Either type in Python

either_type.py

from typing import Generic, Tuple, TypeVar, Iterable, List, Optional, Any, Union

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

# SKI+ Combinators
S = lambda f: lambda g: lambda x: f(x)(g(x))
K = lambda x: lambda y: x
I = S(K)(K(K))
KI = K(I)
B = S(K(S))(K)
T = B(S(I))(K)
V = S(B(B)(B(S)(T)))(K(K))


# Definition of Pair
class Pair(Generic[L, R]):
    def __init__(self) -> None:
        raise NotImplementedError("Should not be used")

    def __call__(self, func):
        return self(func)


def mk_pair(left: L, right: R) -> Pair[L, R]:
    return V(left)(right)


def fst(p: Pair[L, R]) -> L:
    return p(K)


def snd(p: Pair[L, R]) -> R:
    return p(KI)


def unpack(p: Pair[L, R]) -> Tuple[L, R]:
    return fst(p), snd(p)

# Definition of Either type
class Either(Generic[L, R]):
    def __init__(self) -> None:
        raise NotImplementedError("Should not be used")

    def __call__(self, func):
        return self(func)


def either_left(value: L) -> Either[L, R]:
    return mk_pair(K, value)


def either_right(value: R) -> Either[L, R]:
    return mk_pair(KI, value)


def is_left(e: Either[L, R]) -> bool:
    return fst(e)(True)(False)


def is_right(e: Either[L, R]) -> bool:
    return fst(e)(False)(True)


def value_either(e: Either[L, R]) -> Union[L, R]:
    return snd(e)