This file documents the use of MIT Scheme.

Copyright © 1991-95 Massachusetts Institute of Technology

This material was developed by the Scheme project at the Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science.  Permission to copy this document, to redistribute it, and to use it for any purpose is granted, subject to the following restrictions and understandings.

 1. Any copy made of this document must include this copyright notice in full.

 2. Users of this document agree to make their best efforts (a) to return to the MIT Scheme project any improvements or extensions that they make, so that these may be included in future releases; and (b) to inform MIT of noteworthy uses of this document.

 3. All materials developed as a consequence of the use of this document shall duly acknowledge such use, in accordance with the usual standards of acknowledging credit in academic research.

 4. MIT has made no warrantee or representation that the contents of this document will be error-free, and MIT is under no obligation to provide any services, by way of maintenance, update, or otherwise.

 5. In conjunction with products arising from the use of this material, there shall be no use of the name of the Massachusetts Institute of Technology nor of any adaptation thereof in any advertising, promotional, or sales literature without prior written consent from MIT in each case.

Scheme is the UnCommon Lisp.  This Info file is the user's guide for the MIT implementation of Scheme.  It describes how to install and run MIT Scheme, how to execute and compile Scheme programs, and how to use Scheme with Edwin and GNU Emacs.

Installation
************

Unix
====

We will use as an example the installation for HP 9000 series 400 computers.  The installation for other Unix systems is similar.  If you are installing for an HP 9000 series 700 or 800, see also HP-PA Installation.

MIT Scheme is distributed as a compressed `tar' file.  The tar file contains a single directory, called dist-7.3, and that directory contains subdirectories bin, etc, and lib.  The bin subdirectory contains two executable files, scheme and bchscheme.  The etc subdirectory contains several files that are used during the installation process.  The lib subdirectory contains several files and subdirectories that Scheme uses while it is executing.

The goal of the installation is to put the executable files in a directory where they will be executed as commands, and to put the library files in some convenient place where Scheme can find them.

  * First unpack the distribution:

          gzip -cd hp400.tar.gz | tar xvf -

     If you do not have the `gzip' program: we have built executables of this program for each of the machines that we support.  Either use the executable that we have already built, or else get the gzip sources (from us or from numerous other sources) and build it yourself.

  * Next, go to the directory that you just unpacked:

          cd dist-7.3

     The easiest way to install the files is to use the "install.sh" shell script that we've provided in the distribution.  You will need to edit that script to tell it where and how you want Scheme installed.  If you want to do something more complicated, or if the script can't easily be made to work for your system, use the script as a guide to installation.

  * The script assumes there is no pre-existing installation of MIT Scheme on your system.  If there is, you may need to edit the script, or delete the existing version (particularly the contents of /usr/local/lib/mit-scheme, see the later note on editing libdir).  If you have trouble deleting the binary because it is in use, try renaming it to scheme.busy and deleting it later.

  * The installation script normally uses the command cp -pr to copy the files from the distribution directory to their final locations.  You can change this by editing the definition of the install variable in the install.sh shell script.  Some versions of cp do not support the p option; using simply cp -r in this case is acceptable.  Note that most versions of mv require the source and destination to be on the same physical device.

  * The installation script may try to make some symbolic links during the installation.  This is normally done using the command ln -s. If your system doesn't support symbolic links, or if you prefer to copy or hard-link files, change the definition of the link variable in the shell script.

  * For many systems, a standard place to put executable files is the directory /usr/local/bin.  If this is true for your system, we suggest installing the executables there.  Otherwise, you probably have some other directory that serves the same purpose; edit install.sh and change the definition of the shell variable bindir to be this other directory.

  * Normally, MIT Scheme's auxiliary files are stored in the directory /usr/local/lib/mit-scheme/.  If you can, we suggest that you create such a directory and install these files there.  Otherwise, create a directory somewhere else and edit the installation script to change the value of the variable libdir to be this directory.

  * If you have GNU Emacs installed on your system, uncomment the definitions of the variables infodir, emacslisp, and movemail in the installation script.  You will also need to edit these definitions so that they correctly indicate where the GNU Emacs files are located.

  * Once you are satisfied that the install.sh script is correct, execute it:

          ./install.sh

  * After install.sh finishes, you can delete the dist-7.3 directory that you unpacked earlier.

     You should now be able to run MIT Scheme by typing

          scheme

     at the command line (if you use the C shell, you may have to type rehash before scheme will be recognized).

HP-PA Installation
------------------

If you are using an HP 9000 series 700/800 computer (often called an HP Precision Architecture machine, or HP-PA for short), read this section.

Scheme has built-in code that flushes the instruction and data caches of your machine in certain circumstances.  This code is sensitive to your computer's model, because each model has different cache hardware.

This distribution contains a database, called hppacach.mod, that describes the cache structure for each model of computer.  As of this writing, that database contains entries for the following models:

     705, 710, 715, 720, 730, 735, 750, 755, 834, 835, 850, 867

If you have a model that is not in the database, Scheme will not run -- instead it will print an error message informing you of this fact, and stop.  If this happens, you must add an entry to the database. This must be done once, at installation, for each new model.

Here is the procedure for updating the database:

  * Run the program hppacache that is included in the distribution. You must give this program the filename of the database file as an argument.  Normally the database file name is /usr/local/lib/mit-scheme/hppacach.mod, but if you install Scheme in a non-standard place it will be different.  For example, if you install Scheme as /usr/foo/mit-scheme/, the database file is called /usr/foo/mit-scheme/hppacach.mod.

  * Assuming that Scheme is installed in the normal place, update the database by becoming the super-user and executing the following commands on your machine:

          cd /usr/local/lib/mit-scheme
          ./hppacache -update hppacach.mod

     The reason that you must be super-user is that hppacache needs to read the device /dev/kmem to get the information that it needs. Normally, /dev/kmem is readable only by the super-user, or by users in group sys.  Thus, becoming super-user is the easiest way to read this information.  An alternative method for doing this is to change the permissions of the hppacache program so that it is in group sys, and to turn on its ``set group ID on exec'' permission bit, but since this also requires you to be super-user, you might as well just execute the program as the super-user.

     Please note that you must execute this program on the computer whose model you wish to add to the database.  Also, if you wish to add several models to the database, you must execute the program once on each model. If you have several computers that are all of the same model, you need only update the database once from one of the computers; thereafter all of the other computers of that model will work.

  * If you add a new model to the database, we'd appreciate it if you would send us the model information, so that we can update the copy of the database that we distribute to others.  This can be done using the hppacache program, as follows:

          cd /usr/local/lib/mit-scheme
          ./hppacache -print hppacach.mod > model-7xx

     (If you have previously updated the database, you do not need to be super-user to execute this command.)  The resulting file, model-7xx in this example, is the model information for the machine that you executed the command on; it is C code that we can use to update our copy of the database.  Send the model information for each new model to us by electronic mail at bug-cscheme@zurich.ai.mit.edu.  Thanks!

PC Operating Systems
====================

This section describes how to install MIT Scheme on DOS, Windows 3.1, and Windows NT.  We would prefer that the Windows version be used, rather than the DOS version, because we intend to maintain the DOS version only so long as it is convenient.  For the most part the installation on any of these platforms uses the same files, and the procedure is similar.  It is possible to install MIT Scheme so that it will run under all three operating systems on one computer, but this does require some care with the configuration of the system.

Note that we have only tested the DOS version on Microsoft DOS 5.0.

System Requirements
-------------------

MIT Scheme requires at least a 386SX with 8Mb RAM. The bare minimum disk space required is 5.5Mb, which gives you a command line interface for Scheme.  We strongly recommend a more advanced environment.  To build the Edwin editor band requires an additional 4.3Mb.  The whole installation without source code occupies 36Mb of disk.

Manifest
--------

The installation is split into several files according to functional units and the size constraints of a 1.4M high density 3.5" floppy disk. The following files are distributed:

     BIN.ZIP         Scheme binaries for Windows 3.1/Windows NT
     DOSBIN.ZIP      Scheme binaries for DOS
     LIB.ZIP         Smaller files from Scheme library
     RUNTIME.ZIP     runtime.com band
     RUNNOFLO.ZIP    runtime.com band for DOS machines with no FP hardware
     EDDEL.ZIP       eddel.com: a kit to build edwin.com band
     COMPDEL.ZIP     compdel.com: a kit to build compiler.com band

     HELP.ZIP        WinHelp User and Reference Manuals
     BCIRUN1.ZIP     Debugging information for runtime system
     BCIRUN2.ZIP         "          "       "     "      "
     BCIRUN3.ZIP         "          "       "     "      "
     BCIED1.ZIP      Debugging information for Edwin
     BCIED2.ZIP          "          "       "    "
     BCIED3.ZIP          "          "       "    "
     BCINOFLO.ZIP    Extra debugging information for machines with no FP hardware

     SRCRUN.ZIP      Source code for the runtime system
     SRCUC.ZIP       Source code for the microcode (C)
     SRCED.ZIP       Source code for Edwin
     SRCCOMP.ZIP     Source code for i386 compiler

     WIN32S.ZIP      Win32s installation floppy from Microsoft
     INSTALL.TXT     These instructions
     UNZIP.EXE       Program to unpack the .zip files

Minimal installation on Windows NT requires: BIN.ZIP, LIB.ZIP and RUNTIME.ZIP.

For the Edwin editor and the native code compiler add EDDEL.ZIP and COMPDEL.ZIP repectively.

Any configuration for Windows 3.1 is the same as for Windows NT except that Win32S also needs to be installed.

For DOS the minimal installation comprises DOSBIN.ZIP, LIB.ZIP and one of RUNTIME.ZIP or RUNNOFLO.ZIP.

PC Installation
---------------

These installation instructions describe how to install MIT Scheme on one or more of DOS, Windows 3.1, and Windows NT.  If you are installing for DOS and another operating system, you should do the bulk of the installation using the windowing environment.

In each of the following steps the amount of disk space consumed is indicated in square brackets.  These sizes do not include the .zip files which are required only during installation.

 1. MIT Scheme under Windows 3.1 requires the Win32s system (version 1.1) to be installed.  If you have not previously installed the Win32s system then you should create a floppy disk containing the contents of the win32s.zip file, e.g:

          a:
          unzip WHEREVER\win32s.zip

     Run the setup command on the floppy disk.

     If you are not sure whether you have Win32s installed, or what version you have installed, try to install it anyway.  If you have version 1.0 then the Win32s installation disk will upgrade your Win32s system to version 1.1.

          a:setup

 2. Decide on where to install MIT Scheme. We suggest the default: C:\SCHEME. Create the root directory which we from now on refer to as SCHEME. If for example, you choose the default:

          mkdir c:\scheme
          cd \scheme

     SCHEME is the string `C:\scheme'.

 3. In the SCHEME directory unzip the following essential files: bin.zip, lib.zip and runtime.zip [5.5Mb].

          unzip WHEREVER\bin.zip
          unzip WHEREVER\lib.zip
          unzip WHEREVER\runtime.zip

     This will create the directory structures SCHEME\bin, SCHEME\lib and SCHEME\etc, and unpack the essential files. (WHEREVER stands for the place that you have put the .zip files, which might be another directory or a floppy disk.)

     If you have a computer without floating-point hardware (e.g. a 386 machine or a 486SX) and you wish to run the DOS version then you must install the runtime with special floating point support instead of runtime.zip:

          unzip WHEREVER\runnoflo.zip

 4. To install the DOS binaries, unzip the dosbin.zip file.

          unzip WHEREVER\dosbin.zip

     This creates a SCHEME\dos-bin directory containing the DOS versions of the .exe files.  These files are different from the Windows versions, so they are placed in a different directory to allow both versions to co-exist on your computer.  It is only the .exe files that differ between DOS and Windows.  The other parts of the MIT Scheme system are shared.  The DOS version will run under Windows 3.1 but not under NT.  Either running on DOS or Windows 3.1, the DOS version has no graphics support

 5. If you are installing for Windows 3.1 only, do one of the following:

       * Put the following line in the autoexec.bat file to ensure that the SCHEME\bin\win31 directory is on the path:

               path %PATH%;SCHEME\bin\win31

          This command must be in a .bat file to work

       * Copy the files from SCHEME\bin\win31 into SCHEME\bin.

 6. If you are installing for Windows NT only, do one of the following:

       * As system administrator add SCHEME\bin\nt to the PATH in the system environment (using the Registry Editor, regedt32

       * As an individual place PATH=SCHEME\bin\nt in your environment (use the control panel's system option

       * Copy the files from SCHEME\bin\nt into SCHEME\bin.

 7. If you are installing for both Windows 3.1 and Windows NT then you must use use environment variables and the PATH rather than copying files, i.e. you must arrange for Windows 3.1 to be run with SCHEME\bin\win31 on the path and for Windows NT to be run with SCHEME\bin\nt on the path. This can be done by putting

          path %PATH%;SCHEME\bin\win31

     in the autoexec.bat file and adding SCHEME\bin\nt to the Windows NT system environment path.

 8. If you are installing for DOS there is no need to add things to the PATH.

 9. If you did not choose the default installation directory, make sure that the environment variable MITSCHEME_LIBRARY_PATH is defined:

          set MITSCHEME_LIBRARY_PATH=SCHEME\lib

 10. Now test the installation so far. Under either Windows system, you should be able to get a Scheme system running in its terminal window by running the following from the Program Manager or the File Manager

          SCHEME\bin\scheme

     From DOS you should be able to get Scheme running by typing the following at the DOS prompt:

          SCHEME\dos-bin\scheme

     If there are any problems at this stage, review the installation so far. Remember that you might have to restart your machine to get the effect of any changes that you made in autoexec.bat or the NT registry.

