Definite clause grammar

A definite clause grammar (DCG) is a way of expressing grammar, either for natural or formal languages, in a logic programming language such as Prolog. It is closely related to the concept of attribute grammars / affix grammars from which Prolog was originally developed. DCGs are usually associated with Prolog, but similar languages such as Mercury also include DCGs. They are called definite clause grammars because they represent a grammar as a set of definite clauses in first-order logic. The term DCG refers to the specific type of expression in Prolog and other similar languages; not all ways of expressing grammars using definite clauses are considered DCGs. However, all of the capabilities or properties of DCGs will be the same for any grammar that is represented with definite clauses in essentially the same way as in Prolog. The definite clauses of a DCG can be considered a set of axioms where the validity of a sentence, and the fact that it has a certain parse tree can be considered theorems that follow from these axioms. This has the advantage of making it so that recognition and parsing of expressions in a language becomes a general matter of proving statements, such as statements in a logic programming language. The history of DCGs is closely tied to the history of Prolog, and the history of Prolog revolves around several researchers in both Marseille, France, and Edinburgh, Scotland. According to Robert Kowalski, an early developer of Prolog, the first Prolog system was developed in 1972 by Alain Colmerauer and Phillipe Roussel. The first program written in the language was a large natural-language processing system. Fernando Pereira and David Warren at the University of Edinburgh were also involved in the early development of Prolog. Colmerauer had previously worked on a language processing system called Q-systems that was used to translate between English and French. In 1978, Colmerauer wrote a paper about a way of representing grammars called metamorphosis grammars which were part of the early version of Prolog called Marseille Prolog.