Forth Cheat Sheet

Core Forth stack-manipulation words, custom word definitions, conditionals, and loops for this minimalist stack-based language.

1 PageAdvancedApr 8, 2026

Stack Manipulation

Core words for manipulating the data stack.

forth

5 3 + .        \ push 5, push 3, add, print top -> 810 DUP *       \ duplicate 10 then multiply -> 1001 2 SWAP       \ stack becomes: 2 13 4 DROP       \ removes top of stack -> 31 2 OVER       \ copies 2nd item to top -> 1 2 1

Defining Words

Creating new words with : ... ; and stack-effect comments.

forth

: SQUARE ( n -- n^2 ) DUP * ;5 SQUARE .        \ prints 25: GREET ( -- ) ." Hello, World!" CR ;GREET

Conditionals

IF ELSE THEN branching.

forth

: TEST ( n -- )  DUP 0> IF    ." Positive"  ELSE    ." Not positive"  THEN ;5 TEST   \ prints Positive

Loops

DO LOOP for counted iteration, BEGIN UNTIL for conditional iteration.

forth

: COUNT-UP ( n -- )  1 DO    I .  LOOP ;5 COUNT-UP        \ prints 1 2 3 4: COUNTDOWN ( n -- )  BEGIN    DUP .    1-  DUP 0= UNTIL  DROP ;

Pro Tip

Always write the stack-effect comment `( before -- after )` immediately after a word's name — Forth has no type checking, so this comment is the only documentation of what a word expects and leaves on the stack.

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