Programming Clojure, 2nd Edition

Writing a Control Flow Macro

Clojure provides the if special form as part of the language:

(if (= 1 1) (println "yep, math still works today"))
| yep, math still works today

Some languages have an unless, which is (almost) the opposite of if. unless performs a test and then executes its body only if the test is logically false.

Clojure doesn’t have unless, but it does have an equivalent macro called when-not. For the sake of having a simple example to start with, let’s pretend that when-not doesn’t exist and create an implementation of unless. To follow the rules of Macro Club, begin by trying to write unless as a function:

; This is doomed to fail...
(defn unless [expr form]
  (if expr nil form)) ...