forth.r

REBOL [
    subject "FORTH interpreter"
]

forth: make object! [
    stack: []

    dup: [ 'dup (insert stack stack/1) ]
    drop: [ 'drop (remove stack) ]
    drop-n: [ 'drop-n (remove/part stack stack/1) ]
    add: [ '+ (insert stack (take stack) + (take stack)) ]
    sub: [ '- (insert stack (take stack) - (take stack)) ]
    mul: [ '* (insert stack (take stack) * (take stack)) ]
    divider: to-lit-word "/"
    div: [ divider (insert stack (take stack) / (take stack)) ]

    command: [ dup | drop |  drop-n  | add  | sub | mul | div ]
    token: [ set n [ number! | time! ] (insert stack n) | command ]
    rules: [ any token (print mold stack) ]

    run: func [ expr ] [ parse expr rules ]
]