Getting Clojure

Variations on the Theme

In addition to the plain vanilla version of let that we’ve looked at so far, Clojure comes packaged with a couple of handy variations. The most commonly used of these is probably if-let. As you might guess from the name, if-let is an if and a let rolled into one. To see if-let in action, imagine that we decide to represent anonymous books with our now-familiar book map, sans the :author key:

(def anonymous-book
  {:title "Sir Gawain and the Green Knight"})

(def with-author
  {:title "Once and Future King" :author "White"})

Now imagine we needed to write a function that will return the uppercase version of the author’s name, or nil if there is no author. The twist is that we need to avoid computing the uppercase ...