ConstraintKinds and default associated empty constraints
aslatter at gmail.com
Mon Jan 9 07:41:51 CET 2012
On Mon, Jan 9, 2012 at 12:30 AM, Antoine Latter <aslatter at gmail.com> wrote:
> On Sun, Jan 8, 2012 at 11:21 PM, wren ng thornton <wren at freegeek.org> 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
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,
>> Glasgow-haskell-users mailing list
>> Glasgow-haskell-users at haskell.org
More information about the Glasgow-haskell-users