parsec-3.1.3: Monadic parser combinators

Portability | non-portable (uses existentially quantified data constructors)
Stability | provisional
Maintainer | derek.a.elkins@gmail.com
Safe Haskell | Safe-Inferred

Text.Parsec.Perm

Description

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.

Synopsis

- type PermParser tok st a = StreamPermParser String st a
- data StreamPermParser s st a
- permute :: Stream s Identity tok => StreamPermParser s st a -> Parsec s st a
- (<||>) :: Stream s Identity tok => StreamPermParser s st (a -> b) -> Parsec s st a -> StreamPermParser s st b
- (<$$>) :: Stream s Identity tok => (a -> b) -> Parsec s st a -> StreamPermParser s st b
- (<|?>) :: Stream s Identity tok => StreamPermParser s st (a -> b) -> (a, Parsec s st a) -> StreamPermParser s st b
- (<$?>) :: Stream s Identity tok => (a -> b) -> (a, Parsec s st a) -> StreamPermParser s st b

# Documentation

type PermParser tok st a = StreamPermParser String st aSource

Provided for backwards compatibility. The tok type is ignored.

data StreamPermParser s st a Source

The type `StreamPermParser s st a` denotes a permutation parser that, when converted by the `permute` function, parses `s` streams 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 :: Stream s Identity tok => StreamPermParser s st a -> Parsec s st aSource

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)

(<||>) :: Stream s Identity tok => StreamPermParser s st (a -> b) -> Parsec s st a -> StreamPermParser s st bSource

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`.

(<$$>) :: Stream s Identity tok => (a -> b) -> Parsec s st a -> StreamPermParser s st bSource

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.

(<|?>) :: Stream s Identity tok => StreamPermParser s st (a -> b) -> (a, Parsec s st a) -> StreamPermParser s st bSource

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`.

(<$?>) :: Stream s Identity tok => (a -> b) -> (a, Parsec s st a) -> StreamPermParser s st bSource

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.