mypy error for HKT example from docs

Question

mypy error for HKT example from docs

vchernetskyi993 opened this issue a year ago · 5 comments

Bug report

I was going through your documentation and stumbled upon an issue in HKT example: https://returns.readthedocs.io/en/latest/pages/hkt.html#id1.

What's wrong

Trying out your to_str definition:

from typing import TypeVar
from returns.maybe import Maybe
from returns.primitives.hkt import Kind1, kinded
from returns.interfaces.container import Container1, ContainerN

T = TypeVar("T", bound=Container1)

@kinded
def to_str(container: Kind1[T, int]) -> Kind1[T, str]:
    return container.map(str)

reveal_type(to_str(Maybe.from_value(1)))

Produces mypy error: Value of type variable "T" of "to_str" cannot be "Maybe[Any]" [type-var].

Playing around, I found, that replacing Container1 with ContainerN fixes the issue (why?).

T = TypeVar("T", bound=ContainerN)

How is that should be

Examples from the documentation should work :)

I'm not sure whether this is a defect, maybe I'm doing something wrong, but before submitting this one I've rechecked that my code matches the docs closely.

System information

python version: 3.11.1
returns version: 0.20.0
mypy version: 1.3.0
- I'm using config from here: https://github.com/wemake-services/wemake-python-styleguide/blob/master/styles/mypy.toml

Answer 1 · 2023-06-04T16:24:21.000Z

Yes, makes sense. Can you please send a PR? :)

Answer 2 · 2023-06-04T16:55:45.000Z

Uh-oh, need to investigate the issue more for this.

We want T = TypeVar("T", bound=Container1) to work without an error, correct?

Answer 3 · 2023-06-04T17:38:13.000Z

ContainerN is always fine

Answer 4 · 2023-06-04T17:38:28.000Z

Basically, Container1 won't exist soon.

Answer 5 · 2023-06-04T17:42:01.000Z

Understood, thanks! Will create small PR for docs tomorrow.