Keywords & symbols in Clojure

Reads a mixtape track from a map with keywords, then contrasts one with a quoted symbol.

;; keywords evaluate to themselves, which makes them good map keys
(def track {:title "Sea Change" :artist "Wren" :minutes 4})

;; a keyword called as a function looks itself up in a map
(println (:artist track) "-" (:title track))

;; a second argument is the fallback when the key is missing
(println "bpm:" (:bpm track 120))

;; a namespaced keyword keeps keys from colliding across sources
(def tagged (assoc track :mixtape/side :a))
(println (:mixtape/side tagged))

;; a symbol names something else; quoting one stops the lookup
(def side 'flip)
(println (type :a) (type side))

How it works

1. (:artist track) calls a keyword as a function so it looks itself up in the map.
2. (:bpm track 120) supplies a fallback for a key the map does not carry.
3. :mixtape/side shows a namespaced key, and (def side 'flip) binds a symbol so type can tell the two apart.

Keywords and builtins used here

• assoc
• def
• println
• type

The run, in numbers

Lines

16

Characters to type

583

Tokens

71

Three-star pace

75 tpm

At the three-star pace of 75 tokens a minute, this run takes about 57 seconds.

Type this snippet

Step 3 of 3 in Values & bindings, step 3 of 27 in Language basics.

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