More Monads in Python

Simple implementations of the List and Maybe monads.

Photo by marc liu on Unsplash

In the article on the Failure monad we looked at a simple definition of what a monad is:

A monad is a design pattern that allows us to add a context to data values, and also allows us to easily compose existing functions so that they execute in a context aware manner.

In this article we will look at two other common monads:

• List monads
• Maybe monads

List monads

A List monad allows us to process lists of values, without cluttering our code with looping constructs.

How a List monad is used

As with the Failure monad, our List monad should do three main things:

• Allow us to wrap a value (in this case, a list value) in a List monad.
• Allow us to apply an existing function to the entire list, using bind().
• Allow us to retrieve the List from a monad.

So a simple use might be like this:

k = List([1, 2, 3])
n = k.bind(neg)
print(n)              # List([-1, -2, -3])

We will call our monad List because that is the name that is commonly used for similar monads in other languages. Notice that…