Miguel Mitrofanov wrote:
Suppose I want to create a specific monad as a combination of monad transformers - something like "StateT smth1 (ReaderT smth2 Identity)". As you can see, each transformer is parametrized with a type of kind *. I want to abstract these parameters, so that instead of "StateT smth..." I can write something like
Zip (ConsT StateT (ConsT ReaderT NilT)) (ConsA smth1 (ConsA smth2 NilA)) Identity
and it would be a type isomorphic to the first one. I mean, I want (ConsT StateT (ConsT ReaderT NilT))" to be a separate entity of fixed kind, so that I can, say, create a class instance for it or something.
I'd be quite happy if list length appears as a separate parameter, like
Zip (Succ (Succ Zero)) (ConsT ...
I would NOT be happy with something like
Zip (List_2 StateT ReaderT) (Arg_2 smth1 smth2)
If haskell had polymorphic kinds, I'd be able to do it easily; unfortunately, it doesn't have them.
I think the "type families" extension can do this. -- Ashley Yakeley