# Beginners Digest, Vol 118, Issue 6

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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 <mike_k_hough...@yahoo.co.uk>
To: The Haskell-Beginners Mailing List - Discussion of primarily
Message-ID: <57afc9f5-3601-4d18-b860-41352d3f2...@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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