Even ‘better’ instance Monad m => Functor (EitherT m a) where fmap f m = EitherT $ runEitherT m >>= \mv -> return $ fmap f mv :)

On 12 Apr 2018, at 21:32, mike h wrote:

instance Monad m => Functor (EitherT m a) where fmap f m = EitherT $ do mv <- runEitherT m case mv of Left lv -> return $ Left lv Right rv -> return $ Right (f rv)

Thanks all :) I think its correct. The compiler does! Mike

...

On 12 Apr 2018, at 17:27, David McBride mailto:toad3k@gmail.com> wrote:

You will feel like this was obvious in hindsight.

fmap :: (a -> b) -> f a -> f b fmap (a -> b) -> EitherT m c a -> EitherT m c b

Now follow the types in your code. m :: EitherT m c a mv :: Either c a return mv :: EitherT m c a -- <- you are back to the wrong type.

How can you instead return EitherT m c b?

On Thu, Apr 12, 2018 at 6:25 AM, mike h mailto:mike_k_houghton@yahoo.co.uk> wrote: 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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