[Haskell-cafe] pattern match on forall'ed data
David Feuer
david.feuer at gmail.com
Fri Jul 22 09:45:11 UTC 2016
I would be surprised if GHC could derive a Data instance for a GADT.
Deriving generally tends to fall down for GADTs. I also suspect Data
doesn't work for GADTs at all. Generic mechanisms for GADTs in Haskell seem
a bit researchy still.
On Jul 21, 2016 4:27 PM, "Corentin Dupont" <corentin.dupont at gmail.com>
wrote:
> That's great, it works, thanks.
> Now I am blocked on this one:
>
> data Foo a where
> A :: Foo a
> B :: Foo [a]
>
> deriving instance Typeable Foo
> deriving instance (Data a) => Data (Foo a)
>
> Could not deduce (a ~ [a0])
> from the context (Typeable (Foo a), Data a)
>
>
> It would compile only if I comment the B constructor.
> How can I make a Data instance of Foo??
>
>
> On Thu, Jul 21, 2016 at 11:26 AM, Jonas Scholl <
> anselm.scholl at tu-harburg.de> wrote:
>
>> With "just Typeable" I mean using only the Typeable class. As already
>> mentioned by Michael, it is also possible to achieve the same effect
>> with the Data class:
>>
>> data SomeData = forall e. (Data e, Eq e) => SomeData e
>>
>> isNothing'' :: SomeData -> Bool
>> isNothing'' (SomeData a) = toConstr a == toConstr (Nothing :: Maybe ())
>>
>> Depending on your use-case, this may be simpler and it avoids using
>> unsafeCoerce, which may make one feel a little bit uneasy. On the other
>> hand, it adds an additional constraint. Additionally, a programmer can
>> write his own Data instance while Typeable instances are always
>> generated by the compiler (in newer versions of GHC).
>>
>> On 07/21/2016 10:48 AM, Corentin Dupont 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 <mailto: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>
>> > <mailto: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>
>> > > <mailto: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>
>> > <mailto: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> <mailto:
>> 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>
>> > <mailto:Ivan.Miljenovic at gmail.com <mailto:Ivan.Miljenovic at gmail.com
>> >>
>> > > http://IvanMiljenovic.wordpress.com
>> > >
>> > >
>> > >
>> > >
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