<div><div dir="auto">Hi,</div><div dir="auto"><br></div><div dir="auto">Try to unify the types returned by each of the case alternatives, starting from what the compiler has inferred so far.</div><div dir="auto"><br></div><div dir="auto">I’ve made this mistake many times lol.</div><div dir="auto"><br></div><div dir="auto">Best,</div><div dir="auto"><br></div><div dir="auto">toz </div><br><div class="gmail_quote"><div>On Thu, Apr 12, 2018 at 5:38 AM <<a href="mailto:beginners-request@haskell.org">beginners-request@haskell.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Send Beginners mailing list submissions to<br>
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Today's Topics:<br>
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1. EitherT (mike h)<br>
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Message: 1<br>
Date: Thu, 12 Apr 2018 11:25:57 +0100<br>
From: mike h <<a href="mailto:mike_k_houghton@yahoo.co.uk" target="_blank">mike_k_houghton@yahoo.co.uk</a>><br>
To: The Haskell-Beginners Mailing List - Discussion of primarily<br>
beginner-level topics related to Haskell <<a href="mailto:beginners@haskell.org" target="_blank">beginners@haskell.org</a>><br>
Subject: [Haskell-beginners] EitherT<br>
Message-ID: <<a href="mailto:57AFC9F5-3601-4D18-B860-41352D3F2204@yahoo.co.uk" target="_blank">57AFC9F5-3601-4D18-B860-41352D3F2204@yahoo.co.uk</a>><br>
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<br>
Hi,<br>
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 :) <br>
<br>
<br>
This is what I have<br>
<br>
newtype EitherT m a b = EitherT {runEitherT :: m (Either a b)}<br>
instance Monad m => Functor (EitherT m a) where<br>
---- fmap :: (a -> b) -> f a -> f b<br>
fmap f m = EitherT $ do<br>
mv <- runEitherT m<br>
case mv of<br>
Left _ -> return mv<br>
Right rv -> return $ Right (f rv)<br>
<br>
<br>
<br>
and here is the compilers view<br>
Phrase.hs:31:25: error:<br>
• Couldn't match type ‘b’ with ‘a1’<br>
‘b’ is a rigid type variable bound by<br>
the type signature for:<br>
fmap :: forall a1 b. (a1 -> b) -> EitherT m a a1 -> EitherT m a b<br>
at Phrase.hs:27:5-8<br>
‘a1’ is a rigid type variable bound by<br>
the type signature for:<br>
fmap :: forall a1 b. (a1 -> b) -> EitherT m a a1 -> EitherT m a b<br>
at Phrase.hs:27:5-8<br>
Expected type: m (Either a a1)<br>
Actual type: m (Either a b)<br>
• In the expression: return $ Right (f rv)<br>
In a case alternative: Right rv -> return $ Right (f rv)<br>
In a stmt of a 'do' block:<br>
case mv of<br>
Left _ -> return mv<br>
Right rv -> return $ Right (f rv)<br>
• Relevant bindings include<br>
rv :: a1 (bound at Phrase.hs:31:19)<br>
mv :: Either a a1 (bound at Phrase.hs:28:9)<br>
m :: EitherT m a a1 (bound at Phrase.hs:27:12)<br>
f :: a1 -> b (bound at Phrase.hs:27:10)<br>
fmap :: (a1 -> b) -> EitherT m a a1 -> EitherT m a b<br>
(bound at Phrase.hs:27:5)<br>
<br>
<br>
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<br>
<br>
these lines from the compiler are particularly confusing <br>
Expected type: m (Either a a1)<br>
Actual type: m (Either a b)<br>
<br>
as I believe f is f:: a1->b <br>
<br>
So just hints please and I expect I’ll have another duh moment. <br>
<br>
Thanks<br>
<br>
Mike<br>
<br>
<br>
<br>
<br>
<br>
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</blockquote></div></div>