.9.3)
2.46. abort optional argument (7.9.3)
2.47. New inspector commands (7.9.3)
2.48. inspect/object extensions (7.9.3)
2.49. New I/O procedures and support for nonblocking I/O (7.9.3)
2.50. Expression editor delimiter handling (7.9.3)
2.51. file-directory? and file-exists? change under Windows (7.9.3)
2.52. R6RS (7.9.2)
2.53. Interaction environment and R6RS (7.9.2)
2.54. Incompatible Changes (7.9.2)
2.55. C library routines for 32- and 64-bit integers (7.9.2)
2.56. New nonstandard character names (7.9.2)
2.57. Fewer "invalid context for definition" errors (7.9.2)
2.58. Windows library change (7.9.1/7.9.2)
2.59. Saved heaps not currently supported (7.9.1)
2.60. Thread-safe console ports (7.9.1)
2.61. input-port-ready? and char-ready? (7.9.1)
2.62. X86_64 Support (7.9.1)
2.63. Additional byte-vector operations (7.9.1)
2.64. meta-cond generalization (7.9.1)
2.65. Default heap/boot search path (7.9.1)
2.66. Tilde-prefixed paths under Windows (7.9.1)
2.67. Months range from 1 to 12 (7.5)
2.68. Monotonic time (7.5)
2.69. New and altered expression-editor key bindings (7.5)
2.70. Top-level value handling (7.5)
3. Bug Fixes
3.1. assert, let-values, etc., and pattern variables or ellipses (8.0)
3.2. Bug in collector handling of weak pairs (8.0)
3.3. Bogus clear-input and clear-output generic-port messages (8.0)
3.4. Bug current-time and current-date under Windows (8.0)
3.5. Boot file use of scheme-version (7.9.4)
3.6. Work-around for automatic margins on MacOS X Terminal (7.9.3)
3.7. Invalid memory reference using hashtables (7.9.3)
3.8. Bug in format tabulation (7.9.3)
3.9. Expression editor datum comment handling (7.5)
3.10. Expression editor clipboard bug under Windows (7.5)
3.11. Bug in ash (7.5)
3.12. Bug in inexact (7.5)
3.13. Bug in handling of compile (7.5)
4. Performance Enhancements
4.1. Reduced parameter overhead (7.9.4)
4.2. Reduced predicate overhead for sealed record types (7.9.4)
4.3. Optimized procedural record interface (7.9.1/7.9.4)
4.4. Reduced letrec* undefined-variable checking overhead (7.9.3)
4.5. Improved register assignment for x86 (7.9.1)
4.6. Improved recursive bindings (7.9.1)

1. Overview

This document outlines the changes made to Chez Scheme for Version 8.0 since Version 7.4, most of which are focused on converting Chez Scheme into an implementation of the new Scheme standard described in the Revised6 Report on Scheme.

Version 8.0 is available for the following platforms. The Chez Scheme machine type (returned by the machine-type procedure) is given in parentheses.

This document contains three sections describing significant (1) functionality changes, (2) bugs fixed, and (3) performance enhancements. A version number listed in parentheses in the header for a change indicates the first minor release or internal prerelease to support the change. Many other changes, too numerous to list, have been made to improve reliability, performance, and the quality of error messages and source information produced by the system.

More information on Chez Scheme and Petite Chez Scheme can be found at, and extensive documentation is available in The Scheme Programming Language, 4th edition (available directly from MIT Press or from online and local retailers) and the Chez Scheme Version 8 User's Guide. Online versions of both books can be found at

2. Functionality Changes

2.1. Experimental library-group form (8.0)

A library-group form combines multiple libraries and possibly a top-level program into a single compilation unit. This permits various optimizations, like procedure inlining, to occur across library boundaries, which can improve the speed of the generated code significantly depending on the amount of interaction between the procedures in the libraries. The library-group form also simplifies the packaging of applications structured as multiple libraries in that it eliminates the need to distribute libraries separately from the application.

A library-group form contains a sequence of libraries followed optionally by a top-level program:

(library-group library-form ...)
(library-group library-form ... import-form body-form ...)

An include form can be used to insert one or more of the library forms from a file, and an include form can be used to bring in the top-level program portion of the form as well.

This feature is experimental and might be replaced with some other mechanism or altered significantly in future releases.

2.2. import-notify parameter (8.0)

When the new parameter import-notify is set to a true value, import displays messages to the console-output port as it searches for the file containing each library it needs to load. The default value of this parameter is #f.

2.3. Library inspection procedures (8.0)

Several new procedures are available for inspecting libraries:

2.4. compile-program return value (8.0)

compile-program now returns a list of libraries directly invoked by the compiled top-level program. When combined with library-requirements and library-object-filename, this can be used to determine the set of files that need to be distributed with the compiled program file.

2.5. apropos-list, apropos, and library exports (8.0)

The apropos-list and apropos procedures produce additional information identifying matching identifiers from loaded libraries. The list returned by apropos-list can include both symbols representing matching identifiers in the interaction environment (as before) and (now) associations from library names to lists of symbols representing the identifiers that match in that library. The information printed by apropos contains the same information.

2.6. Automatic library compilation (8.0/7.9.4)

When the new parameter compile-imported-libraries is set to #t, the expander automatically calls compile-library on any imported library before it loads it from the file system, as described in the entry below. The default initial value of this parameter is #f. It is set to #t via the command-line option --compile-imported-libraries.

Starting in Version 8.0, when compile-imported-libraries is #t, a library implicitly loaded via an import form is recompiled even when an object file is present and not older than the source file, if a library the object file depends upon has been or must be recompiled.

2.7. FreeBSD x86_64 support (8.0)

Support for running Chez Scheme under FreeBSD on the x86_64 architecture has been added. The nonthreaded version uses the machine type "a6fb," and the threaded version uses the machine type "ta6fb."

2.8. OpenSolaris x86/x86_64 support (8.0)

Support for running Chez Scheme with 32-bit pointers on the i386 architecture or 64-bit pointers on the x86_64 architecture under OpenSolaris has been added, with machine types i3s2 (32-bit), a6s2 (64-bit), ti3s2 (32-bit threaded), and ta6s2 (64-bit threaded). C code intended to be linked with these versions of the system should be compiled using the Gnu C compiler's -m32 or -m64 options as appropriate.

2.9. New and improved pathname operations (8.0/7.9.4)

The new procedure path-absolute? returns #t if its pathname argument is absolute; otherwise, it returns #f. The new procedures path-first and path-rest return the first component of a pathname and the remaining components of the pathname respectively. For example, (path-first "a/b/c") returns "a" and (path-rest "a/b/c") returns "b/c". For pathnames that start with a root directory (including drives and shares under Windows), home directory (e.g., ~/abc or ~user/abc), or a reference to the current or parent directory (. or ..), the first component is that directory. The existing procedures path-last, path-parent path-root, and path-extension have been made more sensitive to the overall structure of a pathname to avoid giving nonsensical or ambiguous results.

