? predicate

A naming convention for functions returning true or false (unenforced). Examples:

• even?
• empty?
• contains?
• nil?

Predicate functions are presumably pure (not having any side-effects on state).

It is sometimes used to name boolean values as well, not just predicate functions.

Create a divisible? predicate:

(defn divisible? [n factor]
  (zero? (mod n factor)))

(divisible? 15 3)
;;=> true

(divisible? 15 2)
;;=> false

(filter #(divisible? 15 %) (range 15))
;;=> (1 3 5)