I see....
The think is, I am interested to know if "e" is "Nothing", whatever the
type of Nothing is!
On Wed, Jul 20, 2016 at 5:44 PM, 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@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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