 11. Windows versions only. Now you should create a Program Manager group for MIT Scheme. This can be done by running a Scheme program from the Program Manager using the File|Run option:

          SCHEME\bin\scheme -load SCHEME\etc\pmgrp

     This program creates a program group called `MIT Scheme' which contains

       * Icons for running scheme in several configurations

       * Windows Help versions of the Scheme Reference Manual and the MIT Scheme User's Manual.

       * `Mincer' icons for building the rest of the system

     At this point only the Scheme icon called `Runtime' will execute. Test it.  The other Scheme icons (`Edwin', `Compiler' and `MIT Scheme' shield) should report that they cannot find their bands.

 12. Windows versions only. To install the help files, unpack help.zip:

          unzip WHEREVER\help.zip

     This installs two windows help files, user.hlp and scheme.hlp, in SCHEME\bin.

 13. To install the Edwin editor you need to build the edwin.com band. First unpack the delta file eddel.com [1.6Mb]:

          cd SCHEME
          unzip WHEREVER\eddel.zip

     To build edwin.com start the `build EDWIN.COM band' icon, or run the following command:

          SCHEME\bin\scheme -large -load SCHEME\etc\build -eval (edwin.com)

     This will load in eddel.com and create the new band [4.3Mb]. After a successful build the program will exit. The edwin.com band can be used by both the DOS and Windows versions, so you only need to do this step once, even if you are installing for more than one of DOS, Windows 3.1, and Windows NT.

     If you are installing only for DOS you will have to build edwin.com from the command line.  Be sure to run the DOS scheme.exe rather than the Windows version:

          SCHEME\dos-bin\scheme -large -load SCHEME\etc\build -eval (edwin.com)

 14. To install the compiler you need to build the compiler.com band. First unpack the compdel.com delta file [2.5Mb]:

          cd SCHEME
          unzip WHEREVER\compdel.zip

     To build compiler.com start the `build COMPILER.COM band' icon, or run the following command:

          SCHEME\bin\scheme -large -load SCHEME\etc\build -eval (compiler.com)

     This will load in compdel.com and create the new compiler.com band [4.8Mb].  After a successful build the program will exit. As for Edwin, this step needs to be done only once.

     If you choose to build this band using the DOS version be sure to run the DOS scheme.exe rather than the Windows version:

          SCHEME\dos-bin\scheme -large -load SCHEME\etc\build -eval (compiler.com)

 15. If you want both Edwin and the compiler in one band you should build the all.com band.  First unpack the delta files [4.1Mb unless already unpacked in previous steps]:

          cd SCHEME
          unzip WHEREVER\eddel.zip
          unzip WHEREVER\compdel.zip

     To build all.com start the `build ALL.COM band' icon, or run the following command:

          SCHEME\bin\scheme -large -load SCHEME\etc\build -eval (all.com)

     This will load in both eddel.com and compdel.com into the runtime band and create the new band [6.7Mb].  After a successful build the program will exit.

     If you choose to build this band under DOS, be sure to run the DOS scheme.exe rather than the Windows version:

          SCHEME\dos-bin\scheme -large -load SCHEME\etc\build -eval (all.com)

 16. Any combination of edwin.com, compiler.com and all.com may be used.  They may be built in any order: it is not necessary to build either of edwin.com and compiler.com before building and using all.com. The bands are shared by all of the supported operating systems so you only have to build the bands once, even if you want to use them from, say, both DOS and Windows 3.1.

     After building the bands you may tidy the MIT Scheme group by removing the mincer icons and recover disk space by deleting the delta files [4.1Mb] and the runtime.com band [2.3Mb] if you do not need it.

     To create icons that use bchscheme instead of scheme copy the icons and edit the command lines to change `scheme' to `bchschem' (that is right: no tailing `e').

 17. Debugging information can be installed by uncompressing the bci*.zip files in the Scheme root directory SCHEME.  The total space required for all of the debugging information is 7.5Mb. The most useful is the runtime debugging info which is in bcirun1.zip through bcirun3.zip [3.6Mb installed].

     If you have installed the DOS version for machines without hardware floating point support then bcinoflo.zip should be uncompressed also.  This must be done after the bcirun*.zip files.

     Debugging information files can be installed in the Scheme root directory or in another directory.  If another directory is chosen then set the MITSCHEME_INF_DIRECTORY environment variable to this directory.  bcied1.zip through bcied3.zip [3.8Mb installed] hold the debugging information files for Edwin.

Release Notes
*************

This chapter describes interesting features and problems for the 7.3 release.

News
====

The last full release of the MIT Scheme system was version 7.1.3 in 1991.  Since that time, there was a partial release of version 7.2 for i386/i486 machines and some MIPS R3000 machines.  This section summarizes the changes that have occurred since version 7.1.3.  The changes are divided into two parts: those that were incorporated in the 7.2 partial release, and those that were subsequently incorporated.

Changes from Release 7.2 to 7.3
-------------------------------

This is an abbreviated list of the changes that have been incorporated in the 7.3 release since the (partial) 7.2 release.

  * Support for MS Windows (3.1 and NT).

  * Native compiled-code support for DEC Alpha.

  * Compiler can now generate C code.  This is used to support the Sun SPARC and IBM RS6000 architectures.

  * Support for dynamic loading of compiled C code on several operating systems, including HP-UX, OSF/1, SunOS 4, and AIX.

  * New data structures: red/black trees and weight-balanced trees.

  * Redesign of hash tables makes them much faster and somewhat more flexible.

  * Completely new random-number generator.  The interface to the random-number generator has been changed to be like that of Common Lisp.

  * Record abstraction now uses a special record type to represent records (previously it used vectors).  define-structure now creates records by default; these records are identical to those created with the record abstraction.

  * call-with-current-continuation is now properly tail-recursive.

  * New command-line switch -no-suspend-file prevents the generation of scheme_suspend files.

  * Uncompression of compiled-code debugging information is now memoized to save time when doing many accesses to the same information (a common occurrence).

  * Constant space now grows automatically when needed.

  * New procedures:

          call-with-values                ;replaces with-values
          current-load-pathname
          error-output-port
          fold-left
          fold-right
          interaction-i/o-port
          notification-output-port
          prompt-for-command-char
          prompt-for-command-expression
          prompt-for-confirmation
          prompt-for-evaluated-expression
          prompt-for-expression
          standard-unparser-method        ;replaces unparser/standard-method
          stream-first
          stream-rest
          symbol<?
          trace-output-port

  * string-pad-left now truncates from the left instead of the right.

  * Argument to make-graphics-device is now a symbol.

  * Each REPL now has its own working directory pathname and pathname defaults.  set-working-directory-pathname! changes only the value for the current REPL.

  * The definitions of the following procedures have been changed to implement more useful behavior:

          string-capitalized?
          string-lower-case?
          string-upper-case?
          substring-capitalized?
          substring-lower-case?
          substring-upper-case?

  * New Edwin features:

          M-x dabbrev-expand
          M-x describe-syntax
          M-x rcs-ci-locked-files
          M-x rcs-diff
          M-x rcs-list-locked-files
          M-x rcs-log
          M-x recover-file
          M-x show-parameter-list
          M-x sort-lines and other sorting commands
          auto-mode-alist variable (as in Emacs)
          Encryption/decryption of files in Dired
          Undo upgraded to match GNU Emacs 18.57
          Buffers grow/shrink with constant amortized time
          Emacs 19 tags support (multiple tags files, inclusion)

  * Edwin compression/uncompression commands changed to use gzip instead of compress.

  * Edwin now has text-properties mechanism that permits implementation of highlighted and mouse-sensitive regions.

Changes from Release 7.1 to 7.2
-------------------------------

This is an abbreviated list of the changes that were incorporated in the (partial) 7.2 release since the 7.1 release.

  * Support for MS DOS.

  * Native compiled-code support for the Intel i386 and i486 architectures.

  * Fixes to MIPS compiled-code support MIPS R3000 with large cache-line sizes.

  * Complete redesign of the error system.

  * X11 graphics interface redesigned.  The new design is smarter about interaction with the window manager (for example, it implements the WM_DELETE_WINDOW protocol) and has some limited support for event handling.  Windows are now closed when they are reclaimed by the garbage collector.

  * Redesign of I/O ports.  New design supports ports that perform both input and output.

  * Ports now have better-defined control over blocking mode and terminal mode.  Flushing of output buffers is not done automatically, except for the console port.

  * New procedures:

          *default-pathname-defaults*
          ->namestring
          ->truename
          close-port
          directory-namestring
          directory-pathname
          directory-pathname-as-file
          enough-namestring
          enough-pathname
          file-access
          file-attributes-direct          ;same as file-attributes
          file-modification-time-direct
          file-modification-time-indirect ;same as file-modification-time
          file-namestring
          file-pathname
          file-readable?
          host-namestring
          i/o-port?
          make-generic-i/o-port
          make-i/o-port
          open-i/o-file
          pathname-simplify
          pathname-wild?
          pathname=?
          port/copy
          port/input-channel
          port/operation
          port/operation-names
          port/output-channel
          port/state
          port?
          record-copy
          record-modifier                 ;replaces record-updater
          set-port/state!

  * with-input-from-file and with-output-to-file no longer close the port when they are exited abnormally.

  * Redefined #!optional and #!rest to be something distinct from symbols.

  * Strings now accept "\NNN" octal character escapes, much like those in C.

  * Redesign of pathname abstraction.  New design is very similar to the Common Lisp pathname abstraction.

       * ->PATHNAME no longer accepts a symbol as an argument.  Only strings and pathnames are valid.

       * Procedures eliminated:

               canonicalize-input-filename
               canonicalize-input-pathname
               canonicalize-output-filename
               canonicalize-output-pathname
               canonicalize-overwrite-filename
               canonicalize-overwrite-pathname
               home-directory-pathname
               init-file-truename
               pathname->absolute-pathname
               pathname->input-truename
               pathname->output-truename
               pathname->overwrite-truename
               pathname->string
               pathname-components
               pathname-default-host
               pathname-directory-path
               pathname-directory-string
               pathname-name-path
               pathname-name-string
               pathname-new-host
               pathname-relative?
               string->pathname
               symbol->pathname

       * pathname-default no longer accepts a HOST argument.

       * Representation of pathname directories changed to match Common Lisp. Directory is now either #f or a list of symbols and strings with first element either absolute or relative.

       * Unix pathnames now set DEVICE and VERSION to unspecific.  unspecific now means that the field is not used by the operating system.

       * Parsing rules for unix filenames changed: the file type is the part of the name after the last dot, if any.  If the dot occurs at the beginning or end of the filename, then it doesn't count -- in that case there is no type.  Thus, names like . and .. have no type.

  * New Edwin features:

          M-x manual-entry
          M-x outline-mode
          M-x shell-resync-dirs
          M-x shell-command
          M-x shell-command-on-region
          M-x telnet
          RMAIL summary
          RMAIL sort
          REPL mode and asynchronous evaluation
          Printing commands
          ``run light'' for evaluation commands
          Reading and writing of compressed files (".Z" suffix)
          Reading and writing of encrypted files (".KY" suffix)
          Compress/uncompress commands for Dired
          Support for X protocols: WM_DELETE_WINDOW and WM_TAKE_FOCUS
          Time, load, and mail notification in mode line

C Back-End Release Notes
========================

This release introduces an important new feature: the compiler can generate output in C, which can then be compiled and linked using the usual C development tools.  This feature is being used to support architectures, such as the SPARC and IBM RS6000, for which the compiler does not have native-code support.

MIT Scheme systems that have been built using this compiler feature are referred to as C back-end systems.  This section provides a brief description of the features and known problems for such systems.

General Notes
-------------

This section describes the general design of C back-end systems.

  * The C back end was written very recently, and we do not use it ourselves for development--we have native back ends for the hardware that we use regularly.  Furthermore, most of the system-building tools and changes were put together and tested immediately preceding the release.  It is not unlikely that there are serious problems and shortcomings.  We'd like to hear about them so we can fix them or bypass them for a future release.  If something does not work, do not assume that we know about it. Please report it.  The worse that can happen is that we hear about it several times.

  * The compiler produces C code that works in the context of the MIT Scheme runtime library.  It is not stand-alone code, and there is currently no way to generate stand-alone executables.

  * We have done some performance testing on machines where we have both a native back end and the C back end.  The C back end code is somewhat larger and slower than the native code.  The speed varies depending primarily on the number of inter-C-procedure calls (even within a single source file).  In some cases the C code is somewhat faster than the native code, but it is usually noticeably slower.  The average is about 2.5 times slower.

  * The output code should work on virtually any modern general-purpose architecture with an ANSI-compliant (perhaps even a traditional K&R) C compiler.  We've tried it on several 32-bit architectures (HP PA, Sparc, Motorola 68040, RS6000/Power) and a 64-bit architecture (Alpha).

     However, there may very well be assumptions of the following form:

       * Memory is byte-addressable.

       * Addresses and ordinary data live in the same memory.  Some special-purpose processors do not satisfy this constraint.

       * unsigned data items and addresses fit into the same memory, and if union-ed, take the same space.

       * Signed integers are represented using 2s-complement representation.

       * The machine/OS uses the ASCII character set.  This is not a property of the compiler per-se, but of the MIT Scheme system.

     In addition, some configuration of the compiler, microcode and build-scripts would have to be done for new architectures/operating systems.  The things that come to mind are:

       * Dynamic loading primitives are OS-dependent.

       * Basic microcode configuration, including:

            * OS interface.

            * Choosing a new machine type (FASL numbers in config. h).

       * Build scripts dispatch on the OS and architecture type, and only understand a handful of types.

       * The compiler has a table of C compiler and linker switches for the configurations we have tried.  It needs to be extended for new configurations, although this can be done easily.

