Text.Megaparsec.Perm

This module implements permutation parsers. 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 s m a

The type PermParser s m a denotes a permutation parser that, when converted by the makePermParser function, produces instance of MonadParsec m that parses s stream 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 makePermParser.

makePermParser :: MonadParsec s m t => PermParser s m a -> m a

The parser makePermParser 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 = makePermParser $
         (,,) <$?> ("", some (char 'a'))
              <||> char 'b'
              <|?> ('_', char 'c')

(<$$>) :: MonadParsec s m t => (a -> b) -> m a -> PermParser s m b infixl 2

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.

(<$?>) :: MonadParsec s m t => (a -> b) -> (a, m a) -> PermParser s m b infixl 2

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 cannot be applied, the default value x will be used instead.

(<||>) :: MonadParsec s m t => PermParser s m (a -> b) -> m a -> PermParser s m b infixl 1

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.

(<|?>) :: MonadParsec s m t => PermParser s m (a -> b) -> (a, m a) -> PermParser s m b infixl 1

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