GitHub - darius/req: A rewrite-rule language

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bertrand		bertrand
test		test
.gitignore		.gitignore
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Makefile		Makefile
Notes		Notes
README		README
bug.h		bug.h
expr.c		expr.c
parse.c		parse.c
prim.c		prim.c
req.c		req.c
req.h		req.h
reqrc		reqrc
rule.c		rule.c
symbol.c		symbol.c

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	 req - a programmable calculator using rewrite rules

To build: 	make
To run: 	cp reqrc ~/.reqrc; ./req

Example runs:

 $ req 2+3
 5
 $ req
 > sphere_area(?r) == 4 * pi * ?r^2
 ok
 > sphere_area(R_earth) / (1 mile)^2   # Earth's surface area in square miles
 197060797.4
 > 

Term rewriting goes beyond defining functions like sphere_area here:
think of manipulating symbolic data in Mathematica (which is of course
much fancier).

This was my first substantial C program, written in the late 1980s. I
do still use it as a calculator (it's handy with the physical
constants and units), but I don't maintain or support it.


Notable files:

 Notes		The beginnings of a tutorial with some sample code

 test/          Directory with more sample code

 req.c		Main program

 reqrc		Sample .reqrc file

 bertrand/      Lame-o beginnings of an augmented term rewriter based on 
		Wm Leler's book _Constraint Programming Languages_ -- 
		I haven't looked at it in a long time and it probably
		doesn't work.

Thanks to Steve Jenson for getting it to build on OS X.

Darius Bacon <darius@wry.me>


P.S. on the history:

In school I took an unusual intro class on algorithms and data
structures from Dave Gillespie, the author of GNU Calc. The
assignments built on each other to develop a basic Mathematica-style
symbolic math program, in Pascal. (Mathematica did not exist yet --
rather, Gillespie was influenced by Wolfram's first system,
https://en.wikipedia.org/wiki/SMP_(computer_algebra_system).)
It was a fun course -- I think any loss of core subject focus for the
symbolic math stuff was outweighed by the interest and realism of the
project. Project courses don't have to be limited to compilers and
OSes.

The next summer, I learned C by working through K&R and then rewriting
this program in it. As a rewrite, it had some changes, but all I
remember is that I left out a few features meant to support symbolic
algebra in particular, leaving a more generic term-rewriting language.

At this point, while thinking about and prototyping some more serious
math support, I started feeling slowed down by C's low level,
especially for debugging. Lisp turned out to answer much better, and
that was the end of active development of req.

The program was complete enough though to be useful to me for physical
calculations, and occasionally handy for the symbolic manipulation
stuff. It's nice to have a tool you understand completely.

I think I posted this code to alt.sources in the 90s.