Clojure: juxt and separate

Juxt is one of those higher-order functions that you never knew you needed. Despite not using it that often I find it can still be surprisingly useful. Let's check out the docs.

(doc juxt)

=>
-------------------------
clojure.core/juxt
([f] [f g] [f g h] [f g h & fs])
  Takes a set of functions and returns a fn that is 
  the juxtapositionof those fns.  The returned fn 
  takes a variable number of args, and returns a 
  vector containing the result of applying each fn 
  to the args (left-to-right).
  ((juxt a b c) x) => [(a x) (b x) (c x)]

So juxt takes a number of functions and returns a function that applies each of those functions to its args.

Sort-by multiple keys

Sort-by returns a sorted sequence of the items in a collection where the sort order is determined by comparing the key function of each item. If you want to have a secondary sort for when the first keys are equal you can use juxt to pass in two or more key functions, with each additional key being: a secondary, tertiary sort, and so on.

(sort-by (juxt :a :b)
 [{:a 3 :b 4} {:a 2 :b 2} {:a 2 :b 1}])

=> ({:a 2 :b 1} {:a 2 :b 2} {:a 3 :b 4})

When you need to sort items by criteria of decreasing importance this comes in handy.

Group-by multiple keys

Group-by returns a map of elements grouped by key, where the supplied function determines the grouping. If you want to group by multiple functions, you can use juxt to pass in two or more functions.

(defn health-status [{:keys [health]}]
  (if (< health 50)
    :below-half-health
    :above-half-health))

(group-by
 (juxt :faction health-status)
  [{:name "Grug" :faction :orcs :health 49}
   {:name "Elendil" :faction :elves :health 98}
   {:name "Gazkral" :faction :orcs :health 65}
   {:name "Varok" :faction :orcs :health 31}
   {:name "Gork" :faction :orcs :health 29}])

=> {[:orcs :below-half-health]
    [{:name "Grug" :faction :orcs :health 49}
     {:name "Varok" :faction :orcs :health 31}
     {:name "Gork" :faction :orcs :health 29}]
    [:elves :above-half-health]
    [{:name "Elendil" :faction :elves :health 98}]
    [:orcs :above-half-health]
    [{:name "Gazkral" :faction :orcs :health 65}]}

(defn cleave-attack [actors]
  (dissoc actors [:orcs :below-half-health]))

(cleave-attack
 {[:orcs :below-half-health]
  [{:name "Grug" :faction :orcs :health 49}
   {:name "Varok" :faction :orcs :health 31}
   {:name "Gork" :faction :orcs :health 29}]
  [:elves :above-half-health]
  [{:name "Elendil" :faction :elves :health 98}]
  [:orcs :above-half-health]
  [{:name "Gazkral" :faction :orcs :health 65}]})

=> {[:elves :above-half-health]
    [{:name "Elendil", :faction :elves, :health 98}]
    [:orcs :above-half-health]
    [{:name "Gazkral", :faction :orcs, :health 65}]}

Now we know how many orcs get cut down by Elendil's cleave attack.

Separate

There was once a function, in the now deprecated clojure.contrib.seq-utils library, called separate. It would return a vector containing a sequence of the items that satisfied the predicate followed by a sequence of items that didn't satisfy the predicate. We can recreate this helpful function like this:

(defn separate [pred s]
  [(filter pred s) (remove pred s)])

(separate odd? [1 2 3 2 1])

=> [(1 3 1) (2 2)]

However, we can make this function more succinct with juxt removing the need to pass the same arguments into two different functions.

(defn separate [pred s]
  ((juxt filter remove) pred s))

(separate odd? [1 2 3 2 1])

=> [(1 3 1) (2 2)]

Hopefully, these examples give you a taste of what juxt can do.