Clojure: High Performance JVM Programming

Writing your first macro

Now that you have a clear understanding of how macros work and what they are for, let's start working with Clojure.

Let me present you with a challenge: write an unless function in Clojure, something that works like this:

(def a 150)

(my-if (> a 200)
  (println"Bigger than 200")
  (println"Smaller than 200"))

Let's give it a first try; maybe with something like the following syntax:

(defn my-if [cond positive negative]
  (if cond
    positive
    negative))

Do you know what would happen if you wrote this code and then ran it? If you test it, you will get the following result:

Bigger than 200
Smaller than 200
Nil

What's happening here? Let's modify it a bit so that we get a value and we can understand what's happening. Let's define it a bit ...