[Haskell-cafe] pattern match on forall'ed data
chpatrick at gmail.com
Wed Jul 20 15:44:15 UTC 2016
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>
> 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?
> Haskell-Cafe mailing list
> To (un)subscribe, modify options or view archives go to:
> Only members subscribed via the mailman list are allowed to post.
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the Haskell-Cafe