Multiple arities in Clojure

A watering function with a default arity, plus a variadic label builder.

(require '[clojure.string :as str])

;; two arities: the short one fills in a default and calls the long one
(defn water
  ([bed] (water bed 2))
  ([bed liters]
   (str "watering " bed " with " liters "L")))

(println (water "tomatoes"))
(println (water "ferns" 5))

;; & gathers the leftover arguments into a seq, or nil when there are none
(defn label [bed & remarks]
  (str bed (when (seq remarks)
             (str " - " (str/join "; " remarks)))))

(println (label "beans"))
(println (label "beans" "shade cloth" "sow again in June"))

How it works

1. ([bed] (water bed 2)) fills in a default and calls the two-argument arity.
2. The longer arity takes [bed liters] and formats the message with str.
3. [bed & remarks] gathers leftover arguments into a seq that (seq remarks) checks before joining.

Keywords and builtins used here

• defn
• label
• println
• require
• seq
• str
• water
• when

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Step 3 of 3 in Functions, step 9 of 27 in Language basics.

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