forth — a Forth interpreter

Run it in the playground ↗ Source on GitHub ↗

The stack-machine counterpart to the Lisp tree-walker. Forth has no parse tree: source is a flat stream of whitespace-separated words, each of which pushes a number or runs a dictionary entry against a shared data stack. : name … ; defines new words from old ones, so the language grows itself from a handful of primitives.

Run it

build/rakupp showcase/forth/forth.raku showcase/forth/examples/demo.fth
build/rakupp showcase/forth/forth.raku               # no file → a REPL
# or compile a standalone binary:
build/rakupp --exe -o forth showcase/forth/forth.raku

A taste

: square ( n -- n*n )  dup * ;
: fact ( n -- n! )  1 swap 1+ 1 do i * loop ;
5 square .      \ prints 25
10 fact .       \ prints 3628800

The included examples/demo.fth walks through arithmetic, word definitions, conditionals, both loop forms, and drawing with emit:

$ build/rakupp showcase/forth/forth.raku showcase/forth/examples/demo.fth
3 4 + 2 * = 14
5 square = 25
3 cube = 27
sign 7: positive
sign -3: negative
sign 0: zero
5! = 120
10! = 3628800
countdown from 5: 5 4 3 2 1
fib 10 = 55
a 10x3 box:
**********
**********
**********

Words it knows

Group	Words
arithmetic	`+ - * / mod /mod negate abs min max 1+ 1- 2* 2/`
comparison	`= <> < > <= >= 0= 0< 0>` (Forth truth: `0` false, `-1` true)
bitwise	`and or xor invert`
stack	`dup ?dup drop swap over rot nip tuck depth`
control	`if … else … then`, `begin … until`, `begin … while … repeat`, `do … loop` (`i`, `j`)
output	`. .s cr space spaces emit bl`
defining	`: name … ;`

How it works

Reader — trivial: strip \ and ( … ) comments, split on whitespace. The interest isn't the parsing, it's the execution model.
Compile — the token stream is parsed once into a small node tree, so the structured control words (if/else/then, the loops) become nested nodes instead of being re-scanned every time.
Execute — walk the nodes against a @stack: a literal pushes, a word runs its dictionary body (recursively) or a primitive block, do/loop tracks the index for i. A user definition is just its node list stored under a name.