dry-python / dry-python/returns
Kinded type behaving differently from non-kinded type in an invariant wrapper
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Description
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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