Standard library map on bound function

从报告的 test.py 复现开始，重点关注 returns.pointfree.bind、ResultE 与 Python 内置 map 之间的交互。将 mypy 推断出的类型与可正常工作的 for 循环和显式的 g 替代方案进行比较；完成的标准是原始的 map(bind(g), map(f, range(3))) 示例能够无错误地通过类型检查。

Bug report

Mypy does not appear to correctly handle the standard library map function over an iterable of containers that returns an iterable of containers, when the function being mapped over the data has had returns.pointfree.bind applied to it. The same code when used without map (by explicit use of a for-loop) or a bind (by explicitly handling a container input) will not result in an error.

What's wrong

Code to reproduce issue

from typing import Iterable

from returns.pointfree import bind
from returns.result import ResultE
from returns.result import safe


@safe
def f(x: int) -> str:
    return str(x)


@safe
def g(x: str) -> int:
    return int(x)


def h(xs: Iterable[ResultE[int]]) -> None:
    for x in xs:
        print(x.unwrap())


if __name__ == "__main__":
    h(map(bind(g), map(f, range(3))))

Mypy output

$ mypy test.py
test.py:24:11: error: Argument 1 to "map" has incompatible type "Kinded[Callable[[KindN[Result[Any, Any], str, Exception, NoReturn]], KindN[Result[Any, Any], int, Exception, NoReturn]]]"; expected "Callable[[Result[str, Exception]], Res
ult[int, Exception]]"  [arg-type]
test.py:24:11: note: "Kinded[Callable[[KindN[Result[Any, Any], str, Exception, NoReturn]], KindN[Result[Any, Any], int, Exception, NoReturn]]].__call__" has type "Callable[[KindN[Result[Any, Any], str, Exception, NoReturn]], KindN[Resul
t[Any, Any], int, Exception, NoReturn]]"
Found 1 error in 1 file (checked 1 source file)

Python output

$ python test.py
0
1
2

How it should be

When the h function is replaced with a for-loop in __main__ no issues are reported by Mypy and the code executes correctly:

if __name__ == "__main__":
    for x in map(f, range(3)):
        print(x.bind(g).unwrap())

Can also fix the issue by swapping out the g function:

def g(x: ResultE[str]) -> ResultE[int]:
    return x.map(int)

<snip>

if __name__ == "__main__":
    h(map(g, map(f, range(3))))

System information

• python version: 3.8.6
• returns version: 0.15.0
• mypy version: 0.800 (also observed in v0.782 due to classess library dependency)