SCM

This manual is for SCM (version 5e5, February 2008), an implementation of the algorithmic language Scheme.

Copyright © 1990-2007 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License.”

1. Overview

SCM is a portable Scheme implementation written in C. SCM provides a machine independent platform for [JACAL], a symbolic algebra system. SCM supports and requires the SLIB Scheme library. SCM, SLIB, and JACAL are GNU projects.

1.1 Features

• Conforms to Revised^5 Report on the Algorithmic Language Scheme [R5RS] and the [IEEE] P1178 specification.
• Support for [SICP], [R2RS], [R3RS], and [R5RS] scheme code.
• Runs under Amiga, Atari-ST, MacOS, MS-DOS, OS/2, NOS/VE, Unicos, VMS, Unix and similar systems. Supports ASCII and EBCDIC character sets.
• Is fully documented in TeXinfo form, allowing documentation to be generated in info, TeX, html, nroff, and troff formats.
• Supports inexact real and complex numbers, 30 bit immediate integers and large precision integers.
• Many Common Lisp functions: logand, logor, logxor, lognot, ash, logcount, integer-length, bit-extract, defmacro, macroexpand, macroexpand1, gentemp, defvar, force-output, software-type, get-decoded-time, get-internal-run-time, get-internal-real-time, delete-file, rename-file, copy-tree, acons, and eval.
• Char-code-limit, most-positive-fixnum, most-negative-fixnum, and internal-time-units-per-second constants. slib:features and *load-pathname* variables.
• Arrays and bit-vectors. String ports and software emulation ports. I/O extensions providing ANSI C and POSIX.1 facilities.
• Interfaces to standard libraries including REGEX string regular expression matching and the CURSES screen management package.
• Available add-on packages including an interactive debugger, database, X-window graphics, BGI graphics, Motif, and Open-Windows packages.
• The Hobbit compiler and dynamic linking of compiled modules.
• User definable responses to interrupts and errors, Process-syncronization primitives. Setable levels of monitoring and timing information printed interactively (the verbose function). Restart, quit, and exec.

1.2 Authors

Aubrey Jaffer (agj @ alum.mit.edu)

Most of SCM.

Radey Shouman

Arrays, gsubrs, compiled closures, records, Ecache, syntax-rules macros, and safeports.

Jerry D. Hedden

Real and Complex functions. Fast mixed type arithmetics.

Hugh Secker-Walker

Syntax checking and memoization of special forms by evaluator. Storage allocation strategy and parameters.

George Carrette

Siod, written by George Carrette, was the starting point for SCM. The major innovations taken from Siod are the evaluator’s use of the C-stack and being able to garbage collect off the C-stack (see section Garbage Collection).

There are many other contributors to SCM. They are acknowledged in the file ‘ChangeLog’, a log of changes that have been made to scm.

1.3 Copyright

Authors have assigned their SCM copyrights to:

Free Software Foundation, Inc.

59 Temple Place, Suite 330, Boston, MA 02111, USA

1.3.1 The SCM License

This program is free software: you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public License along with this program. If not, see http://www.gnu.org/licenses/.

1.3.2 SIOD copyright

COPYRIGHT © 1989 BY

PARADIGM ASSOCIATES INCORPORATED, CAMBRIDGE, MASSACHUSETTS.

ALL RIGHTS RESERVED

Permission to use, copy, modify, distribute and sell this software and its documentation for any purpose and without fee is hereby granted, provided that the above copyright notice appear in all copies and that both that copyright notice and this permission notice appear in supporting documentation, and that the name of Paradigm Associates Inc not be used in advertising or publicity pertaining to distribution of the software without specific, written prior permission.

PARADIGM DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE, INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS, IN NO EVENT SHALL PARADIGM BE LIABLE FOR ANY SPECIAL, INDIRECT OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

gjc@paradigm.com

Phone: 617-492-6079

Paradigm Associates Inc 29 Putnam Ave, Suite 6 Cambridge, MA 02138

1.3.3 GNU Free Documentation License

Version 1.2, November 2002

Copyright © 2000,2001,2002 Free Software Foundation, Inc.
51 Franklin St, Fifth Floor, Boston, MA  02110-1301, USA

Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.

1. PREAMBLE

   The purpose of this License is to make a manual, textbook, or other functional and useful document free in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.

   This License is a kind of “copyleft”, which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.

   We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.

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   A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an “aggregate” if the copyright resulting from the compilation is not used to limit the legal rights of the compilation’s users beyond what the individual works permit. When the Document is included in an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document.

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   Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail.

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ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:

  Copyright (C)  year  your name.
  Permission is granted to copy, distribute and/or modify this document
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If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with…Texts.” line with this:

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If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.

1.4 Bibliography

[IEEE]

IEEE Standard 1178-1990. IEEE Standard for the Scheme Programming Language. IEEE, New York, 1991.

[R4RS]

William Clinger and Jonathan Rees, Editors. <A HREF="r4rs_toc.html"> Revised(4) Report on the Algorithmic Language Scheme. </A> ACM Lisp Pointers Volume IV, Number 3 (July-September 1991), pp. 1-55.

[R5RS]

Richard Kelsey and William Clinger and Jonathan (Rees, editors) <A HREF="r5rs_toc.html"> Revised(5) Report on the Algorithmic Language Scheme. </A> Higher-Order and Symbolic Computation Volume 11, Number 1 (1998), pp. 7-105, and ACM SIGPLAN Notices 33(9), September 1998.

[Exrename]

William Clinger <A HREF="http://www.cs.indiana.edu/scheme-repository/doc.proposals.html"> Hygienic Macros Through Explicit Renaming </A> Lisp Pointers Volume IV, Number 4 (December 1991), pp 17-23.

[SICP]

Harold Abelson and Gerald Jay Sussman with Julie Sussman. Structure and Interpretation of Computer Programs. MIT Press, Cambridge, 1985.

[Simply]

Brian Harvey and Matthew Wright. <A HREF="http://HTTP.CS.Berkeley.EDU/~bh/simply-toc.html"> Simply Scheme: Introducing Computer Science </A> MIT Press, 1994 ISBN 0-262-08226-8

[SchemePrimer]

犬飼大(Dai Inukai) <A HREF="http://www.shuwasystem.co.jp/SchemePrimer/"> 入門Scheme </A> 1999年12月初版 ISBN4-87966-954-7

[SLIB]

Todd R. Eigenschink, Dave Love, and Aubrey Jaffer. <A HREF="slib_toc.html"> SLIB, The Portable Scheme Library. </A> Version 2c8, June 2000.

[JACAL]

Aubrey Jaffer. <A HREF="jacal_toc.html"> JACAL Symbolic Mathematics System. </A> Version 1b0, Sep 1999.

‘scm.texi’

‘scm.info’

Documentation of scm extensions (beyond Scheme standards). Documentation on the internal representation and how to extend or include scm in other programs.

‘Xlibscm.texi’

‘Xlibscm.info’

Documentation of the Xlib - SCM Language X Interface.

2. Installing SCM

2.1 Making SCM

The SCM distribution has Makefile which contains rules for making scmlit, a “bare-bones” version of SCM sufficient for running ‘build’. ‘build’ is used to compile (or create scripts to compile) full featured versions (see section Building SCM).

Makefiles are not portable to the majority of platforms. If ‘Makefile’ works for you, good; If not, I don’t want to hear about it. If you need to compile SCM without build, there are several ways to proceed:

• Use the build web page to create custom batch scripts for compiling SCM.
• Use SCM on a different platform to run ‘build’ to create a script to build SCM;
• Use another implementation of Scheme to run ‘build’ to create a script to build SCM;
• Create your own script or ‘Makefile’.

2.2 SLIB

[SLIB] is a portable Scheme library meant to provide compatibility and utility functions for all standard Scheme implementations. Although SLIB is not neccessary to run SCM, I strongly suggest you obtain and install it. Bug reports about running SCM without SLIB have very low priority. SLIB is available from the same sites as SCM:

• <A HREF="http://swiss.csail.mit.edu/ftpdir/scm/slib-3b1.zip"> http://swiss.csail.mit.edu/ftpdir/scm/slib-3b1.zip </A>
• <A HREF="ftp://ftp.gnu.org/pub/gnu/jacal/slib-3b1.tar.gz"> ftp.gnu.org:/pub/gnu/jacal/slib-3b1.tar.gz </A>

Unpack SLIB (‘tar xzf slib-3b1.tar.gz’ or ‘unzip -ao slib-3b1.zip’) in an appropriate directory for your system; both tar and unzip will create the directory ‘slib’.

Then create a file ‘require.scm’ in the SCM implementation-vicinity (this is the same directory as where the file ‘Init5e5.scm’ is installed). ‘require.scm’ should have the contents:

(define (library-vicinity) "/usr/local/lib/slib/")

where the pathname string ‘/usr/local/lib/slib/’ is to be replaced by the pathname into which you installed SLIB. Absolute pathnames are recommended here; if you use a relative pathname, SLIB can get confused when the working directory is changed (see section chmod). The way to specify a relative pathname is to append it to the implementation-vicinity, which is absolute:

(define library-vicinity
  (let ((lv (string-append (implementation-vicinity) "../slib/")))
    (lambda () lv)))

Alternatively, you can set the (shell) environment variable SCHEME_LIBRARY_PATH to the pathname of the SLIB directory (see section Environment Variables). If set, the environment variable overrides ‘require.scm’. Again, absolute pathnames are recommended.

2.3 Building SCM

The file build loads the file build.scm, which constructs a relational database of how to compile and link SCM executables. ‘build.scm’ has information for the platforms which SCM has been ported to (of which I have been notified). Some of this information is old, incorrect, or incomplete. Send corrections and additions to jaffer @ ai.mit.edu.

2.3.1 Invoking Build

The all method will also work for MS-DOS and unix. Use the all method if you encounter problems with ‘build’.

MS-DOS

From the SCM source directory, type ‘build’ followed by up to 9 command line arguments.

unix

From the SCM source directory, type ‘./build’ followed by command line arguments.

all

From the SCM source directory, start ‘scm’ or ‘scmlit’ and type (load "build"). Alternatively, start ‘scm’ or ‘scmlit’ with the command line argument ‘-ilbuild’.

