Understanding immutability

Another common trait of functional programming is that data structures are typically immutable. This is a tough concept to get around when you are used to imperative programming – the idea of programming without objects that change state over time. Here, we are going to see a simple example of making a function from the previous recipe work in an immutable way: that is, so that no objects are changed, and if we need to pass new information, we create new ones.

This recipe will give a short presentation on immutability from a data structure perspective. It will be, in a sense, the standard presentation that you can find in most books. Our main consideration, though, is to discuss mutability as a code design pattern, the topic of the following recipe. But for this, we need to understand immutability first.

We will be looking at two functions: one that mutates data structures and another that doesn’t. This will be done in the context of...