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

[Patrick Chilton]

You also might want to consider whether this existential approach is
correct to begin with. Could you just use a Maybe SomeData instead? Do you
need the existential at all?
https://lukepalmer.wordpress.com/2010/01/24/haskell-antipattern-existential-typeclass/

On Thu, Jul 21, 2016 at 10:48 AM, Corentin Dupont <corentin.dupont at gmail.com
> wrote:

> That's great, exactly what I need.
> What do you mean by "just Typeable"?
> Do you have another idea in mind?
>
> On Thu, Jul 21, 2016 at 10:23 AM, Jonas Scholl <
> anselm.scholl at tu-harburg.de> wrote:
>
>> If you want to use just Typeable, you can implement your own cast:
>> Extract the TypeRep of the thing in SomeData, get the TyCon, which is
>> the top-level constructor, i.e. Maybe without arguments, and compare it
>> with the TyCon from Maybe. If they match, you coerce the value to Maybe
>> () and use isNothing. While this is not completely safe, we do not
>> evaluate the thing we just coerced to (), and thus are safe, as Maybe
>> should have identical representation regardless of the type parameter.
>>
>> isNothing' :: SomeData -> Bool
>> isNothing' (SomeData a) = tyCon == maybeTyCon
>>     && isNothing (unsafeCoerce a :: Maybe ())
>>     where
>>         tyCon = typeRepTyCon (typeRep (mkProxy a))
>>         maybeTyCon = typeRepTyCon $ typeRep (Proxy :: Proxy (Maybe ()))
>>         mkProxy :: a -> Proxy a
>>         mkProxy = const Proxy
>>
>>
>> On 07/21/2016 09:51 AM, Corentin Dupont wrote:
>> > Hi Ivan,
>> > I could use isNothing, but the data is forall'ed...
>> > I tried but it doesn't work:
>> >
>> > data SomeData = forall e. (Typeable e, Eq e) => SomeData e
>> >
>> > isNothing' :: SomeData -> Bool
>> > isNothing' (SomeData a) = case (cast a) of
>> >    (a :: Maybe a) -> isNothing a
>> >
>> > Could not deduce (Typeable a) arising from a use of ‘cast’
>> >
>> > On Wed, Jul 20, 2016 at 11:21 PM, Ivan Lazar Miljenovic
>> > <ivan.miljenovic at gmail.com <mailto:ivan.miljenovic at gmail.com>> wrote:
>> >
>> >     On 21 July 2016 at 02:30, Corentin Dupont <
>> corentin.dupont at gmail.com
>> >     <mailto: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!
>> >
>> >     Data.Maybe.isNothing ?
>> >
>> >     >
>> >     >
>> >     >
>> >     > On Wed, Jul 20, 2016 at 5:44 PM, Patrick Chilton
>> >     <chpatrick at gmail.com <mailto: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 <mailto: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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>> >
>> >
>> >     --
>> >     Ivan Lazar Miljenovic
>> >     Ivan.Miljenovic at gmail.com <mailto:Ivan.Miljenovic at gmail.com>
>> >     http://IvanMiljenovic.wordpress.com
>> >
>> >
>> >
>> >
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