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

[Corentin Dupont]

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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>
>
>
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