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|
----------------------------------------------------------------------
-- |
-- Module : ErrM
-- Maintainer : AR
-- Stability : (stable)
-- Portability : (portable)
--
-- > CVS $Date: 2005/04/21 16:22:00 $
-- > CVS $Author: bringert $
-- > CVS $Revision: 1.5 $
--
-- hack for BNFC generated files. AR 21/9/2003
-----------------------------------------------------------------------------
module GF.Data.ErrM (Err(..)) where
import Control.Monad (MonadPlus(..))
-- | like @Maybe@ type with error msgs
data Err a = Ok a | Bad String
deriving (Read, Show, Eq)
instance Monad Err where
return = Ok
fail = Bad
Ok a >>= f = f a
Bad s >>= f = Bad s
-- | added 2\/10\/2003 by PEB
instance Functor Err where
fmap f (Ok a) = Ok (f a)
fmap f (Bad s) = Bad s
-- | added by KJ
instance MonadPlus Err where
mzero = Bad "error (no reason given)"
mplus (Ok a) _ = Ok a
mplus (Bad s) b = b
|
