Currently we have
class Functor c where
fmap :: (x -> y) -> (c x -> c y)
This relies on c having kind * -> *. For example, Bytestring cannot be
an instance of Functor.
A cleaner solution would be to have something like
class Container c where
type Element c :: *
I then attempted to write
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. And
even if it did, this type signature still isn't correct: It says that
fmap can transform *any* functor into *any* other functor, which is wrong.
It seems that we have gained the ability to talk about containers
holding any type of value (even hard-coded types or types with class
constraints), but lost the ability to refer to a particular "type of"
functor without also specifying the element type.
Does anybody know a straightforward way to fix this? Or is this just a
dead end?
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