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@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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