LookAhead.hs

{-# LANGUAGE CPP #-}
{-# LANGUAGE UndecidableInstances #-}

#if defined(__GLASGOW_HASKELL__) && __GLASGOW_HASKELL__ >= 704
#define USE_DEFAULT_SIGNATURES
#endif

#ifdef USE_DEFAULT_SIGNATURES
{-# LANGUAGE DefaultSignatures, TypeFamilies #-}
#endif

-----------------------------------------------------------------------------
-- |
-- Module      :  Text.Parser.LookAhead
-- Copyright   :  (c) Edward Kmett 2011-2013
-- License     :  BSD3
--
-- Maintainer  :  ekmett@gmail.com
-- Stability   :  experimental
-- Portability :  non-portable
--
-- Parsers that can 'lookAhead'.
-----------------------------------------------------------------------------
module Text.Parser.LookAhead
  (
  -- * Parsing Combinators
    LookAheadParsing(..)
  ) where

import Control.Monad (MonadPlus(..))
import Control.Monad.Trans.State.Lazy as Lazy
import Control.Monad.Trans.State.Strict as Strict
import Control.Monad.Trans.Writer.Lazy as Lazy
import Control.Monad.Trans.Writer.Strict as Strict
import Control.Monad.Trans.RWS.Lazy as Lazy
import Control.Monad.Trans.RWS.Strict as Strict
import Control.Monad.Trans.Reader
import Control.Monad.Trans.Identity
import qualified Data.Attoparsec.Types as Att
import qualified Data.Attoparsec.Combinator as Att
#if __GLASGOW_HASKELL__ < 710
import Data.Monoid
#endif
import qualified Text.ParserCombinators.ReadP as ReadP
import qualified Text.Parsec as Parsec
import Text.Parser.Combinators

-- | Additional functionality needed to describe parsers independent of input type.
class Parsing m => LookAheadParsing m where
  -- | @lookAhead p@ parses @p@ without consuming any input.
  lookAhead :: m a -> m a

instance (LookAheadParsing m, MonadPlus m) => LookAheadParsing (Lazy.StateT s m) where
  lookAhead (Lazy.StateT m) = Lazy.StateT $ lookAhead . m
  {-# INLINE lookAhead #-}

instance (LookAheadParsing m, MonadPlus m) => LookAheadParsing (Strict.StateT s m) where
  lookAhead (Strict.StateT m) = Strict.StateT $ lookAhead . m
  {-# INLINE lookAhead #-}

instance (LookAheadParsing m, MonadPlus m) => LookAheadParsing (ReaderT e m) where
  lookAhead (ReaderT m) = ReaderT $ lookAhead . m
  {-# INLINE lookAhead #-}

instance (LookAheadParsing m, MonadPlus m, Monoid w) => LookAheadParsing (Strict.WriterT w m) where
  lookAhead (Strict.WriterT m) = Strict.WriterT $ lookAhead m
  {-# INLINE lookAhead #-}

instance (LookAheadParsing m, MonadPlus m, Monoid w) => LookAheadParsing (Lazy.WriterT w m) where
  lookAhead (Lazy.WriterT m) = Lazy.WriterT $ lookAhead m
  {-# INLINE lookAhead #-}

instance (LookAheadParsing m, MonadPlus m, Monoid w) => LookAheadParsing (Lazy.RWST r w s m) where
  lookAhead (Lazy.RWST m) = Lazy.RWST $ \r s -> lookAhead (m r s)
  {-# INLINE lookAhead #-}

instance (LookAheadParsing m, MonadPlus m, Monoid w) => LookAheadParsing (Strict.RWST r w s m) where
  lookAhead (Strict.RWST m) = Strict.RWST $ \r s -> lookAhead (m r s)
  {-# INLINE lookAhead #-}

instance (LookAheadParsing m, Monad m) => LookAheadParsing (IdentityT m) where
  lookAhead = IdentityT . lookAhead . runIdentityT
  {-# INLINE lookAhead #-}

instance (Parsec.Stream s m t, Show t) => LookAheadParsing (Parsec.ParsecT s u m) where
  lookAhead = Parsec.lookAhead

instance Att.Chunk i => LookAheadParsing (Att.Parser i) where
  lookAhead = Att.lookAhead

instance LookAheadParsing ReadP.ReadP where
  lookAhead p = ReadP.look >>= \s ->
                ReadP.choice $ map (return . fst) $ ReadP.readP_to_S p s