Forth

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A low-level functional programming language, mostly concatenative, with imperative features, for a stack-based VM model, invented by Chuck Moore. It is a very interesting language to study, both for its achievements and its shortcomings. The writer won't list those shortcomings of Forth here, and he doesn't list any of its achievements either. You'll just have to guess what he must have been thinking. -- A N Other

Central to Forth is the notion of a "word", corresponding to a function in other languages. A Forth program, after parsing, is nothing but a list of words, executed consecutively by the "inner interpreter", the core of Forth.

Forth is fully reflective and self-containing; the Forth parser (the so-called "outer interpreter") is written in Forth and can be modified at any time. It has also a set of words to access the input buffer (see for example the word PARSE). Thus, at least potentially, Forth can have any syntax.

Forth is usually untyped, much in the same way that Assembly is (though see strongForth below). More generally, Forth is a very low-level language, with manual management of memory, fixed-size buffers, pointer manipulation; but reflection makes it a remarkably powerful language among low-level languages.

See also Forth OSes, Forth is NOT intrinsically slow.

  • "Linear Logic and Permutation Stacks--The Forth Shall Be First" (".ps.gz"), a paper by Henry Baker
    Abstract:
    Girard's linear logic can be used to model programming languages in which each bound variable name has exactly one "occurrence"--i.e., no variable can have implicit "fan-out"; multiple uses require explicit duplication. Among other nice properties, "linear" languages need no garbage collector, yet have no dangling reference problems. We show a natural equivalence between a "linear" programming language and a stack machine in which the top items can undergo arbitrary permutations. Such permutation stack machines can be considered combinator abstractions of Moore's Forth programming language.