Kinded type behaving differently from non-kinded type in an invariant wrapper

まず、Python 3.10.8、returns 0.24.0、mypy 1.15.0 で提供された再現コードを実行し、次に returns.primitives.hkt と mypy の不変リストの処理の挙動を調査します。3 つの呼び出しが示されているとおりに型チェックを通り、それぞれの結果について reveal_type が list[int] を報告すれば完了です。

Hi Nikita, long time! Thanks for the interesting library bringing Haskell-like FP patterns to Python.

Bug report

What's wrong

I'm trying to extract the type T from a Kind1 type (say, Foo[T]) that is wrapped in an invariant type, e.g., list[Foo[T]]. However, that's leading to some unexpected behavior.

Repro:

from typing import Any, TypeVar

from returns.primitives import hkt

T = TypeVar("T")


class Foo(hkt.SupportsKind1["Foo[Any]", T]):
    pass


# These are just helper functions to extract the type `T`.
def expect_list_foo(foo_list: list[Foo[T]]) -> list[T]:
    return []


@hkt.kinded
def expect_list_foo__kinded(foo_list: list[hkt.Kind1[Foo[Any], T]]) -> list[T]:
    return []


@hkt.kinded
def expect_tuple_foo__kinded(foo_tuple: tuple[hkt.Kind1[Foo[Any], T], str]) -> list[T]:
    return []


def inferred_types(foo_list: list[Foo[int]], foo_tuple: tuple[Foo[int], str]) -> None:
    x = expect_list_foo(foo_list)

    y = expect_list_foo__kinded(foo_list)  # <-- Problem

    z = expect_tuple_foo__kinded(foo_tuple)

    reveal_type(x)
    reveal_type(y)
    reveal_type(z)

    print(x, y, z)

How is that should be

I expected all three calls to work and return list[int].

Actual output:

30: error: Argument 1 to "expect_list_foo__kinded" has incompatible type "list[Foo[int]]"; expected "list[KindN[Foo[Any], int, Any, Any]]"  [arg-type]
30: note: "List" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
30: note: Consider using "Sequence" instead, which is covariant
34: note: Revealed type is "builtins.list[builtins.int]"
35: note: Revealed type is "builtins.list[builtins.int]"
36: note: Revealed type is "builtins.list[builtins.int]"
Found 1 error in 1 file (checked 1 source file)

Seems like list[Foo[int]] is not treated as compatible with list[hkt.Kind1[Foo[Any], T]]. tuple[Foo[int], str] is treated as compatible with tuple[hkt.Kind1[Foo[Any], T], str]. So, it seems like this might be due to the invariance of list.

Is this expected behavior? How can I get this to work?

System information

• python version: 3.10.8

• returns version: 0.24.0

• mypy version: 1.15.0

• hypothesis version (if any):

• pytest version (if any):