2.10. Unicode path names in Windows (8.0/7.9.4)

Support for Unicode path names under Windows has been added. This affects file open operations such as open-file-input-port or load, as well as filesystem operations such as delete-file, file-access-time, file-exists?, mkdir, and directory-list. Support for the Windows \\?\ prefix has also been added where it was not previously working, e.g., in path-parent and related routines, in file-exists? and related routines, and in directory-list.

2.11. Library directories and extensions (7.9.4/7.9.3)

The parameter library-directories determines where the files containing library source and object code are located in the file system, and the parameter library-extensions determines the filename extensions for the files holding the code. The values of both parameters are lists of pairs of strings. The first string in each library-directories pair identifies a source-file root directory, and the second identifies the corresponding object-file root directory. Similarly, the first string in each library-extensions pair identifies a source-file extension, and the second identifies the corresponding object-file extension. The full path of a library source or object file consists of the source or object root followed by the components of the library name prefixed by slashes, with the library extension added on the end. For example, for root /usr/lib/scheme, library name (app lib1), and extension .sls, the full path is /usr/lib/scheme/app/lib1.sls.

The initial values of these parameters are shown below.

(library-directories) <graphic> (("." . "."))

(library-extensions) <graphic> ((".chezscheme.sls" . "")
                       (".ss" . ".so")
                       (".sls" . ".so")
                       (".scm" . ".so")
                       (".sch" . ".so"))

As a convenience, when either of these parameters is set, any element of the list can be specified as a single source string, in which case the object string is determined automatically. For library-directories, the object string is the same as the source string, effectively naming the same directory as a source- and object-code root. For library-extensions, the object string is the result of removing the last (or only) extension from the string and appending ".so". The library-directories and library-extensions parameters also accept as input strings in the format described below for the --libdirs and --libexts command-line options.

The --libdirs and --libexts options can be used to set library-directories and library-extensions parameters from the command line. The format of the arguments to these options is the same: a sequence of substrings separated by a single separator character. The separator character is a colon (:), except under Windows where it is a semi-colon (;). Between single separators, the source and object strings, if both are specified, are separated by two separator characters. If a single separator character appears at the end of the string, the specified pairs are added to the existing list; otherwise, the specified pairs replace the existing list. The parameters are set after all boot files have been loaded.

If no --libdirs option appears and the CHEZSCHEMELIBDIRS environment variable is set, the string value of CHEZSCHEMELIBDIRS is treated as if it were specified by a --libdirs option. Similarly, if no --libexts option appears and the CHEZSCHEMELIBEXTS environment variable is set, the string value of CHEZSCHEMELIBEXTS is treated as if it were specified by a --libexts option.

These parameters are consulted by the expander when it encounters an import for a library that has not previously been defined or loaded. By default, the expander searches for the library source or object file in each pair of root directories, in order, trying each of the source extensions then each of the object extensions in turn before moving onto the next pair of root directories. (If the library name forms an absolute pathname, the library source and object directories are ignored.) If the expander finds a source file before it finds an object file, it loads the corresponding object file if the object file exists and is not older than the source file. Otherwise, the expander loads the source file. An exception is raised during this process if a source or object file exists but is not readable or if an object file cannot be created.

When the parameter compile-imported-libraries is set to #t, the library-loading process is modified as follows. If a source file is encountered and the corresponding object file does not exist, or if a source file is encountered and the corresponding object file is older, the source file is compiled via compile-library to define the library and produce the corresponding object file. (Since a library contained within a library file is defined merely by compiling the file, the resulting object file is not loaded. This eliminates the cost of loading the object file and recomputing the transformer expressions, but it does mean that any initialization expressions in the object file outside of a library form are not evaluated. If such expressions are present, it's better to compile and load files explicitly to make sure they are evaluated when desired. On the other hand, such expressions can also cause portability problems, so it's even better to include only library forms in library files.) Otherwise, the object file is loaded. If the object file depends on some library that has been or must be recompiled, the source file is recompiled via compile-library to ensure that the library is propery affected by any changes in the libraries it depends upon.

Whenever the expander determines it must compile a library to a file or loads one from source, it adds the directory in which the file resides to the front of the source-directories list while compiling or loading the library. This allows a library to include files found in or relative to its own directory.

2.12. New top-level begin semantics (7.9.4)

Groups of definitions within a top-level begin are now treated in a manner similar to lambda, library, and top-level program bodies. This eliminates some ordering constraints on definitions appearing within a top-level begin expression. For example:

  (define-syntax a (identifier-syntax 3))
  (define x a))


  (define x a)
  (define-syntax a (identifier-syntax 3)))

are now equivalent. Similarly, the begin form produced by a use of:

(define-syntax define-constant
  (syntax-rules ()
    [(_ x e)
       (define t e)
       (define-syntax x (identifier-syntax t)))]))

and the begin form produced by a use of:

(define-syntax define-constant
  (syntax-rules ()
    [(_ x e)
       (define-syntax x (identifier-syntax t))
       (define t e))]))

are equivalent. More generally, this reduces the difference between groups of forms wrapped in a top-level begin form and groups of forms appearing with a lambda, library, or top-level program body, making it easier to write code or macros that expand into code that operates correctly in all contexts.

2.13. New foreign-data manipulation routines (7.9.4)

Four new procedures have been added for manipulating foreign data: foreign-alloc, which allocates a block of memory outside of the Scheme heap, foreign-free, which frees a block previously allocated by foreign-alloc, foreign-set!, which writes a new value to a foreign memory location, and foreign-ref, which reads a value from a foreign memory location. The type of data written or read from a foreign memory location is specified by a type symbol, one of the machine-dependent "C" types short, unsigned-short, int, unsigned, unsigned-int (an alias for unsigned), long, unsigned-long, long-long, unsigned-long-long, char, wchar, float, double, iptr (an integer the size of a pointer), uptr (an unsigned integer the size of a pointer), or void* (an alias for uptr); one of the fixed-sized types double-float, single-float, integer-8, unsigned-8, integer-16, unsigned-16, integer-32, unsigned-32, integer-64, or unsigned-64; or one of the additional types boolean (a zero or nonzero int on the C side, #f or #t on the Scheme side) or fixnum (an iptr limited to fixnum range).

The new procedure foreign-sizeof can be used to determine the size (in bytes) of a foreign type. It accepts a symbolic type argument and returns the size as an exact nonnegative integer.

