Editing Prolog (section)

== Parsing ==
{{Main|Prolog syntax and semantics#Definite clause grammars|Definite clause grammar}}
There is a special notation called [[definite clause grammar]]s (DCGs). A rule defined via <code>-->/2</code> instead of <code>:-/2</code> is expanded by the preprocessor (<code>expand_term/2</code>, a facility analogous to macros in other languages) according to a few straightforward rewriting rules, resulting in ordinary Prolog clauses. Most notably, the rewriting equips the predicate with two additional arguments, which can be used to implicitly thread state around,{{Clarify|date=October 2012}} analogous to [[monads in functional programming|monads]] in other languages. DCGs are often used to write parsers or list generators, as they also provide a convenient interface to difference lists.