case & condp in Clojure

Subway line colors chosen by case on literals, crowding levels by condp.

;; case dispatches on literal constants and never evaluates them
(defn line-color [line]
  (case line
    ("A" "C" "E") "blue"
    ("1" "2" "3") "red"
    "G" "light green"
    "unknown line"))

;; the trailing expression is the default; without one a miss throws
(println (line-color "C") (line-color "G") (line-color "Q"))

;; condp holds a predicate and a right-hand argument across the clauses
(defn crowding [riders]
  (condp <= riders
    900 "crush"
    500 "heavy"
    "light"))

(println (map crowding [1200 640 40]))

How it works

1. line-color groups literals in lists like ("A" "C" "E") so several keys share one result.
2. The trailing "unknown line" is the default, and without it an unmatched value throws.
3. crowding uses condp <= riders, so the clause 900 "crush" fires when 900 is at or below the count.

Keywords and builtins used here

• case
• condp
• defn
• map
• println

The run, in numbers

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Three-star pace

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Type this snippet

Step 3 of 3 in Flow & truthiness, step 12 of 27 in Language basics.

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