Re: ConstraintKinds and default associated empty constraints

9 Jan 2012

      On Mon, Jan 9, 2012 at 12:30 AM, Antoine Latter  wrote:
...
On Sun, Jan 8, 2012 at 11:21 PM, wren ng thornton  wrote:
...
Couldn't the following work?
   class Functor f where
       type C f :: * -> Constraint
       type C f _ = ()
I get a parse error from that.
The equivalent:
  class Functor f where
      type FC f :: * -> Constraint
      type FC f a = ()
The definitions are accepted by GHC:

   class Functor f where
       type FC f a :: Constraint
       type FC f a = ()

       fmap :: (FC f a, FC f b) => (a -> b) -> f a -> f b

   instance Functor [] where
       fmap = map

But I don't like the 'a' being an index parameter, and then the
following expression:

   fmap (+1) [1::Int]

Gives the error:

    Could not deduce (FC [] Int) arising from a use of `fmap'
    In the expression: fmap (+ 1) [1 :: Int]
    In an equation for `it': it = fmap (+ 1) [1 :: Int]
...
gives the error:
   Number of parameters must match family declaration; expected 1
   In the type synonym instance default declaration for `FC'
   In the class declaration for `Functor'
...
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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