10.19. Functional Pattern Maybe

• Maybe monad

• Continues execution, even, if there is an error

• Final state will be none

• But no intermediate error handling is needed

The concept of monad is a functional programming concept that provides a way to encapsulate and sequence computations. In Python, the concept of monad can be implemented using classes and methods.

One example of a monad in Python is the Maybe monad, which is used to represent computations that may or may not return a value. The Maybe monad is implemented as a class with two subclasses: Just` and ``Nothing. The Just subclass represents a computation that returns a value, while the Nothing subclass represents a computation that does not return a value.

Here is an example implementation of the Maybe monad in Python:

>>> class Maybe:
...     def __init__(self, value):
...         self.value = value
...
...     def bind(self, func):
...         if self.value is None:
...             return Nothing()
...         else:
...             return func(self.value)
>>>
>>> class Just(Maybe):
...     pass
>>>
>>> class Nothing(Maybe):
...     def bind(self, func):
...         return self
>>>
>>> def add_one(x):
...     return Just(x + 1)
>>>
>>> def divide_by_zero(x):
...     return Just(x / 0)
>>>
>>> result = Just(2).bind(add_one).bind(divide_by_zero)  
>>> print(result)  
None

In this example, the Maybe class is used to represent computations that may or may not return a value. The bind method is used to sequence computations by applying a function to the result of a previous computation. The Just and Nothing subclasses are used to represent computations that return a value and computations that do not return a value, respectively.

The add_one function is used to add one to a value, while the divide_by_zero function is used to divide a value by zero. The result variable is used to sequence these computations by first adding one to the value 2, and then attempting to divide the result by zero. Since dividing by zero is not possible, the result is a Nothing object.

10.19.1. Pattern

>>> class Maybe:
...     def __init__(self, value):
...         self.value = value
...
...     def __repr__(self):
...         return f"Maybe({self.value})"
...
...     def unwrap(self):
...         return self.value
...
...     def bind(self, func):
...         if self.value is None:
...             return Maybe(None)
...         return Maybe(func(self.value))

10.19.2. With Value

>>> DATA = 4
>>>
>>> result = (
...     Maybe(DATA)
...     .bind(lambda x: 2*x)
...     .bind(lambda y: y+1)
... )
>>>
>>> print(result)
Maybe(9)
>>>
>>> print(result.unwrap())
9

10.19.3. With Errors

>>> DATA = 4
>>>
>>> result = (
...     Maybe(DATA)
...     .bind(lambda x: None if x < 10 else x)  # this could fail
...     .bind(lambda x: 2*x)
...     .bind(lambda y: y+1)
... )
>>>
>>> result
Maybe(None)
>>>
>>> result.unwrap()