Re: ConstraintKinds and default associated empty constraints

That would be nice. It would also be nice to be able to use _ in type signatures as in: const :: a -> _ -> a const x _ = x During type checking each _ could be replaced by a new unique type variable. Visa versa should also be possible: during type inferencing each unique type variable could be replaced by a _. Bas On Jan 9, 2012 6:22 AM, "wren ng thornton" wrote:

On 1/8/12 8:32 AM, Bas van Dijk wrote:

...

On 23 December 2011 17:44, Simon Peyton-Jones> wrote:

...

My attempt at forming a new understanding was driven by your example.

class Functor f where type C f :: * -> Constraint type C f = ()

sorry -- that was simply type incorrect. () does not have kind * -> Constraint

So am I correct that the `class Empty a; instance Empty a` trick is currently the only way to get default associated empty constraints?

Couldn't the following work?

class Functor f where type C f :: * -> Constraint type C f _ = ()

It seems to me that adding const to the type level (either implicitly or explicitly) is cleaner and simpler than overloading () to be Constraint, *->Constraint, *->*->Constraint,...

-- Live well, ~wren

______________________________**_________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.**org http://www.haskell.org/**mailman/listinfo/glasgow-**haskell-usershttp://www.haskell.org/mailman/listinfo/glasgow-haskell-users