Generic Point Free Function Returns Unexpected Type

[jcsho]

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):