Invoking build without the ‘-F’ option will build or create a shell script with the arrays, inexact, and bignums options as defaults.

bash$ ./build
-|
#! /bin/sh
# unix (linux) script created by SLIB/batch 
# ================ Write file with C defines
rm -f scmflags.h
echo '#define IMPLINIT "Init5e5.scm"'>>scmflags.h
echo '#define BIGNUMS'>>scmflags.h
echo '#define FLOATS'>>scmflags.h
echo '#define ARRAYS'>>scmflags.h
# ================ Compile C source files
gcc -O2 -c continue.c scm.c scmmain.c findexec.c script.c time.c repl.c scl.c eval.c sys.c subr.c debug.c unif.c rope.c
# ================ Link C object files
gcc -rdynamic -o scm continue.o scm.o scmmain.o findexec.o script.o time.o repl.o scl.o eval.o sys.o subr.o debug.o unif.o rope.o -lm -lc

To cross compile for another platform, invoke build with the ‘-p’ or ‘--platform=’ option. This will create a script for the platform named in the ‘-p’ or ‘--platform=’ option.

bash$ ./build -o scmlit -p darwin -F lit
-|
#! /bin/sh
# unix (darwin) script created by SLIB/batch 
# ================ Write file with C defines
rm -f scmflags.h
echo '#define IMPLINIT "Init5e5.scm"'>>scmflags.h
# ================ Compile C source files
cc -O3 -c continue.c scm.c scmmain.c findexec.c script.c time.c repl.c scl.c eval.c sys.c subr.c debug.c unif.c rope.c
# ================ Link C object files
mv -f scmlit scmlit~
cc -o scmlit continue.o scm.o scmmain.o findexec.o script.o time.o repl.o scl.o eval.o sys.o subr.o debug.o unif.o rope.o

2.3.2 Build Options

The options to build specify what, where, and how to build a SCM program or dynamically linked module. These options are unrelated to the SCM command line options.

Build Option: -p platform-name

Build Option: ---platform=platform-name

specifies that the compilation should be for a computer/operating-system combination called platform-name. Note The case of platform-name is distinguised. The current platform-names are all lower-case.

The platforms defined by table platform in ‘build.scm’ are:

Table: platform
name              processor         operating-system  compiler          
#f                processor-family  operating-system  #f                
symbol            processor-family  operating-system  symbol            
symbol            symbol            symbol            symbol            
================= ================= ================= ================= 
*unknown*         *unknown*         unix              cc                
acorn-unixlib     acorn             *unknown*         cc                
aix               powerpc           aix               cc                
alpha-elf         alpha             unix              cc                
alpha-linux       alpha             linux             gcc               
amiga-aztec       m68000            amiga             cc                
amiga-dice-c      m68000            amiga             dcc               
amiga-gcc         m68000            amiga             gcc               
amiga-sas         m68000            amiga             lc                
atari-st-gcc      m68000            atari-st          gcc               
atari-st-turbo-c  m68000            atari-st          tcc               
borland-c         i8086             ms-dos            bcc               
darwin            powerpc           unix              cc                
djgpp             i386              ms-dos            gcc               
freebsd           *unknown*         unix              cc                
gcc               *unknown*         unix              gcc               
gnu-win32         i386              unix              gcc               
highc             i386              ms-dos            hc386             
hp-ux             hp-risc           hp-ux             cc                
irix              mips              irix              gcc               
linux             *unknown*         linux             gcc               
linux-aout        i386              linux             gcc               
linux-ia64        ia64              linux             gcc               
microsoft-c       i8086             ms-dos            cl                
microsoft-c-nt    i386              ms-dos            cl                
microsoft-quick-c i8086             ms-dos            qcl               
ms-dos            i8086             ms-dos            cc                
netbsd            *unknown*         unix              gcc               
openbsd           *unknown*         unix              gcc               
os/2-cset         i386              os/2              icc               
os/2-emx          i386              os/2              gcc               
osf1              alpha             unix              cc                
plan9-8           i386              plan9             8c                
sunos             sparc             sunos             cc                
svr4              *unknown*         unix              cc                
svr4-gcc-sun-ld   sparc             sunos             gcc               
turbo-c           i8086             ms-dos            tcc               
unicos            cray              unicos            cc                
unix              *unknown*         unix              cc                
vms               vax               vms               cc                
vms-gcc           vax               vms               gcc               
watcom-9.0        i386              ms-dos            wcc386p           

Build Option: -f pathname

specifies that the build options contained in pathname be spliced into the argument list at this point. The use of option files can separate functional features from platform-specific ones.

The ‘Makefile’ calls out builds with the options in ‘.opt’ files:

‘dlls.opt’

Options for Makefile targets mydlls, myturtle, and x.so.

‘gdb.opt’

Options for udgdbscm and gdbscm.

‘libscm.opt’

Options for libscm.a.

‘pg.opt’

Options for pgscm, which instruments C functions.

‘udscm4.opt’

Options for targets udscm4 and dscm4 (scm).

‘udscm5.opt’

Options for targets udscm5 and dscm5 (scm).

The Makefile creates options files it depends on only if they do not already exist.

Build Option: -o filename

Build Option: ---outname=filename

specifies that the compilation should produce an executable or object name of filename. The default is ‘scm’. Executable suffixes will be added if neccessary, e.g. ‘scm’ ⇒ ‘scm.exe’.

Build Option: -l libname …

Build Option: ---libraries=libname

specifies that the libname should be linked with the executable produced. If compile flags or include directories (‘-I’) are needed, they are automatically supplied for compilations. The ‘c’ library is always included. SCM features specify any libraries they need; so you shouldn’t need this option often.

Build Option: -D definition …

Build Option: ---defines=definition

specifies that the definition should be made in any C source compilations. If compile flags or include directories (‘-I’) are needed, they are automatically supplied for compilations. SCM features specify any flags they need; so you shouldn’t need this option often.

Build Option: ---compiler-options=flag

specifies that that flag will be put on compiler command-lines.

Build Option: ---linker-options=flag

specifies that that flag will be put on linker command-lines.

Build Option: -s pathname

Build Option: ---scheme-initial=pathname

specifies that pathname should be the default location of the SCM initialization file ‘Init5e5.scm’. SCM tries several likely locations before resorting to pathname (see section File-System Habitat). If not specified, the current directory (where build is building) is used.

Build Option: -c pathname …

Build Option: ---c-source-files=pathname

specifies that the C source files pathname … are to be compiled.

Build Option: -j pathname …

Build Option: ---object-files=pathname

specifies that the object files pathname … are to be linked.

Build Option: -i call …

Build Option: ---initialization=call

specifies that the C functions call … are to be invoked during initialization.

Build Option: -t build-what

Build Option: ---type=build-what

specifies in general terms what sort of thing to build. The choices are:

‘exe’

executable program.

‘lib’

library module.

‘dlls’

archived dynamically linked library object files.

‘dll’

dynamically linked library object file.

The default is to build an executable.

Build Option: -h batch-syntax

Build Option: --batch-dialect=batch-syntax

specifies how to build. The default is to create a batch file for the host system. The SLIB file ‘batch.scm’ knows how to create batch files for:

• unix
• dos
• vms
• amigaos (was amigados)
• system

  This option executes the compilation and linking commands through the use of the system procedure.

• *unknown*

  This option outputs Scheme code.

Build Option: -w batch-filename

Build Option: --script-name=batch-filename

specifies where to write the build script. The default is to display it on (current-output-port).

Build Option: -F feature …

Build Option: ---features=feature

specifies to build the given features into the executable. The defined features are:

array

Alias for ARRAYS

array-for-each

array-map! and array-for-each (arrays must also be featured).

arrays

Use if you want arrays, uniform-arrays and uniform-vectors.

bignums

Large precision integers.

byte

Treating strings as byte-vectors.

byte-number

Byte/number conversions

careful-interrupt-masking

Define this for extra checking of interrupt masking and some simple checks for proper use of malloc and free. This is for debugging C code in ‘sys.c’, ‘eval.c’, ‘repl.c’ and makes the interpreter several times slower than usual.

cautious

Normally, the number of arguments arguments to interpreted closures (from LAMBDA) are checked if the function part of a form is not a symbol or only the first time the form is executed if the function part is a symbol. defining ‘reckless’ disables any checking. If you want to have SCM always check the number of arguments to interpreted closures define feature ‘cautious’.

cheap-continuations

If you only need straight stack continuations, executables compile with this feature will run faster and use less storage than not having it. Machines with unusual stacks need this. Also, if you incorporate new C code into scm which uses VMS system services or library routines (which need to unwind the stack in an ordrly manner) you may need to use this feature.

compiled-closure

Use if you want to use compiled closures.

curses

For the curses screen management package.

debug

Turns on the features ‘cautious’ and ‘careful-interrupt-masking’; uses -g flags for debugging SCM source code.

differ

Sequence comparison

dont-memoize-locals

SCM normally converts references to local variables to ILOCs, which make programs run faster. If SCM is badly broken, try using this option to disable the MEMOIZE_LOCALS feature.

dump

Convert a running scheme program into an executable file.

dynamic-linking

Be able to load compiled files while running.

edit-line

interface to the editline or GNU readline library.

engineering-notation

Use if you want floats to display in engineering notation (exponents always multiples of 3) instead of scientific notation.

generalized-c-arguments

make_gsubr for arbitrary (< 11) arguments to C functions.

i/o-extensions

Commonly available I/O extensions: exec, line I/O, file positioning, file delete and rename, and directory functions.

inexact

Use if you want floating point numbers.

lit

Lightweight – no features

macro

C level support for hygienic and referentially transparent macros (syntax-rules macros).

mysql

Client connections to the mysql databases.

no-heap-shrink

Use if you want segments of unused heap to not be freed up after garbage collection. This may increase time in GC for *very* large working sets.

none

No features

posix

Posix functions available on all Unix-like systems. fork and process functions, user and group IDs, file permissions, and link.

reckless

If your scheme code runs without any errors you can disable almost all error checking by compiling all files with ‘reckless’.

record

The Record package provides a facility for user to define their own record data types. See SLIB for documentation.

regex

String regular expression matching.

rev2-procedures

These procedures were specified in the Revised^2 Report on Scheme but not in R4RS.

sicp

Use if you want to run code from:

Harold Abelson and Gerald Jay Sussman with Julie Sussman. Structure and Interpretation of Computer Programs. The MIT Press, Cambridge, Massachusetts, USA, 1985.

