Re: ConstraintKinds and default associated empty constraints

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