     If you are interested in bringing up the system on a new configuration, please contact us for further instructions, and to obtain the set of crude tools that we use to do this.

  * The performance of the output code could be greatly improved by judicious use of assembly language to achieve faster cross-module tail recursion.  So far, we've given more weight to portability than to performance.

  * The resulting C code can be compiled and statically linked with the MIT Scheme microcode (the runtime library is linked in this way), or compiled and dynamically loaded and linked if the operating system supports this feature.  The C code must be compiled defining the COMPILE_FOR_STATIC_LINKING if it is to be statically linked into the microcode or collected into a larger dynamically-loadable library (e.g. edwin or the compiler).  If you'd like to put together a shared library of a large subsystem that you have written, please contact us for advice on how it can be done.

Problems with the C Back-End
----------------------------

This section details problems and shortcomings of the system that are specific to the C back-end.

  * compile-procedure and compile-scode do not work.  This will be fixed for a future release and is not very hard to bypass in most cases.  compile-procedure gives an error. compile-scode returns something which is not a compiled expression.  cf and compile-bin-file work.

  * Dynamically loaded compiled modules are not unloaded when no longer accessed.  In particular, after a disk-restore, previously loaded modules are still mapped.  In addition, at least in some versions of unix, the shared object file cannot be deleted while it is loaded, so recompilation of a module in place will fail if some process has it already loaded.

  * Multiple loading does not work.  This is bad for development.  After loading a compiled module once, it cannot be reloaded (presumably after recompilation).

  * fasdump does not work for compiled code.  It gives an error. Only bands can be dumped.

  * Several C compilers are unhappy about the size of the resulting C procedures.  They often turn their optimizers off.

  * Some C compilers/optimizers produce non-working code for some of the output.  In particular, we've had serious problems with the RS6000/AIX-3.2 C compiler.

  * Several C compilers give copious ``statement not reached'' warnings about the resulting code.  This arises from the heavy use of macros expanding into the pattern

          do
          {
            BODY
          } while (0)

     Most of these macros are not really necessary, and the code generator could just issue the BODY directly.

  * The mechanism for accreting object files into large dynamically-loadable libraries is currently ad-hoc.

PC Release Notes
================

Known Problems in this Beta Release
-----------------------------------

  * The Windows version of MIT Scheme is more robust under NT than Windows 3.1

  * [NT] Exclusive file access problems.  Edwin sometimes can't write a file that is still open for reading.

     Typically you will have entered (load "foo.scm") in the interaction buffer and got an error, say a mis-matched parenthesis. You then switch to the foo.scm buffer, fix the problem and then try to save the file. Edwin refuses, saying

          Unable to open file "YOUR-PATH\\foo.scm" because: Unknown

     Work-around: In the interaction buffer: Quit to top level.  Do a (gc-flip), which forces the file to be closed when the file port is garbage-collected.  Now you will be able to save the file.

  * MITSCHEME_INF_DIRECTORY sometimes does not work. The runtime system cannot find the debugging information unless it is on the C: drive. To test if the debugging information is available, try

          (pp pp)

     If the debugging information is available the pp procedure pretty-prints procedures as Scheme code. If the information cannot be found then it prints the procedure as an opaque object, similar to this:

          #[compiled-procedure 13 ("pp" #x2) #xF #x646BF7]

  * Edwin autoloads. A few files that are automatically part of Edwin under unix are not part of Edwin in the PC.  These can be loaded manually from the auto-load directory, for example, by putting

          (edwin-load "dabbrev")

     in your edwin.ini file.

  * DOS version 6.  Although we have not tested the DOS version under anythig other than MS-DOS 5.0, we have the following anecdotal evidence:

       * On DOS 6.2 with emm386 you may have to put the following in config.sys:

               device=emm386 noems novcpi

Running Scheme
**************

This chapter describes how to run MIT Scheme on a unix system or a PC running DOS, Windows 3.1, or Windows NT. It also describes how you can customize the behavior of MIT Scheme using command-line options and environment variables.

Basics of Starting Scheme
=========================

Usually, MIT Scheme is invoked by typing

     scheme

at your operating system's command interpreter. (Under Windows 3.1 you must use the Program Manager's RUN command, or an icon.)  Scheme will load itself, clear the screen, and print something like this:

     Scheme saved on Thursday December 2, 1993 at 6:18:35 PM
       Release 7.3.0 (beta)
       Microcode 11.146
       Runtime 14.166

This information, which can be printed again by evaluating

     (identify-world)

tells you the following version information.  ``Release'' is the release number for the entire Scheme system.  This number is changed each time a new version of Scheme is released.  An ``(alpha)'' or ``(beta)'' following the release number indicates that this is a alpha- or beta-test release.  ``Microcode'' is the version number for the part of the system that is written in C.  ``Runtime'' is the version number for the part of the system that is written in Scheme.

Following this there may be additional version numbers for specific subsystems.  SF refers to the scode optimization program sf, Liar is the native-code compiler, Edwin is the Emacs-like text editor, and Student is the S&ICP compatibility package.

If the compiler is supported for your machine, you can invoke it by giving Scheme the -compiler option:

     scheme -compiler

This option causes Scheme to use a larger constant space and heap, and to load the world image containing the compiler.

Customizing Scheme
==================

You can customize your setup by using a variety of tools:

  * Command-line options. Many parameters, like memory usage and the location of libraries, may be varied by command line options.

  * Command scripts or batch files. You might like to write scripts that invoke Scheme with your favourite command line options.  For example, under DOS you might not have enough memory to run edwin or the compiler with its default memory parameters (it will print something like ``Not enough memory for this configuration'' and halt when started), so you can write a shell script (unix) or a .bat file (PC) that will invoke Scheme with the appropriate -heap and other parameters.

  * Scheme supports init files: if the file .scheme.init exists in your home directory, it is loaded immediately after the identification banner, and before the input prompt is printed. In addition, when it starts up, edwin loads the file .edwin from your home directory into the edwin environment. You can use both of these files to define new procedures, commands, or change defaults in the system.

     The -no-init-file command line option causes Scheme to ignore the .scheme.init file (see Command-Line Options).

     On PC systems these initialization files are called scheme.ini and edwin.ini respectively and are searched for in the directory identified by the HOME environment variable.

  * Environment variables. Most microcode parameters, and some runtime system and Edwin parameters, can be specified by means of environment variables.  See Environment Variables.

  * Icons. Under Windows and Windows NT it is possible to create icons in the Program Manager that invoke Scheme with different parameters.

One of the important parameters that can be customized is how much memory Scheme uses and how that memory is used.  Scheme uses four kinds of memory:

  * A stack which is used for recursive procedure calls,

  * A heap which is used for dynamically allocated objects, like cons cells and strings. Storage used for objects in the heap which become unreferenced is eventually reclaimed by garbage collection.

  * A constant space, which is used for allocated objects, like the heap. Unlike the heap, storage used for objects in constant space is not reclaimed by garbage collection. Constant space is used for objects that are essentially permanent, like procedures from the runtime system.

  * An amount of extra storage which is used by the Microcode (the part of the system that is implemented in C).

All aspects except the last may be controlled both by command-line options and by environment variables.  MIT Scheme uses a two-space copying garbage collector for reclaiming storage in the heap.  There are two version of Scheme which handle garbage collection differently. Ordinary scheme has two heaps, one for each `space'. bchscheme has one heap and uses a disk file for the other `space', thus trading memory usage against garbage collection speed.

The total storage required by scheme is:

     STACK + (CONSTANT + 2*HEAP) + EXTRA

where STACK, CONSTANT and HEAP are parameters that may be selected when scheme starts. For bchscheme, which has only one heap in memory, the equation is

     STACK + (CONSTANT + HEAP) + EXTRA

Once the storage is allocated for the constant space and the heap, Scheme will dynamically adjust the proportion of the total that is used for constant space.  The stack and the extra microcode storage is not included in this adjustment.  Previous versions of MIT Scheme needed to be told the amount of constant space that was required when loading your own bands with the -band option.  Dynamic adjustment of the heap and constant space avoids this problem; now all that is required is that the total space is sufficient.

The Scheme procedure (print-gc-statistics) shows how much heap and constant space is available.

Command-Line Options
====================

Scheme accepts the command-line options detailed in the following sections.  The options may appear in any order, with the restriction that the microcode options must appear before the runtime options, and the runtime options must appear before any other arguments on the command line.  (At present, any arguments other than these options will generate a warning message when Scheme starts.  In the future, there will be an advertised mechanism by which the extra arguments can be handled by user code.)

These are the microcode options:

-band FILENAME
     Specifies the initial world image file (band) to be loaded. Searches for FILENAME in the working directory and the library directories, using the full pathname of the first readable file of that name.  If FILENAME is an absolute pathname (on unix, this means it starts with /), then no search occurs --- FILENAME is tested for readability and then used directly.  If this option isn't given, the filename is the value of the environment variable MITSCHEME_BAND, or if that isn't defined, runtime.com; in these cases the library directories are searched, but not the working directory.

-compiler
     This option specifies defaults appropriate for loading the compiler.  It specifies the use of large sizes, exactly like -large.  If the -band option is also specified, that is the only effect of this option.  Otherwise, the default band's filename is the value of the environment variable MITSCHEME_COMPILER_BAND, if defined, or compiler.com; the library directories are searched to locate this file.  Note that the -compiler option is available only on machines with compiled-code support.

-edwin
     This option specifies defaults appropriate for loading the editor.  It specifies the use of large sizes, exactly like -large.  If the -band option is also specified, that is the only effect of this option.  Otherwise, the default band's filename is the value of the environment variable MITSCHEME_EDWIN_BAND, if defined, or edwin.com; the library directories are searched to locate this file.  Note that the -edwin option is available only on machines with compiled-code support.

-large
     Specifies that large heap, constant, and stack sizes should be used. These are specified by the environment variables

          MITSCHEME_LARGE_HEAP
          MITSCHEME_LARGE_CONSTANT
          MITSCHEME_LARGE_STACK

     If this option isn't given, the small sizes are used, specified by the environment variables

          MITSCHEME_SMALL_HEAP
          MITSCHEME_SMALL_CONSTANT
          MITSCHEME_SMALL_STACK

     There are reasonable built-in defaults for all of these environment variables, should any of them be undefined.  Note that any or all of the defaults can be individually overridden by the -heap, -constant, and -stack options.

     Note: the Scheme procedure (print-gc-statistics) shows how much heap and constant space is available and in use.

-heap BLOCKS
     Specifies the size of the heap in 1024-word blocks.  Overrides any default.  Normally two such heaps are allocated; bchscheme allocates only one, and uses a disk file for the other.

-constant BLOCKS
     Specifies the size of constant space in 1024-word blocks.  Overrides any default.  Constant space holds the compiled code for the runtime system and other subsystems.

-stack BLOCKS
     Specifies the size of the stack in 1024-word blocks.  Overrides any default.  This is Scheme's stack, not the unix stack used by C programs.

-option-summary
     Causes Scheme to write an option summary to standard error.  This shows the values of all of the settable microcode option variables.

-emacs
     Specifies that Scheme is running as a subprocess of GNU Emacs.  This option is automatically supplied by GNU Emacs, and should not be given under other circumstances.

-interactive
     If this option isn't specified, and Scheme's standard I/O is not a terminal, Scheme will detach itself from its controlling terminal.  This will prevent it from getting signals sent to the process group of that terminal.  If this option is specified, Scheme will not detach itself from the controlling terminal.

     This detaching behavior is useful for running Scheme as a background job.  For example, using the C shell in unix, the following will run Scheme as a background job, redirecting its input and output to files, and preventing it from being killed by keyboard interrupts or by logging out:

          scheme < /usr/cph/foo.in >& /usr/cph/foo.out &

     This option only makes sense under unix.

-nocore
     Specifies that Scheme should not generate a core dump under any circumstances.  Under unix, if this option is not given, and Scheme terminates abnormally, you will be prompted to decide whether a core dump should be generated.

     This option is ignored on PC versions.

-library PATH
     Sets the library search path to PATH.  This is a list of directories that is searched to find various library files, such as bands.  If this option is not given, the value of the environment variable MITSCHEME_LIBRARY_PATH is used; if that isn't defined, the default is used.

     On unix, the elements of the list are separated by colons, and the default value is /usr/local/lib/mit-scheme.  On PCs, the elements of the list are separated by semicolons, and the default value is c:\scheme.

-utabmd FILENAME

-utab FILENAME
     Specifies that FILENAME contains the microcode tables (the microcode tables are information that informs the runtime system about the microcode's structure).  FILENAME is searched for in the working directory and the library directories.  If this option isn't given, the filename is the value of the environment variable MITSCHEME_UTABMD_FILE, or if that isn't defined, utabmd.bin; in these cases the library directories are searched, but not the working directory.

     -utab is an alternate name for the -utabmd option.  At most one of these options may be given.

-fasl FILENAME
     Specifies that a cold load should be performed, using FILENAME as the initial file to be loaded.  If this option isn't given, a normal load is performed instead.  This option may not be used together with the -band option.  This option is useful only for maintainance and development of the MIT Scheme runtime system.

The following options are runtime options.  They are processed after the microcode options and after the runtime, Edwin or some other band is loaded.

-no-init-file
     This option causes Scheme to ignore the ~/.scheme.init or scheme.ini file, normally loaded automatically when Scheme starts (if it exists).

-no-suspend-file
     Under some circumstances Scheme will write out a file called scheme_suspend in the user's home directory. (On unix, this file is written when Scheme is terminated by the SIGUSR1, SIGHUP, or SIGPWR signals.  On the PC, this file is never written.) This file is a ``band'' containing the complete state of the Scheme process; restoring this file continues the computation that Scheme was performing at the time the file was written.

