Just juxt it!

juxt is one remarkably useful core Clojure function, that doesn’t seem to be widely used (or understood for that matter), but is part of the arsenal of every experienced Clojure hacker.

Looking at the official docs you’ll see that juxt takes a set of functions and returns a function that is the juxtaposition of those functions. The returned function takes a variable number of arguments, and returns a vector containing the result of applying each function to the arguments (from left to right). Basically:

((juxt a b c) x) => [(a x) (b x) (c x)]

At first glace that probably doesn’t seem particularly useful. Let’s see some practical applications of juxt. What if we wanted to split a sequence into two sequences - one with the values that satisfy some predicate and one with the values that don’t. While there are many ways to do so, juxt offers one particularly elegant:

;; illustration of the general idea
((juxt filter remove) pred coll)

;; separate the even from the odd numbers
((juxt filter remove) even? (range 1 10))
;; => [(2 4 6 8) (1 3 5 7 9)]

juxt is also quite helpful when dealing with multiple map keys:

;; extract the values of a couple of maps keys
((juxt :alias :name) {:alias "Batman" :name "Bruce Wayne"})
;; => ["Batman" "Bruce Wayne"]

;; sort a vector of maps by a composite criteria
(sort-by (juxt :alias :name)
         [{:alias "Batman" :name "Bruce Wayne"}
          {:alias "Robin" :name "Jason Todd"}
          {:alias "Robin" :name "Tim Drake"}
          {:alias "Robin" :name "Dick Grayson"}])

;; => ({:name "Bruce Wayne", :alias "Batman"} {:name "Dick Grayson", :alias "Robin"} {:name "Jason Todd", :alias "Robin"} {:name "Tim Drake", :alias "Robin"})

Hopefully this short article has whetted your appetite and you’ll find even more elegant uses for juxt in your code.