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parser combinator
In functional programming, a parser combinator is a higher-order function that accepts several parsers as input and returns a new parser as its output. In this context, a parser is a function accepting strings as input and returning some structure as output, typically a parse tree or a set of indices representing locations in the string where parsing stopped successfully. Parser combinators enable a recursive descent parsing strategy that facilitates modular piecewise construction and testing. This parsing technique is called combinatory parsing.
Parsers built using combinators are straightforward to construct, ‘readable,’ modular, well-structured and easily maintainable. They have been used extensively in the prototyping of compilers and processors for domain-specific languages such as natural-language interfaces to databases, where complex and varied semantic actions are closely integrated with syntactic processing. In 1989, Richard Frost and John Launchbury demonstrated use of parser combinators to construct natural-language interpreters. Graham Hutton also used higher-order functions for basic parsing in 1992. S.D. Swierstra also exhibited the practical aspects of parser combinators in 2001. In 2008, Frost, Hafiz and Callaghan described a set of parser combinators in Haskell that solve the long-standing problem of accommodating left recursion, and work as a complete top-down parsing tool in polynomial time and space.
==Basic idea==
In functional programming, parser combinators can be used to combine basic parsers to construct parsers for more complex rules. For example, a production rule of a context-free grammar (CFG) may have one or more ‘alternatives’ and each alternative may consist of a sequence of non-terminal(s) and/or terminal(s), or the alternative may consist of a single non-terminal or terminal or the empty string. If a simple parser is available for each of these alternatives, a parser combinator can be used to combine each of these parsers, returning a new parser which can recognise any or all of the alternatives.
A parser combinator can take the form of an infix operator, used to ‘glue’ different parsers to form a complete rule. Parser combinators thereby enable parsers to be defined in an embedded style, in code which is similar in structure to the rules of the grammar. As such, implementations can be thought of as executable specifications with all the associated advantages.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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