Hi, Try to unify the types returned by each of the case alternatives, starting from what the compiler has inferred so far. I’ve made this mistake many times lol. Best, toz On Thu, Apr 12, 2018 at 5:38 AM wrote:

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Today's Topics:

1. EitherT (mike h)

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Message: 1 Date: Thu, 12 Apr 2018 11:25:57 +0100 From: mike h To: The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell Subject: [Haskell-beginners] EitherT Message-ID: <57AFC9F5-3601-4D18-B860-41352D3F2204@yahoo.co.uk> Content-Type: text/plain; charset=utf-8

Hi, I’m trying to write EitherT from first principles and I’m stuck at the first hurdle - Functor. I’m looking for a hint rather than a complete answer :)

This is what I have

newtype EitherT m a b = EitherT {runEitherT :: m (Either a b)} instance Monad m => Functor (EitherT m a) where ---- fmap :: (a -> b) -> f a -> f b fmap f m = EitherT $ do mv <- runEitherT m case mv of Left _ -> return mv Right rv -> return $ Right (f rv)

and here is the compilers view Phrase.hs:31:25: error: • Couldn't match type ‘b’ with ‘a1’ ‘b’ is a rigid type variable bound by the type signature for: fmap :: forall a1 b. (a1 -> b) -> EitherT m a a1 -> EitherT m a b at Phrase.hs:27:5-8 ‘a1’ is a rigid type variable bound by the type signature for: fmap :: forall a1 b. (a1 -> b) -> EitherT m a a1 -> EitherT m a b at Phrase.hs:27:5-8 Expected type: m (Either a a1) Actual type: m (Either a b) • In the expression: return $ Right (f rv) In a case alternative: Right rv -> return $ Right (f rv) In a stmt of a 'do' block: case mv of Left _ -> return mv Right rv -> return $ Right (f rv) • Relevant bindings include rv :: a1 (bound at Phrase.hs:31:19) mv :: Either a a1 (bound at Phrase.hs:28:9) m :: EitherT m a a1 (bound at Phrase.hs:27:12) f :: a1 -> b (bound at Phrase.hs:27:10) fmap :: (a1 -> b) -> EitherT m a a1 -> EitherT m a b (bound at Phrase.hs:27:5)

what I think I need to do is fmap over the right value after pulling if out of the monad m by doing mv <- runEitherT m

these lines from the compiler are particularly confusing Expected type: m (Either a a1) Actual type: m (Either a b)

as I believe f is f:: a1->b

So just hints please and I expect I’ll have another duh moment.

Thanks

Mike

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