Functor instance for Set?

[Gábor Lehel]

On Thu, Mar 1, 2012 at 1:20 PM, Twan van Laarhoven <twanvl at gmail.com> wrote:
> On 01/03/12 09:31, Bas van Dijk wrote:
>>
>> class Functor f where
>>     type FunctorConstraint f :: * ->  Constraint
>>     type FunctorConstraint f = Empty
>>
>>     fmap :: (FunctorConstraint f b) =>  (a ->  b) ->  f a ->  f b
>>
>> class Empty a
>> instance Empty a
>
>
> Do you really need this Empty class?

Depends on what you want. You do need it if you want to be able to use
'FunctorConstraint f' itself as a type of kind * -> Constraint, for
example to say type FunctorWithoutConstraint f = (Functor f,
FunctorConstraint f ~ Empty).

> That seems inconvenient. I had hoped
> you would be able to write something like
>
>
>    type FunctorConstraint f :: * ->  Constraint
>    type FunctorConstraint f a = ()
>

See this thread:
http://www.mail-archive.com/glasgow-haskell-users@haskell.org/msg20792.html