Extensions
Since DCGs were introduced by Pereira and Warren, several extensions have been proposed. Pereira himself proposed an extension called extraposition grammars (XGs). This formalism was intended, in part to make it easier to express certain grammatical phenomena, such as left-extraposition. Pereira states, "The difference between XG rules and DCG rules is then that the left-hand side of an XG rule may contain several symbols." This makes it easier to express rules for context-sensitive grammars.
Another, more recent, extension was made by researchers at NEC Corporation called Multi-Modal Definite Clause Grammars (MM-DCGs) in 1995. Their extensions were intended to allow the recognizing and parsing expressions that include non-textual parts such as pictures.
Another extension, called definite clause translation grammars (DCTGs) was described by in 1984. DCTG notation looks very similar to DCG notation; the major difference is that one uses ::= instead of --> in the rules. It was devised to handle grammatical attributes conveniently. The translation of DCTGs into normal Prolog clauses is like that of DCGs, but 3 arguments are added instead of 2.
Read more about this topic: Definite Clause Grammar
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