Keywords and Symbols

Keywords

A keyword starts with a colon: :name, :status, :http/get. Keywords evaluate to themselves and are used everywhere as map keys and as named constants.

(def user {:name "Ada" :role :admin})

(println (:name user))            ; a keyword is a function that looks itself up
(println (:email user))           ; missing key: nil
(println (:email user "none"))    ; with a default
(println (= :admin (:role user)))
(println (keyword "status") (name :status))
(println :shop/order (namespace :shop/order))

Ada
nil
none
true
:status status
:shop/order shop

• (:key map) reads a value — the most common way to access maps.
• Keywords compare very fast, so they're ideal for statuses and options (:pending, :done).
• Qualified keywords like :shop/order include a namespace, which avoids clashes between libraries. ::order means "qualified with the current namespace".

Symbols

A symbol is a name that refers to something — println, user, str/join. When evaluated, a symbol is replaced by its value. Quote it to use the symbol itself as data:

(def answer 42)

(println answer)          ; the value
(println 'answer)         ; the symbol
(println (type 'answer) (type :answer))
(println (symbol "x") (resolve 'println))

42
answer
clojure.lang.Symbol clojure.lang.Keyword
x #'clojure.core/println

You rarely work with symbols directly until you write macros (Level 3); for data, use keywords.