A "concatenative language"

Predefined operations

dup

Duplicates the top value on the stack

[1, 2, 3] dup

[1, 1, 2, 3]

drop

Drops the top value on the stack

[1, 2, 3] drop

[2, 3]

swap

Swaps the top two values on the stack

[1, 2, 3] swap

[2, 1, 3]

skip

Skips forward in the program by the amount on the top of the stack. If the value on the top of the stack is 0 or negative, the value is dropped and the program does not skip ahead.

[1, 2, 3] skip drop dup

[2, 3] dup

[2, 2, 3]

[-1, 2, 3] skip drop dup

[2, 3] drop dup

[3] dup

[3, 3]

: word ... ;

Creates a new definition.

Example: ": double 2 * ;" defines a new word called double. Any subsequent mention of double will replace the word with its definition.

[] : double 2 * ; 3 double

[] 3 double

[3] double

[3] 2 *

[2, 3] *

[6] <==

Example programs

Factorial

Replace program

                : counter   dup 2 swap - ;
                : !   dup 1 - counter skip ! * ;
                7 !