why does “(def vowel? (set ”aeiou“))” work?

Question

I\'m taking a look at the excellent Clojure tutorial here. In one of the examples it has Clojure code along the following lines:

(def vowel? (set \"aeiou\"))
         


        

Answer 1

Aha! I ended up finding it out myself. It doesn't actually return true or false, rather it returns the first occurrence in the set, or nil if it doesn't occur.

And because you can use this as a condition (nil being handled as false, and non-nil as true), this works as a nice little hack for checking if a string contains a letter.

(vowel? (first "abc")) ; => "a"
(vowel? (first "cba")) ; => nil

(if (vowel? (first "abc"))
       (println "yay")
       (println "oops"))  ; => "yay"