Differences from R5RS are:

• (eq? ’() ’#f)
• (define a 25) returns the symbol a.
• (set! a 36) returns 36.

single-precision-only

Use if you want all inexact real numbers to be single precision. This only has an effect if SINGLES is also defined (which is the default). This does not affect complex numbers.

socket

BSD socket interface. Socket addr functions require inexacts or bignums for 32-bit precision.

tick-interrupts

Use if you want the ticks and ticks-interrupt functions.

turtlegr

Turtle graphics calls for both Borland-C and X11 from sjm@ee.tut.fi.

unix

Those unix features which have not made it into the Posix specs: nice, acct, lstat, readlink, symlink, mknod and sync.

wb

WB database with relational wrapper.

windows

Microsoft Windows executable.

x

Alias for Xlib feature.

xlib

Interface to Xlib graphics routines.

2.3.3 Compiling and Linking Custom Files

A correspondent asks:

How can we link in our own c files to the SCM interpreter so that we can add our own functionality? (e.g. we have a bunch of tcp functions we want access to). Would this involve changing build.scm or the Makefile or both?

(see section Changing Scm has instructions describing the C code format). Suppose a C file foo.c has functions you wish to add to SCM. To compile and link your file at compile time, use the ‘-c’ and ‘-i’ options to build:

bash$ ./build -c foo.c -i init_foo
-|
#! /bin/sh
rm -f scmflags.h
echo '#define IMPLINIT "/home/jaffer/scm/Init5e5.scm"'>>scmflags.h
echo '#define COMPILED_INITS init_foo();'>>scmflags.h
echo '#define BIGNUMS'>>scmflags.h
echo '#define FLOATS'>>scmflags.h
echo '#define ARRAYS'>>scmflags.h
gcc -O2 -c continue.c scm.c findexec.c script.c time.c repl.c scl.c \
        eval.c sys.c subr.c unif.c rope.c foo.c
gcc -rdynamic -o scm continue.o scm.o findexec.o script.o time.o \
        repl.o scl.o eval.o sys.o subr.o unif.o rope.o foo.o -lm -lc

To make a dynamically loadable object file use the -t dll option:

bash$ ./build -t dll -c foo.c
-|
#! /bin/sh
rm -f scmflags.h
echo '#define IMPLINIT "/home/jaffer/scm/Init5e5.scm"'>>scmflags.h
echo '#define BIGNUMS'>>scmflags.h
echo '#define FLOATS'>>scmflags.h
echo '#define ARRAYS'>>scmflags.h
echo '#define DLL'>>scmflags.h
gcc -O2 -fpic -c foo.c
gcc -shared -o foo.so foo.o -lm -lc

Once ‘foo.c’ compiles correctly (and your SCM build supports dynamic-loading), you can load the compiled file with the Scheme command (load "./foo.so"). See Configure Module Catalog for how to add a compiled dll file to SLIB’s catalog.

2.4 Installing Dynamic Linking

Dynamic linking has not been ported to all platforms. Operating systems in the BSD family (a.out binary format) can usually be ported to DLD. The dl library (#define SUN_DL for SCM) was a proposed POSIX standard and may be available on other machines with COFF binary format. For notes about porting to MS-Windows and finishing the port to VMS VMS Dynamic Linking.

DLD is a library package of C functions that performs dynamic link editing on GNU/Linux, VAX (Ultrix), Sun 3 (SunOS 3.4 and 4.0), SPARCstation (SunOS 4.0), Sequent Symmetry (Dynix), and Atari ST. It is available from:

<A HREF="ftp://ftp.gnu.org/pub/gnu/dld-3.3.tar.gz"> ftp.gnu.org:pub/gnu/dld-3.3.tar.gz </A>

These notes about using libdl on SunOS are from ‘gcc.info’:

On a Sun, linking using GNU CC fails to find a shared library and reports that the library doesn’t exist at all.

This happens if you are using the GNU linker, because it does only static linking and looks only for unshared libraries. If you have a shared library with no unshared counterpart, the GNU linker won’t find anything.

We hope to make a linker which supports Sun shared libraries, but please don’t ask when it will be finished–we don’t know.

Sun forgot to include a static version of ‘libdl.a’ with some versions of SunOS (mainly 4.1). This results in undefined symbols when linking static binaries (that is, if you use ‘-static’). If you see undefined symbols ‘_dlclose’, ‘_dlsym’ or ‘_dlopen’ when linking, compile and link against the file ‘mit/util/misc/dlsym.c’ from the MIT version of X windows.

2.5 Configure Module Catalog

The SLIB module catalog can be extended to define other require-able packages by adding calls to the Scheme source file ‘mkimpcat.scm’. Within ‘mkimpcat.scm’, the following procedures are defined.

Function: add-link feature object-file lib1 …

feature should be a symbol. object-file should be a string naming a file containing compiled object-code. Each libn argument should be either a string naming a library file or #f.

If object-file exists, the add-link procedure registers symbol feature so that the first time require is called with the symbol feature as its argument, object-file and the lib1 … are dynamically linked into the executing SCM session.

If object-file exists, add-link returns #t, otherwise it returns #f.

For example, to install a compiled dll ‘foo’, add these lines to ‘mkimpcat.scm’:

(add-link 'foo (in-vicinity (implementation-vicinity) "foo" link:able-suffix))

Function: add-alias alias feature

alias and feature are symbols. The procedure add-alias registers alias as an alias for feature. An unspecified value is returned.

add-alias causes (require 'alias) to behave like (require 'feature).

Function: add-source feature filename

feature is a symbol. filename is a string naming a file containing Scheme source code. The procedure add-source registers feature so that the first time require is called with the symbol feature as its argument, the file filename will be loaded. An unspecified value is returned.

Remember to delete the file ‘slibcat’ after modifying the file ‘mkimpcat.scm’ in order to force SLIB to rebuild its cache.

2.6 Saving Images

In SCM, the ability to save running program images is called dump (see section Dump). In order to make dump available to SCM, build with feature ‘dump’. dumped executables are compatible with dynamic linking.

Most of the code for dump is taken from ‘emacs-19.34/src/unex*.c’. No modifications to the emacs source code were required to use ‘unexelf.c’. Dump has not been ported to all platforms. If ‘unexec.c’ or ‘unexelf.c’ don’t work for you, try using the appropriate ‘unex*.c’ file from emacs.

The ‘dscm4’ and ‘dscm5’ targets in the SCM ‘Makefile’ save images from ‘udscm4’ and ‘udscm5’ executables respectively.

Recent GNU/Linux innovations interfere with dump. For:

Fedora-Core-1

Remove the ‘#’ from the line ‘#SETARCH = setarch i386’ in the ‘Makefile’.

Fedora-Core-3

http://jamesthornton.com/writing/emacs-compile.html writes: [For FC3] combreloc has become the default for recent GNU ld, which breaks the unexec/undump on all versions of both Emacs and XEmacs...

Override by adding the following to ‘udscm5.opt’: ‘--linker-options="-z nocombreloc"’

Kernels later than 2.6.11

http://www.opensubscriber.com/message/emacs-devel@gnu.org/1007118.html mentions the exec-shield feature. Kernels later than 2.6.11 must do (as root):

echo 0 > /proc/sys/kernel/randomize_va_space

before dumping. ‘Makefile’ has this ‘randomize_va_space’ stuffing scripted for targets ‘dscm4’ and ‘dscm5’. You must either set ‘randomize_va_space’ to 0 or run as root to dump.

2.7 Automatic C Preprocessor Definitions

These ‘#defines’ are automatically provided by preprocessors of various C compilers. SCM uses the presence or absence of these definitions to configure include file locations and aliases for library functions. If the definition(s) corresponding to your system type is missing as your system is configured, add -Dflag to the compilation command lines or add a #define flag line to ‘scmfig.h’ or the beginning of ‘scmfig.h’.

#define         Platforms:
-------         ----------
ARM_ULIB        Huw Rogers free unix library for acorn archimedes
AZTEC_C         Aztec_C 5.2a
__CYGWIN__      Cygwin
__CYGWIN32__    Cygwin
_DCC            Dice C on AMIGA
__GNUC__        Gnu CC (and DJGPP)
__EMX__         Gnu C port (gcc/emx 0.8e) to OS/2 2.0
__HIGHC__       MetaWare High C
__IBMC__        C-Set++ on OS/2 2.1
_MSC_VER        MS VisualC++ 4.2
MWC             Mark Williams C on COHERENT
__MWERKS__      Metrowerks Compiler; Macintosh and WIN32 (?)
_POSIX_SOURCE   ??
_QC             Microsoft QuickC
__STDC__        ANSI C compliant
__TURBOC__      Turbo C and Borland C
__USE_POSIX     ??
__WATCOMC__     Watcom C on MS-DOS
__ZTC__         Zortech C

_AIX            AIX operating system
__APPLE__       Apple Darwin
AMIGA           SAS/C 5.10 or Dice C on AMIGA
__amigaos__     Gnu CC on AMIGA
atarist         ATARI-ST under Gnu CC
__DragonflyBSD__ DragonflyBSD
__FreeBSD__     FreeBSD
GNUDOS          DJGPP (obsolete in version 1.08)
__GO32__        DJGPP (future?)
hpux            HP-UX
linux           GNU/Linux
macintosh       Macintosh (THINK_C and __MWERKS__ define)
MCH_AMIGA       Aztec_c 5.2a on AMIGA
__MACH__        Apple Darwin
__MINGW32__     MinGW - Minimalist GNU for Windows
MSDOS           Microsoft C 5.10 and 6.00A
_MSDOS          Microsoft CLARM and CLTHUMB compilers.
__MSDOS__       Turbo C, Borland C, and DJGPP
__NetBSD__      NetBSD
nosve           Control Data NOS/VE
__OpenBSD__     OpenBSD
SVR2            System V Revision 2.
sun             SunOS
__SVR4          SunOS
THINK_C         developement environment for the Macintosh
ultrix          VAX with ULTRIX operating system.
unix            most Unix and similar systems and DJGPP (!?)
__unix__        Gnu CC and DJGPP
_UNICOS         Cray operating system
vaxc            VAX C compiler
VAXC            VAX C compiler
vax11c          VAX C compiler
VAX11           VAX C compiler
_Windows        Borland C 3.1 compiling for Windows
_WIN32          MS VisualC++ 4.2 and Cygwin (Win32 API)
_WIN32_WCE      MS Windows CE
vms             (and VMS) VAX-11 C under VMS.

