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Forth

Language type:

T - Threaded or stack-based

Description:

Forth is an interpreted stack-based language with a very simple syntax and elegant abstract exection model. Designed for efficiency and simplicity, Forth is noted for the very small size of the language system.
A Forth program is a sequence of words. Each word is independent, and causes some specified action. Even constants are words: the word "12" is just a command to push the number 12 onto the data stack. Sequences of words can be grouped as procedures, and employed to build up modular programs. In order for this to work, of course, operators and operands must be given in reverse polish order (operands followed by operator). All data manipulation takes place on the data stack, so Forth offers several operations for manipulating the stack.
Forth implementations vary in the range of data types they provide, but most provide a conventional set of numeric and string types, plus arrays. Forth offers regular control structures: If-Else, While-Do, and For-Next, albeit with peculiar ordering of the keywords. Most dialects of Forth provides simple file handling and I/O support.
Forth is also meant to be extensible at every level. Programmers can define new words, and implement them in Forth itself, in some other high-level language, or in machine code.
Information about Forth is widely available on the Internet. There are several commercial and implementations, many available from here.

Origin:

Charles H. Moore, 1969-1971.

See Also:

Postscript   

Remarks:

Forth is often employed for controlling micro-computers and embedded systems, especially at boot-up prior to loading of a more complex OS. Because Forth is small and fast, it is well-suited for embedding in ROM. Forth is also used for creating general-purpose applications, even CGI scripts.
Forth is standardized as ANS-Forth; the standard is available here. There is also an IEEE standard in draft for using Forth as a standard firmware language, IEEE 1275.
Forth's stream-of-conciousness syntax, polymorphism, and typically terse style make reading real Forth code a challenging exercise.
The first real application of what we call Forth was controlling telescopes at the NRAO at Kitt Peak, in 1971. Forth became very popular in the astronomy community, and was adopted as the standard programming language by the International Astronomical Union in 1976.

Links:

Forth FAQ Lists

Forth Information page at Taygeta Scientific

Prof. Koopman's Stack Computers and Forth pages

Extensive Forth Tutorial

Date:

Last updated 2/28/98

Sample code:

\ Forth implementation of Newton's method for finding
\ roots, simplified. (c) Copyright 1994 Everett F. Carter.

: z1 ( i -- ) ( f: -- z1 )
      z F@ xn{ SWAP } F@ F-
;

: Newton ( i -- ) ( f: e d p -- e d p )
             \ calculate new D
             DUP z1 FROT F* FOVER F+

             \ calculate new P
             FSWAP DUP z1 F* dif{ OVER } F@ F+

             \ calculate new E
             FROT z1 FABS F* FOVER FABS F+

             \ restore stack order
             FROT FROT
;

: FNewt ( &xn &dif n -- ) ( f: z -- e d p)
        >R
        & dif{ &!
        & xn{  &!
        R>

        z F! 0.0e0  0.0e0 0.0e0
        0 DO
             I Newton
          LOOP
;

Descriptions in this dictionary are ©1997-99 Neal Ziring. Some examples copyright of their respective authors. Some technologies and languages are trademarked. Permission to copy descriptions is granted as long as authorship credit is preserved.

Comments on this dictionary, corrections and suggestions, are all welcome. Please use email, the address is ziring@home.com