src/Distribution/ReadE.hs

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Distribution.ReadE
    -- Copyright   :  Jose Iborra 2008
    -- License     :  BSD3
    --
    -- Maintainer  :  cabal-devel@haskell.org
    -- Portability :  portable
    --
    -- Simple parsing with failure

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

    import Distribution.Compat.Prelude
    import Prelude ()

    import Distribution.Parsec
    import Distribution.Parsec.FieldLineStream

    -- | 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

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

    parsecToReadE :: (String -> ErrorMsg) -> ParsecParser a -> ReadE a
    parsecToReadE err p = ReadE $ \txt ->
        case runParsecParser p "<parsecToReadE>" (fieldLineStreamFromString txt) of
            Right x -> Right x
            Left _e -> Left (err txt)
    -- TODO: use parsec error to make 'ErrorMsg'.