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Let’s implement the classic “Maybe” monad, for simple functions.

#!/usr/local/bin/python

class Undefined:
    pass

class Maybe:
    def __init__(self, x=Undefined):
        self.value = x

    def apply(self, f):
        if not self.is_defined:
            return Maybe()
        else:
            return Maybe(f(self.value))

    def __repr__(self):
        if not self.is_defined:
            return '<Undefined>'
        else:
            return repr(self.value)

    @property
    def is_defined(self):
        return not self.value is Undefined

def monad_aware(f):
    def fprime(x):
        return x.apply(f)
    return fprime

if __name__ == '__main__':

    @monad_aware
    def double(x):
        return 2*x

    x = Maybe()
    print double(x)
    assert(not double(x).is_defined)

    x = Maybe(2)
    print double(x)
    assert(double(x).is_defined)
    assert(double(x).value == 4)

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