2.14. Extended foreign-procedure types (7.9.4)

The set of foreign-procedure and foreign-callable argument and result types has been extended to include all of the types listed above for foreign-ref and foreign-set!. That is, to the set supported by previous versions, Version 8.0 adds short, unsigned-short, long-long, unsigned-long-long, char, wchar, float, double, void*, integer-8, unsigned-8, integer-16, and unsigned-16. It also makes integer-64 and unsigned-64 available even on 32-bit machines. Version 8.0 also adds ptr as an alias for scheme-object.

A set of new types for passing Scheme bytevectors and strings between Scheme and C have been added: u8*, u16*, u32*, utf-8, utf-16le, utf-16be, utf-32le, and utf-32be.

The types u8*, u16*, and u32* are used to pass or return the address of a bytevector's data from Scheme to C or to pass or return a null-terminated string of 8-, 16-, or 32-bit integers from C to Scheme. Null termination is required only when passing or returning data from C to Scheme and is used to determine the length of the returned sequence of integers. Going the other direction, if the receiving C code requires the data to be null-terminated, the null terminator should be included explicitly in the bytevector. Because Scheme bytevectors and strings can include zero elements, it is often better to pass the length as an explicit additional argument.

The types utf-8, utf-16le, utf-16be, utf-32le, and utf-32be are used to pass Unicode strings between C and Scheme. When going from Scheme to C, the Scheme string is converted to a Scheme bytevector, using the appropriate conversion with the appropriate null terminator, and a pointer to the bytevector's data is passed to C. When going from C to Scheme, the value should be a null-terminated sequence of 8-, 16-, or 32-bit integers and is converted via the appropriate conversion to a Scheme string.

The old string type is now an alias for utf-8. While previous releases did not allow string to be used as a foreign-callable return type, Version 8.0 allows string and all of the other bytevector and string types to be used as foreign-callable return types. Because the C code receives a pointer into a Scheme object, and Scheme objects can be moved or destroyed, with their storage reclaimed for other purposes by the Scheme garbage collector, the C code should not retain pointers it receives via any of the bytevector and string types after it returns to or calls into Scheme.

A new wstring type has also been added and is an alias for utf-16le, utf-16be, utf-32le, or utf-32be depending on the size of a C wchar_t and the endianness of the host machine.

The valid set of result types also still includes void.

In an incompatible change from previous prereleases, bytevectors are no longer permissible as string inputs, and bytevectors are no longer implicitly null terminated.

2.15. UTF-16LE, UTF-16BE codecs and changes to utf-16-codec (7.9.4)

Support for UTF-16LE and UTF-16BE codecs has been added. The procedure utf-16le-codec returns a codec that can be used to read or write files in the little-endian form of UTF-16, with no byte-order mark (BOM), and the procedure utf-16be-codec returns a codec that can be used to read or write files in the big-endian form of UTF-16, also with no BOM.

For the UTF-16 codec returned by utf-16-codec, Version 8.0 handles byte-order marks differently from earlier prereleases. First, Version 8.0 emits a BOM just before the first character written to a port transcoded with a transcoder based on the UTF-16 codec. For textual ports created via transcoded-port, the BOM appears at the beginning of the underlying data stream or file only if the binary port passed to transcoded-port is positioned at the start of the data stream or file. Second, when the system can determine that a transcoded port is positioned at the start of a data stream or file, it automatically positions the port at the start of the data beyond the BOM when an attempt is made to reposition the port at the beginning of the file, so that the BOM is preserved. Finally, for input-output ports, the BOM is not emitted if the file is read before written, and a BOM is not looked for if the file is written before read.

The utf-16-codec procedure can now be called with an optional argument that specifies the default endiannes. The argument defaults to the symbol big but can also be the symbol little. The codec returned by (utf-16-codec 'little) behaves just like its statndard counterpart, but if no BOM is present when the file is first read, the file is assumed to be encoded in little-endian format rather than big-endian format. This allows programs to read files claimed to be Unicode files but actually encoded in UTF-16LE with no BOM, while still handling those that might have BOMs. It also allows programs to create BOM-prefixed files that use little-endian rather than the default big-endian format.

2.16. Support for non-Unicode character sets (7.9.4)

Support for handling non-Unicode character sets has been added.

For non-Windows systems that provide built-in support for iconv and for Windows systems upon which an iconv dynamic-link library (DLL) can be loaded, the iconv-codec procedure takes a string naming a code page and returns a codec that can be used as the basis of a transcoder used to transcode files or convert between bytevectors and strings. The set of supported code pages depends on the version of iconv available; consult the iconv documentation or use the shell command iconv --list to obtain a list of supported code pages.

Two additional procedures are available under Windows. The procedure multibyte->string is a wrapper for the Windows API MultiByteToWideChar function. It takes a code-page identifier and a bytevector containing a sequence of multibyte characters and converts them to a Scheme string containing the corresponding sequence of whole characters. Similarly, string->multibyte is a wrapper for the Windows API WideCharToMultiByte function. It takes a code-page identifier and a string and returns a bytevector containing a sequence of the corresponding multibyte characters. A code-page identifier is an exact nonnegative integer or one of the symbols cp-acp, cp-maccp, cp-oemcp, cp-symbol, cp-thread-acp, cp-utf7, or cp-utf8, which have the same meanings as the like-named API constants.

2.17. New current-transcoder parameter (7.9.4)

The new parameter current-transcoder, which defaults to the value of (native-transcoder), determines the transcoder used when textual files are opened by operations for which an explicit transcoder is not specified, e.g., open-input-file, open-output-file, load, compile-file, and include.

2.18. New transcoder? predicate (7.9.4)

The new procedure transcoder?, which should be standard but is not, returns #t if its argument is a transcoder and #f otherwise.

2.19. New port and file operations (7.9.4)

The new procedure file-port? returns true if its port argument is a file port, which includes any port based directly on an O/S file descriptor (e.g., one created by open-file-input-port, open-output-port, open-fd-input-port, etc., but not a string port). The new procedure port-file-descriptor returns the file descriptor assocated with a file port.

The new procedure get-mode retrieves the permissions on the file named by its string argument and returns them as a exact integer in the same form as the mode argument to chmod. If the optional follows? argument is supplied and is #f, If follow? is specified and #f, symbolic links are not followed; othewise, they are followed.

Three procedures have been added to allow the times recorded for a file to be retrieved:

Each of these takes a string pathname or file port. Each also takes an optional follow? argument, as for get-mode. The follow? argument is ignored when the first argument is a port.

