Text.ParserCombinators.Parsec.Perm

This module implements permutation parsers. The algorithm used is fairly complex since we push the type system to its limits :-) The algorithm is described in:

Parsing Permutation Phrases, by Arthur Baars, Andres Loh and Doaitse Swierstra. Published as a functional pearl at the Haskell Workshop 2001.

Documentation

data PermParser tok st a

The type PermParser tok st a denotes a permutation parser that, when converted by the permute function, parses tok tokens with user state st and returns a value of type a on success.

Normally, a permutation parser is first build with special operators like (<||>) and than transformed into a normal parser using permute.

permute :: PermParser tok st a -> GenParser tok st a

The parser permute perm parses a permutation of parser described by perm. For example, suppose we want to parse a permutation of: an optional string of a's, the character b and an optional c. This can be described by:

  test  = permute (tuple <$?> ("",many1 (char 'a'))
                         <||> char 'b'
                         <|?> ('_',char 'c'))
        where
          tuple a b c  = (a,b,c)

(<||>) :: PermParser tok st (a -> b) -> GenParser tok st a -> PermParser tok st b

The expression perm <||> p adds parser p to the permutation parser perm. The parser p is not allowed to accept empty input - use the optional combinator (<|?>) instead. Returns a new permutation parser that includes p.

(<$$>) :: (a -> b) -> GenParser tok st a -> PermParser tok st b

The expression f <$$> p creates a fresh permutation parser consisting of parser p. The the final result of the permutation parser is the function f applied to the return value of p. The parser p is not allowed to accept empty input - use the optional combinator (<$?>) instead.

If the function f takes more than one parameter, the type variable b is instantiated to a functional type which combines nicely with the adds parser p to the (<||>) combinator. This results in stylized code where a permutation parser starts with a combining function f followed by the parsers. The function f gets its parameters in the order in which the parsers are specified, but actual input can be in any order.

(<|?>) :: PermParser tok st (a -> b) -> (a, GenParser tok st a) -> PermParser tok st b

The expression perm <||> (x,p) adds parser p to the permutation parser perm. The parser p is optional - if it can not be applied, the default value x will be used instead. Returns a new permutation parser that includes the optional parser p.

(<$?>) :: (a -> b) -> (a, GenParser tok st a) -> PermParser tok st b

The expression f <$?> (x,p) creates a fresh permutation parser consisting of parser p. The the final result of the permutation parser is the function f applied to the return value of p. The parser p is optional - if it can not be applied, the default value x will be used instead.