Lisp in Forth

Original preface by Mark Probst

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.

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.

DEFECT

There was a defect removed in name look up, a spurious UNLOOP that crashed any Forth except gforth.

Remark AH

In struct.fs I have made the structs more portable and removed more portability issues.

• The struct's needed are pretty simple and are defined and understood by most Forth's.
• I replaced all LOCAL with VALUE (outside the definition).
• I defined the word ENDIF
• I also removed the word %allocate. Either the Forth knows allocate or it isn't. Renaming is silly.
• the words CELL is well established (no %cell). Now you can compile lisp.fs from most ISO-compliant Forth's, as long as they are case-insensitive, or are put into that mode.

The bottomline is that it now runs on pforth, SwiftForth, and mpeforth, and of course gforth. For lina/wina and other version of ciforth you must include preambule.frt first.

Remark AH

Using easy.fs will save you a lot of key strokes in using the three words above.

I added .l "dot L", not .1

The examples above become

    ev (+1 2 3) OK
    \ stack now contains a lisp object
    .l
    => 6 OK
    inc test.scm
    => 4 120 OK
    \ The stack now contains the empty list, c.q. false.
    .l
    => () OK

I added a function similar to VLIST/WORDS .symtab ==> eq? cdr car ..

Remark AH

Even the simple example in coins.scm cannot be run by this lisp. In scheme this calculates the number of ways to change one dollar.

    scheme -load coins.scm

#ADVANCED

The advanced subdirectory contains a version that relies on - classes/ objects - anonymous functions - parsing lisp directly It workes on all versions of ciforth lina/wina 32/64 .

This foregoes the need for any values, locals and variables, except one, (the ancre for the linked list env).