Little example of how to have a typed ADT-ish compose in Python

compose_python.py

import dataclasses
import typing

import typing_extensions

T = typing.TypeVar("T")
U = typing.TypeVar("U")
V = typing.TypeVar("V")


# Need this so that function attributes are not typed as methods
# https://github.com/python/mypy/issues/5485#issuecomment-441935633
@dataclasses.dataclass
class Box(typing.Generic[T]):
    inner: T

    @property
    def unboxed(self) -> T:
        return self.inner


@dataclasses.dataclass
class Compose(typing.Generic[T, U, V]):
    left: Box[typing.Callable[[U], V]]
    right: Box[typing.Callable[[T], U]]

    def __call__(self, arg: T) -> V:
        return self.left.unboxed(self.right.unboxed(arg))


def int_to_str(i: int) -> str:
    return str(i)


def str_to_int(s: str) -> int:
    return int(s)


identity = Compose(Box(str_to_int), Box(int_to_str))
print(identity)
h: int = identity(123)
print(h)