dry-python / dry-python/returns

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

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bug
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Python
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Descrizione

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:

```py
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):

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