adts3.py

adts3.py

from typing import Generic, TypeVar, final
from typing import Callable
from dataclasses import dataclass

L = TypeVar("L")
R = TypeVar("R")

class Either(Generic[L,R]): ...

@final
@dataclass(frozen=True)
class Left(Either[L,R]):
    val: L

@final
@dataclass(frozen=True)
class Right(Either[L,R]):
    val: R

A = TypeVar("A")

def either(l: Callable[[L], A], r: Callable[[R], A], e: Either[L,R]) -> A:
    match e:
        case Left(x):
            return l(x)
        case Right(y):
            return r(y)
    assert False

# either(lambda x: print(x + 1), lambda y: print(y + "_"), Left(2))       # falsely rejected by mypy 1.8.0    
# either(lambda x: print(x + 1), lambda y: print(y + "_"), Right("hi"))   # correctly accepted by mypy 1.8.0

# either(lambda x: print(x + 1), lambda y: print(y + "_"), Right(2))      # correctly rejected by mypy 1.8.0
# either(lambda x: print(x + 1), lambda y: print(y + "_"), Left("hi"))    # correctly rejected by mypy 1.8.0

def pr_l(x: int):...
def pr_r(x: str):...

either(pr_l, pr_r, Left(23))
either(pr_l, pr_r, Right(""))
# either(pr_l, pr_r, Right(23))  # correctly rejected
# either(pr_l, pr_r, Left(""))   # correctly rejected