Some monads in python

monads.py

class Monad:
  def __init__(self, value):
    self.value = value

  def __iter__(self):
    yield self

  def do(self):
    raise NotImplementedError()

  def join(self):
    if isinstance(self.value, self.__class__):
      return self.value.join()
    return self

class IO(Monad):
  def __init__(self, value):
    if callable(value) and not isinstance(value, self.__class__):
      self.predicate = value
      self.value = None
    else:
      self.value = value
      self.predicate = lambda: self.value

  def __repr__(self):
    if not self.value:
      return f'{self.__class__.__name__}(<uncalculated>)'
    return f'{self.__class__.__name__}({self.value})'

  def do(self):
    self.value = self.join().predicate()
    return self.value

  def __add__(self, value):
    return self.__class__(lambda: self.do() + value)

  def __eq__(self, value):
    return self.__class__(lambda: self.do() == value)

  def __ne__(self, value):
    return self.__class__(lambda: self.do() != value)

  def __iadd__(self, value):
    return self + value

  def __getattr__(self, name):
    return self.__class__(lambda: getattr(self.do(), name))

  def __getitem__(self, key, default=None):
    return self.__class__(lambda: self.do().get(key, default))

  def __call__(self, *args, **kwargs):
    return self.__class__(lambda: self.predicate()(*args, **kwargs))


class Option(Monad):
  def __repr__(self):
    if self.value:
      return f'Some({self.value})'
    return 'None'

  def __bool__(self):
    return self.value

  def __add__(self, value):
    if not self.value:
      return self
    return self.__class__(self.value + value)

  def __eq__(self, value):
    if not self.value:
      return self
    return self.__class__(self.do() == value)

  def __ne__(self, value):
    if not self.value:
      return self
    return self.__class__(self.do() != value)

  def __iadd__(self, value):
    if not self.value:
      return self
    return self + value

  def __getattr__(self, name):
    if not self.value:
      return self
    return self.__class__(getattr(self.do(), name))

  def __getitem__(self, key, default=None):
    if not self.value:
      return self
    return self.__class__(self.do().get(key, default))

  def __call__(self, *args, **kwargs):
    if not self.value:
      return self
    return self.__class__(self.do()(*args, **kwargs))

  def do(self):
    return self.join().value