1+ {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies ,FlexibleInstances #-}
12module EitherT where
23
34import Control.Monad
4- import Control.Monad.Trans
55
66newtype MyEitherT l m r = MyEitherT { runMyEitherT :: m (Either l r )}
77
8+ class MonadTrans t where
9+ lift :: Monad m => m a -> t m a
10+ 11+ class (Show e , Monad m ) => (MonadError e m ) where
12+ eFail :: e -> m a
13+ eHandle :: m a -> (e -> m a ) -> m a
14+ 815instance (Monad m ) => Functor MyEitherT l m ) where
916 fmap = liftM
1017
1118instance (Monad m ) => Applicative MyEitherT l m ) where
12- pure = MyEitherT . pure . Right
19+ pure = MyEitherT . pure . Right
20+ (<*>) = ap
1321
1422instance (Monad m ) => Monad MyEitherT l m ) where
1523 return = pure
@@ -23,3 +31,11 @@ instance MonadTrans (MyEitherT l) where
2331 lift m = MyEitherT $ do
2432 m' <- m
2533 return $ Right
34+ 35+ instance (Show l , Monad m ) => MonadError l (MyEitherT l m ) where
36+ eFail l = MyEitherT $ return $ Left
37+ eHandle m1 m2 = MyEitherT $ do
38+ m1' <- runMyEitherT m1
39+ case m1' of
40+ Right -> return m1'
41+ Left -> runMyEitherT $ m2 err
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