Support Higher Kinded Types

[sobolevn]

This feature is crutial before 1.0 release.

Currently, our interfaces are not universal. Many functions and methods do require to explicitly annotate all containers that they can work with. As a result users cannot create their own containers.

And we struggle to build useful abstractions (like MonadIO on top of IO and Future, see https://github.com/gcanti/fp-ts/blob/master/src/MonadIO.ts). Let's see this example: https://returns.readthedocs.io/en/latest/pages/context.html#requirescontextfutureresult-container

It can possibly work with both IO and Future containers. In the first case, it would be sync. But, in the second one it would be async. And the source code won't be changed at all! We would only need to substitute our monad stack.

def _fetch_post(
      post_id: int,
      client_get: Callable[[str], MonadIO[Response]],
  ) -> MonadIO[_Post]:
      return client_get(_URL.format(post_id)).bind_result(
          safe(tap(httpx.Response.raise_for_status)),
      ).map(
          lambda response: response.json(),
      )

Passing IOResult will run this code in sync, while FutureResult will run like real async code.

To make this happen, we need HKT!
I have spent a lot of time working on the initial implementation, but still no luck.

Related:

• Lightweight higher-kinded polymorphism: https://www.cl.cam.ac.uk/~jdy22/papers/lightweight-higher-kinded-polymorphism.pdf
• Swift: https://bow-swift.io/docs/fp-concepts/higher-kinded-types/
• TypeScript: https://github.com/gcanti/fp-ts/blob/master/docs/guides/HKT.md

Native blockers:

• Higher-Kinded TypeVars python/typing#548
• Feature request: plugins that have access to both semanal and typechecker APIs python/mypy#8848

[sobolevn]

That's exactly the same example I had in How Async Should Have Been: https://dev.to/gcanti/functional-design-tagless-final-332k

[sobolevn]

Blocked by python/mypy#9001
Because I need a TypeQuery instance to find Kind[] types in the function signature.

[antonagestam]

Just lurking on this PR which seems really promising 🙌 Has there been any interest from mypy to internalize support for this further on?

[sobolevn]

@antonagestam thanks! 👍

When this will be released, I will work with mypy team! I am pretty sure that we can incorporate some parts of our API into the standard API.

[sobolevn]

BTW, your https://github.com/antonagestam/phantom-types project looks amazing!

[antonagestam]

@sobolevn Thanks! I'm finding it hard to find the time to get it off the ground but I'm currently making some changes to make it much more based around predicate functions which I think will make it a lot more useful and adaptable to different contexts :)

Exciting times in the world of typed Python!