__alpha         DEC Alpha processor
__alpha__       DEC Alpha processor
hp9000s800      HP RISC processor
__ia64          GCC on IA64
__ia64__        GCC on IA64
_LONGLONG       GCC on IA64
__i386__        DJGPP
i386            DJGPP
_M_ARM          Microsoft CLARM compiler defines as 4 for ARM.
_M_ARMT         Microsoft CLTHUMB compiler defines as 4 for Thumb.
MULTIMAX        Encore computer
ppc             PowerPC
__ppc__         PowerPC
pyr             Pyramid 9810 processor
__sgi__         Silicon Graphics Inc.
sparc           SPARC processor
sequent         Sequent computer
tahoe           CCI Tahoe processor
vax             VAX processor
__x86_64        AMD Opteron

2.8 Problems Compiling

2.9 Problems Linking

2.10 Problems Running

2.11 Testing

Loading ‘r4rstest.scm’ in the distribution will run an [R4RS] conformance test on scm.

> (load "r4rstest.scm")
-|
;loading "r4rstest.scm"
SECTION(2 1)
SECTION(3 4)
 #<primitive-procedure boolean?>
    #<primitive-procedure char?>
       #<primitive-procedure null?>
          #<primitive-procedure number?>
…

Loading ‘pi.scm’ in the distribution will enable you to compute digits of pi.

> (load "pi")
;loading "pi"
;done loading "pi.scm"
;Evaluation took 20 ms (0 in gc) 767 cells work, 233.B other
#<unspecified>
> (pi 100 5)
00003 14159 26535 89793 23846 26433 83279 50288 41971 69399
37510 58209 74944 59230 78164 06286 20899 86280 34825 34211
70679
;Evaluation took 550 ms (60 in gc) 36976 cells work, 1548.B other
#<unspecified>

Loading ‘bench.scm’ will compute and display performance statistics of SCM running ‘pi.scm’. ‘make bench’ or ‘make benchlit’ appends the performance report to the file ‘BenchLog’, facilitating tracking effects of changes to SCM on performance.

Sparc(SUN-4) heap is growing out of control

You are experiencing a GC problem peculiar to the Sparc. The problem is that SCM doesn’t know how to clear register windows. Every location which is not reused still gets marked at GC time. This causes lots of stuff which should be collected to not be. This will be a problem with any conservative GC until we find what instruction will clear the register windows. This problem is exacerbated by using lots of call-with-current-continuations. A possible fix for dynthrow() is commented out in ‘continue.c’.

2.12 Reporting Problems

Reported problems and solutions are grouped under Compiling, Linking, Running, and Testing. If you don’t find your problem listed there, you can send a bug report to agj @ alum.mit.edu. The bug report should include:

1. The version of SCM (printed when SCM is invoked with no arguments).
2. The type of computer you are using.
3. The name and version of your computer’s operating system.
4. The values of the environment variables SCM_INIT_PATH and SCHEME_LIBRARY_PATH.
5. The name and version of your C compiler.
6. If you are using an executable from a distribution, the name, vendor, and date of that distribution. In this case, corresponding with the vendor is recommended.

3. Operational Features

3.1 Invoking SCM

 scm  [-a kbytes] [-muvbiq] [--version] [--help]
[[-]-no-init-file] [--no-symbol-case-fold]
[-p int] [-r feature] [-h feature]
[-d filename] [-f filename] [-l filename]
[-c expression] [-e expression] [-o dumpname]
[-- | - | -s] [filename] [arguments …]

Upon startup scm loads the file specified by by the environment variable SCM_INIT_PATH.

If SCM_INIT_PATH is not defined or if the file it names is not present, scm tries to find the directory containing the executable file. If it is able to locate the executable, scm looks for the initialization file (usually ‘Init5e5.scm’) in platform-dependent directories relative to this directory. See File-System Habitat for a blow-by-blow description.

As a last resort (if initialization file cannot be located), the C compile parameter IMPLINIT (defined in the makefile or ‘scmfig.h’) is tried.

Unless the option -no-init-file or --no-init-file occurs in the command line, or if scm is being invoked as a script, ‘Init5e5.scm’ checks to see if there is file ‘ScmInit.scm’ in the path specified by the environment variable HOME (or in the current directory if HOME is undefined). If it finds such a file, then it is loaded.

‘Init5e5.scm’ then looks for command input from one of three sources: From an option on the command line, from a file named on the command line, or from standard input.

This explanation applies to SCMLIT or other builds of SCM.

Scheme-code files can also invoke SCM and its variants. See section #!.

3.2 Options

The options are processed in the order specified on the command line.

Command Option: -a k

specifies that scm should allocate an initial heapsize of k kilobytes. This option, if present, must be the first on the command line. If not specified, the default is INIT_HEAP_SIZE in source file ‘setjump.h’ which the distribution sets at 25000*sizeof(cell).

Command Option: -no-init-file

Command Option: ---no-init-file

Inhibits the loading of ‘ScmInit.scm’ as described above.

Command Option: --no-symbol-case-fold

Symbol (and identifier) names will be case sensitive.

Command Option: ---help

prints usage information and URI; then exit.

Command Option: ---version

prints version information and exit.

Command Option: -r feature

requires feature. This will load a file from [SLIB] if that feature is not already provided. If feature is 2, 2rs, or r2rs; 3, 3rs, or r3rs; 4, 4rs, or r4rs; 5, 5rs, or r5rs; scm will require the features neccessary to support [R2RS]; [R3RS]; [R4RS]; or [R5RS], respectively.

Command Option: -h feature

provides feature.

Command Option: -l filename

Command Option: -f filename

loads filename. Scm will load the first (unoptioned) file named on the command line if no -c, -e, -f, -l, or -s option preceeds it.

Command Option: -d filename

Loads SLIB databases feature and opens filename as a database.

Command Option: -e expression

Command Option: -c expression

specifies that the scheme expression expression is to be evaluated. These options are inspired by perl and sh respectively. On Amiga systems the entire option and argument need to be enclosed in quotes. For instance ‘"-e(newline)"’.

Command Option: -o dumpname

saves the current SCM session as the executable program ‘dumpname’. This option works only in SCM builds supporting dump (see section Dump).

If options appear on the command line after ‘-o dumpname’, then the saved session will continue with processing those options when it is invoked. Otherwise the (new) command line is processed as usual when the saved image is invoked.

Command Option: -p level

sets the prolixity (verboseness) to level. This is the same as the scm command (verobse level).

Command Option: -v

(verbose mode) specifies that scm will print prompts, evaluation times, notice of loading files, and garbage collection statistics. This is the same as -p3.

Command Option: -q

(quiet mode) specifies that scm will print no extra information. This is the same as -p0.

Command Option: -m

specifies that subsequent loads, evaluations, and user interactions will be with syntax-rules macro capability. To use a specific syntax-rules macro implementation from [SLIB] (instead of [SLIB]’s default) put -r macropackage before -m on the command line.

Command Option: -u

specifies that subsequent loads, evaluations, and user interactions will be without syntax-rules macro capability. Syntax-rules macro capability can be restored by a subsequent -m on the command line or from Scheme code.

Command Option: -i

specifies that scm should run interactively. That means that scm will not terminate until the (quit) or (exit) command is given, even if there are errors. It also sets the prolixity level to 2 if it is less than 2. This will print prompts, evaluation times, and notice of loading files. The prolixity level can be set by subsequent options. If scm is started from a tty, it will assume that it should be interactive unless given a subsequent -b option.

Command Option: -b

specifies that scm should run non-interactively. That means that scm will terminate after processing the command line or if there are errors.

Command Option: -s

specifies, by analogy with sh, that scm should run interactively and that further options are to be treated as program aguments.

Command Option: -

Command Option: ---

specifies that further options are to be treated as program aguments.

3.3 Invocation Examples

% scm foo.scm

Loads and executes the contents of ‘foo.scm’ and then enters interactive session.

% scm -f foo.scm arg1 arg2 arg3

Parameters arg1, arg2, and arg3 are stored in the global list *argv*; Loads and executes the contents of ‘foo.scm’ and exits.

% scm -s foo.scm arg1 arg2

Sets *argv* to ("foo.scm" "arg1" "arg2") and enters interactive session.

% scm -e `(write (list-ref *argv* *optind*))' bar

Prints ‘"bar"’.

% scm -rpretty-print -r format -i

Loads pretty-print and format and enters interactive session.

% scm -r5

Loads dynamic-wind, values, and syntax-rules macros and enters interactive (with macros) session.

% scm -r5 -r4

Like above but rev4-optional-procedures are also loaded.

3.4 Environment Variables

Environment Variable: SCM_INIT_PATH

is the pathname where scm will look for its initialization code. The default is the file ‘Init5e5.scm’ in the source directory.

Environment Variable: SCHEME_LIBRARY_PATH

is the [SLIB] Scheme library directory.

Environment Variable: HOME

is the directory where ‘Init5e5.scm’ will look for the user initialization file ‘ScmInit.scm’.

Environment Variable: EDITOR

is the name of the program which ed will call. If EDITOR is not defined, the default is ‘ed’.

3.5 Scheme Variables

Variable: *argv*

contains the list of arguments to the program. *argv* can change during argument processing. This list is suitable for use as an argument to [SLIB] getopt.

Variable: *syntax-rules*

controls whether loading and interaction support syntax-rules macros. Define this in ‘ScmInit.scm’ or files specified on the command line. This can be overridden by subsequent -m and -u options.

Variable: *interactive*

controls interactivity as explained for the -i and -b options. Define this in ‘ScmInit.scm’ or files specified on the command line. This can be overridden by subsequent -i and -b options.

3.6 SCM Session

• Options, file loading and features can be specified from the command line. See (scm)System interface section ‘System interface’ in SCM. See (slib)Require section ‘Require’ in SLIB.
• Typing the end-of-file character at the top level session (while SCM is not waiting for parenthesis closure) causes SCM to exit.
• Typing the interrupt character aborts evaluation of the current form and resumes the top level read-eval-print loop.

Function: quit

Function: quit n

Function: exit

Function: exit n

Aliases for exit (see exit: (slib)System section ‘System’ in SLIB). On many systems, SCM can also tail-call another program. See section execp.

Callback procedure: boot-tail dumped?

boot-tail is called by scm_top_level just before entering interactive top-level. If boot-tail calls quit, then interactive top-level is not entered.

Function: program-arguments

Returns a list of strings of the arguments scm was called with.

Function: getlogin

Returns the (login) name of the user logged in on the controlling terminal of the process, or #f if this information cannot be determined.

For documentation of the procedures getenv and system See (slib)System Interface section ‘System Interface’ in SLIB.

Function: vms-debug

If SCM is compiled under VMS this vms-debug will invoke the VMS debugger.

3.7 Editing Scheme Code

Function: ed arg1 …

Function: ed filename

The value of the environment variable EDITOR (or just ed if it isn’t defined) is invoked as a command with arguments arg1 ….

