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Forth language

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Overview

Syntax

( comment... requires a space after first left-paren )

( define a word with colon and semicolon )
: timesTwo 2 * ;

( everything else is either a number, which pushes to the stack, or a word )
5 2 + 3 * 10 - timesTwo

Words are often “documented” by indicating how the stack is transformed: ( n -- n n ) means n was on the top of the stack, and n n is now at the top of the stack (n is duplicated).

Interesting words:

.s ( show stack )
. ( pop off top of stack, show it )
clearstack
invert ( -1/0 -- 0/-1 // instead of "not" )
see [word] ( show definition of a word )
dup ( n -- n n )
2dup ( a b -- a b a b )
drop ( n -- )
swap ( a b -- b a )
over ( a b -- a b a )
2over ( a b c d -- a b c d a b )
rot ( a b c -- b c a )
2rot ( a b c d e f -- c d e f a b )

Conditionals

A -1 (true) or 0 (false) value should be at the top of the stack.

if [words to execute when if is true] else [alternative words] then

The else part is not required.

Loops

A do loop expands at compile time, which is nice because you do not need to keep any values on the stack. The words inside the loop are repeated num1-num2+1 times.

[num1] [num2] do [words to repeat] loop

Example follows. In this case, the words inside the loop are repeated five times.

5 1 do [words to repeat] loop

A more sophisticated loop is begin/until. In this case, the word just before until must return -1 (true) or 0 (false) to indicate if the loop should stop. -1 means stop, 0 means repeat. Because the code inside the loop can decide whether or not it repeats, this loop is maximally general. The do/loop, on the other hand, repeats a fixed number of times (determined by what’s on the stack just before the loop starts).

begin [words to repeat] [-1/0 value, or word to produce that value] until
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