Re: [Haskell-beginners] EitherT

12 Apr 2018

      On Thu, Apr 12, 2018 at 11:25:57AM +0100, mike h wrote:
...
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)
Tricky error! The signature for this fmap is:

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

The offending line is:

    Left _   -> return mv

You *think* you are returning `Either a c`, but are you really?

Type HINTS for more :P
-F

Re: [Haskell-beginners] EitherT

Francesco Ariis