This file documents the installation and use of UMB Scheme.

Copyright (C) 1988, 1990 William R Campbell.

Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies.

Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the section entitled ``GNU General Public License'' is included exactly as in the original, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the section entitled ``GNU General Public License'' and this permission notice may be included in translations approved by the Free Software Foundation instead of in the original English.

Introduction

This manual documents how to install and use UMB Scheme as well as its new features and incompatibilities, and how to report bugs.

GNU GENERAL PUBLIC LICENSE
**************************

Version 1, February 1989

     Copyright © 1989 Free Software Foundation, Inc.
     675 Mass Ave, Cambridge, MA 02139, USA

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

Preamble
========

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END OF TERMS AND CONDITIONS

Why We Implemented UMB Scheme
*****************************

Our reasons form implementing yet another version of Scheme are the following:

 1. We use Scheme for modeling the operational semantics of programming languages in our undergraduate languages course.  We wanted a Scheme system that was easily ported to the various architectures on campus and that gave reasonable run-time performance.

 2. We wanted to produce a relatively large piece if code as an example of good program organization for our undergraduate and graduate software engineering students.

 3. We wanted to involve students in all phases of the implementation effort, including initial development, testing, extensions, and performance tuning.

 4. We wanted to illustrate that object-oriented design might successfully be applied to programs written in vanilla programming languages such as C.

Although this effort was meant as an exercise in design and programming, we have ended up with what we think to be a rather nice interpreter.  It runs relatively fast (performance comparisons with MIT Scheme are made below) and we (as well as others not involved in development) have found the code very easy to maintain and extend.

What's Supported by UMB Scheme
******************************

UMB Scheme is an implementation of the language described in the IEEE STANDARD FOR THE SCHEME PROGRAMMING LANGUAGE (December, 1990).

All syntax, variables and procedures are implemented. Integers are implemented as fixnums and bignums, rationals as pairs of integers, (inexact) reals as double-precision floats, and (inexact) complex numbers as pairs of double-precision floats.

The following files are loaded in order at startup:

If the variable SCHEME_INIT is set in the user's environment by executing

             setenv SCHEME_INIT file

then file is loaded.

If SCHEME_INIT is not set and if a file .SCHEME exists in the user's home directory then it is loaded.

The files named as optional arguments are loaded from left to right.

The primitive procedure EDIT may be used for editing files during a scheme session.

     	(edit filename)
     	(edit)

Filename is a string object specifying the file to be edited. If no filename is given then that file that was most recently edited is assumed. The editor used is taken from the shell variable, EDITOR, in the user's environment; if this variable is not set then VI(1) is used by default. The user can make sure EDITOR is always set by putting a setenv in his .LOGIN file; e.g.

     	setenv EDITOR /usr/ucb/emacs

Upon leaving the editor, that file specified by filename is automatically loaded using the primitive procedure LOADV, which causes the interpreter output to be sent to the current output port, normally the user's terminal. If verbose loading is not desired use the commands

     	(edits filename)
     	(edits)

causing filename to be loaded silently by the primitive procedure LOAD.

LOAD and LOADV can be used to load any file in silent or verbose mode respectively:

     	(load filename)
     	(loadv filename).

UMB Scheme has PROPERTY LISTS:

     	(put symbol property-name object)
     	(get symbol property-name)

where PROPERTY-NAMES are symbols.

UMB Scheme has a simple
macro facility:

             (macro name transformer)

defines a macro with the given name. Transformer is a procedure of one argument (bound to the calling expression list upon invocation) that defines the translation.  For example, assuming quasi-quote notation, the definition

             (macro := (lambda (form) `(set! ,(cadr form) ,(caddr form))))

defines a macro := that translates forms like

             (:= a b)    to    (set! a b)

An alternative form of the macro definition is

             (defmacro (name arg) body)

allowing one to write, for example,

             (defmacro (:= form)
                 `(set! ,(cadr form) ,(caddr form)))

UMB Scheme has a simple debugger.  Throughout a session one is in one of two modes: TOP-LEVEL MODE or DEBUGGING MODE. In general, one works in top-level mode.  If the debugger has been turned on, an error raised during an evaluation (or an explicit call to break) causes a BREAK which puts the user into debugging mode. The user can place explicit calls to break or error in his code:

     	(break obj ...)	; Print the objects and break the evaluation.
     	(error obj ...)	; Print the objects and raise an error.

In debugging mode, certain primitives apply for finding the cause of the offense. Notice that syntax errors cause a RESET, returning the user to top-level mode.  When the debugger is turned off, all errors simply cause a reset to top-level mode.

     	(debug)	; turn on the debugger
     	(debug-off)	; turn off the debugger.