2.20. Console error port (7.9.4)

The new parameter console-error-port determines the port to which conditions and other messages are displayed by default-exception-handler, which is initial value of the base-exception-handler parameter that determines how uncaught exceptions are handled. When the system determines that the standard output and standard error streams are directed to the same place (tty/console, file, pipe, etc.) the initial value of this parameter is the same as the initial value of console-output-port. Otherwise, the initial value of this parameter is a different port tied to the standard error stream of the Scheme process. The initial value of current-error-port is the same as the initial value of console-error-port.

2.21. Debug changes (7.9.4)

The continuation of a failed assert or a call to assertion-violation, assertion-violationf, error, errorf, or syntax-error is now included within the conditions they pass to raise. Conditions encapsulating raise continuations (conditions of type &continuation) can also be created explicitly by the procedure make-continuation-condition. The predicate continuation-condition? returns #t if its argument is a continuation condition, and the procedure condition-continuation returns the continuation encapsulated within a continuation condition.

When the default exception handler is invoked with any serious or non-warning condition, including one that encapsulates a continuation, it displays the condition and also saves it for possible later use by the debugger in the debug-condition parameter. The debug procedure in turn allows the condition and/or the continuation encapsulated within it to be inspected.

In previous prereleases, the continuation made available to the debugger was the continuation of the call to the default exception handler, but this is often the wrong continuation. For example, if a guard has caught and reraised the exception, perhaps after performing some clean-up action like closing a file, the continuation is that of the raise call in the guard, not of the original raise.

Ordinarily, the user must type debug explicitly to enter the debugger and inspect the last debug condition or its continuation. If, however, the parameter debug-on-exception is set to #t, the default exception handler directly enters the debugger, from which the continuation (control stack) of the exception can be inspected. The --debug-on-exception option may be used to set debug-on-exception to #t from the command line, which is particularly useful when debugging scripts or top-level programs run via the --script or --program options.

2.22. More programmer control over collection (7.9.4)

The storage manager now gives the programmer more control over the garbage collector. First, the collect procedure allows the programmer to specify the target generation for each collection, and even cause objects to be collected into a static generation previously used only when compacting the heap. Second, collect-maximum-generation is now a parameter that can be used to alter the number of generations used by the collector. Third, bytes-allocated now takes an optional generation argument that can be used to monitor generation sizes.

When called without arguments, collect behaves as it did before. That is, it collects generations g and younger every rg times it is called, where r is the value of the parameter collect-generation-radix. The target generation into which generations g and younger are collected is g + 1, except when g is the maximum nonstatic generation, in which case the target generation is g itself. (Younger generations have lower numbers, with generation 0 being the youngest generation into which objects are placed when they are created.)

When called with one argument, which must be a generation g in the range 0 through m, where m is the maximum nonstatic generation, the oldest generation collected is g. The target generation is g for the maximum nonstatic generation and g + 1 otherwise. In previous releases, the oldest generation collected might be greater than g, if the older generation would ordinarily have been collected the next time g is collected.

When called with two arguments, the first, g, determines the oldest generation to be collected, as if only one argument were provided. The second, gtarget, determines the target generation. When g is the maximum nonstatic generation, gtarget must be g or the symbol static. Otherwise, gtarget must be g or g + 1. When the target generation is the symbol static, all data in the nonstatic generations are moved to the static generation, from which objects are never collected. This is primarily useful after an application's permanent code and data structures have been loaded and initialized, to reduce the overhead of subsequent collections.

The procedure collect-maximum-generation is now a parameter. When called without arguments, it returns the current maximum nonstatic generation. This is a change from previous releases, in which (collect-maximum-generation) returns a number one greater than the maximum nonstatic generation, i.e., the number assigned to the static generation.

A more significant change is that collect-maximum-generation can now also be used to change the number of generations. When called with one argument, g, which must be an exact integer in the range 1 through 254, collect-maximum-generation changes the maximum nonstatic generation to g. When set to 1, only two nonstatic generations are used; when set to 2, three nonstatic generations are used, and so on. When set to 254, 255 nonstatic generations are used, plus the single static generation for a total of 256 generations. Increasing the number of generations effectively decreases how often old objects are collected, potentially decreasing collection overhead but potentially increasing the number of inaccessible objects retained in the system and thus the total amount of memory required.

Finally, the bytes-allocated procedure now takes an optional generation argument, which must be an exact nonnegative integer no grater than the value of (collect-maximum-generation) or the symbol static. When a generation argument is supplied, bytes-allocated reports the number of bytes allocated in that generation, rather than the total number of bytes allocated in all generations. This information might be used to help guide an adaptive collection strategy based on the sizes of the generations.

2.23. New system querying procedures (7.9.4/7.5)

The procedure scheme-version-number returns three exact nonnegative integer values: the major version number, the minor version number, and the sub-minor version number. For example, in Chez Scheme Version 7.9.4, (scheme-version-number) returns the three values 7, 9, and 4.

The procedure scheme-version returns a string that identifies the system (Petite Chez Scheme or Chez Scheme) and version number.

The procedure petite? returns #t when called in Petite Chez Scheme and #f when called in Chez Scheme

The procedure threaded? returns #t when called in a threaded version of Chez Scheme and #f otherwise.

The procedure get-process-id returns the process id of the process. It is the same for all threads running in the same process in threaded versions of Chez Scheme.

The procedure get-thread-id returns the Chez Scheme thread id for the current thread. It is different for each thread and is not related to the process id or any other O/S process or thread id.

2.24. New date/time conversion procedures (7.9.4)

The new procedures date->time-utc and time-utc->date convert between time objects with type time-utc and date objects.

2.25. New sleep procedure (7.9.4)

The sleep procedure takes a time object with type time-duration, which can be created using make-time, and causes the invoking thread to suspend operation for at least the amount of time indicated by the time object, unless the process receives a signal that interrupts the sleep operation. The actual time slept depends on the granularity of the system clock and how busy the system is running other threads and processes.

2.26. Profile palette change (7.9.4)

Profile palettes must now contain three or more (rather than two or more) entries. The first entry is used for unprofiled code, the second for unexecuted profiled code, and the remainder for executed profiled code. By default, a black background is used for unprofiled code, and a gray background is used for unexecuted profiled code, while background colors ranging from purple to red are used for executed profiled code, depending on frequency of execution, with red for the most frequently executed code.

2.27. New gensym printing option (7.9.4)

When the parameter print-gensym is set to pretty/suffix, the printer prints the gensym's "pretty" name along with a suffix based on the gensym's "unique" name, separated by a dot ( "." ). If the gensym's unique name is generated automatically during the current session, the suffix is that portion of the unique name that is not common to all gensyms created during the current session. Otherwise, the suffix is the entire unique name.

