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

On 21 July 2016 at 02:30, Corentin Dupont wrote:

I see.... The think is, I am interested to know if "e" is "Nothing", whatever the type of Nothing is!

Data.Maybe.isNothing ?

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 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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