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

[Lyle Kopnicky]

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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