If SCM is compiled under VMS ed will invoke the editor with a single the single argument filename.

Gnu Emacs:

Editing of Scheme code is supported by emacs. Buffers holding files ending in .scm are automatically put into scheme-mode.

If your Emacs can run a process in a buffer you can use the Emacs command ‘M-x run-scheme’ with SCM. Otherwise, use the emacs command ‘M-x suspend-emacs’; or see “other systems” below.

Epsilon (MS-DOS):

There is lisp (and scheme) mode available by use of the package ‘LISP.E’. It offers several different indentation formats. With this package, buffers holding files ending in ‘.L’, ‘.LSP’, ‘.S’, and ‘.SCM’ (my modification) are automatically put into lisp-mode.

It is possible to run a process in a buffer under Epsilon. With Epsilon 5.0 the command line options ‘-e512 -m0’ are neccessary to manage RAM properly. It has been reported that when compiling SCM with Turbo C, you need to ‘#define NOSETBUF’ for proper operation in a process buffer with Epsilon 5.0.

One can also call out to an editor from SCM if RAM is at a premium; See “under other systems” below.

other systems:

Define the environment variable ‘EDITOR’ to be the name of the editing program you use. The SCM procedure (ed arg1 …) will invoke your editor and return to SCM when you exit the editor. The following definition is convenient:

(define (e) (ed "work.scm") (load "work.scm"))

Typing ‘(e)’ will invoke the editor with the file of interest. After editing, the modified file will be loaded.

3.8 Debugging Scheme Code

The cautious option of build (see section Build Options) supports debugging in Scheme.

CAUTIOUS

If SCM is built with the ‘CAUTIOUS’ flag, then when an error occurs, a stack trace of certain pending calls are printed as part of the default error response. A (memoized) expression and newline are printed for each partially evaluated combination whose procedure is not builtin. See Memoized Expressions for how to read memoized expressions.

Also as the result of the ‘CAUTIOUS’ flag, both error and user-interrupt (invoked by <C-c>) to print stack traces and conclude by calling breakpoint (see (slib)Breakpoints section ‘Breakpoints’ in SLIB) instead of aborting to top level. Under either condition, program execution can be resumed by (continue).

In this configuration one can interrupt a running Scheme program with <C-c>, inspect or modify top-level values, trace or untrace procedures, and continue execution with (continue).

If verbose (see section verbose) is called with an argument greater than 2, then the interpreter will check stack size periodically. If the size of stack in use exceeds the C #define STACK_LIMIT (default is HEAP_SEG_SIZE), SCM generates a ‘stack’ segment violation.

There are several SLIB macros which so useful that SCM automatically loads the appropriate module from SLIB if they are invoked.

Macro: trace proc1 …

Macro: trace

Traces the top-level named procedures given as arguments.

With no arguments, makes sure that all the currently traced identifiers are traced (even if those identifiers have been redefined) and returns a list of the traced identifiers.

Macro: untrace proc1 …

Macro: untrace

Turns tracing off for its arguments.

With no arguments, untraces all currently traced identifiers and returns a list of these formerly traced identifiers.

The routines I use most frequently for debugging are:

Function: print arg1 …

Print writes all its arguments, separated by spaces. Print outputs a newline at the end and returns the value of the last argument.

One can just insert ‘(print '<label>’ and ‘)’ around an expression in order to see its values as a program operates.

Function: pprint arg1 …

Pprint pretty-prints (see (slib)Pretty-Print section ‘Pretty-Print’ in SLIB) all its arguments, separated by newlines. Pprint returns the value of the last argument.

One can just insert ‘(pprint '<label>’ and ‘)’ around an expression in order to see its values as a program operates. Note pretty-print does not format procedures.

When typing at top level, pprint is not a good way to see nested structure because it will return the last object pretty-printed, which could be large. pp is a better choice.

Procedure: pp arg1 …

Pprint pretty-prints (see (slib)Pretty-Print section ‘Pretty-Print’ in SLIB) all its arguments, separated by newlines. pp returns #<unspecified>.

Syntax: print-args name

Syntax: print-args

Writes name if supplied; then writes the names and values of the closest lexical bindings enclosing the call to Print-args.

(define (foo a b) (print-args foo) (+ a b)) (foo 3 6) -| In foo: a = 3; b = 6; ⇒ 9

Sometimes more elaborate measures are needed to print values in a useful manner. When the values to be printed may have very large (or infinite) external representations, (slib)Quick Print section ‘Quick Print’ in SLIB, can be used.

When trace is not sufficient to find program flow problems, <A HREF="http://www.cs.tut.fi/staff/pk/scheme/psd/article/article.html"> SLIB-PSD, the Portable Scheme Debugger </A> offers source code debugging from GNU Emacs. PSD runs slowly, so start by instrumenting only a few functions at a time.

http://swiss.csail.mit.edu/ftpdir/scm/slib-psd1-3.tar.gz
swiss.csail.mit.edu:/pub/scm/slib-psd1-3.tar.gz
ftp.maths.tcd.ie:pub/bosullvn/jacal/slib-psd1-3.tar.gz
ftp.cs.indiana.edu:/pub/scheme-repository/utl/slib-psd1-3.tar.gz

3.9 Debugging Continuations

These functions are defined in ‘debug.c’, all operate on captured continuations:

Procedure: frame-trace cont n

Prints information about the code being executed and the environment scopes active for continuation frame n of continuation CONT. A "continuation frame" is an entry in the environment stack; a new frame is pushed when the environment is replaced or extended in a non-tail call context. Frame 0 is the top of the stack.

Procedure: frame->environment cont n

Prints the environment for continuation frame n of continuation cont. This contains just the names, not the values, of the environment.

Procedure: scope-trace env

will print information about active lexical scopes for environment env.

Procedure: frame-eval cont n expr

Evaluates expr in the environment defined by continuation frame n of continuation CONT and returns the result. Values in the environment may be returned or SET!.

stack-trace also now accepts an optional continuation argument. stack-trace differs from frame-trace in that it truncates long output using safeports and prints code from all available frames.

(define k #f)
(define (foo x y)
   (set! k (call-with-current-continuation identity))
   #f)
(let ((a 3) (b 4))
  (foo a b)
  #f)
(stack-trace k)
-|
;STACK TRACE
1; ((#@set! #@k (#@call-with-current-continuation #@identity)) #f ...
2; (#@let ((a 3) (b 4))  (#@foo #@a #@b) #f)
…
#t

(frame-trace k 0)
-|
(#@call-with-current-continuation #@identity)
; in scope:
;   (x y)  procedure foo#<unspecified>

(frame-trace k 1)
-|
((#@set! #@k (#@call-with-current-continuation #@identity)) #f)
; in scope:
;   (x y)  procedure foo#<unspecified>

(frame-trace k 2)
-|
(#@let ((a 3) (b 4))  (#@foo #@a #@b) #f)
; in scope:
;   (a b . #@let)#<unspecified>

(frame-trace k 3)
-|
(#@let ((a 3) (b 4))  (#@foo #@a #@b) #f)
; in top level environment.

(frame->environment k 0)
-|
((x y) 2 foo)

(scope-trace (frame->environment k 0))
-|
; in scope:
;   (x y)  procedure foo#<unspecified>

(frame-eval k 0 'x) ⇒ 3

(frame-eval k 0 '(set! x 8))
(frame-eval k 0 'x) ⇒ 8

3.10 Errors

A computer-language implementation designer faces choices of how reflexive to make the implementation in handling exceptions and errors; that is, how much of the error and exception routines should be written in the language itself. The design of a portable implementation is further constrained by the need to have (almost) all errors print meaningful messages, even when the implementation itself is not functioning correctly. Therefore, SCM implements much of its error response code in C.

The following common error and conditions are handled by C code. Those with callback names after them can also be handled by Scheme code (see section Interrupts). If the callback identifier is not defined at top level, the default error handler (C code) is invoked. There are many other error messages which are not treated specially.

ARGn

Wrong type in argument

ARG1

Wrong type in argument 1

ARG2

Wrong type in argument 2

ARG3

Wrong type in argument 3

ARG4

Wrong type in argument 4

ARG5

Wrong type in argument 5

WNA

Wrong number of args

OVFLOW

numerical overflow

OUTOFRANGE

Argument out of range

NALLOC

(out-of-storage)

THRASH

GC is (thrashing)

EXIT

(end-of-program)

HUP_SIGNAL

(hang-up)

INT_SIGNAL

(user-interrupt)

FPE_SIGNAL

(arithmetic-error)

BUS_SIGNAL

bus error

SEGV_SIGNAL

segment violation

ALRM_SIGNAL

(alarm-interrupt)

VTALRM_SIGNAL

(virtual-alarm-interrupt)

PROF_SIGNAL

(profile-alarm-interrupt)

Variable: errobj

When SCM encounters a non-fatal error, it aborts evaluation of the current form, prints a message explaining the error, and resumes the top level read-eval-print loop. The value of errobj is the offending object if appropriate. The builtin procedure error does not set errobj.

errno and perror report ANSI C errors encountered during a call to a system or library function.

Function: errno

Function: errno n

With no argument returns the current value of the system variable errno. When given an argument, errno sets the system variable errno to n and returns the previous value of errno. (errno 0) will clear outstanding errors. This is recommended after try-load returns #f since this occurs when the file could not be opened.

Function: perror string

Prints on standard error output the argument string, a colon, followed by a space, the error message corresponding to the current value of errno and a newline. The value returned is unspecified.

warn and error provide a uniform way for Scheme code to signal warnings and errors.

Function: warn arg1 arg2 arg3 …

Alias for slib:warn: (slib)System section ‘System’ in SLIB. Outputs an error message containing the arguments. warn is defined in ‘Init5e5.scm’.

Function: error arg1 arg2 arg3 …

Alias for slib:error: (slib)System section ‘System’ in SLIB. Outputs an error message containing the arguments, aborts evaluation of the current form and resumes the top level read-eval-print loop. Error is defined in ‘Init5e5.scm’.

If SCM is built with the ‘CAUTIOUS’ flag, then when an error occurs, a stack trace of certain pending calls are printed as part of the default error response. A (memoized) expression and newline are printed for each partially evaluated combination whose procedure is not builtin. See Memoized Expressions for how to read memoized expressions.

Also as the result of the ‘CAUTIOUS’ flag, both error and user-interrupt (invoked by <C-c>) are defined to print stack traces and conclude by calling breakpoint (see (slib)Breakpoints section ‘Breakpoints’ in SLIB). This allows the user to interract with SCM as with Lisp systems.

