Vector destructuring in Clojure

Pulls trail waypoints and a coordinate pair apart by position inside let.

;; Destructuring a vector binds names to positions from left to
;; right, and & gathers everything still unnamed into a sequence.
(def waypoints [:trailhead :bridge :ridge :summit])

(let [[start next-stop & later] waypoints]
  (println (format "start %s, then %s, %d to go"
                   (name start) (name next-stop) (count later))))

;; :as keeps a name for the whole vector while the parts stay bound.
(let [[lat lon :as fix] [45.52 -122.68]]
  (println (format "%.2f/%.2f from %d parts" lat lon (count fix))))

;; Nesting works at any depth, and _ marks a slot you do not need.
(def leg [[45.52 -122.68] [47.61 -122.33]])
(let [[[from-lat _] [to-lat _]] leg]
  (println (format "north by %.2f degrees" (- to-lat from-lat))))

How it works

1. (let [[start next-stop & later] waypoints] names the first two elements and collects everything after them into later.
2. The binding [lat lon :as fix] splits a coordinate pair while fix still refers to the whole vector.
3. [[from-lat _] [to-lat _]] reaches into a vector of vectors, and _ marks the longitudes this code ignores.

Keywords and builtins used here

• count
• def
• format
• let
• name
• println

The run, in numbers

Lines

16

Characters to type

709

Tokens

115

Three-star pace

70 tpm

At the three-star pace of 70 tokens a minute, this run takes about 99 seconds.

Type this snippet

Step 1 of 3 in Destructuring, step 19 of 27 in Language basics.

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