     If the -no-suspend-file option is given, Scheme will not write a scheme_suspend file under any circumstances.

-eval
     This option causes Scheme to evaluate the expressions following it on the command line, up to (but not including) the next option that starts with a hyphen.  The expressions are evaluated in the user-initial-environment. Unless explicitly handled, errors during evaluation are silently ignored.

-load
     This option causes Scheme to load the files (or lists of files) following it on the command line, up to (but not including) the next option that starts with a hyphen.  The files are loaded in the user-initial-environment using the default syntax table. Unless explicitly handled, errors during loading are silently ignored.

In addition to the above, bchscheme recognises the following command line options, all of which specify parameters affecting how bchscheme uses disk storage to do garbage collection:

-gc-directory DIRECTORY
     Specifies that DIRECTORY should be used to create files for garbage collection.  If the option is not given, the value of environment variable MITSCHEME_GC_DIRECTORY is used instead, and if that is not defined, /tmp is used.

-gc-end-position NUMBER
     It specifies the last byte position in -gc-file at which this invocation of scheme can write.  If the option is not given, the value of environment variable MITSCHEME_GC_END_POSITION is used instead, and if that is not defined, it is computed from the start position (as provided with -gc-start-position) and the heap size.  The area of the file used (and locked if possible) is the region between -gc-start-position and -gc-end-position.

-gc-file FILENAME
-gcfile FILENAME
     Specifies that FILENAME should be used for garbage collection.  If the option is not given, the value of environment variable MITSCHEME_GC_FILE is used, and if this is not defined, a unique filename is generated in the directory specified with -gc-directory.

     -gcfile is an alias for -gc-file.  At most one of these options should be specified.

-gc-keep
     Specifies that the gc file used for garbage collection should not be deleted when scheme terminates. The gc file is deleted only if the file was created by this invocation of scheme, and this option is not set.

-gc-start-position NUMBER
     It specifies the first byte position in -gc-file at which this invocation of scheme can write.  If the option is not given, the value of environment variable MITSCHEME_GC_START_POSITION is used instead, and if that is not defined, 0 is used, meaning the beginning of the file.  The area of the file used (and locked if possible) is the region between -gc-start-position and -gc-end-position.

-gc-window-size BLOCKS
     Specifies the size of the windows into new space during garbage collection.  If this option is not given, the value of environment variable MITSCHEME_GC_WINDOW_SIZE is used instead, and if that is not defined, the value 16 is used.

The following command line options are only used by an experimental version of bchscheme that uses Unix System V-style shared memory, and only then if the gcdrone program is installed in the lib directory.

-gc-drone PROGRAM
     Specifies that PROGRAM should be used as the drone program for overlapped I/O during garbage collection.  This option is recognized only by bchscheme.  If the option is not given, the value of environment variable MITSCHEME_GC_DRONE is used instead, and if that is not defined, gcdrone is used.

-gc-read-overlap N
     Specifies that scheme should delegate at most N simultaneous disk read operations during garbage collection.  This option is recognized only by bchscheme.  If the option is not given, the value of environment variable MITSCHEME_GC_READ_OVERLAP is used instead, and if that is not defined, 0 is used, disabling overlapped reads.

-gc-write-overlap N
     Specifies that scheme should delegate at most N simultaneous disk write operations during garbage collection.  This option is recognized only by bchscheme.  If the option is not given, the value of environment variable MITSCHEME_GC_WRITE_OVERLAP is used instead, and if that is not defined, 0 is used, disabling overlapped writes.

Environment Variables
=====================

There are many environment variables that Scheme (and Edwin, etc.) look for.  Environment variables that affect the microcode must be defined before you start Scheme, but others can be defined or overwritten within Scheme by using the set-environment-variable! procedure, e.g.

     (set-environment-variable! "EDWIN_FOREGROUND" "32")

The rest of this section is a summary of the environment variables that are specific to MIT Scheme.  The environment variables are organised according to the parts of MIT Scheme that they affect.

Environment Variables Affecting the Microcode
---------------------------------------------

These environment variables are referred to by the microcode (the executable C programs called scheme and bchscheme).

MITSCHEME_BAND (default: runtime.com on the library path)
     The initial band to be loaded.  Overridden by -band, -compiler, or -edwin.

MITSCHEME_COMPILER_BAND (default: compiler.com on the library path)
     The initial band to be loaded if the -compiler option is given. Overridden by -band.

MITSCHEME_EDWIN_BAND (default: edwin.com on the library path)
     The initial band to be loaded if the -edwin option is given. Overridden by -band.

MITSCHEME_LARGE_CONSTANT (default: 1000)
     The size of constant space, in 1024-word blocks, if the -large, -compiler, or -edwin options are given.  Overridden by -constant.  Note: default is somewhat larger on RISC machines.

MITSCHEME_LARGE_HEAP (default: 1000)
     The size of the heap, in 1024-word blocks, if the -large, -compiler, or -edwin options are given.  Overridden by -heap.

MITSCHEME_LARGE_STACK (default: 100)
     The size of the stack, in 1024-word blocks, if the -large, -compiler, or -edwin options are given.  Overridden by -stack.

MITSCHEME_LIBRARY_PATH
     A list of directories.  These directories are searched, left to right, to find bands and various other files. On Unix systems the list is colon separated, with the default /usr/local/lib/mit-scheme. On PC systems the list is semi-colon separated with the default c:\scheme\lib.

MITSCHEME_INF_DIRECTORY
     Directory containing the debugging information files for the system. It should contain subdirectories corresponding to the subdirectories in the source tree.  For example, if its value is f:\random, then runtime system debugging files will be expected in f:\random\runtime, while edwin debugging files will be expected in f:\random\edwin.

MITSCHEME_SMALL_CONSTANT (default: 400)
     The size of constant space, in 1024-word blocks, if the size options are not given.  Overridden by -constant, -large, -compiler, or -edwin.  Note: default is somewhat larger on RISC machines.

MITSCHEME_SMALL_HEAP (default: 250)
     The size of the heap, in 1024-word blocks, if the size options are not given.  Overridden by -heap, -large, -compiler, or -edwin.

MITSCHEME_SMALL_STACK (default: 100)
     The size of the stack, in 1024-word blocks, if the size options are not given.  Overridden by -stack, -large, -compiler, or -edwin.

MITSCHEME_UTABMD_FILE (default: utabmd.bin in the library path)
     The file containing the microcode tables.  Overridden by -utabmd and -utab.  It is only necessary when re-building runtime.com.

Environment Variables for Bchscheme
-----------------------------------

These environment variables are referred to by bchscheme (not by scheme).

MITSCHEME_GC_DIRECTORY (default: /tmp)
     The directory where to write gc files.  Overridden by -gc-directory.

MITSCHEME_GC_END_POSITION (default: start-position + heap-size)
     The last position in the gc file to use.  Overridden by -gc-end-position.

MITSCHEME_GC_FILE (default: GCXXXXXX)
     The name of the file to use for garbage collection.  If it ends in 6 Xs, the Xs are replaced by a letter and process id of the scheme process, thus generating a unique name.  Overridden by -gc-file.

MITSCHEME_GC_START_POSITION (default: 0)
     The first position in the gc file to use.  Overridden by -gc-start-position.

MITSCHEME_GC_WINDOW_SIZE (default: 16)
     The size in blocks of windows into new space (in the gc file). Overridden by -gc-window-size.

The following environment variables are only used by an experimental version of Bchscheme that uses Unix System V-style shared memory, and only then if the gcdrone program is installed:

MITSCHEME_GC_DRONE (default: gcdrone)
     The program to use as the I/O drone during garbage collection. Overridden by -gc-drone.

MITSCHEME_GC_READ_OVERLAP (default: 0)
     The maximum number of simultaneous read operations.  Overridden by -gc-read-overlap.

MITSCHEME_GC_WRITE_OVERLAP (default: 0)
     The maximum number of simultaneous write operations.  Overridden by -gc-write-overlap.

Environment Variables for the PC
--------------------------------

These environment variables are referred to by the Windows, Windows NT, and DOS versions of MIT Scheme.

MITSCHEME_DPMI_EXT_KBD (default: false)
     DOS only. Boolean option specifying whether Scheme inserts its own keyboard handling routine when running under DPMI (DOS Protected Mode Interface) or Windows.

MITSCHEME_X32_EXT_KBD (default: false)
     DOS only. Boolean option specifying whether Scheme inserts its own keyboard handling routine when not running under DPMI or Windows.

MITSCHEME_TRAP_ALT_TAB (default: false)
MITSCHEME_TRAP_ALT_ESCAPE (default: false)
     Windows and Windows NT only. Boolean option specifying the handling of system command accelerators. These options do not actually work.

MITSCHEME_GEOMETRY (default: -1,-1,-1,-1)
     Windows and Windows NT only.  Four integers separated by commas or spaces that specify the placement and size of the MIT Scheme window as a LEFT,TOP,WIDTH,HEIGHT quadruple.  The units are screen pixels, and -1 means allow the system to choose this parameter. E.g.  -1,-1,500,300 places a 500 by 300 pixel window at some system determined position on the screen.  The width and height include the window border and title.

MITSCHEME_FOREGROUND (default: according to desktop color scheme)
     Windows and Windows NT only.  An value specifying the window text color.  The color is specified as hex blue, green and red values (not RGB): e.g. 0xff0000 for blue.

MITSCHEME_BACKGROUND (default: according to desktop color scheme)
     Windows and Windows NT only.  A value specifying the window background color.  See MITSCHEME_FOREGROUND.

Environment Variables Affecting the Runtime System
--------------------------------------------------

These environment variables are referred to by the runtime system.

HOME
     Directory where to look for init files.  E.g. c:\users\joe or /homes/joe.

TEMP, TMP
     Directory for various temporary files.  TEMP is given preference to TMP.

Environment Variables Affecting Edwin
-------------------------------------

These environment variables are referred to by Edwin.

EDWIN_BINARY_DIRECTORY (default: edwin/autoload on the library path)
     Directory where edwin expects to find files providing autoloaded facilities.

EDWIN_INFO_DIRECTORY (default: edwin/info on the library path)
     Directory where edwin expects to find files for the `info' documentation subsystem.

EDWIN_ETC_DIRECTORY (default: edwin/etc on the library path)
     Directory where edwin expects to find utility programs and documentation strings.

EDWIN_FOREGROUND (default: none (white))
     DOS version only. ANSI foreground color specifier. Must be a two-digit sequence in the range 30-37.  E.g. 32 (green).

EDWIN_BACKGROUND (default: none (black))
     DOS version only. ANSI background color specifier. Must be a two-digit sequence in the range 40-47. E.g. 40 (black).

TERM
     Terminal type.  For DOS, should be ansi.sys or ibm_pc_bios.  For Windows and Windows NT it should be ansi.sys, which is the default if not set.

LINES (default: auto-sense)
     Unix only. Number of text lines on the screen, depending on the terminal type.

MITSCHEME_LINES (default: auto-sense or 25)
     DOS only.  Number of text lines on the screen, depending on the video adapter and support software.  E.g. 43.

COLUMNS (default: auto-sense)
     Unix only. Number of text columns on the screen, depending on the terminal type (Unix) or video adapter and support software (DOS).  E.g. 132.

MITSCHEME_COLUMNS (default: auto-sense, or 80)
     DOS only.  Number of text columns on the screen, depending on the video adapter and support software.  E.g. 132.

Starting Scheme from Windows 3.1/NT
===================================

There are two distinct versions of MIT Scheme that run on IBM `compatible' PCs: the DOS version is a character-mode only implementation, which can also run under Windows 3.1 as a DOS application. The Windows version runs as a graphics-based application under Windows 3.1 or Windows NT. The DOS version does not run under Windows NT.

Under Windows 3.1, Scheme must be run from the Program Manager or the File Manager.  Scheme cannot be run from the command line, because only DOS programs can be run from the command line. (This is the case even with WXSERVER as it appears not to work with win32s-based programs). Windows NT overcomes this restriction, but it is still useful to know how to run Scheme from the Program Manager.

Once an icon is set up to run Scheme with some particular command line options, Scheme is run by double-clicking that icon. The rest of this section gives some tips on how to set up icons in the Program Manager that run Scheme.  If you are unfamiliar with this concept you should read about it under the help topic of the Program Manager.

Under Windows NT program manager groups can be common or personal.  When setting up icons in a common group it is important to make the icons independent of the vagaries of the environment of the user who is running them.  It is often worthwhile doing this under Windows 3.1 and for personal groups too:

  * Give the icon an accurate DESCRIPTION.

  * Include absolute pathnames to scheme.exe and bchscheme.exe in the icon COMMAND LINE if these executables are not in a directory on the default PATH.

  * Set the icon's WORKING DIRECTORY to: %HOMEPATH%

  * There are several icons available --- choose one that best represents the options given on the command line.

  * Specifying a band that contains Edwin is not sufficient to invoke the editor.  You also have to put

          -eval (edit)

     at the end of the command line.

Leaving Scheme
==============

There are two ways you can leave Scheme.  The first is to evaluate

     (exit)

which will halt the Scheme system, after first requesting confirmation. Any information that was in the environment is lost, so this should not be done lightly.

The second way to leave Scheme is to suspend it; when this is done you may later restart where you left off.  Unfortunately this is not possible in all operating systems --- currently it works only on unix versions that support job control (i.e. all of the unix versions for which we distribute Scheme), but not on PCs.

Scheme is suspended by evaluating

     (quit)

If your system supports suspension, this will cause Scheme to stop, and you will be returned to the operating system's command interpreter. Scheme remains stopped, and can be continued using the job-control commands of your command interpreter.  If your system doesn't support suspension, this procedure does nothing.  (Calling the quit procedure is analogous to typing Control-Z, but it allows Scheme to respond by typing a prompt when it is unsuspended.)

