    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Distribution.ReadE
    -- Copyright   :  Jose Iborra 2008
    --
    -- Maintainer  :  cabal-devel@haskell.org
    -- Portability :  portable
    --
    -- Simple parsing with failure

    {- Copyright (c) 2007, Jose Iborra
    All rights reserved.

    Redistribution and use in source and binary forms, with or without
    modification, are permitted provided that the following conditions are
    met:

        * Redistributions of source code must retain the above copyright
          notice, this list of conditions and the following disclaimer.

        * Redistributions in binary form must reproduce the above
          copyright notice, this list of conditions and the following
          disclaimer in the documentation and/or other materials provided
          with the distribution.

        * Neither the name of Isaac Jones nor the names of other
          contributors may be used to endorse or promote products derived
          from this software without specific prior written permission.

    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
    "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
    LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
    A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
    OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
    SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
    LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
    DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
    THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
    (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
    OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. -}

    module Distribution.ReadE (
       -- * ReadE
       ReadE(..), succeedReadE, failReadE,
       -- * Projections
       parseReadE, readEOrFail,
       readP_to_E
      ) where

    import Distribution.Compat.ReadP
    import Data.Char ( isSpace )

    -- | Parser with simple error reporting
    newtype ReadE a = ReadE {runReadE :: String -> Either ErrorMsg a}
    type ErrorMsg   = String

    instance Functor ReadE where
      fmap f (ReadE p) = ReadE $ \txt -> case p txt of
                                           Right a  -> Right (f a)
                                           Left err -> Left err

    succeedReadE :: (String -> a) -> ReadE a
    succeedReadE f = ReadE (Right . f)

    failReadE :: ErrorMsg -> ReadE a
    failReadE = ReadE . const Left

    parseReadE :: ReadE a -> ReadP r a
    parseReadE (ReadE p) = do
      txt <- look
      either fail return (p txt)

    readEOrFail :: ReadE a -> (String -> a)
    readEOrFail r = either error id . runReadE r

    readP_to_E :: (String -> ErrorMsg) -> ReadP a a -> ReadE a
    readP_to_E err r =
        ReadE $ \txt -> case [ p | (p, s) <- readP_to_S r txt
                             , all isSpace s ]
                        of [] -> Left (err txt)
                           (p:_) -> Right p