Function: stack-trace

Prints information describing the stack of partially evaluated expressions. stack-trace returns #t if any lines were printed and #f otherwise. See ‘Init5e5.scm’ for an example of the use of stack-trace.

3.11 Memoized Expressions

SCM memoizes the address of each occurence of an identifier’s value when first encountering it in a source expression. Subsequent executions of that memoized expression is faster because the memoized reference encodes where in the top-level or local environment its value is.

When procedures are displayed, the memoized locations appear in a format different from references which have not yet been executed. I find this a convenient aid to locating bugs and untested expressions.

• The names of memoized lexically bound identifiers are replaced with #@<m>-<n>, where <m> is the number of binding contours back and <n> is the index of the value in that binding countour.
• The names of identifiers which are not lexiallly bound but defined at top-level have #@ prepended.

For instance, open-input-file is defined as follows in ‘Init5e5.scm’:

(define (open-input-file str)
  (or (open-file str open_read)
      (and (procedure? could-not-open) (could-not-open) #f)
      (error "OPEN-INPUT-FILE couldn't open file " str)))

If open-input-file has not yet been used, the displayed procedure is similar to the original definition (lines wrapped for readability):

open-input-file ⇒
#<CLOSURE (str) (or (open-file str open_read)
 (and (procedure? could-not-open) (could-not-open) #f)
 (error "OPEN-INPUT-FILE couldn't open file " str))>

If we open a file using open-input-file, the sections of code used become memoized:

(open-input-file "r4rstest.scm") ⇒ #<input-port 3>
open-input-file ⇒
#<CLOSURE (str) (#@or (#@open-file #@0+0 #@open_read)
 (and (procedure? could-not-open) (could-not-open) #f)
 (error "OPEN-INPUT-FILE couldn't open file " str))>

If we cause open-input-file to execute other sections of code, they too become memoized:

(open-input-file "foo.scm") ⇒

ERROR: No such file or directory
ERROR: OPEN-INPUT-FILE couldn't open file  "foo.scm"

open-input-file ⇒
#<CLOSURE (str) (#@or (#@open-file #@0+0 #@open_read)
 (#@and (#@procedure? #@could-not-open) (could-not-open) #f)
 (#@error "OPEN-INPUT-FILE couldn't open file " #@0+0))>

3.12 Internal State

Variable: *interactive*

The variable *interactive* determines whether the SCM session is interactive, or should quit after the command line is processed. *interactive* is controlled directly by the command-line options ‘-b’, ‘-i’, and ‘-s’ (see section Invoking SCM). If none of these options are specified, the rules to determine interactivity are more complicated; see ‘Init5e5.scm’ for details.

Function: abort

Resumes the top level Read-Eval-Print loop.

Function: restart

Restarts the SCM program with the same arguments as it was originally invoked. All ‘-l’ loaded files are loaded again; If those files have changed, those changes will be reflected in the new session.

Note When running a saved executable (see section Dump), restart is redefined to be exec-self.

Function: exec-self

Exits and immediately re-invokes the same executable with the same arguments. If the executable file has been changed or replaced since the beginning of the current session, the new executable will be invoked. This differentiates exec-self from restart.

Function: verbose n

Controls how much monitoring information is printed. If n is:

0

no prompt or information is printed.

>= 1

a prompt is printed.

>= 2

messages bracketing file loading are printed.

>= 3

the CPU time is printed after each top level form evaluated; notifications of heap growth printed; the interpreter checks stack depth periodically.

>= 4

a garbage collection summary is printed after each top level form evaluated;

>= 5

a message for each GC (see section Garbage Collection) is printed; warnings issued for top-level symbols redefined.

Function: gc

Scans all of SCM objects and reclaims for further use those that are no longer accessible.

Function: room

Function: room #t

Prints out statistics about SCM’s current use of storage. (room #t) also gives the hexadecimal heap segment and stack bounds.

Constant: *scm-version*

Contains the version string (e.g. ‘5e5’) of SCM.

3.12.1 Executable path

In order to dump a saved executable or to dynamically-link using DLD, SCM must know where its executable file is. Sometimes SCM (see section Executable Pathname) guesses incorrectly the location of the currently running executable. In that case, the correct path can be set by calling execpath with the pathname.

Function: execpath

Returns the path (string) which SCM uses to find the executable file whose invocation the currently running session is, or #f if the path is not set.

Function: execpath #f

Function: execpath newpath

Sets the path to #f or newpath, respectively. The old path is returned.

For other configuration constants and procedures See (slib)Configuration section ‘Configuration’ in SLIB.

3.13 Scripting

3.13.1 Unix Scheme Scripts

In reading this section, keep in mind that the first line of a script file has (different) meanings to SCM and the operating system (execve).

file: #! interpreter \ …

On unix systems, a Shell-Script is a file (with execute permissions) whose first two characters are ‘#!’. The interpreter argument must be the pathname of the program to process the rest of the file. The directories named by environment variable PATH are not searched to find interpreter.

When executing a shell-script, the operating system invokes interpreter with a single argument encapsulating the rest of the first line’s contents (if not just whitespace), the pathname of the Scheme Script file, and then any arguments which the shell-script was invoked with.

Put one space character between ‘#!’ and the first character of interpreter (‘/’). The interpreter name is followed by ‘ \’; SCM substitutes the second line of file for ‘\’ (and the rest of the line), then appends any arguments given on the command line invoking this Scheme-Script.

When SCM executes the script, the Scheme variable *script* will be set to the script pathname. The last argument before ‘!#’ on the second line should be ‘-’; SCM will load the script file, preserve the unprocessed arguments, and set *argv* to a list of the script pathname and the unprocessed arguments.

Note that the interpreter, not the operating system, provides the ‘\’ substitution; this will only take place if interpreter is a SCM or SCSH interpreter.

Read syntax: #! ignored !#

When the first two characters of the file being loaded are #! and a ‘\’ is present before a newline in the file, all characters up to ‘!#’ will be ignored by SCM read.

This combination of interpretatons allows SCM source files to be used as POSIX shell-scripts if the first line is:

#! /usr/local/bin/scm \

The following Scheme-Script prints factorial of its argument:

#! /usr/local/bin/scm \ %0 %*
- !#

(define (fact.script args)
  (cond ((and (= 1 (length args))
              (string->number (car args)))
         => (lambda (n) (print (fact n)) #t))
        (else (fact.usage))))

(define (fact.usage)
  (print *argv*)
  (display "\
Usage: fact N
  Returns the factorial of N.
"
           (current-error-port))
  #f)

(define (fact n) (if (< n 2) 1 (* n (fact (+ -1 n)))))

(if *script* (exit (fact.script (list-tail *argv* *optind*))))

./fact 32
⇒
263130836933693530167218012160000000

If the wrong number of arguments is given, fact prints its argv with usage information.

./fact 3 2
-|
("./fact" "3" "2") 
Usage: fact N
  Returns the factorial of N.

3.13.2 MS-DOS Compatible Scripts

It turns out that we can create scheme-scripts which run both under unix and MS-DOS. To implement this, I have written the MS-DOS programs: #!.bat and !#.exe, which are available from: http://swiss.csail.mit.edu/ftpdir/scm/sharpbang.zip

With these two programs installed in a PATH directory, we have the following syntax for <program>.BAT files.

file: #! interpreter \ %0 %*

The first two characters of the Scheme-Script are ‘#!’. The interpreter can be either a unix style program path (using ‘/’ between filename components) or a DOS program name or path. The rest of the first line of the Scheme-Script should be literally ‘\ %0 %*’, as shown.

If interpreter has ‘/’ in it, interpreter is converted to a DOS style filename (‘/’ ⇒ ‘\’).

In looking for an executable named interpreter, #! first checks this (converted) filename; if interpreter doesn’t exist, it then tries to find a program named like the string starting after the last ‘\’ (or ‘/’) in interpreter. When searching for executables, #! tries all directories named by environment variable PATH.

Once the interpreter executable path is found, arguments are processed in the manner of scheme-shell, with all the text after the ‘\’ taken as part of the meta-argument. More precisely, #! calls interpreter with any options on the second line of the Scheme-Script up to ‘!#’, the name of the Scheme-Script file, and then any of at most 8 arguments given on the command line invoking this Scheme-Script.

The previous example Scheme-Script works in both MS-DOS and unix systems.

3.13.3 Unix Shell Scripts

Scheme-scripts suffer from two drawbacks:

• Some Unixes limit the length of the ‘#!’ interpreter line to the size of an object file header, which can be as small as 32 bytes.
• A full, explicit pathname must be specified, perhaps requiring more than 32 bytes and making scripts vulnerable to breakage when programs are moved.

The following approach solves these problems at the expense of slower startup. Make ‘#! /bin/sh’ the first line and prepend every subsequent line to be executed by the shell with :;. The last line to be executed by the shell should contain an exec command; exec tail-calls its argument.

/bin/sh is thus invoked with the name of the script file, which it executes as a *sh script. Usually the second line starts ‘:;exec scm -f$0’, which executes scm, which in turn loads the script file. When SCM loads the script file, it ignores the first and second lines, and evaluates the rest of the file as Scheme source code.

The second line of the script file does not have the length restriction mentioned above. Also, /bin/sh searches the directories listed in the ‘PATH’ environment variable for ‘scm’, eliminating the need to use absolute locations in order to invoke a program.

The following example additionally sets *script* to the script argument, making it compatible with the scheme code of the previous example.

#! /bin/sh
:;exec scm -e"(set! *script* \"$0\")" -l$0 "$@"

(define (fact.script args)
  (cond ((and (= 1 (length args))
              (string->number (car args)))
         => (lambda (n) (print (fact n)) #t))
        (else (fact.usage))))

(define (fact.usage)
  (print *argv*)
  (display "\
Usage: fact N
  Returns the factorial of N.
"
           (current-error-port))
  #f)

(define (fact n) (if (< n 2) 1 (* n (fact (+ -1 n)))))

(if *script* (exit (fact.script (list-tail *argv* *optind*))))

./fact 6
⇒ 720 

4. The Language

4.1 Standards Compliance

Scm conforms to the [IEEE], IEEE Standard 1178-1990. IEEE Standard for the Scheme Programming Language. and [R5RS], <A HREF="r5rs_toc.html"> Revised(5) Report on the Algorithmic Language Scheme. </A> All the required features of these specifications are supported. Many of the optional features are supported as well.

