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

[Corentin Dupont]

That's right, there are hints that my design is overly complex, I'm looking
at simplifying it.

On Thu, Jul 21, 2016 at 11:50 AM, Patrick Chilton <chpatrick at gmail.com>
wrote:

> 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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>>
>>
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