Functor instance for Set?
Bas van Dijk
v.dijk.bas at gmail.com
Thu Mar 1 17:33:16 CET 2012
On 1 March 2012 13:20, 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? That seems inconvenient. I had hoped
> you would be able to write something like
> type FunctorConstraint f :: * -> Constraint
> type FunctorConstraint f a = ()
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In GHC-7.4.1 that will give the following error:
Number of parameters must match family declaration; expected 1
In the type synonym instance default declaration for `FunctorConstraint'
However, I believe SPJ fixed this in HEAD.
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