comp & partial in Clojure

Weather station readings converted by functions built with comp and partial.

;; comp glues functions right to left: the rightmost one runs first
(def celsius->f #(+ 32 (* 1.8 %)))
(def tenths #(/ (Math/round (* 10.0 %)) 10.0))
(def station-reading (comp tenths celsius->f))
(println (station-reading 21.5) (station-reading -3.0))

;; partial pre-fills the leftmost arguments and returns a new fn
(def scale-by (partial * 100))
(def prefix (partial str "station "))
(println (scale-by 0.42) (prefix "KB7"))

(def readings [21.5 -3.0 14.25 30.0])

;; both compose cleanly with map, which is where they earn their keep
(println (map station-reading readings))
(println (map (comp prefix str) [1 2 3]))
(println (map (partial max 0) [-4 7 -1 12]))

How it works

1. (comp tenths celsius->f) runs celsius->f first and rounds what comes back.
2. (partial * 100) and (partial str "station ") pre-fill the leftmost arguments.
3. Both go straight into map, as in (map (comp prefix str) [1 2 3]) and (map (partial max 0) [-4 7 -1 12]).

Keywords and builtins used here

• comp
• def
• map
• max
• partial
• prefix
• println
• str

The run, in numbers

Lines

17

Characters to type

666

Tokens

134

Three-star pace

70 tpm

At the three-star pace of 70 tokens a minute, this run takes about 115 seconds.

Type this snippet

Step 3 of 3 in Seq toolkit, step 18 of 27 in Language basics.

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