The Clojure function complement can be used to created a new function that returns the opposite truth value of the old function. The new function accepts the same number of arguments as the old function. Also when we invoke the new function created by the complement the old function is actually invoked and the result is used as argument for the not function to return the opposite truth value. So if the original function returns false or nil the result for the new function is true.
In the following example code we create a new function bad-weather that is the complement of good-weather:
(ns mrhaki.core.complement
(:require [clojure.test :refer [is]]))
;; List with some temperatures for dates.
(def weather [{:date #inst "2020-06-05" :temperature 21}
{:date #inst "2020-06-06" :temperature 17}
{:date #inst "2020-06-07" :temperature 19}
{:date #inst "2020-06-08" :temperature 25}
{:date #inst "2020-06-09" :temperature 26}])
(defn good-weather
"'Good' weather is when the temperature is between 20 and 30 degrees Celcius."
[{temp :temperature}]
(< 20 temp 30))
;; The opposite can easily be turned into a
;; new function with complement.
;; The bad-weather takes the same argument (temperature)
;; and returns false when good-weather would return true
;; and true when good-weather returns false.
(def bad-weather (complement good-weather))
(is (= [{:date #inst "2020-06-05" :temperature 21}
{:date #inst "2020-06-08" :temperature 25}
{:date #inst "2020-06-09" :temperature 26}]
(filter good-weather weather)))
(is (= [{:date #inst "2020-06-06" :temperature 17}
{:date #inst "2020-06-07" :temperature 19}]
(filter bad-weather weather)))
;; Filtering on the good-weather and complement bad-weather
;; should return the same number of items as in the original collection.
(is (= 5
(count weather)
(count (concat (filter good-weather weather) (filter bad-weather weather)))))
Written with Clojure 1.10.1.
