Forth

Elements of syntax:

Links:

Examples:

Hello, World!:

Word ." reads a double-quotes-delimited string and outputs it. The space character that separates ." and Hello doesn’t count as part of the string, it’s necessary to recognize ." as a word.

cr outputs a new line character and is an equivalent of nl in Prolog and endl in C++.

." Hello, World!" cr

Factorial:

This example uses recursive factorial definition. Forth is a stack-oriented language, so all commands (words) perform stack manipulations. Thus, dup duplicates the topmost element of the stack, a constant pushes itself on the top of the stack, > compares second-topmost element to the topmost element and pushes the result, - subtracts topmost element from second-topmost, etc.

: fac recursive
 dup 1 > IF
 dup 1 - fac *
 else
 drop 1
 endif ;
 
: lp
 swap 1 + swap
 do
 i . i ." ! = " i fac . cr
 loop ;
 
16 0 lp

Fibonacci numbers:

This example uses iterative definition of Fibonacci numbers.

: fib-iter
 0 1 rot 0 ?do over + swap loop drop ;
 
: lp
 1 do
 i dup fib-iter . ." , "
 loop drop ;
 
17 lp
." ..."

Fibonacci numbers:

This example uses recursive definition of Fibonacci numbers.

: fib-rec
 dup 2 u< if exit then
 1- dup recurse swap 1- recurse + ;
 
: lp
 1 do
 i dup fib-rec . ." , "
 loop drop ;
 
17 lp
." ..."