(define g1 (gensym))
(define g2 '#{g0 qlw4di6s4woz0sy-0})
g1 <graphic> #{g0 ql8sto9wbe3de7u-0}
g2 <graphic> #{g0 qlw4di6s4woz0sy-0}
(print-gensym 'pretty/suffix)
g1 <graphic> g0.0
g2 <graphic> g0.qlw4di6s4woz0sy-0

This new option is particularly useful as alternative to the pretty option for programs that generate many gensyms with the same pretty name, by calling gensym with a string argument, e.g., (gensym "x"), as illustrated below.

(print-gensym 'pretty/suffix)
(map gensym (make-list 5 "x")) <graphic> (x.0 x.1 x.2 x.3 x.4)

2.28. Compile-time Values and Properties (7.9.4)

Two mechanisms are now available for attaching information to identifiers in the compile-time environment: compile-time values and compile-time properties. These mechanisms are useful for communicating information between macros. For example, a record-definition macro might use one of these mechanisms to store information about the record type in the compile-time environment for possible use by record definitions that define subtypes of the record type.

A compile-time value is a kind of transformer, created with the procedure compile-time-value, that can be associated with an identifier via define-syntax, let-syntax, letrec-syntax, and fluid-let-syntax. When an identifier is associated with a compile-time value, it cannot also have any other meaning, and an attempt to reference it as an ordinary identifier results in a syntax error.

A compile-time property, on the other hand, is maintained alongside an existing binding, providing additional information about the binding. Properties are ignored when ordinary references to an identifier occur. Compile-time properties are defined with define-property, which is a definition and can appear wherever any definition can appear.

The mechanisms used by a macro to obtain compile-time values and properties are similar. In both cases, the macro's transformer returns a procedure p rather than a syntax object. The expander invokes p with one argument, an environment-lookup procedure lookup, which p can then use to obtain compile-time values and properties for one or more identifiers before it constructs the macro's final output. lookup accepts one or two identifier arguments. With one argument, id, lookup returns the compile-time value of id, or #f if id has no compile-time value. With two arguments, id and key, lookup returns the value of id's key property, or #f if id has no key property.

2.29. New define-values syntax (7.9.4)

The new define-values form can be used to define multiple variables to the multiple values produced by an expression. (define-values formals expr) is similar to (define var expr) except that the left-hand side is structured as a lambda formals list. It evaluates expr and binds the variables specified by formals to the resulting values.

2.30. create-exception-state procedure (7.9.4)

The new procedure create-exception-state creates an exception state whose stack of exception handlers is empty except for, in effect, an infinite number of occurrences of a given handler at its base. If no handler is supplied, a handler that invokes the value of the base-exception-handler parameter is used.

2.31. New environment-mutable? predicate (7.9.4)

The new procedure environment-mutable? accepts one argument, an environment. It returns #t if the environment is mutable, otherwise #f.

2.32. with-interrupts-disabled (7.9.4)

The new syntactic form with-interrupts-disabled is identical in syntax and behavior to the existing critical-section form. While the critical-section is still supported, it may be removed in a future release, because it does not actually provide a critical section when multiple native threads are used in threaded versions of the Chez Scheme. Rather, as the new name suggests, the form disables interrupts, which are per-thread, but when used in one thread has no affect on what other threads can do.

2.33. custom-port-buffer-size parameter (7.9.4)

A new parameter, custom-port-buffer-size, has been added. This parameter controls the sizes of the buffers associated with custom ports. The initial value is currently 128.

2.34. make-boot-file (7.9.4)

The new procedure make-boot-file is used to combine a set of source and object files into a boot file. The procedure takes an output filename, a list of strings naming base boot files, and zero or more input filenames. It writes to the output file a boot header naming the base boot files (as with make-boot-header) followed by the object code for each input file. If an input file is not already compiled, make-boot-file compiles the file as it proceeds.

If the list of strings naming base boot files is empty, the first named input file should be a base boot file, i.e., petite.boot or some boot file derived from petite.boot. If no input files are specified, make-boot-file creates a boot header; thus, make-boot-file subsumes the functionality of make-boot-header.

2.35. Expression editor enabled for scripts (7.9.4)

For scripts that call new-cafe, the expression editor is now used unless it has been disabled via the --eedisable command-line option. The history file is not read until the first call to the prompt-and-read procedure installed by the expression editor.

2.36. Windows heap search path (7.9.4)

Within a heap-search path under Windows, the two-character sequence %x now expands into the executable's directory, and the default heap-search path now consists only of %x. The system thus looks for boot files only, by default, in the same directory as the executable. The registry key HeapSearchPath in HKLM\SOFTWARE\Chez Scheme\csvversion, where version is the Chez Scheme version number, e.g., 7.9.4, can be set to override the default search path, and the environment variable SCHEMEHEAPDIRS can be set to override both the default and the registry setting, if any.

2.37. New (chezscheme) and (chezscheme csv7) libraries (7.9.4/7.9.3)

The new (chezscheme) library is identical to the existing (scheme) library, and the new (chezscheme csv7) library is identical to the existing (scheme csv7) library. The new names can be used to make clear that Chez Scheme extensions are involved.

2.38. Procedures for manipulating top-level keyword bindings (7.9.4/7.9.3)

The new procedure define-top-level-syntax, which accepts a symbol representing a keyword, a transformer, and an optional environment defaulting to the current interaction environment, can be used to establish a top-level keyword binding in an interactive environment, just as define-top-level-value can be used to establish a top-level variable binding. The new procedure top-level-syntax, which accepts a symbol and an optional environment, can be used to retrieve the transformer associated with a keyword in an environment. The new procedure top-level-syntax?, which accepts a symbol and an optional environment, can be used to ask whether an identifier represented by a symbol is bound as a keyword in an environment.

2.39. Compiling and loading programs and libraries (7.9.4/7.9.3/7.5)

New procedures for compiling and loading programs and libraries have been added. compile-program is similar to compile-script but differs in that it implements the semantics of RNRS top-level programs, while compile-script implements the semantics of the interactive top-level. The resulting compiled program will also run faster than if compiled via compile-file or compile-script. load-program is similar to load but also differs in that it implements the semantics of RNRS top-level programs.

compile-library and load-library are identical to compile-file and load except they treat the input file as if prefixed by an implicit #!r6rs, disabling any Chez Scheme lexical extensions, such as explicit vector lengths. More precisely, Chez Scheme lexical extensions can be used within a library compiled or loaded with these procedures only if the #!chezscheme marker appears before the first use of those extensions.