Optionals of [R5RS] Supported by SCM

- and / of more than 2 arguments

exp

log

sin

cos

tan

asin

acos

atan

sqrt

expt

make-rectangular

make-polar

real-part

imag-part

magnitude

angle

exact->inexact

inexact->exact

See (r5rs)Numerical operations section ‘Numerical operations’ in Revised(5) Scheme.

with-input-from-file

with-output-to-file

See (r5rs)Ports section ‘Ports’ in Revised(5) Scheme.

load

transcript-on

transcript-off

See (r5rs)System interface section ‘System interface’ in Revised(5) Scheme.

Optionals of [R5RS] not Supported by SCM

numerator

denominator

rationalize

See (r5rs)Numerical operations section ‘Numerical operations’ in Revised(5) Scheme.

[SLIB] Features of SCM and SCMLIT

delay

full-continuation

ieee-p1178

object-hash

rev4-report

source

See SLIB file ‘Template.scm’.

current-time

See (slib)Time and Date section ‘Time and Date’ in SLIB.

defmacro

See (slib)Defmacro section ‘Defmacro’ in SLIB.

getenv

system

See (slib)System Interface section ‘System Interface’ in SLIB.

hash

See (slib)Hashing section ‘Hashing’ in SLIB.

logical

See (slib)Bit-Twiddling section ‘Bit-Twiddling’ in SLIB.

multiarg-apply

See (slib)Multi-argument Apply section ‘Multi-argument Apply’ in SLIB.

multiarg/and-

See (slib)Multi-argument / and - section ‘Multi-argument / and -’ in SLIB.

rev4-optional-procedures

See (slib)Rev4 Optional Procedures section ‘Rev4 Optional Procedures’ in SLIB.

string-port

See (slib)String Ports section ‘String Ports’ in SLIB.

tmpnam

See (slib)Input/Output section ‘Input/Output’ in SLIB.

transcript

See (slib)Transcripts section ‘Transcripts’ in SLIB.

vicinity

See (slib)Vicinity section ‘Vicinity’ in SLIB.

with-file

See (slib)With-File section ‘With-File’ in SLIB.

[SLIB] Features of SCM

array

See (slib)Arrays section ‘Arrays’ in SLIB.

array-for-each

See (slib)Array Mapping section ‘Array Mapping’ in SLIB.

bignum

complex

inexact

rational

real

See (slib)Require section ‘Require’ in SLIB.

4.2 Storage

Function: vector-set-length! object length

Change the length of string, vector, bit-vector, or uniform-array object to length. If this shortens object then the remaining contents are lost. If it enlarges object then the contents of the extended part are undefined but the original part is unchanged. It is an error to change the length of literal datums. The new object is returned.

Function: copy-tree obj

Function: @copy-tree obj

See copy-tree: (slib)Tree Operations section ‘Tree Operations’ in SLIB. This extends the SLIB version by also copying vectors. Use @copy-tree if you depend on this feature; copy-tree could get redefined.

Function: acons obj1 obj2 obj3

Returns (cons (cons obj1 obj2) obj3).

(set! a-list (acons key datum a-list))

Adds a new association to a-list.

Callback procedure: gc-hook …

Allows a Scheme procedure to be run shortly after each garbage collection. This procedure will not be run recursively. If it runs long enough to cause a garbage collection before returning a warning will be printed.

To remove the gc-hook, (set! gc-hook #f).

Function: add-finalizer object finalizer

object may be any garbage collected object, that is, any object other than an immediate integer, character, or special token such as #f or #t, See section Immediates. finalizer is a thunk, or procedure taking no arguments.

finalizer will be invoked asynchronously exactly once some time after object becomes eligible for garbage collection. A reference to object in the environment of finalizer will not prevent finalization, but will delay the reclamation of object at least until the next garbage collection. A reference to object in some other object’s finalizer will necessarily prevent finalization until both objects are eligible for garbage collection.

Finalizers are not run in any predictable order. All finalizers will be run by the time the program ends.

This facility was based on the paper by Simon Peyton Jones, et al, “Stretching the storage manager: weak pointers and stable names in Haskell”, Proc. 11th International Workshop on the Implementation of Functional Languages, The Netherlands, September 7-10 1999, Springer-Verlag LNCS.

4.3 Time

Constant: internal-time-units-per-second

Is the integer number of internal time units in a second.

Function: get-internal-run-time

Returns the integer run time in internal time units from an unspecified starting time. The difference of two calls to get-internal-run-time divided by internal-time-units-per-second will give elapsed run time in seconds.

Function: get-internal-real-time

Returns the integer time in internal time units from an unspecified starting time. The difference of two calls to get-internal-real-time divided by interal-time-units-per-second will give elapsed real time in seconds.

Function: current-time

Returns the time since 00:00:00 GMT, January 1, 1970, measured in seconds. See current-time: (slib)Time and Date section ‘Time and Date’ in SLIB. current-time is used in (slib)Time and Date section ‘Time and Date’ in SLIB.

4.4 Interrupts

Function: ticks n

Returns the number of ticks remaining till the next tick interrupt. Ticks are an arbitrary unit of evaluation. Ticks can vary greatly in the amount of time they represent.

If n is 0, any ticks request is canceled. Otherwise a ticks-interrupt will be signaled n from the current time. ticks is supported if SCM is compiled with the ticks flag defined.

Callback procedure: ticks-interrupt …

Establishes a response for tick interrupts. Another tick interrupt will not occur unless ticks is called again. Program execution will resume if the handler returns. This procedure should (abort) or some other action which does not return if it does not want processing to continue.

Function: alarm secs

Returns the number of seconds remaining till the next alarm interrupt. If secs is 0, any alarm request is canceled. Otherwise an alarm-interrupt will be signaled secs from the current time. ALARM is not supported on all systems.

Function: milli-alarm millisecs interval

Function: virtual-alarm millisecs interval

Function: profile-alarm millisecs interval

milli-alarm is similar to alarm, except that the first argument millisecs, and the return value are measured in milliseconds rather than seconds. If the optional argument interval is supplied then alarm interrupts will be scheduled every interval milliseconds until turned off by a call to milli-alarm or alarm.

virtual-alarm and profile-alarm are similar. virtual-alarm decrements process execution time rather than real time, and causes SIGVTALRM to be signaled. profile-alarm decrements both process execution time and system execution time on behalf of the process, and causes SIGPROF to be signaled.

milli-alarm, virtual-alarm, and profile-alarm are supported only on systems providing the setitimer system call.

Callback procedure: user-interrupt …

Callback procedure: alarm-interrupt …

Callback procedure: virtual-alarm-interrupt …

Callback procedure: profile-alarm-interrupt …

Establishes a response for SIGINT (control-C interrupt) and SIGALRM, SIGVTALRM, and SIGPROF interrupts. Program execution will resume if the handler returns. This procedure should (abort) or some other action which does not return if it does not want processing to continue after it returns.

Interrupt handlers are disabled during execution system and ed procedures.

To unestablish a response for an interrupt set the handler symbol to #f. For instance, (set! user-interrupt #f).

Callback procedure: out-of-storage …

Callback procedure: could-not-open …

Callback procedure: end-of-program …

Callback procedure: hang-up …

Callback procedure: arithmetic-error …

Establishes a response for storage allocation error, file opening error, end of program, SIGHUP (hang up interrupt) and arithmetic errors respectively. This procedure should (abort) or some other action which does not return if it does not want the default error message to also be displayed. If no procedure is defined for hang-up then end-of-program (if defined) will be called.

To unestablish a response for an error set the handler symbol to #f. For instance, (set! could-not-open #f).

4.5 Process Synchronization

An exchanger is a procedure of one argument regulating mutually exclusive access to a resource. When a exchanger is called, its current content is returned, while being replaced by its argument in an atomic operation.

Function: make-exchanger obj

Returns a new exchanger with the argument obj as its initial content.

(define queue (make-exchanger (list a)))

A queue implemented as an exchanger holding a list can be protected from reentrant execution thus:

(define (pop queue) (let ((lst #f)) (dynamic-wind (lambda () (set! lst (queue #f))) (lambda () (and lst (not (null? lst)) (let ((ret (car lst))) (set! lst (cdr lst)) ret))) (lambda () (and lst (queue lst)))))) (pop queue) ⇒ a (pop queue) ⇒ #f

Function: make-arbiter name

Returns an object of type arbiter and name name. Its state is initially unlocked.

Function: try-arbiter arbiter

Returns #t and locks arbiter if arbiter was unlocked. Otherwise, returns #f.

Function: release-arbiter arbiter

Returns #t and unlocks arbiter if arbiter was locked. Otherwise, returns #f.

4.6 Files and Ports

These procedures generalize and extend the standard capabilities in (r5rs)Ports section ‘Ports’ in Revised(5) Scheme.

4.6.1 Opening and Closing

Function: open-file string modes

Function: try-open-file string modes

Returns a port capable of receiving or delivering characters as specified by the modes string. If a file cannot be opened #f is returned.

Internal functions opening files callback to the SCM function open-file. You can extend open-file by redefining it. try-open-file is the primitive procedure; Do not redefine try-open-file!

Constant: open_read

Constant: open_write

Constant: open_both

Contain modes strings specifying that a file is to be opened for reading, writing, and both reading and writing respectively.

Both input and output functions can be used with io-ports. An end of file must be read or a two-argument file-position done on the port between a read operation and a write operation or vice-versa.

Function: _ionbf modestr

Returns a version of modestr which when open-file is called with it as the second argument will return an unbuffered port. An input-port must be unbuffered in order for char-ready? and wait-for-input to work correctly on it. The initial value of (current-input-port) is unbuffered if the platform supports it.

Function: _tracked modestr

Returns a version of modestr which when open-file is called with it as the second argument will return a tracked port. A tracked port maintains current line and column numbers, which may be queried with port-line and port-column.

Function: _exclusive modestr

Returns a version of modestr which when open-file is called with it as the second argument will return a port only if the named file does not already exist. This functionality is provided by calling try-create-file See section I/O-Extensions, which is not available for all platforms.

Function: open-ports

Returns a list of all currently open ports, excluding string ports, see See (slib)String Ports section ‘String Ports’ in SLIB. This may be useful after a fork See section Posix Extensions, or for debugging. Bear in mind that ports that would be closed by gc will be kept open by a reference to this list.

Function: close-port port

Closes port. The same as close-input-port and close-output-port.

4.6.2 Port Properties

Function: port-closed? port

Returns #t if port is closed.

