schani / forthlisp

A Small Lisp in Forth

Lisp in Forth

This is an interpreter for a really simple dynamically scoped Scheme dialect. It only runs with Gforth, because it uses Gforth's structs to implement its data structures. One of the more involved parts of this interpreter is the reader, where I had to do quite a lot of stack juggling to keep everything in line. It doesn't look very involved now but I remember spending quite some time thinking about the stack layout for the reader routines. Read here why the way I did it is most certainly wrong.

How to use it

Assuming you have Gforth installed (if you're on MacOS you can get it via Brew), start it with

gforth lisp.fs

There are mainly three words of interest:

• lisp-load-from-string reads and evaluates a string.

• lisp-load-from-file reads and evaluates a file, given a filename.

• lisp-display prints a lisp value.

Example:

s" (+ 1 2 3)" lisp-load-from-string lisp-display
=> 6 ok

s" test.scm" lisp-load-from-file lisp-display
=> 4 120 () ok

In this example, 4 and 120 are printed in test.scm, and () is the result of the evaluation of the file.

forthlisp's Issues

License?

Hi! Was curious - is there a license assigned to this code?

unloop, a trifle bizarre

I see an unloop in symtab-lookup. My conclusion is that it goes up two levels in the call chain.
A somewhat bizarre tail-call optimization.

You've have a very nice and portable program. With this you have triple environmental dependancy

1. a classical usage of the return stack for call addresses
2. the return stack is used for do loop parameters
3. do loop parameters occupy exactly two places

I had a hard time debugging it in lina because of 3. lina has 3 places for the loop parameters

In terms of clarity you program scores 9 or 10. The unloop makes it somewhere near 4.
So my advice, just leave out this unloop .

No comparison between special's

The dispatch table for eq? misses an entry for special.
(eq? cond define )
leads to a crash.

No comparison foreseen with lisp-false

If two flags are compared and they are unequal this leads to a crash
(eq? (eq? 1 2 ) (eq? 1 1 ) )