mirror of
https://github.com/mrkkrp/megaparsec.git
synced 2024-12-20 23:01:30 +03:00
134 lines
5.2 KiB
Haskell
134 lines
5.2 KiB
Haskell
-- |
|
||
-- Module : Text.Megaparsec.Perm
|
||
-- Copyright : © 2015 Megaparsec contributors
|
||
-- © 2007 Paolo Martini
|
||
-- © 1999–2001 Daan Leijen
|
||
-- License : BSD3
|
||
--
|
||
-- Maintainer : Mark Karpov <markkarpov@opmbx.org>
|
||
-- Stability : experimental
|
||
-- Portability : non-portable (uses existentially quantified data constructors)
|
||
--
|
||
-- 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.
|
||
|
||
module Text.Megaparsec.Perm
|
||
( StreamPermParser -- abstract
|
||
, permute
|
||
, (<||>)
|
||
, (<$$>)
|
||
, (<|?>)
|
||
, (<$?>) )
|
||
where
|
||
|
||
import Control.Monad.Identity
|
||
|
||
import Text.Megaparsec
|
||
|
||
infixl 1 <||>, <|?>
|
||
infixl 2 <$$>, <$?>
|
||
|
||
-- Building a permutation 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@.
|
||
|
||
(<||>) :: Stream s Identity tok => StreamPermParser s st (a -> b) ->
|
||
Parsec s st a -> StreamPermParser s st b
|
||
(<||>) = add
|
||
|
||
-- | 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 => (a -> b) ->
|
||
Parsec s st a -> StreamPermParser s st b
|
||
f <$$> p = newperm f <||> p
|
||
|
||
-- | 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 => StreamPermParser s st (a -> b) ->
|
||
(a, Parsec s st a) -> StreamPermParser s st b
|
||
perm <|?> (x, p) = addopt perm x p
|
||
|
||
-- | The expression @f \<$?> (x, p)@ creates a fresh permutation parser
|
||
-- consisting of parser @p@. The the final result of the permutation parser
|
||
-- is the function @f@ applied to the return value of @p@. The parser @p@ is
|
||
-- optional — if it can not be applied, the default value @x@ will be used
|
||
-- instead.
|
||
|
||
(<$?>) :: Stream s Identity tok => (a -> b) ->
|
||
(a, Parsec s st a) -> StreamPermParser s st b
|
||
f <$?> xp = newperm f <|?> xp
|
||
|
||
-- The permutation tree
|
||
|
||
-- | The type @StreamPermParser s st a@ denotes a permutation parser that,
|
||
-- when converted by the 'permute' function, parses @s@ streams with user
|
||
-- state @st@ and returns a value of type @a@ on success.
|
||
--
|
||
-- Normally, a permutation parser is first build with special operators like
|
||
-- ('<||>') and than transformed into a normal parser using 'permute'.
|
||
|
||
data StreamPermParser s st a = Perm (Maybe a) [StreamBranch s st a]
|
||
|
||
data StreamBranch s st a =
|
||
forall b. Branch (StreamPermParser s st (b -> a)) (Parsec s st b)
|
||
|
||
-- | The parser @permute perm@ parses a permutation of parser described
|
||
-- by @perm@. For example, suppose we want to parse a permutation of: an
|
||
-- optional string of @a@'s, the character @b@ and an optional @c@. This can
|
||
-- be described by:
|
||
--
|
||
-- > test = permute (tuple <$?> ("", some (char 'a'))
|
||
-- > <||> char 'b'
|
||
-- > <|?> ('_', char 'c'))
|
||
-- > where tuple a b c = (a, b, c)
|
||
|
||
-- Transform a permutation tree into a normal parser
|
||
|
||
permute :: Stream s Identity tok => StreamPermParser s st a -> Parsec s st a
|
||
permute (Perm def xs) = choice (fmap branch xs ++ empty)
|
||
where empty = case def of
|
||
Nothing -> []
|
||
Just x -> [return x]
|
||
branch (Branch perm p) = flip ($) <$> p <*> permute perm
|
||
|
||
-- Build permutation trees
|
||
|
||
newperm :: Stream s Identity tok => (a -> b) -> StreamPermParser s st (a -> b)
|
||
newperm f = Perm (Just f) []
|
||
|
||
add :: Stream s Identity tok => StreamPermParser s st (a -> b) ->
|
||
Parsec s st a -> StreamPermParser s st b
|
||
add perm@(Perm _mf fs) p = Perm Nothing (first : fmap insert fs)
|
||
where first = Branch perm p
|
||
insert (Branch perm' p') = Branch (add (mapPerms flip perm') p) p'
|
||
|
||
addopt :: Stream s Identity tok => StreamPermParser s st (a -> b) ->
|
||
a -> Parsec s st a -> StreamPermParser s st b
|
||
addopt perm@(Perm mf fs) x p = Perm (fmap ($ x) mf) (first : map insert fs)
|
||
where first = Branch perm p
|
||
insert (Branch perm' p') = Branch (addopt (mapPerms flip perm') x p) p'
|
||
|
||
mapPerms :: Stream s Identity tok => (a -> b) ->
|
||
StreamPermParser s st a -> StreamPermParser s st b
|
||
mapPerms f (Perm x xs) = Perm (fmap f x) (map mapBranch xs)
|
||
where mapBranch (Branch perm p) = Branch (mapPerms (f.) perm) p
|