module ErrM where
import Control.Monad (MonadPlus(..), liftM)
import Control.Applicative (Applicative(..), Alternative(..))
data Err a = Ok a | Bad String
deriving (Read, Show, Eq, Ord)
instance Monad Err where
return = Ok
Ok a >>= f = f a
Bad s >>= _ = Bad s
instance Applicative Err where
pure = Ok
(Bad s) <*> _ = Bad s
(Ok f) <*> o = liftM f o
instance Functor Err where
fmap = liftM
instance MonadPlus Err where
mzero = Bad "Err.mzero"
mplus (Bad _) y = y
mplus x _ = x
instance Alternative Err where
empty = mzero
(<|>) = mplus