@@ -9,19 +9,29 @@ newtype MyErrorT m a = MyErrorT { runMyErrorT :: m (Either String a) }
99instance Monad m => Monad MyErrorT m ) where
1010 return = pure
1111 x >>= y = MyErrorT $ do
12- res <- x
12+ res <- runMyErrorT x
1313 case res of
14- Right -> return $ runMyErrorT $ y res
14+ Right -> runMyErrorT $ y val
1515 Left -> return $ Left
1616
1717instance Monad m => Functor MyErrorT m ) where
1818 fmap = liftM
1919
2020instance Monad m => Applicative MyErrorT m ) where
21- pure a = MyErrorT $ return $ Right a
21+ pure = MyErrorT . return . Right
2222 (<*>) = ap
2323
2424instance MonadTrans MyErrorT where
25- lift m = MyErrorT $ liftM Right
26- 27- 25+ lift m = MyErrorT $ do
26+ m' <- m
27+ return $ Right
28+ 29+ throw :: Monad m => String -> MyErrorT m a
30+ throw = MyErrorT . return . Left
31+ 32+ catch :: Monad m => MyErrorT m a -> (String -> MyErrorT m a ) -> MyErrorT m a
33+ catch m c = MyErrorT $ do
34+ m' <- runMyErrorT m
35+ case m' of
36+ Left -> runMyErrorT $ c err
37+ Right -> runMyErrorT m
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