Matt Harden writes:
> That doesn't work either: Haskell98 doesn't allow type synonyms to be
> used to define instances. GHC supposedly lifts this restriction, but it
> complains: (I'm using version 4.02)
>
> > type synonym `Func2' should have 3 arguments, but has been given 2
> > When checking kinds in `Func2 a b'
> > When checking kinds in `Functor (Func2 a b)'
>
> So I guess GHC only accepts type synonyms as instances when the kind of
> the class is *.
The actual restriction is that type synonyms must always be fully applied; they can be
at any kind. Thus in your first example,
type Func a = (->) a
instance Functor (Func a) where
...
works fine (in GHC).
This doesn't solve your problem, however. I believe (someone will correct me if I'm
wrong) that you're stuck here. What you really need is a type lambda:
type Func2 a b = /\c. a->(b->c)
but this isn't provided in Haskell.
--KW 8-)
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: Keith Wansbrough, MSc, BSc(Hons) (Auckland) -------------------:
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