ConstraintKinds and default associated empty constraints

[Bas van Dijk]

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" <wren at freegeek.org> wrote:

> On 1/8/12 8:32 AM, Bas van Dijk wrote:
>
>> On 23 December 2011 17:44, Simon Peyton-Jones<simonpj@**microsoft.com<simonpj at microsoft.com>>
>>  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
>
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