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

Arguments

:: MonadParsec e s m

=> PermParser s m a

Given permutation parser

-> m a

Normal parser built from it

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')

(<$$>) infixl 2

Arguments

:: MonadParsec e s m

=> (a -> b)

Function to use on result of parsing

-> m a

Normal parser

-> PermParser s m b

Permutation parser build from it

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.

(<$?>) infixl 2

Arguments

:: MonadParsec e s m

=> (a -> b)

Function to use on result of parsing

-> (a, m a)

Default value and parser

-> PermParser s m b

Permutation parser

The expression f <$?> (x, p) creates a fresh permutation parser consisting of parser p. 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.

(<||>) infixl 1

Arguments

:: MonadParsec e s m

=> PermParser s m (a -> b)

Given permutation parser

-> m a

Parser to add (should not accept empty input)

-> PermParser s m b

Resulting parser

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.

(<|?>) infixl 1

Arguments

:: MonadParsec e s m

=> PermParser s m (a -> b)

Given permutation parser

-> (a, m a)

Default value and parser

-> PermParser s m b

Resulting parser

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.