[Haskell-beginners] EitherT

[David McBride]

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 <mike_k_houghton at 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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