也许Python中的monad具有方法链接

这样做的缺点是它故意抑制错误,这在python中通常被认为是一个坏主意。

但是,您可以捕获并存储 Maybe 实例并将其报告回去。

例如:

class Maybe(object):
    def __init__(self, val, error=None):
        self.val = val
        self.error = error

    def __repr__(self):
        if self.val is not None:
            return repr(self.val)
        else:
            return repr(self.error)

    def do(self, func):
        if self.val is None:
            return self
        try:
            return Maybe(func(self.val))
        except Exception as e:
            return Maybe(None, e)

def squared(x):
    return x * x

def addone(x):
    return x + 1

result1 = Maybe(5).do(squared).do(addone)
result2 = Maybe('a').do(squared).do(addone)
print result1
print result2

这产生了:

26
TypeError("can't multiply sequence by non-int of type 'str'",)

这与DanD的答案类似,但其优点是存储发生的错误,而不是完全抑制错误。

无论你如何划分它,这个习惯用法都会感觉有些“不和谐”,但这是一种稍微更稳健的处理方法。

以下是您所要求的,并让函数更加简洁。还更新了以正确捕获异常:

class Maybe:
    def __init__(self, val, err=None):
        self.val = val
        self.err = err

    def __repr__(self):
        if self.err is not None:
           return 'Maybe('+repr(self.val)+', '+repr(self.err)+')'
        else:
           return 'Maybe('+repr(self.val)+')'

    def do(self, func):  # Bind function
        if self.val is not None:
            try:
                val = func(self.val)
            except Exception as e:
                return Maybe(None, e)
            if not isinstance(val, Maybe):
                return Maybe(val)
            else:
                 return val
        else:
            return Maybe(None, self.err)


def double(number):
    result = number * 2
    return result

def square(number):
    result = number * number
    return result

result = Maybe(5).do(double).do(square)
print(result.val)
print(result)
result2 = Maybe('a').do(double).do(square)
print(result2.val)
print(result2)

打印:

100
Maybe(100)
None
Maybe(None, TypeError("can't multiply sequence by non-int of type 'str'",))

我可能会考虑一些调整:

• 按照其他人的建议,存储异常。
• 制作 Maybe 可调用以消除 do .
• 通过 xs 和 kws 通过调用,以防任何函数采用其他参数。

例如:

class Maybe:
    def __init__(self, val, error = None):
        self.val = val
        self.error = error

    def __call__(self, func, *xs, **kws):
        v = None
        e = None
        if self.val is not None:
            try:
                v = func(self.val, *xs, **kws)
            except Exception as e:
                pass
        return Maybe(v, error = e)

def double(n):
    return n * 2

def square(n):
    return n * n

def multiply_by(n, by):
    return n * by

def message(n, *xs, **kws):
    return 'Message: {} {} {}'.format(n, xs, kws)

ms = [
    Maybe(5)(double)(square),
    Maybe(4)(square)(square)(double),
    Maybe(3)(square)(square),
    Maybe(7)(multiply_by, 3),
    Maybe(9)(message, 1, 2, 3, foo = 'blah'),
    Maybe('hi')(double)(square),
]

for m in ms:
    print(dict(val = m.val, error = m.error))

这个问题已经得到了回答,但我觉得使用Maybe monad的界面可以变得更易读。允许左右调用的接口,例如:

 square(double(Maybe(5)))

此外,最好向用户返回一个Maybe类型以供使用,这样他们就可以选择进一步链接,而不是立即检索值。该功能类似于Swift的可选类型。

class Maybe(object):
  def __init__(self, val=None)
     self.val

  def unwrap(self):
     return self.val

  def calc(self, func):
     try:
       return func(self.val)
     except:
       return None

def double(number):
    return Maybe(maybe.calc(lambda x: x*2))

def square(number):
    return Maybe(maybe.calc(lambda x: x**2))

print square(double(Maybe(5))) # prints Maybe type
print square(double(Maybe(5))).unwrap() # prints '100'
print square(double(None)) # prints 'None'

异常不会存储在我的回答中,因为调用者需要 maybe monad 检查异常本身,并在连锁操作失败时转换为“无”(即None)。这允许用户在使用链的条件下轻松处理故障。