Recursive descent parser: Difference between revisions

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A '''recursive descent parser''' is a top-down [[parser]] built from a set of [[Mutual recursion|mutually-recursive]] procedures (or a non-recursive equivalent) where each such [[procedure]] usually implements one of the production rules of the [[formal grammar|grammar]]. Thus the structure of the resulting program closely mirrors that of the grammar it recognises.

A '''predictive parser''' is a recursive descent parser with no backup. Predictive parsing is possible only for the class of [[LL parser|LL(k)]] grammars, which are defined as the context-free grammars 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 parser|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 parser|LL(k)]] grammar.) A predictive parser runs in [[linear time]].

Recursive descent with backup is a technique that determines which production to use by trying each production in turn. Recursive descent with backup is not limited to [[LL parser|LL(k)]] grammars, but is not guaranteed to terminate even when the grammar is [[LL parser|LL(k)]]. Even when they terminate, parsers that use recursive descent with backup may run in [[exponential time]].