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Forth Programming Tutorial

Forth is a stack-based, concatenative programming language invented by Charles Moore in 1970. It is beloved for its simplicity, efficiency, and close relationship with the machine.

The Stack

Everything in Forth revolves around a data stack. Numbers are pushed onto the stack; words (functions) pop and push values.

5 3 +      \ Push 5, push 3, add -> 8 on stack
.          \ Print top of stack: 8

Basic Arithmetic

10 4 + .    \ 14
10 4 - .    \ 6
10 4 * .    \ 40
20 4 / .    \ 5
17 5 MOD .  \ 2  (remainder)

Stack Manipulation

Word Effect Description
DUP ( n -- n n ) Duplicate top
DROP ( n -- ) Discard top
SWAP ( a b -- b a ) Swap top two
OVER ( a b -- a b a ) Copy second
ROT ( a b c -- b c a ) Rotate top 3 
5 DUP  . .      \ 5 5
3 7 SWAP . .    \ 3 7  -> prints 3, then 7
2 4 OVER . . .  \ 2 4 2

Defining Words

\ Simple word definition
: GREET ." Hello from Forth!" CR ;
GREET

\ Word with computation
: SQUARE DUP * ;
5 SQUARE .    \ 25

: CUBE DUP DUP * * ;
3 CUBE .      \ 27

\ Factorial (recursive)
: FACTORIAL
    DUP 1 <= IF DROP 1
    ELSE DUP 1 - FACTORIAL *
    THEN ;

6 FACTORIAL .    \ 720

Control Flow

\ IF ... THEN
5 3 > IF ." 5 is greater" THEN CR

\ IF ... ELSE ... THEN
10 DUP 0 > IF ." positive" ELSE ." non-positive" THEN CR

\ DO ... LOOP (definite loop)
10 1 DO
    I .
LOOP CR

\ BEGIN ... UNTIL (indefinite loop)
1
BEGIN
    DUP .
    2 *
    DUP 100 >
UNTIL DROP CR

String Output

\ Print string literals
." Hello, World!" CR
." Time Warp Studio" CR

\ CR outputs a newline
: SEPARATOR
    40 0 DO ." - " LOOP CR ;

SEPARATOR

Stack Comments Convention

By convention words are documented with stack effect comments:

( before -- after )

: SQUARE ( n -- n^2 )   DUP * ;
: + ( a b -- sum )       \ built-in

Working Example

\ Print a multiplication table
: .TABLE
    10 1 DO
        10 1 DO
            I J * 4 .R
        LOOP CR
    LOOP ;

.TABLE

Further Reading