[Haskell-cafe] pattern match on forall'ed data

[Patrick Chilton]

It's because you're doing === Nothing and the type of the Nothing is
ambiguous (Maybe a1).

On Wed, Jul 20, 2016 at 5:18 PM, Corentin Dupont <corentin.dupont at gmail.com>
wrote:

> Hi all,
> I'm surprised this doesn't work:
>
> data SomeData = forall e. (Typeable e, Eq e) => SomeData e
>
> (===) :: (Typeable a, Typeable b, Eq a, Eq b) =>  a ->  b ->  Bool
> (===) x y = cast x == Just y
>
> test :: SomeData' ->  Bool
> test (SomeData' e) | e === Nothing = True
> test _ = False
>
> It says
>  Could not deduce (Eq a1) arising from a use of ‘===’
>
> How can I achieve something of the same effect?
>
> Thanks
> Corentin
>
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