NB: When the debugger is turned on, UMB Scheme is no longer properly tail-recursive as required by the language definition.  For this reason, the debugger is turned OFF by default.  One can insure the debugger is always turned on by putting a call to debug in the Scheme Init file (e.g. .scheme).

Any scheme expression may be evaluated in debugging mode.  It is evaluated in the environment that existed when the break occurred in the top level computation; this makes it easy to find the bindings for local variables. In addition, the following primitives apply.

     	(reset)	; Return to the top-level read-eval-print loop.
     	Control-D	; Typing Control-D causes a (reset).

     	(show-env)	; Show the bindings of all local environments.
     	(show-env k)	; Show-env for only the k most recent frames.
     	(show-globals)	; Show bindings for all user-defined globals.
     	(show-proc-env proc)	; Show a procedure's environment.
     	(how symbol)	; Show the expression causing symbol's binding.

     	(where)		; Show an enumerated backtrace of the
     				; computation being debugged.
     	(where k)	; Show the most recent k steps of the backtrace.

     	(go obj)	; Resume the computation being debugged,
     				; substituting the value of obj for the most recent
     				; step (as indicated by a call to where).
     	(go k obj)	; Likewise, but obj is substituted for the k-th step
     				; as enumerated in the backtrace by (where).

Tracing a procedure involves interrupting evaluation when either the procedure is about to be applied or the procedure is about to return with a value. Upon such an interruption, the call or the returned value is printed, and the user is put in debugging mode.

     	(trace proc ...)	; Trace named procedures.
     	(trace)	; Trace all procedures.
     	(untrace proc ...)	; Cancel tracing for named procedures.
     	(untrace)	; Cancel tracing for all procedures.

     	Control-D	; Resume the computation interrupted in the trace.

Stepping through a computation involves interrupting evaluation at every k-th expression, for a given k.  The expression in question is printed and the user is put in debugging mode.

     	(step k)	; Interrupt evaluation at every k-th expression.
     	(step 0)	; Inhibit stepping altogether.

     	Control-D	; Resume the evaluation broken in the stepping.

Notice that, when stepping or tracing is in effect, any one of a number of events (the application of a procedure or the k-th expression being reached) will interrupt evaluation.  Since typing Control-D resumes the interrupted computation, one can step through such a computation by repeatedly typing Control-D.

Finally, errors or explicit calls to break arising while in debugging mode simply leave the user in debugging mode.  Unlike some other implementations, UMB Scheme does not support nested debugging sessions.  (Keep it simple.)

Remarks on Performance
**********************

So far as speed is concerned, UMB Scheme loses to MIT's C Scheme in pure evaluation but performs significantly better than C Scheme when it comes to compile-load-and-go.

For example, consider the following TAK benchmark code from Richard P Gabriel, PERFORMANCE AND EVALUATION OF LISP SYSTEMS, MIT Press, 1985.

     ; tak
     ; function-call-heavy; test of function call and recursion

     (define (tak x y z)
       (if (not (< y x))
           z
           (tak (tak (- x 1) y z)
     	   (tak (- y 1) z x)
     	   (tak (- z 1) x y))))

     (tak 18 12 6)

On our Sun 3/140, C Scheme gets through this in 75 seconds while UMB Scheme takes 98 seconds; in this instance C Scheme runs faster.

But consider a definition-heavy file of code such as the following.

     (define (fac x) (if (= x 0) 1 (* x (fac (- x 1))) ))
     (fac 5)

     (define (fac x) (if (= x 0) 1 (* x (fac (- x 1))) ))
     (fac 5)

     (define (fac x) (if (= x 0) 1 (* x (fac (- x 1))) ))
     (fac 5)

     ;; ...  1000 copies

To get through these 1000 definitions of (and calls to) FAC, C Scheme takes 432 seconds but UMB Scheme takes only 19 seconds; in this instance UMB Scheme runs 22 times faster than C Scheme.

Therefore, UMB Scheme appears to do much better in a heavy compile-load-and-go environment (such as one would find in an undergraduate languages course) than in raw computation.

Space is more difficult to address when comparing Scheme systems since it depends on one's choice for a heap size.  UMB Scheme's load module has a size of 216K bytes; C Scheme's load module is 279K bytes.  As they are currently configured, UMB Scheme runs in a 1600K region as compared to C Scheme's 3944K (as measured by top(1) on Unix). UMB Scheme's storage allocator begins with a small (200K byte) heap and increases heap size as necessary (and as permitted by the operating system) at garbage collection time.

