[Haskell-cafe] More experiments with ATs
andrewcoppin at btinternet.com
Sun Jul 4 05:31:34 EDT 2010
Sjoerd Visscher wrote:
> On Jul 3, 2010, at 4:39 PM, Andrew Coppin wrote:
>> class Container c => Functor c where
>> fmap :: (Functor cx, Functor cy, Element cx ~ x, Element cy ~ y) => (x -> y) -> (cx -> cy)
>> However, this fails horribly: The type signature fails to mention c.
> You have to mention c, this means an extra argument to fmap. But if you do that you also get the opportunity to restrict what x and y can be.
Well, you can say
class Container cy => Functor cy where
fmap :: (Functor cx, Element cx ~ x, Element cy ~ y) => (x -> y) ->
(cx -> cy)
But that's still wrong.
> As you'll have to pass around this extra argument, it will usually be easier to just pass around the map function though.
>> type family F f a :: *
>> class RFunctor f where
>> (%) :: f a b -> (a -> b) -> F f a -> F f b
I have literally no idea what a type family is. I understand ATs (I
think!), but TFs make no sense to me.
(For this reason, most if not all of the rest of this post doesn't make
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