[Haskell-cafe] pattern match on forall'ed data
Corentin Dupont
corentin.dupont at gmail.com
Thu Jul 21 20:27:50 UTC 2016
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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