Contributors to UMB Scheme
**************************

  * The interpreter is based on the Explicit-Control Evaluator described in the textbook: Harold Abelson and Gerald Jay Sussman, STRUCTURE AND INTERPRETATION OF COMPUTER PROGRAMS, MIT Press, Cambridge, Massachusetts, 1985.

  * Bill Campbell was the primary author and project manager.

  * The idea of organizing the control of the interpreter around objects comes from Richard Schooler.

  * Karl Berry and Kathy Hargreaves helped with the initial programming.

  * Bill McCabe coded some performance improvements, particularly lexical variable addressing.

  * Mary Glaser, Tim Holt, Long Nguyen and Thang Quoc Tran worked on numbers.

  * Ira Gerstein and Jeyashree Sivasubram help bring UMB Scheme up to R4RS standard.

  * Barbara Dixey, Susan Quina and Bela Sohoni coded some additional performance improvements, handling varying numbers of arguments to functions in C.

  * Many undergraduate students at UMB wrote test programs and offered suggestions for improvement.

Compiling and Installing UMB Scheme
***********************************

UMB Scheme has been installed successfully using both CC and GCC (the Gnu C compiler) on the following machines and operating systems:

  * Sun 3 and Sun 4 running Unix.

  * Sun 386i running Sun Unix 4.0.1.

  * DEC Vax 11/750 running Unix 4.3 BSD.

  * DEC Decstation 5000 running Ultrix.

UMB Scheme was written so as to be easily ported to other systems. Some modifications may be required.

To install UMB Scheme one of the systems listed above, simply go into the distribution directory and do the following.

 1. Carefully read the file portable.h file.  Any minor modifications will want to be made here.

     Of particular importance are ALIGNMENT and NEGATIVE_ADDRESSES; see portable.h for details.

 2. Compile UMB Scheme using the Makefile provided.  If you have the Gnu C Compiler, GCC, then simply type

          	make

     or,

          	make CC=gcc

     Otherwise, to use CC, type

          	make CC=cc CFLAGS=-O

     This will compile all necessary source files and create the executable file scheme.

 3. Install the executable file in some public bin, e.g.

          	cp scheme /usr/local/bin/scheme

 4. Install the UMB Scheme STANDARD PRELUDE in the public library.

          	mkdir /usr/local/lib/scheme
          	cp prelude.scheme /usr/local/lib/scheme

     This pathname is wired into UMB Scheme.  If necessary, you can change the definition of STANDARD_PRELUDE_FILENAME in the source file steering.c.

 5. Copy the manual page to the appropriate directory, e.g.

          	cp scheme.l /usr/spool/man/man1

     Some shops maintain a separate manl directory for local manual pages; consult your local system administrator.

That should do it!

Using UMB Scheme
****************

Invoking UMB Scheme is straightforward; simply type

     	scheme [names of any scheme files to be loaded]

To get out of UMB Scheme type Control-D.

See the man pages (scheme.l) for details.

What To Do About Bugs
*********************

UMB Scheme has been used by many undergraduate students and several bugs have been exposed and repaired.  No doubt, bugs remain.  The author would appreciate hearing about any bugs found (and any fixes made).

Send reports to

     	bill@cs.umb.edu

or, by postal service, to

     	Bill Campbell
     	Department of Mathematics and Computer Science
     	University of Massachusetts at Boston
     	Harbor Campus
     	Boston, MA 02125

     	617-287-6449

Distribution files
******************

     Distribution Files for UMB Scheme Version 1.1

     README               This list of files.
     scheme.texinfo       UMB Scheme Release Notes.
     Makefile             For compiling UMB Scheme.
     prelude.scheme       The Scheme prelude, primitives loaded at start.
     scheme.l             UMB Scheme manual page (in man(1) format).

     Source files:

     portable.h           Portability considerations.

     steering.c           UMB Scheme steering -- including main().
     steering.h

     debug.c              Debugger steering and debug primitives.
     debug.h

     architecture.c       Registers, stacks, heap, name handling.
     architecture.h

     object.c             Scheme objects.
     object.h

     io.c                 Read, display, print, file handling.
     io.h

     compiler.c           Compiler: Expression -> Graph.
     compiler.h

     eval.c               Eval().
     eval.h

     primitive.c          (Non-numeric) primitives implemented in C.
     primitive.h

     number.c             Number primitives.
     number.h

     bignum.c             Bignum (integers of arbitrary magnitude).
     bignum.h

     complex.c            Complex number support.
     complex.h

     fixnum.c             Fixnum (smaller integers) support.
     fixnum.h

     real.c               Real number (C double) support.
     real.h

     rational.c           NB: rationals not yet implemented (skeletons).
     rational.h

     See the Release Notes (scheme.texinfo) for installation.