The Read-Eval-Print Loop
************************

When you first start up Scheme from the command line (i.e not under Edwin), you will be typing at a program called the Read-Eval-Print Loop (abbreviated REPL).  It displays a prompt at the left hand side of the screen whenever it is waiting for input.  You then type an expression (terminating it with RET).  Scheme evaluates the expression, prints the result, and gives you another prompt.

The Prompt and Level Number
===========================

The REPL prompt normally has the form

     1 ]=>

The 1 in the prompt is a level number, which is always a positive integer.  This number is incremented under certain circumstances, the most common being an error.  For example, here is what you will see if you type f o o RET after starting Scheme:

     ;Unbound variable: foo
     ;To continue, call RESTART with an option number:
     ; (RESTART 3) => Specify a value to use instead of foo.
     ; (RESTART 2) => Define foo to a given value.
     ; (RESTART 1) => Return to read-eval-print level 1.

     2 error>

In this case, the level number has been incremented to 2, which indicates that a new REPL has been started (also the prompt string has been changed to remind you that the REPL was started because of an error).  The 2 means that this new REPL is ``over'' the old one.  The original REPL still exists, and is waiting for you to return to it, for example, by entering (restart 1).  Furthermore, if an error occurs while you are in this REPL, yet another REPL will be started, and the level number will be increased to 3.  This can continue ad infinitum, but normally it is rare to use more than a few levels.

The normal way to get out of an error REPL and back to the top level REPL is to use the C-g interrupt.  This is a single-keystroke command executed by holding down the CTRL key and pressing the G key.  C-g always terminates whatever is running and returns you to the top level REPL immediately.

Note: The appearance of the error> prompt does not mean that Scheme is in some weird inconsistent state that you should avoid.  It is merely a reminder that your program was in error: an illegal operation was attempted, but it was detected and avoided.  Often the best way to find out what is in error is to do some poking around in the error REPL.  If you abort out of it, the context of the error will be destroyed, and you may not be able to find out what happened.

Interrupting
============

Scheme has two interrupt keys under unix: C-g and C-c. Other systems, like the PC, may have more than two. The PC version has C-b, C-x, and C-u as well as C-g and C-c. The C-g key stops any Scheme evaluation that is running and returns you to the top level REPL. C-c prompts you for another character and performs some action based on that character.  It is not necessary to type RET after C-g or C-c, nor is it needed after the character that C-c will ask you for.

Here are the more common options for C-c.

C-c i
     Ignore the interrupt.  Type this if you made a mistake and didn't really mean to type C-c.

C-c ?
     Print help information.  This will describe any other options not documented here.

C-c q
     Similar to typing (exit) at the REPL, except that it works even if Scheme is running an evaluation, and does not request confirmation.

C-c z
     Similar to typing (quit) at the REPL, except that it works even if Scheme is running an evaluation.

C-c C-c
     Identical to typing C-g.  If no evaluation is running, this is equivalent to evaluating

          (cmdl-interrupt/abort-top-level)

     The options C-c C-g and C-c g, supplied for compatibility with older implementations, are equivalent to C-c C-c.

C-c C-x
     Abort whatever Scheme evaluation is currently running and return to the ``current'' REPL.  If no evaluation is running, this is equivalent to evaluating

          (cmdl-interrupt/abort-nearest)

     The option C-c x, supplied for compatibility with older implementations, is equivalent to C-c C-x.

     On the PC version C-x is equivalent to C-c C-x.

C-c C-u
     Abort whatever Scheme evaluation is running and go up one level.  If you are already at level number 1, it just aborts the evaluation, leaving you at level 1.  If no evaluation is running, this is equivalent to evaluating

          (cmdl-interrupt/abort-previous)

     The option C-c u, supplied for compatibility with older implementations, is equivalent to C-c C-u.

     On the PC version C-u is equivalent to C-c C-u.

C-c C-b
     Suspend whatever Scheme evaluation is running and start a breakpoint REPL.  The evaluation can be resumed by evaluating

          (proceed)

     in that REPL at any time.

     The option C-c b, supplied for compatibility with older implementations, is equivalent to C-c C-b.

     On the PC version C-b is equivalent to C-c C-b.

Restarting
==========

Another way of exiting a REPL is to use the restart procedure:

 -- procedure+: restart [K]
     This procedure selects and invokes a restart method.  The list of restart methods is different for each REPL; in the case of an error REPL, this list is printed when the REPL is started:

          ;Unbound variable: foo
          ;To continue, call RESTART with an option number:
          ; (RESTART 3) => Specify a value to use instead of foo.
          ; (RESTART 2) => Define foo to a given value.
          ; (RESTART 1) => Return to read-eval-print level 1.

          2 error>

     If the K argument is given, it must be a positive integer index into the list (in this example it must be between one and three inclusive).  The integer K selects an item from the list and invokes it.  If K is not given, restart prints the list and prompts for the integer index:

          2 error> (restart)
          ;Choose an option by number:
          ;  3: Specify a value to use instead of foo.
          ;  2: Define foo to a given value.
          ;  1: Return to read-eval-print level 1.

          Option number:

     The simplest restart methods just perform their actions.  For example:

          2 error> (restart 1)
          ;Abort!

          1 ]=>

     Other methods will prompt for more input before continuing:

          2 error> (restart)
          ;Choose an option by number:
          ;  3: Specify a value to use instead of foo.
          ;  2: Define foo to a given value.
          ;  1: Return to read-eval-print level 1.

          Option number: 3

          Value to use instead of foo: '(a b)
          ;Value: (a b)

          1 ]=>

The Current REPL Environment
============================

Every REPL has a current environment, which is the place where expressions are evaluated and definitions are stored.  When Scheme is started, this environment is the value of the variable user-initial-environment.  There are a number of other environments in the system, for example system-global-environment, where the runtime system's bindings are stored.

You can get the current REPL environment by evaluating

     (nearest-repl/environment)

There are several other ways to obtain environments.  For example, if you have a procedure object, you can get a pointer to the environment in which it was closed by evaluating

     (procedure-environment PROCEDURE)

Your programs create new environments whenever a procedure is called.

Here is the procedure that changes the REPL's environment:

 -- procedure+: ge environment
     Changes the current REPL environment to be ENVIRONMENT (ge stands for ``Goto Environment'').  ENVIRONMENT is allowed to be a procedure as well as an environment object.  If it is a procedure, then the closing environment of that procedure is used in its place.

 -- procedure+: gst syntax-table
     In addition to the current environment, each REPL maintains a current syntax table.  The current syntax table tells the REPL which keywords are used to identify special forms (e.g. if, lambda).  If you write macros, often you will want to make your own syntax table, in which case it is useful to be able to make that syntax table be the current one.  gst allows you to do that.

Debugging
*********

This chapter is out of date and currently under revision.

This chapter is adapted from Don't Panic: A 6.001 User's Guide to the Chipmunk System, by Arthur A. Gleckler.

Even computer software that has been planned carefully and written well may not always work correctly.  Mysterious creatures called bugs may creep in and wreak havoc, leaving the programmer to clean up the mess.  Some have theorized that a program fails only because its author made a mistake, but experienced computer programmers know that bugs are always to blame.  This is why the task of fixing broken computer software is called debugging.

It is impossible to prove the correctness of any non-trivial program; hence the Cynic's First Law of Debugging:

     Programs don't become more reliable as they are debugged; the bugs just get harder to find.

Scheme is equipped with a variety of special software for finding and removing bugs.  The debugging tools include facilities for tracing a program's use of specified procedures, for examining Scheme environments, and for setting breakpoints, places where the program will pause for inspection.

Many bugs are detected when programs try to do something which is impossible, like adding a number to a symbol, or using a variable which does not exist; this type of mistake is called an error. Whenever an error occurs, Scheme prints an error message and starts a new REPL.  For example, using a nonexistent variable foo will cause Scheme to respond

     1 ]=> foo

     ;Unbound variable: foo
     ;To continue, call RESTART with an option number:
     ; (RESTART 3) => Specify a value to use instead of foo.
     ; (RESTART 2) => Define foo to a given value.
     ; (RESTART 1) => Return to read-eval-print level 1.

     2 error>

Sometimes, a bug will never cause an error, but will still cause the program to operate incorrectly.  For instance,

     (prime? 7)   =>   #f

In this situation, Scheme does not know that the program is misbehaving. The programmer must notice the problem and, if necessary, start the debugging tools manually.

There are several approaches to finding bugs in a Scheme program:

  * Inspect the original Scheme program.

  * Use the debugging tools to follow your program's progress.

  * Edit the program to insert checks and breakpoints.

Only experience can teach how to debug programs, so be sure to experiment with all these approaches while doing your own debugging. Planning ahead is the best way to ward off bugs, but when bugs do appear, be prepared to attack them with all the tools available.

Subproblems and Reductions
==========================

Understanding the concepts of reduction and subproblem is essential to good use of the debugging tools.  The Scheme interpreter evaluates an expression by reducing it to a simpler expression. In general, Scheme's evaluation rules designate that evaluation proceeds from one expression to the next by either starting to work on a subexpression of the given expression, or by reducing the entire expression to a new (simpler, or reduced) form.  Thus, a history of the successive forms processed during the evaluation of an expression will show a sequence of subproblems, where each subproblem may consist of a sequence of reductions.

For example, both (+ 5 6) and (+ 7 9) are subproblems of the following combination:

     (* (+ 5 6) (+ 7 9))

If (prime? n) is true, then (cons 'prime n) is a reduction for the following expression:

     (if (prime? n)
         (cons 'prime n)
         (cons 'not-prime n))

This is because the entire subproblem of the if expression can be reduced to the problem (cons 'prime n), once we know that (prime?  n) is true; the (cons 'not-prime n) can be ignored, because it will never be needed.  On the other hand, if (prime? n) were false, then (cons 'not-prime n) would be the reduction for the if expression.

The subproblem level is a number representing how far back in the history of the current computation a particular evaluation is.  Consider factorial:

     (define (factorial n)
       (if (< n 2)
           1
           (* n (factorial (- n 1)))))

If we stop factorial in the middle of evaluating (- n 1), the (- n 1) is at subproblem level 0.  Following the history of the computation ``upwards,'' (factorial (- n 1)) is at subproblem level 1, and (* n (factorial (- n 1))) is at subproblem level 2. These expressions all have reduction number 0.  Continuing upwards, the if expression has reduction number 1.

Moving backwards in the history of a computation, subproblem levels and reduction numbers increase, starting from zero at the expression currently being evaluated.  Reduction numbers increase until the next subproblem, where they start over at zero.  The best way to get a feel for subproblem levels and reduction numbers is to experiment with the debugging tools, especially debug.

The Debugger
============

There are three debuggers available with MIT Scheme.  Two of them require and run under Edwin, and are described in that section of this document (see Edwin).  The third is command oriented, does not require Edwin, and is described here.

The debugger, called debug, is the tool you should use when Scheme signals an error and you want to find out what caused the error. When Scheme signals an error, it records all the information necessary to continue running the Scheme program that caused the error; the debugger provides you with the means to inspect this information.  For this reason, the debugger is sometimes called a continuation browser.

Here is the transcript of a typical Scheme session, showing a user evaluating the expression (fib 10), Scheme responding with an unbound variable error for the variable fob, and the user starting the debugger:

     1 ]=> (fib 10)

     ;Unbound variable: fob
     ;To continue, call RESTART with an option number:
     ; (RESTART 3) => Specify a value to use instead of fob.
     ; (RESTART 2) => Define fob to a given value.
     ; (RESTART 1) => Return to read-eval-print level 1.

     2 error> (debug)

     There are 6 subproblems on the stack.

     Subproblem level: 0 (this is the lowest subproblem level)
     Expression (from stack):
         fob
     Environment created by the procedure: FIB
      applied to: (10)
     The execution history for this subproblem contains 1 reduction.
     You are now in the debugger.  Type q to quit, ? for commands.

     3 debug>

This tells us that the error occurred while trying to evaluate the expression fob while running (fib 10).  It also tells us this is subproblem level 0, the first of 8 subproblems that are available for us to examine.  The expression shown is marked ``(from stack)'', which tells us that this expression was reconstructed from the interpreter's internal data structures.  Another source of information is the execution history, which keeps a record of expressions evaluated by the interpreter.  The debugger informs us that the execution history has recorded some information for this subproblem, specifically a description of one reduction.

Debugging Aids
==============

An important step in debugging is to locate the piece of code from which the error is signalled.  The Scheme debugger contains a history examiner and an environment examiner to aid the user in locating a bug.

 -- special form+: bkpt message irritant
     Sets a breakpoint.  When the breakpoint is encountered, MESSAGE and IRRITANT are typed and a read-eval-print loop is entered in the current environment.  To exit from the breakpoint and proceed with the interrupted process, call the procedure proceed.  Sample usage:

          1 ]=> (begin (write-line 'foo)
                       (bkpt 'test-2 'test-3)
                       (write-line 'bar)
                       'done)

          foo
           test-2 test-3
          ;To continue, call RESTART with an option number:
          ; (RESTART 2) => Return from BKPT.
          ; (RESTART 1) => Return to read-eval-print level 1.

          2 bkpt> (+ 3 3)

          ;Value: 6

          2 bkpt> (proceed)

          bar
          ;Value: done

 -- procedure+: where [obj]
     The procedure where enters the environment examination system. This allows environments and variable bindings to be examined and modified.  where accepts one letter commands.  The commands can be found by typing ? to the where> prompt.  The optional argument, OBJ, is an object with an environment associated with it: an environment, a procedure, or a promise.  If OBJ is omitted, the environment examined is the read-eval-print environment from which where was called (or an error or breakpoint environment if called from the debugger).  If a compound procedure is supplied, where lets the user examine the environment of definition of the procedure. This is useful for debugging procedure arguments and values.

