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

[Michael Burge]

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 <corentin.dupont at gmail.com>
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 <chpatrick at gmail.com>
> 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 at 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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>>
>>
>
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