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

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 mailto:ivan.miljenovic@gmail.com> wrote:

On 21 July 2016 at 02:30, Corentin Dupont mailto:corentin.dupont@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 mailto:chpatrick@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 >> mailto:corentin.dupont@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 >>> >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> To (un)subscribe, modify options or view archives go to: >>> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >>> Only members subscribed via the mailman list are allowed to post. >> >> > > > _______________________________________________ > Haskell-Cafe mailing list > To (un)subscribe, modify options or view archives go to: > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > Only members subscribed via the mailman list are allowed to post.

-- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com mailto:Ivan.Miljenovic@gmail.com http://IvanMiljenovic.wordpress.com

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