Distribution/Compat/MonadFail.hs

    {-# LANGUAGE CPP #-}

    -- | Compatibility layer for "Control.Monad.Fail"
    module Distribution.Compat.MonadFail ( MonadFail(fail) ) where
    #if __GLASGOW_HASKELL__ >= 800
    -- provided by base-4.9.0.0 and later
    import Control.Monad.Fail (MonadFail(fail))
    #else
    -- the following code corresponds to
    -- http://hackage.haskell.org/package/fail-4.9.0.0
    import qualified Prelude as P
    import Distribution.Compat.Prelude hiding (fail)

    import Text.ParserCombinators.ReadP
    import Text.ParserCombinators.ReadPrec

    class Monad m => MonadFail m where
        fail :: String -> m a

    -- instances provided by base-4.9

    instance MonadFail Maybe where
        fail _ = Nothing

    instance MonadFail [] where
        fail _ = []

    instance MonadFail P.IO where
        fail = P.fail

    instance MonadFail ReadPrec where
        fail = P.fail -- = P (\_ -> fail s)

    instance MonadFail ReadP where
        fail = P.fail
    #endif