Generic Point Free Function Returns Unexpected Type

test_generic2 の mypy の出力を再現し、エントリポイント bind_result とレポートに示されているジェネリックな Callable シグネチャに焦点を当てます。推論された型を、期待される IOResult[T4, Exception] 型の結果と比較し、ネストされた IOResult エラーなしで例の型チェックが成功すれば問題が解決されたことを確認します。

Bug report

What's wrong

Trying to compose a generic function that converts between different container types fails to type check correctly when using generics. Hopefully I am just using this wrong.

Code:

from typing import Callable, TypeVar

from returns.io import IOResultE
from returns.pointfree import bind_result
from returns.result import ResultE
from typing_extensions import reveal_type

T1 = TypeVar("T1")
T2 = TypeVar("T2")
T3 = TypeVar("T3")
T4 = TypeVar("T4")


# this works correctly
def test(
    i: int,
    f1: Callable[[int], str],
    f2: Callable[[str], IOResultE[int]],
    f3: Callable[[int], ResultE[str]],
) -> IOResultE[str]:
    f1_res = f1(i)
    f2_res = f2(f1_res)
    f3_res = bind_result(f3)(f2_res)
    reveal_type(f3_res)
    return f3_res


# this passes typecheck with returns.io.IOResult[T3`-3, builtins.Exception]
def test_generic(
    i: T1,
    f1: Callable[[T1], IOResultE[T2]],
    f2: Callable[[T2], ResultE[T3]],
) -> IOResultE[T3]:
    f1_res = f1(i)
    f2_res = bind_result(f2)(f1_res)
    reveal_type(f2_res)
    return f2_res


# this fails typecheck with returns.io.IOResult[returns.io.IOResult[Any, Any], builtins.Exception]
def test_generic2(
    i: T1,
    f1: Callable[[T1], T2],
    f2: Callable[[T2], IOResultE[T3]],
    f3: Callable[[T3], ResultE[T4]],
) -> IOResultE[T4]:
    f1_res = f1(i)
    f2_res = f2(f1_res)
    f3_res = bind_result(f3)(f2_res)
    reveal_type(f3_res)
    return f3_res

Running mypy:

$ python3 -m mypy test.py
test.py:23: note: Revealed type is "returns.io.IOResult[builtins.str, builtins.Exception]"
test.py:34: note: Revealed type is "returns.io.IOResult[T3`-3, builtins.Exception]"
test.py:47: note: Revealed type is "returns.io.IOResult[returns.io.IOResult[Any, Any], builtins.Exception]"
test.py:48: error: Incompatible return value type (got "IOResult[IOResult[Any, Any], Exception]", expected "IOResult[T4, Exception]")
Found 1 error in 1 file (checked 1 source file)

How is that should be

I would have expected the return type for test_generic2 to be returns.io.IOResult[T4`-3, builtins.Exception]

System information

• python version: 3.9.12

• returns version: 0.19.0

• mypy version: 0.95.0

• hypothesis version (if any):

• pytest version (if any):