2.40. #!fold-case and #!no-fold-case (7.9.3)

The reader now recognizes two new comment directives, analogous to the existing #!r6rs and #!chezscheme directives. If #!fold-case has been seen (more recently than #!no-fold-case) in an input stream, subsequent reads from port are case-insensitive, i.e., symbols and character names are case-folded, as if by string-foldcase. If #!no-fold-case has been seen (more recently than #!fold-case) in an input stream, subsequent reads from the port are case-sensitive. If neither has been seen in an input stream, case-sensitivity is determined as before by the case-sensitive parameter, whose value defaults to #t. The case sensitive parameter is not consulted if either #!fold-case or #!no-fold-case has been seen. #!fold-case, #!no-fold-case, and case-sensitive are all ignored when vertical bars or slashes (excluding Unicode hex escapes) are seen. Like #!r6rs and #!chezscheme, #!fold-case and #!no-fold-case do not require delimiting, so #!fold-caseaBc reads as the symbol abc.

2.41. New --optimize-level command-line option (7.9.3)

The new --optimize-level option sets the initial value of the optimize-level parameter to 0, 1, 2, or 3. The value is 0 by default.

2.42. file-buffer-size parameter (7.9.3)

A new parameter, file-buffer-size, has been added. This parameter controls the size of a buffer used when a file is opened with anything by buffer mode none. The default value of this parameter is currently set at the minimum of 4096 and the value of the underlying operating system's stdio BUFSIZ constant.

2.43. current-exception-state parameter (7.9.3)

The new current-exception-state may be used to save and later restore the state of the exception system.

2.44. fork-thread and multiple values (7.9.3)

fork-thread now permits its thunk argument to return zero values or more than one value.

2.45. system return value (7.9.3)

The system procedure, which runs a command in a newly forked process and waits for it to terminate, now raises an exception if the subprocess creation fails, if the exit status of the forked process cannot be determined, or if the forked process is terminated by a signal. Otherwise, it returns the exit status of the child process.

2.46. abort optional argument (7.9.3)

The abort procedure now takes an optional argument, like exit, that may be used to specify the exit code for the Scheme process.

2.47. New inspector commands (7.9.3)

The interactive inspector now supports reset (r) and abort (a) commands, which may be used to reset to the current REPL or abort from the system.

For characters and strings, the interactive inspector also supports a new unicode command that displays the Unicode scalar values of the character or string elements.

2.48. inspect/object extensions (7.9.3)

The continuation and code inspector-object source-path message now returns a position as well as a file name if the file name is known but the (unmodified) file cannot be found. Previously, it returned a single value in this case, the file name.

2.49. New I/O procedures and support for nonblocking I/O (7.9.3)

New input procedures get-string-some, get-string-some!, and get-bytevector-some! have been added. get-string-some is like the R6RS get-bytevector-some but operates on textual input ports and returns a string rather than a bytevector. get-string-some! and get-bytevector-some! are like the R6RS get-string-n! and get-bytevector-n! except they may return less than the requested count.

The new procedures put-string-some and put-bytevector-some are like put-string and put-bytevector except they do not guarantee to write the entire string or bytevector, and they return a count of the actual number of elements written.

The procedure set-port-nonblocking! may be used to put a port into nonblocking mode. When a port is in nonblocking mode, get-string-some may return an empty string, get-bytevector-some may return an empty bytevector, and get-string-some! and get-bytevector-some! may return 0, in each case indicating that no input is available, i.e., an attempt to read would block. Similarly, put-string-some and put-bytevector-some may return 0, indicating that an attempt write to the port would block.

The procedure port-nonblocking? may be used to determine if a port is in nonblocking mode. The predicates port-has-port-nonblocking?? and port-has-set-port-nonblocking!? return #t if the nonblocking status of a port may be queried or set and #f otherwise. They should be called before attempting to use the set-port-nonblocking! or port-nonblocking? procedures.

2.50. Expression editor delimiter handling (7.9.3)

The expression editor no longer changes the corresponding close delimiter when an open delimiter is inserted into existing code, since this often led to surprising results.

2.51. file-directory? and file-exists? change under Windows (7.9.3)

The file-directory? and file-exists? predicates now return #t under Windows for existing drive names like c:, mounted volumes like //server/mount, and directories specified with (or without) a trailing slash (and regardless of whether forward or backward slashes are used).

2.52. R6RS (7.9.2)

(Initial support was included in Version 7.9.1.)

Version 8.0 implements the entire language of the Revised6 Report on Scheme (R6RS) and associated libraries, including all of the syntactic requirements and each export of the standard libraries, namely:

(rnrs arithmetic bitwise)
(rnrs arithmetic fixnums)
(rnrs arithmetic flonums)
(rnrs base)
(rnrs bytevectors)
(rnrs conditions)
(rnrs control)
(rnrs enums)
(rnrs eval)
(rnrs exceptions)
(rnrs files)
(rnrs hashtables)
(rnrs io ports)
(rnrs io simple)
(rnrs lists)
(rnrs mutable-pairs)
(rnrs mutable-strings)
(rnrs programs)
(rnrs r5rs)
(rnrs records procedural)
(rnrs records syntactic)
(rnrs records inspection)
(rnrs sorting)
(rnrs syntax-case)
(rnrs unicode)

Top-level programs are supported via the --program command-line option, i.e., the shell command:

scheme --program pathname

runs the top-level program contained in the file named by pathname. To create an executable R6RS top-level program on Unix-based system, insert:

#! /usr/bin/scheme --program

at the front of the top-level program (adjusting the path for scheme or replacing scheme with petite as appropriate) and give the file execute permissions. To make use of existing executable top-level programs containing the "shebang" line

#! /usr/bin/env scheme-script

recommended by R6RS nonnormative appendix D.1, create a copy or symbolic link of the Scheme executable to scheme-script and copies or symbolic links of the "scheme" or "petite" boot file to scheme-script.boot. Place the former somewhere in the path searched by /usr/bin/env and the latter in a standard directory for Chez Scheme boot files, e.g., the same place where scheme.boot and/or petite.boot already reside. (This may already be done as part of the installation process.)

2.53. Interaction environment and R6RS (7.9.2)

The default interaction environment used for any code that occurs outside of an R6RS top-level program or library (including such code typed at the REPL or loaded from a file) contains all of the bindings of the (scheme) library (or scheme module, which exports the same set of bindings). This set contains a number of bindings that are not in the R6RS libraries. It also contains a number of bindings that extend the R6RS counterparts in some way and are thus not strictly compatible with the R6RS bindings for the same identifiers. To replace these with bindings strictly compatible with R6RS, simply import the rnrs libraries into the interaction environment by typing the following into the REPL or loading it from a file:

  (rnrs eval)
  (rnrs mutable-pairs)
  (rnrs mutable-strings)
  (rnrs r5rs))

To obtain an interaction environment that contains all and only R6RS bindings, use the following.

