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

I've used toConstr from Data.Data when I only care which constructor a value was constructed with. The specific type 'a' in 'Maybe a' is effectively a phantom type for the Nothing constructor, which I haven't used before. But it looks like it still works, so you could try comparing the constructors: Prelude Data.Typeable Data.Data> toConstr (Nothing :: Maybe Int) == toConstr (Nothing :: Maybe String) True Most of the time, you should be able to use a typeclass for this. On Wed, Jul 20, 2016 at 9:30 AM, Corentin Dupont wrote:

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 wrote:

...

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

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.