[Haskell-cafe] Existentials for multi-parameter type classes?

[David Feuer]

And in GADT syntax with crazy records, that's

data Collision where
    Collision :: Collidable a b => {collider1 :: a, collider2 :: b} ->
Collision

Or (much cleaner) without the crazy record syntax,

data Collision where
    Collision :: Collidable a b => a -> b -> Collision
On Mar 20, 2015 1:15 AM, "Michael Sloan" <mgsloan at gmail.com> wrote:

> There is!
>
> data Collision = forall a b. Collidable a b => Collision {collider1 ::
> a, collider2 :: b}
>
> On Thu, Mar 19, 2015 at 10:11 PM, Lyle Kopnicky <lists at qseep.net> wrote:
> > Hi folks,
> >
> > I know how to write existentially quantified data constructors for
> > single-parameter type classes, but I'm having trouble doing it for
> > multi-parameter type classes. Is it not possible, or am I just using the
> > wrong syntax?
> >
> > I have this class:
> >
> > class Collidable a b where
> > collideWith :: a -> b -> String
> >
> > And I tried to define an existentially quantified data constructor
> thusly:
> >
> > data Collision = forall a. (forall b. Collidable a b => Collision
> {collider1
> > :: a, collider2 :: b})
> >
> > I get a parse error. So then I tried:
> >
> > data Collision = forall a. forall b. Collidable a b => Collision
> {collider1
> > :: a, collider2 :: b}
> >
> > I get a different parse error. Is there any way to make this work?
> >
> > Thanks,
> > Lyle
> >
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> >
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