Threading macros in Clojure

One elevation pipeline written nested, then with ->>, plus -> over a map.

(def elevations [820 1450 610 1980 1230])

;; nested calls read inside out: the last thing written happens first
(println (apply + (take 3 (sort > (filter #(> % 700) elevations)))))

;; ->> threads the value into the last slot, which suits seq functions
(println (->> elevations
              (filter #(> % 700))
              (sort >)
              (take 3)
              (apply +)))

;; -> threads into the first slot, which suits maps and assoc/update
(def trail {:name "ridge loop" :miles 6.2})
(println (-> trail
             (assoc :gain 1450)
             (update :miles + 0.8)
             (assoc :steep? true)))

How it works

1. The nested (apply + (take 3 (sort > (filter #(> % 700) elevations)))) has to be read inside out.
2. ->> threads the value into the last slot, so (filter #(> % 700)) and (take 3) line up in reading order.
3. -> threads into the first slot, letting (assoc :gain 1450) and (update :miles + 0.8) walk the trail map.

Keywords and builtins used here

• apply
• assoc
• def
• filter
• println
• sort
• take
• update

The run, in numbers

Lines

18

Characters to type

525

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100

Three-star pace

70 tpm

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Type this snippet

Step 3 of 3 in Sequences, step 15 of 27 in Language basics.

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