Function: port-type obj

If obj is not a port returns false, otherwise returns a symbol describing the port type, for example string or pipe.

Function: port-filename port

Returns the filename port was opened with. If port is not open to a file the result is unspecified.

Function: file-position port

Function: file-position port #f

Returns the current position of the character in port which will next be read or written. If port is open to a non-file then #f is returned.

Function: file-position port k

Sets the current position in port which will next be read or written. If successful, #f is returned. If port is open to a non-file, then file-position returns #f.

Function: port-line port

Function: port-column port

If port is a tracked port, return the current line (column) number, otherwise return #f. Line and column numbers begin with 1. The column number applies to the next character to be read; if that character is a newline, then the column number will be one more than the length of the line.

Function: freshline port

Outputs a newline to optional argument port unless the current output column number of port is known to be zero, ie output will start at the beginning of a new line. port defaults to current-output-port. If port is not a tracked port freshline is equivalent to newline.

Function: isatty? port

Returns #t if port is input or output to a serial non-file device.

procedure: char-ready?

procedure: char-ready? port

Returns #t if a character is ready on the input port and returns #f otherwise. If char-ready? returns #t then the next read-char operation on the given port is guaranteed not to hang. If the port is at end of file then char-ready? returns #t. Port may be omitted, in which case it defaults to the value returned by current-input-port.

Rationale Char-ready? exists to make it possible for a program to accept characters from interactive ports without getting stuck waiting for input. Any input editors associated with such ports must ensure that characters whose existence has been asserted by char-ready? cannot be rubbed out. If char-ready? were to return #f at end of file, a port at end of file would be indistinguishable from an interactive port that has no ready characters.

procedure: wait-for-input x

procedure: wait-for-input x port1 …

Returns a list those ports port1 … which are char-ready?. If none of port1 … become char-ready? within the time interval of x seconds, then #f is returned. The port1 … arguments may be omitted, in which case they default to the list of the value returned by current-input-port.

4.6.3 Port Redirection

Function: current-error-port

Returns the current port to which diagnostic output is directed.

Function: with-error-to-file string thunk

thunk must be a procedure of no arguments, and string must be a string naming a file. The file is opened for output, an output port connected to it is made the default value returned by current-error-port, and the thunk is called with no arguments. When the thunk returns, the port is closed and the previous default is restored. With-error-to-file returns the value yielded by thunk.

Function: with-input-from-port port thunk

Function: with-output-to-port port thunk

Function: with-error-to-port port thunk

These routines differ from with-input-from-file, with-output-to-file, and with-error-to-file in that the first argument is a port, rather than a string naming a file.

Function: call-with-outputs thunk proc

Calls the thunk procedure while the current-output-port and current-error-port are directed to string-ports. If thunk returns, the proc procedure is called with the output-string, the error-string, and the value returned by thunk. If thunk does not return a value (perhaps because of error), proc is called with just the output-string and the error-string as arguments.

4.6.4 Soft Ports

A soft-port is a port based on a vector of procedures capable of accepting or delivering characters. It allows emulation of I/O ports.

Function: make-soft-port vector modes

Returns a port capable of receiving or delivering characters as specified by the modes string (see section open-file). vector must be a vector of length 5. Its components are as follows:

1. procedure accepting one character for output
2. procedure accepting a string for output
3. thunk for flushing output
4. thunk for getting one character
5. thunk for closing port (not by garbage collection)

For an output-only port only elements 0, 1, 2, and 4 need be procedures. For an input-only port only elements 3 and 4 need be procedures. Thunks 2 and 4 can instead be #f if there is no useful operation for them to perform.

If thunk 3 returns #f or an eof-object (see eof-object?: (r5rs)Input section ‘Input’ in Revised(5) Scheme) it indicates that the port has reached end-of-file. For example:

If it is necessary to explicitly close the port when it is garbage collected, (see section add-finalizer).

(define stdout (current-output-port)) (define p (make-soft-port (vector (lambda (c) (write c stdout)) (lambda (s) (display s stdout)) (lambda () (display "." stdout)) (lambda () (char-upcase (read-char))) (lambda () (display "@" stdout))) "rw")) (write p p) ⇒ #<input-output-soft#\space45d10#\>

4.7 Eval and Load

Function: try-load filename

If the string filename names an existing file, the try-load procedure reads Scheme source code expressions and definitions from the file and evaluates them sequentially and returns #t. If not, try-load returns #f. The try-load procedure does not affect the values returned by current-input-port and current-output-port.

Variable: *load-pathname*

Is set to the pathname given as argument to load, try-load, and dyn:link (see (hobbit)Compiling And Linking section ‘Compiling And Linking’ in Hobbit). *load-pathname* is used to compute the value of program-vicinity: (slib)Vicinity section ‘Vicinity’ in SLIB.

Function: eval obj

Alias for eval: (slib)System section ‘System’ in SLIB.

Function: eval-string str

Returns the result of reading an expression from str and evaluating it. eval-string does not change *load-pathname* or line-number.

Function: load-string str

Reads and evaluates all the expressions from str. As with load, the value returned is unspecified. load-string does not change *load-pathname* or line-number.

Function: line-number

Returns the current line number of the file currently being loaded.

4.7.1 Line Numbers

Scheme code defined by load may optionally contain line number information. Currently this information is used only for reporting expansion time errors, but in the future run-time error messages may also include line number information.

Function: try-load pathname reader

This is the primitive for loading, pathname is the name of a file containing Scheme code, and optional argument reader is a function of one argument, a port. reader should read and return Scheme code as list structure. The default value is read, which is used if reader is not supplied or is false.

Line number objects are disjoint from integers or other Scheme types. When evaluated or loaded as Scheme code, an s-expression containing a line-number in the car is equivalent to the cdr of the s-expression. A pair consisting of a line-number in the car and a vector in the cdr is equivalent to the vector. The meaning of s-expressions with line-numbers in other positions is undefined.

Function: read-numbered port

Behaves like read, except that

• bullet Load (read) sytnaxes are enabled.
• bullet every s-expression read will be replaced with a cons of a line-number object and the sexp actually read. This replacement is done only if port is a tracked port See See section Files and Ports.

Function: integer->line-number int

Returns a line-number object with value int. int should be an exact non-negative integer.

Function: line-number->integer linum

Returns the value of line-number object linum as an integer.

Function: line-number? obj

Returns true if and only if obj is a line-number object.

Function: read-for-load port

Behaves like read, except that load syntaxes are enabled.

Variable: *load-reader*

Variable: *slib-load-reader*

The value of *load-reader* should be a value acceptable as the second argument to try-load (note that #f is acceptable). This value will be used to read code during calls to scm:load. The value of *slib-load-reader* will similarly be used during calls to slib:load and require.

In order to disable all line-numbering, it is sufficient to set! *load-reader* and *slib-load-reader* to #f.

4.8 Lexical Conventions

4.8.1 Common-Lisp Read Syntax

Read syntax: #\token

If token is a sequence of two or more digits, then this syntax is equivalent to #.(integer->char (string->number token 8)).

If token is C-, c-, or ^ followed by a character, then this syntax is read as a control character. If token is M- or m- followed by a character, then a meta character is read. c- and m- prefixes may be combined.

Read syntax: #+ feature form

If feature is provided? then form is read as a scheme expression. If not, then form is treated as whitespace.

Feature is a boolean expression composed of symbols and and, or, and not of boolean expressions.

For more information on provided?, See (slib)Require section ‘Require’ in SLIB.

Read syntax: #- feature form

is equivalent to #+(not feature) expression.

Read syntax: #| any thing |#

Is a balanced comment. Everything up to the matching |# is ignored by the read. Nested #|…|# can occur inside any thing.

Load sytax is Read syntax enabled for read only when that read is part of loading a file or string. This distinction was made so that reading from a datafile would not be able to corrupt a scheme program using ‘#.’.

Load syntax: #. expression

Is read as the object resulting from the evaluation of expression. This substitution occurs even inside quoted structure.

In order to allow compiled code to work with #. it is good practice to define those symbols used inside of expression with #.(define …). For example:

#.(define foo 9) ⇒ #<unspecified> '(#.foo #.(+ foo foo)) ⇒ (9 18)

Load syntax: #' form

is equivalent to form (for compatibility with common-lisp).

4.8.2 Load Syntax

#! is the unix mechanism for executing scripts. See Unix Scheme Scripts for the full description of how this comment supports scripting.

Load syntax: #?line

Load syntax: #?column

Return integers for the current line and column being read during a load.

Load syntax: #?file

Returns the string naming the file currently being loaded. This path is the string passed to load, possibly with ‘.scm’ appended.

4.8.3 Documentation and Comments

procedure: procedure-documentation proc

Returns the documentation string of proc if it exists, or #f if not.

If the body of a lambda (or the definition of a procedure) has more than one expression, and the first expression (preceeding any internal definitions) is a string, then that string is the documentation string of that procedure.

(procedure-documentation (lambda (x) "Identity" x)) ⇒ "Identity" (define (square x) "Return the square of X." (* x x)) ⇒ #<unspecified> (procedure-documentation square) ⇒ "Return the square of X."

Function: comment string1 …

Appends string1 … to the strings given as arguments to previous calls comment.

Function: comment

Returns the (appended) strings given as arguments to previous calls comment and empties the current string collection.

Load syntax: #;text-till-end-of-line

Behaves as (comment "text-till-end-of-line").

4.8.4 Modifying Read Syntax

Callback procedure: read:sharp c port

If a <#> followed by a character (for a non-standard syntax) is encountered by read, read will call the value of the symbol read:sharp with arguments the character and the port being read from. The value returned by this function will be the value of read for this expression unless the function returns #<unspecified> in which case the expression will be treated as whitespace. #<unspecified> is the value returned by the expression (if #f #f).

Callback procedure: load:sharp c port

Dispatches like read:sharp, but only during loads. The read-syntaxes handled by load:sharp are a superset of those handled by read:sharp. load:sharp calls read:sharp if none of its syntaxes match c.

Callback procedure: char:sharp token

If the sequence <#\> followed by a non-standard character name is encountered by read, read will call the value of the symbol char:sharp with the token (a string of length at least two) as argument. If the value returned is a character, then that will be the value of read for this expression, otherwise an error will be signaled.

Note When adding new <#> syntaxes, have your code save the previous value of load:sharp, read:sharp, or char:sharp when defining it. Call this saved value if an invocation’s syntax is not recognized. This will allow #+, #-, and Uniform Arrays to still be supported (as they dispatch from read:sharp).

4.