      '(rnrs eval) 
      '(rnrs mutable-pairs) 
      '(rnrs mutable-strings) 
      '(rnrs r5rs))

The read-eval-print loop (REPL) and files loaded using load or included using include also support various Chez Scheme lexical extensions, by default. To disable these extensions, #!r6rs can be inserted at the front of a file to be loaded or included or before the start of an expression typed into the REPL. The #!r6rs mode is implicit for libraries loaded as a result of an import form and for R6RS top-level programs. To enable Chez Scheme extensions in libraries and R6RS top-level programs, the prefix #!chezscheme can be used.

To be useful for most purposes, library and import should probably also be included, from the (scheme) library.

      '(rnrs eval) 
      '(rnrs mutable-pairs) 
      '(rnrs mutable-strings) 
      '(rnrs r5rs)
      '(only (scheme) library import))

It might also be useful to include debug in the set of identifiers imported from (scheme) to allow the debugger to be entered after an exception is raised.

Most of the identifiers bound in the default interaction environment that are not strictly compatible with the R6RS are variables bound to procedures with extended interfaces, i.e., optional arguments or extended argument domains. The others are keywords bound to transformers that extend the R6RS syntax in some way. This should not be a problem except for R6RS programs that count on exceptions being raised in cases that coincide with the extensions. For example, if a program passes the = procedure a single numeric argument and expects an exception to be raised, it will fail in the initial interaction environment because = returns #t when passed a single numeric argument.

The procedures that are not strictly compatible include the following, which return #t for one argument (while the R6RS versions require two or more):

<, <=, =, >, >=, char<=?, char<?, char=?, char>=?, char>?, string<=?, string<?, string=?, string>=?, string>?, char-ci<=?, char-ci<?, char-ci=?, char-ci>=?, char-ci>?, string-ci<=?, string-ci<?, string-ci=?, string-ci>=?, string-ci>?;

the following, which are extended to accept zero or more arguments:

fx*, fx+, exit;

the following, which is extended to accept one or more arguments:


the following, which allow radixes 3, 5-7, 9-15, and 17-36:

string->number, number->string;

the following, which accepts either one or two arguments and allows the first argument to represent a definition:


the following, for which the argument is optional and defaults to the current output port:


the following, which are parameters and thus may be used to alter the encapsulated value:

command-line, current-error-port, current-input-port, current-output-port,
standard-error-port, standard-input-port, standard-output-port;

the following, which accept an "options" symbol or list:

call-with-input-file, call-with-output-file, open-input-file, open-output-file,
with-input-from-file, with-output-to-file;

and the following, which accept optional arguments:

delete-file, dynamic-wind, file-exists?, record?.

The only keyword that is not strictly compatible is syntax-rules, which allows fenders.

For details on the (scheme) library versions of these procedures and syntactic forms, see the Chez Scheme Version 7 User's Guide.

2.54. Incompatible Changes (7.9.2)

Although we have strived to maintain backward compatibility with earlier versions of Scheme wherever possible, the following incompatible changes have been made to support R6RS.

2.55. C library routines for 32- and 64-bit integers (7.9.2)

Four new C library operators have been added for converting 32- and 64-bit integers from their Scheme representations:

An exception is raised if a 32-bit value is not in the range -231 through 232 - 1 or a 64-bit values is not in the range -263 through 264 - 1.

Similarly, four new C library operators have been added for converting 32- and 64-bit integers to their Scheme representations:

Each returns a Scheme exact integer whose value is x. The arguments are signed or unsigned 32- or 64-bit C integers, as appropriate.

2.56. New nonstandard character names (7.9.2)

The characters #\nel and #\ls, representing the Unicode next-line and line-separator characters, are recognized by the reader (except after #!r6rs).

2.57. Fewer "invalid context for definition" errors (7.9.2)

The expander now expands library body forms left-to-right even after discovering an apparent expression so that a spelling error in a definition keyword (e.g., defnie for define) results in an "unbound identifier" error rather than an "invalid context for definition" error for some subsequent definition.

2.58. Windows library change (7.9.1/7.9.2)

Windows builds of Chez Scheme now link against msvcr90.dll. Static libraries built using libcmt are also available.

2.59. Saved heaps not currently supported (7.9.1)

None of the operating systems upon which Chez Scheme runs provide the guarantees about storage allocation needed to restore a saved heap with absolute addresses, and while restoration used to work on some operating systems even without the guarantee, this is generally no longer the case as the operating systems are randomizing the addresses of data and even code for security purposes. We are looking into ways to address this problem, including relocatable saved heaps, but for the present, support for saved heaps has been removed from the system.

2.60. Thread-safe console ports (7.9.1)

Unbuffered file output ports are no longer thread-safe. The default console output port remains thread-safe, however, and the default console input port is now thread-safe as well. There is a substantial performance cost in both cases, so programs that need faster I/O should create their own ports and either avoid using them from multiple threads or arrange for appropriate synchronization. (This is relevant only for the threaded versions of Chez Scheme.)

2.61. input-port-ready? and char-ready? (7.9.1)

A new input-port-ready? procedure has been that is similar to the old char-ready? but works on both binary and textual ports. The char-ready? procedure still works for textual ports.

Under Windows, these procedures properly handle files and ports, while in previous releases, char-ready? always returned #t. In cases where the system cannot determine if input is ready, e.g., for a console port that has no data already buffered, each of these procedures raises an exception so that the application can control what happens in such cases. Thus, these procedures either (a) return #f if no input is ready and the port is not at end of file, (b) return #t if input is ready or the port is at end of file, or (c) raise an exception if the ready status cannot be determined.

2.62. X86_64 Support (7.9.1)

Support for running Chez Scheme with 64-bit pointers on the x86_64 architecture under Linux, MacOS X, and OpenBSD with machine types a6le, a6osx, and a6ob for nonthreaded and ta6le, ta6osx, and ta6ob for threaded, has been added. C code intended to be linked with these versions of the system should be compiled using the Gnu C compiler's -m64 option, which may or may not be the default.

2.63. Additional byte-vector operations (7.9.1)

In addition to the standard R6RS bytevector operations, Chez Scheme also supports the following procedures:

(bytevector->s8-list bytevector<graphic> list
(s8-list->bytevector s8-list<graphic> bytevector
(bytevector fill ...) <graphic> bytevector

where each element of s8-list is an exact integer value ranging from -128 through +128 and each fill is an exact integer value ranging from -128 through +255.

As with vectors and bytevectors, the reader allows an optional length to appear in the bytevector prefix, e.g.:

(bytevector? #10vu8(0))

is a bytevector containing 10 zero bytes. (This cannot be used following #!r6rs, which limits the syntax recognized by the reader to R6RS-only features.)

The read-token procedure returns token types vu8paren and vu8nparen for bytevector prefixes, analogous to the vparen and vnparen token types for vector prefixes.

