namespaced-keywords.org

Namespaced Keywords

Clojure has namespaced keywords so that you can use keywords that avoid name collisions. :start is an example of an un-namespaced keyword, and :donut.system/start is an example of a namespaced keyword.

Clojure has some conveniences for writing more compact code when you’re using namespaced keywords. If you require a namespace and specify an alias, you can use that alias for namespaced keywords:

(ns donut.examples.printer
  (:require [donut.system :as ds]))

;; use two colons to expand a namespace alias
::ds/start
;; => :donut.system/start

;; when creating maps, you can specify a namespace for all the keys in the map:
:donut.system{:start "start component"}
;; this is the same as {:donut.system/start "start component"}

;; You can also use a namespace alias
#::ds{:start "start component"}
;; => {:donut.system/start "start component"}

#::ds{:start "start component"
      :stop  "stop component"}
;; => {:donut.system/start "start component", :donut.system/stop "stop component"}

donut.system heavily relies on namespaced keywords to avoid name collisions. When you define a component, you use keys like :donut.system/start and :donut.system/config. By using namespaced keywords like this the library is more open to being extended in unforeseen ways.

You can use namespaced keywords when destructuring arguments:

(def Component
  {::ds/start "start component"})

(let [{:keys [::ds/start]} Component]
  (prn "start is: " start))
;; prints "start component"