Clojure libraries and dependencies

The Clojure language provides some keywords to handle libraries in each file:

• :require loads a Clojure library so it can be used in the current file or in the REPL
• :use brings definitions to current namespace via aliases
• :import gives access to native Java classes and interfaces

Note: in ClojureScript the native JavaScript code can be usually accessed using :require, but sometimes :import is needed (eg. for Google Closure library).

How to use these keywords

Load foo.bar and invoke a function from that library with the full namespace:

(ns user
  (:require
    [foo.bar]
    [foo.baz]))

(foo.bar/a-function)

Load a library making an alias to simplify access to a function:

(ns user
  (:require
    [foo.bar :as bar]
    [foo.baz :as baz]))

(bar/a-function)

Load with :refer only the definitions we are interested in:

(ns user
  (:require
    [foo.bar :refer [a-function]]))

(a-function)

Load with :refer only some definitions and also make aliases with :rename for our convenience:

(ns user
  (:require
    [foo.bar :refer [a-function]
             :rename [a-function func]]))

(func)

The :use keyword can be applied with :only to indicate which definitions will be provided:

(ns user
  (:use
    [foo.bar :only [a-function]]))

(a-function)

The :use keyword can also be applied without :only, but beware! It can cause conflicts, eg. the following snippet will have a problem if foo.bar and foo.baz namespaces provide definitions with the same name:

(ns user
  (:use
    [foo.bar]   ;; avoid :use without :only, it can cause conflicts!
    [foo.baz]))

(a-function)

Import a native Java class of java.util package and invoke the Date. method:

(ns user
  (:import
    (java.util Date)))

(Date.) ; call a Java method to get the current date