Portability | non-portable |
---|---|

Stability | provisional |

Maintainer | ekmett@gmail.com |

Safe Haskell | None |

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

The type `Perm m a`

denotes a permutation parser that,
when converted by the `permute`

function, parses
using the base parsing monad `m`

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 :: Alternative m => Perm m a -> m 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 <$?> ("",some (char 'a')) <||> char 'b' <|?> ('_',char 'c')) where tuple a b c = (a,b,c)

(<||>) :: Functor m => Perm m (a -> b) -> m a -> Perm m 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`

.

(<$$>) :: Functor m => (a -> b) -> m a -> Perm m 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.

(<|?>) :: Functor m => Perm m (a -> b) -> (a, m a) -> Perm m 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`

.

(<$?>) :: Functor m => (a -> b) -> (a, m a) -> Perm m 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.