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April 17, 2020

Clojure Goodness: Getting Results From Multiple Functions With Juxt Function

The function juxt can be used to get results from multiple functions on the same argument in one go. We pass the functions we want to invoke as arguments to the juxt function. This results in a new function that will return a vector with the results from each original function that is passed to the juxt function. So the first element in the result vector is the result of the first function, the second element is the result of the second function and so on. The documentation of the juxt function shows this clearly as ((juxt a b c)) => [a(x) b(x) c(x)].

In the following example we use the juxt function to apply multiple functions on a string, collection and map:

(ns mrhaki.core.juxt
  (:require [clojure.test :refer [is]]))

;; The functions first, last and count are applied
;; to the string value and we get a vector
;; with the result of each function.
(is (= [\C \e 18]
       ((juxt first last count) "Clojure is awesome")))

(defn sum 
  "Calculate sum of values in collection"
  (apply + coll))

(defn avg 
  "Calculate average of values in collection"
  (/ (sum coll) (count coll)))

;; Create new function summarize to give back statistics
;; on a collection based on count, min, max, sum and average.
(def summarize (juxt count #(apply min %) #(apply max %) sum avg))

(is (= [4 1 108 171 171/4]
       (summarize [1 20 42 108])))

(let [[count min max sum avg] (summarize [1 20 42 108])]
  (is (= 4 count))
  (is (= 1 min))
  (is (= 108 max))
  (is (= 171 sum))
  (is (= 171/4 avg)))

;; As keywords are functions we can use them to get values
;; for keys in a map.
(is (= ["Hubert" "mrhaki"]
       ((juxt :name :alias) {:alias "mrhaki" :name "Hubert" :location "Tilburg"})))

Written with Clojure 1.10.1.