Functor instance for Set?
illissius at gmail.com
Thu Mar 1 14:26:25 CET 2012
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:
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