Advising Procedures
===================

Giving advice to procedures is a powerful debugging technique. trace and break are useful examples of advice-giving procedures. Note that the advice system only works for interpreted procedures.

 -- procedure+: trace-entry proc
     Causes an informative message to be printed whenever the procedure PROC is entered.  The message is of the form

          [Entering #[compound-procedure 1 foo]
              Args: VAL1
                    VAL2
                    ...]

     where VAL1, VAL2 etc. are the evaluated arguments supplied to the procedure.

 -- procedure+: trace-exit proc
     Causes and informative message to be printed when procedure PROC terminates.  The message contains the procedure, its argument values, and the value returned by the procedure.

 -- procedure+: trace-both proc
 -- procedure+: trace proc
     trace-both is the same as calling both trace-entry and trace-exit on PROC. trace is the same as trace-both.

 -- procedure+: untrace-entry [proc]
     untrace-entry stops tracing the entry of PROC. If PROC is not given, the default is to stop tracing the entry of all entry-traced procedures.

 -- procedure+: untrace-exit [proc]
     Stops tracing the exit of PROC.  If PROC is not included, the default is all exit-traced procedures.

 -- procedure+: untrace [proc]
     Stops tracing both the entry to and the exit from PROC.  If PROC is not given, the default is all traced procedures.

 -- procedure+: break-entry proc
     Like trace-entry with the additional effect that a breakpoint is entered when procedure PROC is invoked.  Both PROC and its arguments can be accessed by calling the procedures *proc* and *args*, respectively. Use restart or proceed to continue from a breakpoint.

 -- procedure+: break-exit proc
     Like trace-exit, except that a breakpoint is entered just prior to leaving the procedure PROC.  PROC, its arguments, and the result can be accessed by calling the procedures *proc*, *args*, and *result*, respectively. Use restart or proceed to continue from a breakpoint.

 -- procedure+: break-both proc
 -- procedure+: break proc
     Sets a breakpoint at the beginning and end of PROC.  This is break-entry and break-exit combined.

 -- procedure+: unbreak [proc]
     Discontinues the entering of a breakpoint on the entry to and exit from the procedure PROC.  If PROC is not given, the default is all breakpointed procedures.

 -- procedure+: unbreak-entry [proc]
     Discontinues the entering of a breakpoint on the entry to the procedure PROC.  If PROC is not given, the default is all entry-breakpointed procedures.

 -- procedure+: unbreak-exit [proc]
     Discontinues the entering of a breakpoint on the exit from the procedure PROC.  If PROC is not given, the default is all exit-breakpointed procedures.

 -- procedure+: advise-entry proc advice
     General entry-advising procedure.  trace-entry and break-entry are examples of entry-advising procedures. advise-entry gives ADVICE to PROC. When PROC is invoked, ADVICE is passed three arguments: PROC, a list of PROC's argument values, and the current environment.

 -- procedure+: advise-exit proc advice
     The general exit-advising procedure.  trace-exit and break-exit are examples of exit-advising procedures. ADVICE is a procedure that should accept four arguments: PROC, its argument values, the result computed by PROC, and the current environment.  ADVICE is responsible for returning a value on behalf of PROC.  That is, the value returned by ADVICE is the value returned by the advised procedure.

 -- procedure+: advice proc
     Returned the advice, if any, given to PROC.

 -- procedure+: unadvise-entry [proc]
     Removes entry advice from PROC. If PROC is not given, the default is all entry-advised procedures.

 -- procedure+: unadvise-exit [proc]
     Removes exit advice from PROC. If PROC is not given, the default is all exit-advised procedures.

 -- procedure+: unadvise [proc]
     Removes all advice from PROC. This is a combination of unadvise-entry and unadvise-exit. If PROC is not given, the default is all advised procedures.

 -- procedure+: *proc*
     *proc* is a procedure which returns as its value the ``broken'' procedure.  It is used only at a breakpoint set by the procedures break-exit and break-entry or procedures defined in terms of these procedures (like break-both and break).

 -- procedure+: *args*
     *args* is a procedure which returns as its value a list of the arguments supplied to the ``broken'' procedure.  It is used only at a breakpoint set by the procedures break-exit and break-entry or procedures defined in terms of these procedures (like break-both and break).

 -- procedure+: *result*
     *result* is a procedure which returns as its value the result that was about to be returned by the ``broken'' procedure. It is used only at a breakpoint set by the procedure break-exit or procedures defined in terms of this procedure (like break-both and break).

Loading Files
*************

To load files of Scheme code, use the procedure load:

 -- procedure: load filename [environment [syntax-table]]
     FILENAME may be a string naming a file, or a list of strings naming many files.  ENVIRONMENT, if given, is the environment to evaluate the file in; if not given the current REPL environment is used.  Likewise SYNTAX-TABLE is the syntax table to use.

     load determines whether the file to be loaded is binary or source code, and performs the appropriate action.  By convention, files of source code have a pathname type of "scm", and files of binary SCode have pathname type "bin".  Native-code binaries have pathname type "com".  (See the description of pathname-type in the reference manual.)

 -- variable+: load-noisily?
     If load-noisily? is set to #t, load will print the value of each expression in the file as it is evaluated.  Otherwise, nothing is printed except for the value of the last expression in the file.  (Note: the noisy loading feature is implemented for source-code files only.)

 -- variable+: load/default-types
     When load is given a pathname without a type, it uses the value of this variable to determine what pathname types to look for and how to load the file.  load/default-types is a list of associations that maps pathname types (strings) to loader procedures.  load tries the pathname types in the order that they appear in the list.  The initial value of this variable has pathname types in this order:

          "com" "so" "sl" "bin" "scm"

     This means that, for example, (load "foo") will try to load foo.com first, and foo.scm only after looking for and failing to find the other pathname types.

All pathnames are interpreted relative to a working directory, which is initialized when Scheme is started.  The working directory can be obtained from the procedure pwd or modified by the procedure cd; see the reference manual for details.  Files may be loaded when Scheme first starts; see the -load command line option for details.

World Images
************

A world image is a file that contains a complete Scheme system, perhaps additionally including user application code.  Scheme provides two methods for saving and restoring world images.  The first method writes a file containing all of the Scheme code in the world, which is called a band.  The file runtime.com that is loaded by the microcode is just such a band. To make your own band, use the procedure disk-save.

 -- procedure+: disk-save filename [identify]
     Causes a band to be written to the file specified by FILENAME. The optional argument IDENTIFY controls what happens when that band is restored, as follows:

     not specified
          Start up in the top-level REPL, identifying the world in the normal way.

     a string
          Do the same thing except print that string instead of Scheme when restarting.

     the constant #t
          Restart exactly where you were when the call to disk-save was performed.  This is especially useful for saving your state when an error has occurred and you are not in the top-level REPL.

     the constant #f
          Just like #t, except that the runtime system will not perform normal restart initializations; in particular, it will not load your init file.

To restore a saved band, give the -band option when starting Scheme.  Alternatively, evaluate (disk-restore FILENAME) from a running Scheme, which will destroy the current world, replacing it with the saved world.  The argument to disk-restore may be omitted, in which case it defaults to the filename from which the current world was last restored.

Note: with the C back-end, disk-save is not very useful.  The reason is that compiled procedures are compiled C code that has been dynamically linked in, and disk-save does not save any C procedures.  If you need to build a band for a C back-end system, please contact us.  Your system is a C back-end system if the following expression does not evaluate to #f:

     (system-library-directory-pathname "shared")

Note: when restoring a saved band, the Scheme executable must be configured with a large enough constant space and heap to hold the band's contents.  If you attempt to restore a band using the -band option, and the band is too large, Scheme will write an error message that tells you the appropriate command-line options needed to load that band.  If you attempt restore a too-large band using disk-restore, Scheme will signal an error, but will not provide the configuration information.  In general, the configuration that was used to save a band is sufficiently large to restore it.

Another method for saving the world is the dump-world procedure, which accepts the same arguments as disk-save and works in much the same way.  However, rather than dumping a band, dump-world saves an executable image, which is started just like any other program. This has the advantage of being considerably faster to start on some systems, but the image file is typically much larger than the corresponding band.  However, dump-world is only supported for a few operating systems, and is not built into the distributed executable files --- if you wish to use dump-world, you must build your own executable file from the source code. Note that dump-world is unlikely to work with this release as MIT Scheme now uses shared libraries.

Garbage Collection
******************

This section describes procedures that control garbage collection.  See Customizing Scheme for a discussion of how MIT Scheme uses memory.

 -- procedure+: gc-flip [safety-margin]
     Forces a garbage collection to occur.  Returns the number of words of available storage after collection.

     SAFETY-MARGIN determines the number of words of storage available for system tasks in-between detecting the need for a garbage collection and entering the garbage collector proper.  An example of such a system task is changing the run-light to show `gc' when scheme is running under Emacs.

 -- procedure+: set-gc-notification! on?
     Controls whether the user is notified of garbage collections.  If ON? is #F, the user is not notified, otherwise the user is notified.  The default is no notification.

     The notification appears as a single line like the following, showing how many garbage collections have occured, the time taken to perform the garbage collection and the free storage remaining (in words) after collection.

          GC 5: took:   0.50   (8%) CPU time,   0.70   (2%) real time; free: 364346

     To operate comfortably, the amount of free storage after garbage collection should be a substantial proportion of the heap size.  If the percentage CPU time is consistently high (over 20%), you should consider running with a larger heap.  A rough rule of thumb to halve the GC overhead is to take the amount of free storage, divide by 1000, and add this figure to the current value used for the -heap command line option.  Unfortunately there is no way to adjust the heap size without restarting Scheme.

 -- purify: item [pure-space? [queue?]]
 -- flush-purification-queue!:
     ** We should say something about these **

Compiling Files
***************

Note: the procedures described in this section are only available in the compiler.com world image.  Furthermore, cf is only available on machines that support native-code compilation.

Compilation Procedures
======================

 -- procedure+: cf filename [destination]
     This is the program that transforms a source-code file into native-code binary form.  If DESTINATION is not given, as in

          (cf "foo")

     cf compiles the file foo.scm, producing the file foo.com (incidentally it will also produce foo.bin, foo.bci, and possibly foo.ext).  If you later evaluate

          (load "foo")

     foo.com will be loaded rather than foo.scm.

     If DESTINATION is given, it says where the output files should go. If this argument is a directory, they go in that directory, e.g.:

          (cf "foo" "../bar/")

     will take foo.scm and generate the file ../bar/foo.com. If DESTINATION is not a directory, it is the root name of the output:

          (cf "foo" "bar")

     takes foo.scm and generates bar.com.

About the .bci files: these files contain the debugging information that Scheme uses when you call debug to examine compiled code.  When you load a .com file, Scheme remembers where it was loaded from, and when the debugger (or pp) looks at the compiled code from that file, it attempts to find the .bci file in the same directory from which the .com file was loaded.  Thus it is a good idea to leave these files together.

.bci files are stored in a compressed format. The debugger has to uncompress the files when it looks at them, and on a slow machine this can take a noticeable time. The system takes steps to reduce the impact of this behaviour: debugging information is cached in memory, and uncompressed versions of .bci files are kept around. The default behavior is that a temporary file is created and the .bci file is uncompressed into it.  The temporary file is kept around for a while afterwards, and during that time if the uncompressed .bci file is needed the temporary file is used.  Each such reference updates an `access time' that is associated with the temporary file.  The garbage collector checks the access times of all such temporary files, and deletes any that have not been accessed in five minutes or more.  All of the temporaries are deleted automatically when the Scheme process is killed.

Two other behaviors are available.  One of them uncompresses the .bci file each time it is referenced, and the other uncompresses the .bci file and writes it back out as a .bif file (the old default). The .bif file remains after Scheme exits. The time interval and the behavior are controlled by variables.  (These variables are not in the global environment; perhaps they should be. They are in the (runtime compiler-info) package environment.)

 -- variable+: *save-uncompressed-files?*
     This variable affects what happens when .bci files are uncompressed.  It allows a trade-off between performance and disk space. There are three possible values:

     #f
          The uncompressed versions of .bci files are never saved.  Each time the information is needed the .bci file is uncompressed. This option requires the minimum amount of disk space and is the slowest.

     automatic
          Uncompressed versions of .bci files are kept as temporary files. The temporary files are deleted when Scheme exits, and if they have not been used for a while.  This is the default.

     #t
          The .bci files are uncompressed to permanent .bif files. These files remain on disk after Scheme exits, and are rather large - about twice the size of the corresponding .bci files.  If you choose this option and you are running out of disk space you may delete the .bif files.  They will be regenerated as needed.

 -- variable+: *uncompressed-file-lifetime*
     The minimum length of time that a temporary uncompressed version of a .bci file will stay on disk after it is last used. The time is in microseconds; the default is 300000 (five minutes).

 -- variable+: load-debugging-info-on-demand?
     If this variable is #f, then printing a compiled procedure will print the procedure's name only if the debugging information for that procedure is already loaded.  Otherwise, it will force the loading of the debugging information. The default value is #f.

 -- procedure+: sf filename [destination]
     sf is the program that transforms a source-code file into binary SCode form; it is used on machines that do not support native-code compilation.  It performs numerous optimizations that can make your programs run considerably faster than unoptimized interpreted code. Also, the binary files that it generates load very quickly compared to source-code files.

     The simplest way to use sf is just to say:

          (sf FILENAME)

     This will cause your file to be transformed, and the resulting binary file to be written out with the same name, but with pathname type "bin".  If you do not specify a pathname type on the input file, "scm" is assumed.

