Introduction

Data in our programs doesn't always take the nice, linear form for which functions such as map or reduce are particularly adapted. None of the techniques we've discussed in the last two chapters will work for traversing non-linear structures such as trees or graphs. And while it's possible to do a lot by being creative with reduce, the strong guard rails that reduce provides can sometimes get in the way of writing expressive code. There are situations that call for tools that give the programmer more control. Clojure has other resources for these kinds of problems and that is what we are going to look at in this chapter.

Recursion plays a major role when functions such as map and reduce are no longer adapted to the task at hand. Thinking recursively is an important Clojure skill to learn. Because functional programming languages tend to emphasize recursion, this might seem unfamiliar if your background is in more procedural languages. Most programming...