Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!caen!ox.com!emv
From: gjc@mitech.COM
Newsgroups: comp.archives
Subject: [scheme] Alternative scheme syntax: PRATT.SCM in Siod V2.4
Message-ID: <1991Feb4.003611.204@ox.com>
Date: 4 Feb 91 00:36:11 GMT
References: <9102030731.AA04629@schizo>
Sender: emv@ox.com (Edward Vielmetti)
Reply-To: gjc@mitech.com
Followup-To: comp.lang.scheme
Organization: The Internet
Lines: 73
Approved: emv@ox.com (Edward Vielmetti)
X-Original-Newsgroups: comp.lang.scheme

Archive-name: languages/scheme/siod/1991-02-02
Archive: bu.edu:/users/gjc/siod-v2.4-shar [128.197.2.6]
Original-posting-by: gjc@mitech.COM
Original-subject: Alternative scheme syntax: PRATT.SCM in Siod V2.4
Reposted-by: emv@ox.com (Edward Vielmetti)

One alternative syntax used in the days of Maclisp at MIT
was called CGOL. A parser that uses the same techniques as
CGOL is available in Siod Version 2.4

{"Advertisement" for siod follows, skip to closing bracket to
get a discussion of how to hook up alternative syntax

-----------------------------------------------------------------

What is SIOD? It is an extremely small scheme implementation in C
arranged as set of subroutines that can be called from any main
program for the purpose of introducing an interpreted extension
language. 

How small is it? Compiling using the VAX/VMS C compiler results in a
total of 16333 bytes of executable code.

What features? 
 * Lisp calls C and C calls lisp transparently
 * no hassle in C coded functions due to GC or EVAL considerations.
 * ability to hook into the read-eval-print loop without modifying the code.
 * ability to extend the datatypes without modifying the code.
 * mark-and-sweep or stop-and-copy GC selectable at runtime.
 * readmacros
 * macros
 * backquote (quasi-quote)
 * optionally loadable pratt (infix-language) parser

Platforms?
 * VAX/VMS * VAX UNIX * SUN-3 * SUN-4 * AMIGA * MACINTOSH * MIPS * CRAY

Availability?

Anonymous FTP to BU.EDU (128.197.2.6), cd to users/gjc and get siod-v2.4-shar
(Get *HQX* files for Macintosh).

Copyright? Has MIT/X-WINDOWS style copyright notice. Free for any purpose,
including commercial use. 

}

Q:  Once you have a parser how do you turn off and turn on the
    alternative syntax?
A1: In MACLISP and COMMON-LISP, one technique was to create
    a readtable where every character, except for whitespace
    perhaps, was set up to invoked the parser.
    Change of syntax could be enabled by setting and/or binding
    the readtable.
A2: A less pervasive technique was to use a single readmacro combination,
    for example: #$, to introduce a segment of alternative syntax
    inside a section of lisp code.

Q:  Is there any provision in the ieee standard or R^4 for alternative
    syntax hooks (e.g. readmacros).

(define (fact x) #$ if x = 0 then 1 else x * fact(x) $)

#$ fact(x) := if x = 0 then 1 else x * fact(x) $

A clever trick/convention on the lispmachine was that the readtable,
package, and the input radix, could be *bound* before a file was loaded,
by a specification at the very start of a file. The old "file mode line"
hack. Without something like that it is a bit dangerous to modify
the readtable in such a drastic way.


-gjc