     Like load, the first argument to sf may be a list of filenames rather than a single filename.

     sf takes an optional second argument, which is the filename of the output file.  If this argument is a directory, then the output file has its normal name but is put in that directory instead.

Declarations
============

Several declarations can be added to your programs to help cf and sf make them more efficient.

Standard Names
--------------

Normally, all files have a line

     (declare (usual-integrations))

near their beginning, which tells the compiler that free variables whose names are defined in system-global-environment will not be shadowed by other definitions when the program is loaded.  If you redefine some global name in your code, for example car, cdr, and cons, you should indicate it in the declaration:

     (declare (usual-integrations car cdr cons))

You can obtain an alphabetically-sorted list of the names that the usual-integrations declaration affects by evaluating the following expression:

     (eval '(sort (append usual-integrations/constant-names
                          usual-integrations/expansion-names)
                  (lambda (x y)
                    (string<=? (symbol->string x)
                               (symbol->string y))))
           (->environment '(scode-optimizer)))

In-line Coding
--------------

Another useful facility is the ability to in-line code procedure definitions.  In fact, the compiler will perform full beta conversion, with automatic renaming, if you request it.  Here are the relevant declarations:

 -- declaration+: integrate name ...
     The variables NAMEs must be defined in the same file as this declaration.  Any reference to one of the named variables that appears in the same block as the declaration, or one of its descendant blocks, will be replaced by the corresponding definition's value expression.

 -- declaration+: integrate-operator name ...
     Similar to the integrate declaration, except that it only substitutes for references that appear in the operator position of a combination.  All other references are ignored.

 -- declaration+: integrate-external filename
     Causes the compiler to use the top-level integrations provided by FILENAME.  FILENAME should not specify a file type, and the source-code file that it names must have been previously processed by the compiler.

     If FILENAME is a relative filename (the normal case), it is interpreted as being relative to the file in which the declaration appears.  Thus if the declaration appears in file /usr/cph/foo.scm, then the compiler looks for a file called /usr/cph/FILENAME.ext.

     Note: When the compiler finds top-level integrations, it collects them and outputs them into an auxiliary file with extension .ext. This .ext file is what the integrate-external declaration refers to.

Note that the most common use of this facility, in-line coding of procedure definitions, requires a somewhat complicated use of these declarations.  Because this is so common, there is a special form, define-integrable, which is like define but performs the appropriate declarations.  For example:

     (define-integrable (foo-bar foo bar)
       (vector-ref (vector-ref foo bar) 3))

Here is how you do the same thing without this special form: there should be an integrate-operator declaration for the procedure's name, and (internal to the procedure's definition) an integrate declaration for each of the procedure's parameters, like this:

     (declare (integrate-operator foo-bar))

     (define foo-bar
       (lambda (foo bar)
         (declare (integrate foo bar))
         (vector-ref (vector-ref foo bar) 3)))

The reason for this complication is as follows: the integrate-operator declaration finds all the references to foo-bar and replaces them with the lambda expression from the definition.  Then, the integrate declarations take effect because the combination in which the reference to foo-bar occurred supplies code which is substituted throughout the body of the procedure definition.  For example:

     (foo-bar (car baz) (cdr baz))

First use the integrate-operator declaration:

     ((lambda (foo bar)
        (declare (integrate foo bar))
        (vector-ref (vector-ref foo bar) 3))
      (car baz)
      (cdr baz))

Next use the internal integrate declaration:

     ((lambda (foo bar)
        (vector-ref (vector-ref (car baz) (cdr baz)) 3))
      (car baz)
      (cdr baz))

Next notice that the variables foo and bar are not used, and eliminate them:

     ((lambda ()
        (vector-ref (vector-ref (car baz) (cdr baz)) 3)))

Finally, remove the ((lambda () ...)) to produce

     (vector-ref (vector-ref (car baz) (cdr baz)) 3)

Useful tip: to see the effect of integration declarations (and of macros) on a source file, pretty-print the .bin file like this (be prepared for a lot of output).

     (sf "foo.scm")
     (pp (fasload "foo.bin"))

Operator Replacement
--------------------

The replace-operator declaration is provided to inform the compiler that certain operators may be replaced by other operators depending on the number of arguments. For example:

Declaration:

     (declare (replace-operator (map (2 map-2) (3 map-3))))

Replacements:

     (map F X Y Z) ==> (map F X Y Z)
     (map F X Y) ==> (map-3 F X Y)
     (map F X) ==> (map-2 F X)
     (map F) ==> (map F)
     (map) ==> (map)

Presumably map-2 and map-3 are efficient versions of map that are written for exactly two and three arguments respectively.  All the other cases are not expanded but are handled by the original, general map procedure, which is less efficient because it must handle a variable number of arguments.

 -- declaration+: replace-operator name ...

     The syntax of this declaration is

          (replace-operator
            (NAME
              (NARGS1 VALUE1)
              (NARGS2 VALUE2)
              ...))

     where

       * NAME is a symbol.

       * NARGS1, NARGS2 etc. are non-negative integers, or one of the following symbols:  any, else or otherwise.

       * VALUE1, VALUE2 etc. are simple expressions in one of these forms:

          'CONSTANT
               A constant.

          VARIABLE
               A variable.

          (primitive PRIMITIVE-NAME [ARITY])
               The primitive procedure named PRIMITIVE-NAME.  The optional element ARITY, a non-negative integer, specifies the number of arguments that the primitive accepts.

          (global VAR)
               A global variable.

     The meanings of these fields are:

       * NAME is the name of the operator to be reduced.  If is is not shadowed (for example, by a let) then it may be replaced according to the following rules.

       * If the operator has NARGSN arguments then it is replaced with a call to VALUEN with the same arguments.

       * If the number of arguments is not listed, and one of the NARGSN is any, else or otherwise, then the operation is replaced with a call to the corresponding VALUEN. Only one of the NARGSN may be of this form.

       * If the number of arguments is not listed and none of the NARGSN is any, else or otherwise, then the operation is not replaced.

Operator Reduction
------------------

The reduce-operator declaration is provided to inform the compiler that certain names are n-ary versions of binary operators. Here are some examples:

Declaration:

     (declare (reduce-operator (cons* cons)))

Replacements:

     (cons* X Y Z W) ==> (cons X (cons Y (cons Z W))),
     (cons* X Y) ==> (cons X Y)
     (cons* X) ==> X
     (cons*) error--> too few arguments

Declaration:

     (declare (reduce-operator (list cons (null-value '() any))))

Replacements:

     (list X Y Z W) ==> (cons X (cons Y (cons Z (cons W '()))))
     (list X Y) ==> (cons X (cons Y '()))
     (list X) ==> (cons X '())
     (list) ==> '()

Declaration:

     (declare (reduce-operator (- %- (null-value 0 single) (group left))))

Replacements:

     (- X Y Z W) ==> (%- (%- (%- X Y) Z) W)
     (- X Y) ==> (%- X Y)
     (- X) ==> (%- 0 X)
     (-) ==> 0

Declaration:

     (declare (reduce-operator (+ %+ (null-value 0 none) (group right))))

Replacements:

     (+ X Y Z W) ==> (%+ X (%+ Y (%+ Z W)))
     (+ X Y) ==> (%+ X Y)
     (+ X) ==> X
     (+) ==> 0

Note: This declaration does not cause an appropriate definition of %+ (in the last example) to appear in your code.  It merely informs the compiler that certain optimizations can be performed on calls to + by replacing them with calls to %+.  You should provide a definition of %+ as well, although it is not required.

Declaration:

     (declare (reduce-operator (apply (primitive cons)
                                      (group right)
                                      (wrapper (global apply) 1))))

Replacements:

     (apply F X Y Z W) ==> ((access apply ()) F (cons X (cons Y (cons Z W))))
     (apply F X Y) ==> ((access apply ()) F (cons X Y))
     (apply F X) ==> (apply F X)
     (apply F) ==> (apply F)
     (apply) ==> (apply)

 -- declaration+: reduce-operator name ...
     The general format of the declaration is (brackets denote optional elements):

          (reduce-operator
            (NAME
              BINOP
              [(group ORDERING)]
              [(null-value VALUE NULL-OPTION)]
              [(singleton UNOP)]
              [(wrapper WRAP [n])]
              [(maximum M)]
            ))

     where

       * N and M are non-negative integers.

       * NAME is a symbol.

       * BINOP, VALUE, UNOP, and WRAP are simple expressions in one of these forms:

          'CONSTANT
               A constant.

          VARIABLE
               A variable.

          (primitive PRIMITIVE-NAME [ARITY])
               The primitive procedure named PRIMITIVE-NAME.  The optional element ARITY specifies the number of arguments that the primitive accepts.

          (global VAR)
               A global variable.

       * NULL-OPTION is either always, any, one, single, none, or empty.

       * ORDERING is either left, right, or associative.

     The meaning of these fields is:

       * NAME is the name of the n-ary operation to be reduced.

       * BINOP is the binary operation into which the n-ary operation is to be reduced.

       * The group option specifies whether NAME associates to the right or left.

       * The null-value option specifies a value to use in the following cases:

          none
          empty
               When no arguments are supplied to NAME, VALUE is returned.

          one
          single
               When a single argument is provided to NAME, VALUE becomes the second argument to BINOP.

          any
          always
               BINOP is used on the ``last'' argument, and VALUE provides the remaining argument to BINOP.

          In the above options, when VALUE is supplied to BINOP, it is supplied on the left if grouping to the left, otherwise it is supplied on the right.

       * The singleton option specifies a function, UNOP, to be invoked on the single argument left.  This option supersedes the null-value option, which can only take the value none.

       * The wrapper option specifies a function, WRAP, to be invoked on the result of the outermost call to BINOP after the expansion. If N is provided it must be a non-negative integer indicating a number of arguments that are transferred verbatim from the original call to the wrapper.  They are passed to the left of the reduction.

       * The maximum option specifies that calls with more than M arguments should not be reduced.

GNU Emacs Interface
*******************

There is an interface library, called xscheme, distributed with MIT Scheme and GNU Emacs, which facilitates running Scheme as a subprocess of Emacs.  If you wish to use this interface, please install the version of xscheme.el that comes with MIT Scheme, as it is guaranteed to be correct for your version of Scheme.

To invoke Scheme from Emacs, use M-x run-scheme, which is defined when either of the libraries scheme or xscheme is loaded. You may give run-scheme a prefix argument, in which case it will allow you to edit the command line that is used to invoke Scheme. Do not remove the -emacs option!

Scheme will be started up as a subprocess in a buffer called *scheme*.  This buffer will be in scheme-interaction-mode and all output from the Scheme process will go there.  The mode line for the *scheme* buffer will have this form:

     --**-*scheme*: 1 [Evaluator]           (Scheme Interaction: input)------

The first field, showing 1 in this example, is the level number.

The second field, showing [Evaluator] in this example, describes the type of REPL that is running.  Other values include:

     [Debugger]
     [Where]

The mode after Scheme Interaction is one of:

input
     Scheme is waiting for input.

run
     Scheme is running an evaluation.

gc
     Scheme is garbage collecting.

When xscheme is loaded, scheme-mode is extended to include commands for evaluating expressions (do C-h m in any scheme-mode buffer for the most up-to-date information):

ESC o
     Evaluates the current buffer (xscheme-send-buffer).

ESC z
     Evaluates the current definition (xscheme-send-definition).  This is also bound to ESC C-x.

ESC C-z
     Evaluates the current region (xscheme-send-region).

C-x C-e
     Evaluates the expression to the left of point (xscheme-send-previous-expression).  This is also bound to ESC RET.

C-c C-s
     Selects the *scheme* buffer and places you at its end (xscheme-select-process-buffer).

C-c C-y
     Yanks the most recently evaluated expression, placing it at point (xscheme-yank-previous-send).  This works only in the *scheme* buffer.

The following commands provide interrupt capability:

C-c C-c
     Like typing C-g when running Scheme without Emacs (xscheme-send-control-g-interrupt).

C-c C-x
     Like typing C-c C-x when running Scheme without Emacs (xscheme-send-control-x-interrupt).

C-c C-u
     Like typing C-c C-u when running Scheme without Emacs (xscheme-send-control-u-interrupt).

C-c C-b
     Like typing C-c C-b when running Scheme without Emacs (xscheme-send-breakpoint-interrupt).

C-c C-p
     Like evaluating (proceed) (xscheme-send-proceed).

Edwin
*****

This chapter describes how to start Edwin, the MIT Scheme text editor. Edwin is very similar to GNU Emacs --- you should refer to the GNU Emacs manual for information about Edwin's commands and key bindings --- except that Edwin's extension language is MIT Scheme, while GNU Emacs extensions are written in Emacs Lisp.  This manual does not discuss customization of Edwin.

Starting Edwin
==============

To use Edwin, start Scheme with a world image containing Edwin (for example by giving the -edwin command-line option), then call the procedure edit:

 -- procedure+: edit
 -- procedure+: edwin
     Enter the Edwin text editor.  If entering for the first time, the editor is initialized (by calling create-editor with no arguments). Otherwise, the previously-initialized editor is reentered.

     This procedure is sometimes evaluated from the command line to start Scheme with Edwin running:

          scheme -edwin -eval (edit)

     The procedure edwin is an alias for edit.

 -- variable+: inhibit-editor-init-file?
     When Edwin is first initialized, it loads your init file (called ~/.edwin on unix, edwin.ini on PCs) if you have one.  If the Scheme variable inhibit-editor-init-file? is true, however, your init file will not be loaded even if it exists.  By default, this variable is false.

 -- procedure+: create-editor arg ...
     Initializes Edwin, or reinitializes it if already initialized. create-editor is normally invoked automatically by edit.

