Recursive descent parser: Difference between revisions

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A ''predictive parser'' is a recursive descent parser that does not require [[backtracking]].<ref name="Watson2017">{{cite book|author=Des Watson|title=A Practical Approach to Compiler Construction|url=https://books.google.com/books?id=05B0DgAAQBAJ&printsec=frontcover#v=onepage&q=%22predictive%20parser%22&f=false|date=22 March 2017|publisher=Springer|isbn=978-3-319-52789-5}}</ref> 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 grammar]]s, 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]].

Although predictive parsers are widely used, and are frequently chosen if writing a parser by hand, programmers often prefer to use a table-based parser produced by a [[parser generator]]{{Citation needed|date=February 2018}}, either for an LL(''k'') language or using an alternative parser, such as [[LALR parser|LALR]] or [[LR parser|LR]]. This is particularly the case if a grammar is not in [[LL parser|LL(''k'')]] form, as transforming the grammar to LL to make it suitable for predictive parsing is involved. Predictive parsers can also be automatically generated, using tools like [[ANTLR]].