[Haskell-beginners] EitherT
mike h
mike_k_houghton at yahoo.co.uk
Thu Apr 12 20:32:05 UTC 2018
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 <toad3k at 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 <mike_k_houghton at yahoo.co.uk <mailto: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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