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" wrote:

There is!

data Collision = forall a b. Collidable a b => Collision {collider1 :: a, collider2 :: b}

...

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

On Thu, Mar 19, 2015 at 10:11 PM, Lyle Kopnicky wrote: 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

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe