Truthiness & nil in Clojure

Only nil and false are falsey; or and and turn that into defaults.

;; only nil and false are falsey: 0, "" and [] are all truthy
(defn truthy? [x] (if x true false))
(println (truthy? 0) (truthy? "") (truthy? []) (truthy? nil))

;; or returns its first truthy value; and returns the last one
(def jar {:label "cumin" :grams 0})
(def shelf-name (or (:name jar) (:label jar) "unlabeled"))
(println shelf-name (and (:grams jar) (:label jar)) (and nil :skipped))

;; if-let binds and tests in one step; the else branch sees no binding
(defn jar-note [rack spice]
  (if-let [grams (get rack spice)]
    (str spice " holds " grams " g")
    (str spice " is missing")))

;; when-let skips its body entirely when the binding comes up nil
(when-let [low (seq (filter #(< (val %) 5) {:mace 2 :dill 40}))]
  (println "low jars:" (map key low)))
(println (jar-note {"mace" 2} "mace") (jar-note {} "anise"))

How it works

1. truthy? reports that 0, "" and [] all count as true.
2. or picks the first truthy value for shelf-name, while and returns the last value when every step holds.
3. if-let binds grams only when the lookup succeeds, and when-let skips its body when seq gives nil.

Keywords and builtins used here

• and
• def
• defn
• filter
• get
• if
• key
• map
• or
• println
• seq
• str
• val

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Step 2 of 3 in Flow & truthiness, step 11 of 27 in Language basics.

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