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
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