Web Development with Clojure, 2nd Edition

Threading Expressions

By this point you’ve probably noticed that nested expressions can be difficult to read. Fortunately, Clojure provides a couple of helper forms to deal with this problem. Let’s say we have a range of numbers, and we want to increment each number, interpose the number 5 between them, and then sum the result. We could write the following code to do that:

(reduce + (interpose 5 (map inc (range 10))))

It’s a little difficult to tell what’s happening in the preceding example at a glance. With a few more steps in the chain, we’d be really lost. On top of that, if we wanted to rearrange any of the steps, such as interposing 5 before incrementing, then we’d have to re-nest all our expressions. An alternative way to write this ...