Check.hs

-- | A simple exception monad.
module DDC.Control.Monad.Check
        ( CheckM (..)
        , throw
        , result)
where


data CheckM err a
        = CheckM (Either err a)


instance Monad (CheckM err) where
 return !x   
  = CheckM (Right x)
 {-# INLINE return #-}

 (>>=) !m !f  
  = case m of
          CheckM (Left err)     -> CheckM (Left err)
          CheckM (Right x)      -> x `seq` f x
 {-# INLINE (>>=) #-}


-- | Throw a type error in the monad.
throw :: err -> CheckM err a
throw !e        = CheckM $ Left e


-- | Take the result from a check monad.
result :: CheckM err a -> Either err a
result (CheckM r)       = r