深入理解Maybe Monad及其在Python中的实现挑战

from typing import Callable, TypeVar, Generic, Union

# 定义类型变量
T = TypeVar('T')
U = TypeVar('U')

class Just(Generic[T]):
    """
    表示Maybe Monad中包含一个有效值的状态。
    """
    def __init__(self, value: T):
        self.value = value

    def __repr__(self) -> str:
        return f'Just({self.value})'

    def __eq__(self, other) -> bool:
        if isinstance(other, Just):
            return self.value == other.value
        return False

class Nothing:
    """
    表示Maybe Monad中没有值（空）的状态。
    使用单例模式，确保所有Nothing实例都是同一个对象。
    """
    _instance = None

    def __new__(cls):
        if cls._instance is None:
            cls._instance = super(Nothing, cls).__new__(cls)
        return cls._instance

    def __repr__(self) -> str:
        return 'Nothing'

    def __eq__(self, other) -> bool:
        return isinstance(other, Nothing)

# 定义Maybe类型为Just[T]或Nothing的联合
Maybe = Union[Just[T], Nothing]

def unit(value: T) -> Maybe[T]:
    """
    Maybe Monad的unit操作，将一个普通值封装到Just中。
    """
    return Just(value)

def bind(f: Callable[[U], Maybe[T]], x: Maybe[U]) -> Maybe[T]:
    """
    Maybe Monad的bind操作。
    如果Maybe值是Just，则将内部值应用到函数f，并返回结果。
    如果Maybe值是Nothing，则直接返回Nothing。
    """
    if isinstance(x, Nothing):
        return x
    else:
        # f应该返回一个Maybe类型的值
        return f(x.value)

# 示例函数
def add_one(n: int) -> Maybe[int]:
    """一个将数字加1的函数，返回Maybe类型。"""
    if isinstance(n, int): # 可以在这里添加更多逻辑来决定是否返回Nothing
        return Just(n + 1)
    return Nothing()

def get_length(s: str) -> Maybe[int]:
    """获取字符串长度的函数，返回Maybe类型。"""
    if isinstance(s, str):
        return Just(len(s))
    return Nothing()

def safe_divide_by_two(n: int) -> Maybe[float]:
    """安全地将数字除以2的函数，处理奇数情况。"""
    if n % 2 == 0:
        return Just(n / 2)
    return Nothing()

# 演示
print("--- Maybe Monad 示例 ---")

# 1. 使用 unit 创建 Maybe 值
maybe_one = unit(1)
print(f"unit(1): {maybe_one}") # 输出: Just(1)

maybe_none = unit(None) # 注意：unit(None) 仍会创建 Just(None)，这不是我们想要的 Nothing 语义
print(f"unit(None): {maybe_none}") # 输出: Just(None)

# 2. bind 操作链
result1 = bind(add_one, Just(1))
print(f"bind(add_one, Just(1)): {result1}") # 输出: Just(2)

result2 = bind(add_one, Nothing())
print(f"bind(add_one, Nothing()): {result2}") # 输出: Nothing

# 链式操作：如果任何一步返回Nothing，整个链条都会短路
chained_result_success = bind(add_one, Just(1))
chained_result_success = bind(add_one, chained_result_success)
chained_result_success = bind(get_length, Just("hello")) # 这是一个不兼容的类型，但bind本身不阻止
print(f"Chained (success): {chained_result_success}") # 输出: Just(5)

chained_result_failure = bind(add_one, Just(1))
chained_result_failure = bind(lambda x: Nothing(), chained_result_failure) # 中途返回Nothing
chained_result_failure = bind(add_one, chained_result_failure)
print(f"Chained (failure): {chained_result_failure}") # 输出: Nothing

# 结合 safe_divide_by_two
initial_value = Just(4)
step1 = bind(safe_divide_by_two, initial_value) # Just(2.0)
step2 = bind(add_one, step1) # Just(3.0)
print(f"Just(4) -> safe_divide_by_two -> add_one: {step2}")

initial_value_odd = Just(3)
step1_odd = bind(safe_divide_by_two, initial_value_odd) # Nothing
step2_odd = bind(add_one, step1_odd) # Nothing
print(f"Just(3) -> safe_divide_by_two -> add_one: {step2_odd}")

# 类型提示的限制：Python的类型检查器会在这里发出警告，因为它期望add_one接收int，但step1_odd是Maybe[float]
# 但在运行时，由于短路效应，并不会真正执行add_one(Nothing)
# 这突显了Python在编译时强制Monad法则的局限性