Can we do something similar with functions? The answer is yes: we will represent a tree (or any other structure) with a function itself—not with a data structure that is processed by a set of functions, nor with an object with some methods, but by just a function. Furthermore, we will get a functional data structure that's 100% immutable, which, if updated, produces a new copy of itself. We will do all this without using objects; here, closures will provide the desired results.
How can this work? We shall be applying similar concepts to the ones we looked at earlier in this chapter, so the function will act as a container and it will produce, as its result, a mapping of its contained values. Let's walk backward and ...
