[Haskell-cafe] Dynamic and equality

[Alberto G. Corona]

You can define:

data EqDyn= forall a.(Typeable a, Eq a)=> EqDyn a

instance Eq EqDyn where
    (EqDyn x) == (EqDyn y)= typeOf x== typeOf y && x== unsafeCoerce y

unsafeCoerce is safe synce the expression assures that types are equal


2013/7/20 adam vogt <vogt.adam at gmail.com>

> On Sat, Jul 20, 2013 at 12:31 AM, Carter Schonwald
> <carter.schonwald at gmail.com> wrote:
> > the tricky part then is to add support for other types.
> >
> > another approach to existentially package type classes with the data
> type!
> >
> > eg
> > data HasEq  = forall a . HasEq ( Eq a => a)
> > or its siblinng
> > data HasEq a = Haseq (Eq a => a )
> >
> > note this requires more planning in how you structure your program, but
> is a
> > much more pleasant approach than using dynamic when you can get it to
> suite
> > your application needs.
> >
> > note its also late, so I've not type checked these examples ;)
>
> Hi Carter,
>
> It doesn't seem like the existential one will work as-is, since ghc
> rejects this:
>
> {-# LANGUAGE ExistentialQuantification #-}
> data HEQ = forall a. Eq a => HEQ a
> usingHEQ :: HEQ -> HEQ -> Bool
> usingHEQ (HEQ a) (HEQ b) = a == b
>
>
> I think you were hinting at this option which is better than my first
> suggestion:
>
> {-# LANGUAGE ExistentialQuantification #-}
> import Data.Typeable
> data DYN = forall a. Typeable a => DYN (a, DYN -> Bool)
>
> mkDyn :: (Eq a, Typeable a) => a -> DYN
> mkDyn x = DYN (x, \(DYN (y, eq2)) -> case cast y of
>                 Just y' -> x == y'
>                 _ -> False)
>
> mkDyn' :: Typeable a => a -> DYN
> mkDyn' x = DYN (x, \_ -> False)
>
> eqDyn :: DYN -> DYN -> Bool
> eqDyn x@(DYN (_, fx)) y@(DYN (_,fy)) = fx y || fy x
>
>
> Maybe there's some way to get mkDyn' and mkDyn as the same function,
> without having to re-write all of the Eq instances as a 2-parameter
> class like <http://www.haskell.org/haskellwiki/GHC/AdvancedOverlap>.
>
>
> --
> Adam
>
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>



-- 
Alberto.
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