Recursive descent parser: Difference between revisions

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In [[computer science]], a '''recursive descent parser''' is a kind of [[top-down parsing|top-down parser]] built from a set of [[mutual recursion|mutually recursive]] procedures (or a non-recursive equivalent) where each such [[procedure (computer science)|procedure]] implements one of the [[Terminal and nonterminal symbols|nonterminals]] of the [[formal grammar|grammar]]. Thus the structure of the resulting program closely mirrors that of the grammar it recognizes.<ref>{{cite book | title=Recursive Programming Techniques | author=Burge, W.H. | year=1975 | isbn=0-201-14450-6}}</ref>

A ''predictive parser'' is a recursive descentdescen parser that does not require [[backtracking]]. Predictive parsing is possible only for the class of [[LL parser|LL(''k'')]] grammars, which are the [[context-free grammar]]s for which there exists some positive integer ''k'' that allows a recursive descent parser to decide which production to use by examining only the next ''k'' tokens of input. The LL(''k'') grammars therefore exclude all ambiguous grammars, as well as all grammars that contain [[left recursion]]. Any context-free grammar can be transformed into an equivalent grammar that has no left recursion, but removal of left recursion does not always yield an LL(''k'') grammar. A predictive parser runs in [[linear time]].

Recursive descent with backtracking is a technique that determines which [[Production rule (formal languages)|production]] to use by trying each production in turn. Recursive descent with backtracking is not limited to LL(k) grammars, but is not guaranteed to terminate unless the grammar is LL(k). Even when they terminate, parsers that use recursive descent with backtracking may require [[exponential time]].