     If no ARGs are given, the value of create-editor-args is used instead.  In other words, the following are equivalent:

          (create-editor)
          (apply create-editor create-editor-args)

     On the other hand, if ARGs are given, they are used to update create-editor-args, making the following equivalent:

          (apply create-editor ARGS)
          (begin (set! create-editor-args ARGS) (create-editor))

 -- variable+: create-editor-args
     This variable controls the initialization of Edwin.  The following values are defined:

     (console)
          This says to run Edwin on Scheme's console, or in unix terminology, the standard input and output.  If the console is not a terminal device, or is not powerful enough to run Edwin, an error will be signalled at initialization time.

     (x)
          This says to create an X window and run Edwin on it.  This requires the DISPLAY environment variable to have been set to the appropriate value before Scheme was started.

     (x GEOMETRY)
          This is like (x) except that GEOMETRY specifies the window's geometry in the usual way.  GEOMETRY must be a character string whose contents is an X geometry specification.

     (#f)
          This is the default.  It says to try running Edwin on Scheme's console, and failing that, to create an X window and run Edwin on that.  This signals an error if neither the console nor the X display is usable.

Leaving Edwin
=============

Once Edwin has been entered, it can be exited in the following ways:

C-x z
     Stop Edwin and return to Scheme (suspend-edwin).  The call to the procedure edit that entered Edwin returns normally.  A subsequent call to edit will resume Edwin where it was stopped.

C-x c
     Offer to save any modified buffers, then kill Edwin, returning to Scheme (save-buffers-kill-edwin).  This is like the suspend-edwin command, except that a subsequent call to edit will reinitialize the editor.

C-x C-z
     Stop Edwin and suspend Scheme, returning control to the operating system's command interpreter (suspend-scheme).  When Scheme is resumed (using the command interpreter's job-control commands), Edwin is automatically restarted where it was stopped.  This command is identical to the C-x C-z command of GNU Emacs.

C-x C-c
     Offer to save any modified buffers, then kill both Edwin and Scheme (save-buffers-kill-scheme).  Control is returned to the operating system's command interpreter, and the Scheme process is terminated. This command is identical to the C-x C-c command of GNU Emacs.

Last Resorts
============

When Scheme exits abnormally it tries to save any unsaved Edwin buffers. The buffers are saved in an auto-save file in case the original is more valuable than the unsaved version.  You can use the editor command M-x recover-file to recover the auto-saved version.  The auto-save files are named so: under unix, foo.scm will be saved as #foo.scm#; on PCs it will be saved as foo.s00, a backup file with version 00 (which is never used as an actual version).

The following Scheme procedures are useful for recovering from bugs in Edwin's implementation.  All of them are designed for use when Edwin is not running --- they should not be used when Edwin is running. These procedures are designed to help Edwin's implementors deal with bugs during the implementation of the editor; they are not intended for casual use, but as a means of recovering from bugs that would otherwise require reloading the editor's world image from the disk.

 -- procedure+: save-editor-files
     Examines Edwin, offering to save any unsaved buffers.  This is useful if some bug caused Edwin to die while there were unsaved buffers, and you want to save the information without restarting the editor.

 -- procedure+: reset-editor
     Resets Edwin, causing it to be reinitialized the next time that edit is called.  If you encounter a fatal bug in Edwin, a good way to recover is to first call save-editor-files, and then to call reset-editor.  That should completely reset the editor to its initial state.

 -- procedure+: reset-editor-windows
     Resets Edwin's display structures, without affecting any of the buffers or their contents.  This is useful if a bug in the display code causes Edwin's internal display data structures to get into an inconsistent state that prevents Edwin from running.

Comparison of Edwin 3.82 to Emacs 18.57
=======================================

This section documents all known differences between Edwin 3.82 and Emacs 18.57.  A reference to a ``documented'' feature of Emacs means that the feature is documented in the GNU Emacs version 18 manual.

Incompatibilities
-----------------

These are differences in design, unlikely to be `fixed'.

  * Edwin's keyboard character set is a superset of Emacs'.  Edwin has support for real MIT-style bucky bits.  Under X windows (and perhaps other systems) Edwin can distinguish between C-a and C-a, C-SPC and C-@, etc.  When running on a terminal or a PC/DOS console, the input set is limited to 8-bit ASCII as in Emacs.

  * Edwin is case-insensitive for command and variable names.

  * Edwin additionally defines C-z and C-^ as prefix keys. C-z is the CTRL-META prefix, and C-^ is the CTRL prefix, both as in TOPS-20 EMACS.

  * C-z doesn't suspend Edwin.  C-x C-z suspends both Emacs and Edwin, but isn't defined in the Emacs Manual.  Under X, Emacs produces an error message when C-x C-z is typed; Edwin suspends itself if it was started from a shell with job control, otherwise it dies.

  * C-h f doesn't work for anything but editor commands, i.e. it only documents editor commands, not ordinary Scheme procedures like car or call-with-current-continuation.

  * C-h v doesn't work for anything but editor variables, i.e. it only documents editor variables, not ordinary Scheme variables.

  * In Edwin, C-w and C-y don't skip to next occurrence in reverse i-search.  This is intentional.

  * comment-indent-hook is defined differently.  In Emacs, it is called with no arguments and with point at the beginning of the comment; in Edwin, it is called with a single argument, the mark at the beginning of the comment, and with point undefined.

  * Edwin's completion of Scheme variables is environment dependent.  In Scheme and Scheme Interaction modes, M-TAB completes the Scheme variable name.  Completion is environment-dependent, so a prefix may complete differently (or not at all) in two different buffers with different associated environments.

     DOS Note: Since Alt+TAB is an MS Windows hot key, you can get the same effect (without disabling it) by typing Alt+Ctrl+I.

  * M-x load-file and M-x load-library behave somewhat differently in Edwin.

  * The evaluation commands are much different in Edwin.

  * Edwin doesn't recognize Local Variables: at the end of a file; instead it recognizes Edwin Variables:.  This is done because it cannot support the full syntax of Emacs local variables.

  * Variable text-mode-hook is an event distributor rather than a procedure of no arguments.

Deficiencies
------------

Deficiencies are shortcomings of Edwin that are likely to be fixed.

  * C-x ^ can be used to grow or shrink the minibuffer window in Emacs but not in Edwin.

  * C-h in Edwin is a subset of that in Emacs. Missing subcommands:

          n   view-emacs-news
          c-c describe-copying
          c-d describe-distribution
          c-w describe-warranty

  * Dired is missing these commands in Edwin: v and B. Under DOS it is missing the following:

          v   view this file.
          B   byte-compile this file.
          M   change this file's permissions.
          G   change this file's group.
          O   change this file's owner.
          C   compress this file.
          U   uncompress this file.

  * Dired has the following subcommands that Emacs does not have:

          k   mark file for copying
          y   copy marked files

  * C-x n is not disabled by default.

  * Edwin does not have the notion of ``novice''.

Missing Subsystems
------------------

The following documented subsystems are implemented by Emacs but not by Edwin.

  * abbrev mode (edwin has dynamic abbreviations)

  * emulation modes: edt, vi, vip

  * file locks

  * language modes: emacs-lisp, fortran, lisp, modula-2, prolog

  * overwrite mode

  * picture mode

  * slow terminal incremental search

  * spelling

  * tab stops

  * text formatter modes: nroff, scribe, tex, latex

DOS Note:

Some modes are available under Unix, but are not included in the standard DOS edwin binary to reduce its size or because they don't work under DOS:

     Missing Command or mode                     Reason
     -----------------------                     ------

     * dabbrev                                   reduce size
       compile                                   needs subprocess support
       shell                                     needs subprocess support
                                                 + there is a DOS replacement
       techinfo                                  specific to MIT & Unix
       telnet                                    needs subprocess support
     * midas mode (assembly language editing)    reduce size
     * pascal mode                               reduce size
       texinfo                                   reduce size
       man                                       specific to unix
       print                                     does not work yet
     * run-notifier                              reduce size
     * outline mode                              reduce size
     * info                                      reduce size
       rcs                                       needs subprocess support
       sendmail                                  needs subprocess support
       malias                                    useless without sendmail
     * occur (and list-matching-lines)           reduce size
       rmail                                     needs subprocess support
                                                 + specific to Unix ?
       rmailsrt                                  useless without rmail

Many of the missing modes and commands (compile, shell, rcs, sendmail) require subprocess support.  They should be easy to bring up if subprocesses become available, however the code (or even the concept) may be Unix specific.  There is a pseudo-shell mode for DOS.

Many of the subsystems listed above have not been tried under DOS. The ones that are known to work are marked with an asterisk (*), although for some of them variables have to be set appropriately before use (e.g. info-directory for info).

Missing Commands
----------------

These commands are implemented by Emacs but not by Edwin.  The commands marked with an asterisk are implemented by the unix version of Edwin but not by the PC version.  Some of the asterisked comands can work in the PC version but the code to implement them is not loaded in order to save space; others are unix-specific and are not implemented on the PC.

       abbrev-mode
       abbrev-prefix-mark
       add-change-log-entry
       add-change-log-entry-other-window
       add-global-abbrev
       add-mode-abbrev
       add-name-to-file
       append-to-buffer
       byte-compile-file
       byte-recompile-directory
       cancel-debug-on-entry
     * compile
       convert-mocklisp-buffer
       copy-to-buffer
       debug-on-entry
       define-abbrevs
     * delete-matching-lines
     * delete-non-matching-lines
       describe-copying
       describe-distribution
       describe-no-warranty
       describe-syntax
       disable-command
       disassemble
       display-time                          (run-notifier is similar)
       dissociated-press
       doctor
       edit-abbrevs
       edit-abbrevs-redefine
       edit-options
       edit-picture
       edit-tab-stops
       edit-tab-stops-note-changes
       edt-emulation-on
       emacs-lisp-mode
       emacs-version
       enable-command
       expand-abbrev
       expand-region-abbrevs
       flush-lines
       fortran-mode
       global-set-key                        (set-key is similar)
       grep
       hanoi
       indent-c-exp                          c-indent-expression
       insert-abbrevs
       insert-kbd-macro                      write-kbd-macro
       insert-parentheses
       inverse-add-global-abbrev
       inverse-add-mode-abbrev
     * keep-lines
       kill-all-abbrevs
       latex-mode                            Note: BAL has one
       lisp-complete-symbol                  (scheme-complete-variable is similar)
       lisp-interaction-mode                 (inferior-repl-mode is similar)
       lisp-mode
       lisp-send-defun
       list-abbrevs
       list-command-history
       list-options
       list-tags
       local-set-key
     * lpr-buffer
     * lpr-region
       make-symbolic-link
       make-variable-buffer-local
     * manual-entry
       modify-syntax-entry
       move-past-close-and-reindent
       next-error
       next-file
       nroff-mode
     * occur
     * occur-mode-goto-occurrence
       open-dribble-file
       open-termscript
       overwrite-mode
       plain-tex-mode
       prepend-to-buffer
     * print-buffer
     * print-region
       read-abbrev-file
       recover-file
       run-lisp
       save-buffers-kill-emacs               save-buffers-kill-scheme
       set-gosmacs-bindings
     * shell-command
     * shell-command-on-region
       spell-buffer
       spell-region
       spell-string
       spell-word
       suspend-emacs                         suspend-scheme
       tab-to-tab-stop
       tags-apropos
       tex-mode
       top-level
       unexpand-abbrev
       vi-mode
       view-buffer
       view-emacs-news
       view-file
       vip-mode
       write-abbrev-file
       yow
       zap-to-char

Missing Variables
-----------------

These documented variables are implemented by Emacs but not Edwin.  The variables marked with an asterisk are implemented by the unix version of Edwin, but not by the PC version.

       abbrev-all-caps
       abbrev-file-name
       blink-matching-paren
       blink-matching-paren-distance
       buffer-read-only
       c-tab-always-indent
       comment-start-skip
     * compile-command
       completion-ignore-case                (not documented)
       ctl-arrow
       debug-on-error                        debug-on-editor-error
                                             debug-on-internal-error
                                             debug-on-evaluation-error
                                             are similar, but more specific
       debug-on-quit
       default-directory
       default-major-mode
       echo-keystrokes
       initial-major-mode                    Scheme variable initial-buffer-mode
       insert-default-directory
       inverse-video
       kill-ring-max
       load-path
     * lpr-switches
       major-mode
       mark-ring
       mark-ring-max                         mark-ring-maximum
       meta-flag
       no-redraw-on-recenter
       save-abbrevs
       selective-display-ellipses
     * shell-file-name
       tab-stop-list
       tags-file-name                        (tags-table-pathname is similar)
       track-eol
       visible-bell

Per-Buffer Variables
--------------------

These documented variables are per-buffer in Emacs but not in Edwin.

     abbrev-mode
     auto-fill-hook
     buffer-auto-save-file-name
     buffer-backed-up
     buffer-file-name
     buffer-offer-save
     buffer-read-only
     buffer-saved-size
     buffer-undo-list
     ctl-arrow
     default-directory
     local-abbrev-table
     major-mode
     mark-ring
     mode-name
     overwrite-mode
     selective-display
     selective-display-ellipses
     shell-prompt-pattern

Edwin Bugs
----------

Incorrect behavior of Edwin that will be fixed:

  * Negative argument to M-u, M-l, and M-c seems to work by accident.

  * -*- foo -*- is valid if preceded only by blank lines; in Edwin this works only if it is on a file's the first line.

  * Closing paren doesn't check for mismatched delimiters and issue warning message.  In c-mode, ] isn't treated specially, and don't flash the matching delimiter.

  * C-u C-x ; isn't handled right in Edwin.

  * M-x indent-for-comment shouldn't reindent existing comment in column zero.