The C library interface supports four new bytevector operators:

2.64. meta-cond generalization (7.9.1)

The meta-cond syntax has been extended to allow it to be used in definition contexts. meta-cond still expands into (void) if no clause's test evaluates to true and no else clause is present, however, which is not suitable in a definition context. To avoid this problem, use an explicit else clause and expand into (begin).

2.65. Default heap/boot search path (7.9.1)

The default heap and boot search path on Unix-based systems now starts with ~/lib/csv%v/%m if the home directory can be determined. The default heap and boot search path is still determined by a registry setting under Windows.

2.66. Tilde-prefixed paths under Windows (7.9.1)

The filename prefix ~/ now expands to the user's home directory under Windows, as on Unix-based systems, if the HOMEDRIVE and HOMEPATH environment variables are set.

2.67. Months range from 1 to 12 (7.5)

The month field of a date record, e.g., one returned by current-date, runs from 1 through 12 rather than 0 through 11, for compatibility with SRFI 19.

2.68. Monotonic time (7.5)

Requesting monotonic no longer fails on Linux systems that do not support monotonic clocks; on such systems, the real-time clock is now used instead. All of the high-precision time procedures now fall back on older time mechanisms when requesting high-precision timers fails.

2.69. New and altered expression-editor key bindings (7.5)

The expression editor now binds the common delete-key sequence (Esc[3~) to ee-delete-char and escape followed by the same sequence to ee-delete-sexp. The effect of this is that the delete key should now delete forward rather than backward. The backspace key should still delete backward.

New bindings have also been added for the home and end keys of certain terminal emulators, including the Gnome terminal.

2.70. Top-level value handling (7.5)

The top-level-value and top-level-bound? procedures now recognize as bound a variable assigned by a file compiled in one session and loaded into another session---even if the variable was not defined.

3. Bug Fixes

3.1. assert, let-values, etc., and pattern variables or ellipses (8.0)

A bug that caused various problems when pattern variables or ellipses appeared within an assert form or within a right-hand-side of a let-values, let*-values, or define-values form has been fixed. [This bug dated back to Version 7.5.]

3.2. Bug in collector handling of weak pairs (8.0)

A collector bug that could result in a nonrecoverable invalid memory reference, triggered when large numbers of weak pairs are used by an application, has been fixed. [This bug dated back to Version 7.5.]

3.3. Bogus clear-input and clear-output generic-port messages (8.0)

The implementations of clear-input-port and clear-output-port now properly send clear-input-port and clear-output-port messages to generic ports, rather than clear-input and clear-output. [This bug dated back to Version 7.5.]

3.4. Bug current-time and current-date under Windows (8.0)

The procedures current-time and current-date were improperly reporting microseconds as nanoseconds under Windows. This bug has been fixed. [This bug dated back to Version 7.4.]

3.5. Boot file use of scheme-version (7.9.4)

The procedure scheme-version now works properly when called during the loading of a boot file. [This bug dated back to Version 7.5.]

3.6. Work-around for automatic margins on MacOS X Terminal (7.9.3)

The expression editor disables automatic margins (i.e., automatic line wrapping) when possible so it can use the last column without scrolling the display. Unfortunately, when automatic margins are disabled in the MacOS X Terminal application, they are disabled simultaneously for all windows, not just for the current window, causing problems when the expression editor is running in one window and long lines are printed in another. The expression editor now disables automatic margins only temporarily while writing text to the last column, reducing (though not entirely eliminating) the possibility that other windows are affected. [This bug dated back to Version 7.4.]

3.7. Invalid memory reference using hashtables (7.9.3)

A bug that caused corruption of the heap and typically resulted in an "unrecoverable invalid memory reference" while using the R6RS eq hashtable interface has been fixed. [This bug dated back to Version 7.5.]

3.8. Bug in format tabulation (7.9.3)

A bug resulting in an "undefined for zero" error when the optional column width specifier in a tabulate directive is zero, e.g., ~1,0@t, has been fixed. [This bug dated back to Version 7.5.]

3.9. Expression editor datum comment handling (7.5)

The expression editor no longer returns end-of-file for an entry containing only a single commented-out datum. [This bug dated back to Version 7.4.]

3.10. Expression editor clipboard bug under Windows (7.5)

The expression editor now properly closes the clipboard after a paste (control-v) operation, i.e., no longer prevents new items from being copied to the clipboard. [This bug dated back to Version 7.4.]

3.11. Bug in ash (7.5)

A bug that caused ash to return the wrong result for word-sized or greater negative (right) shifts of large powers of two has been fixed. [This bug dated back to Version 7.0.]

3.12. Bug in inexact (7.5)

A bug that could cause inexact (aka exact->inexact) to improperly round an exact value halfway between two floating-point values has been fixed. [This bug dated back to Version 7.0.]

3.13. Bug in handling of compile (7.5)

A bug in the compiler that caused it to treat the return value of compile as true in test context, no matter what it actually returns, has been fixed. [This bug dated back to Version 6.0.]

4. Performance Enhancements

4.1. Reduced parameter overhead (7.9.4)

References to locally defined parameters, including thread parameters, are now inlined, thus reducing the cost of such references.

4.2. Reduced predicate overhead for sealed record types (7.9.4)

The code generated by record-predicate for sealed record types is now more efficient than the more general code produced for non-sealed record types.

4.3. Optimized procedural record interface (7.9.1/7.9.4)

Record constructors, predicates, accessors, and mutators created with the procedural record interface are often faster than before because the compiler now tracks record-type information through to the sites where constructors, predicates, accessors, and mutators are created in order to generate specific and inlinable code for them. The effects of this optimization can be seen with expand/optimize. (This optimization is not performed by Petite Chez Scheme.)

Initial support for this optimization was implemented in Version 7.9.1, with additional improvements in Version 7.9.4.

4.4. Reduced letrec* undefined-variable checking overhead (7.9.3)

An improvement in the handling of letrec*, which also affects internal definitions, including library and top-level program definitions, causes the compiler to produce fewer checks for undefined variables. Improvements in safe code can be substantial (up to 15% or so), but the amount of improvement is highly dependent on the structure of the code. Unsafe code (code compiled at optimize-level 3) is not affected, since these checks are not performed in unsafe code.

4.5. Improved register assignment for x86 (7.9.1)

A better choice of register assignments has resulted in measurable improvement in x86 performance. Actual improvements differ from one program to another, and we've seen anything from -9% decreases in speed to 36% increases, with increases of 5-15% appearing typical.

4.6. Improved recursive bindings (7.9.1)

The compiler now generates better code for some letrec expressions, letrec* expressions, and bodies containing internal definitions by taking advantage of the R6RS restriction that right-hand-side expressions should not return to their continuations multiple times.