This guide covers:

  • An overview of Clojure namespaces and vars
  • How to define namespaces
  • How to use functions in other namespaces
  • require, refer and use
  • Common compilation errors and typical problems that cause them
  • Namespaces and their relation to code compilation in Clojure

This work is licensed under a Creative Commons Attribution 3.0 Unported License (including images & stylesheets). The source is available on Github.

What Version of Clojure Does This Guide Cover?

This guide covers Clojure 1.5.


Clojure functions are organized into namespaces. Clojure namespaces are very similar to Java packages and Python modules. Namespaces are basically maps (dictionaries) that map names to vars. In many cases, those vars store functions in them.

Defining a Namespace

Namespaces are usually defined using the clojure.core/ns macro. In its basic form, it takes a name as a symbol:

(ns superlib.core)

Namespaces can have multiple segments, separated by a dot:

(ns megacorp.service.core)

It is highly recommended to avoid using single segment namespaces (e.g. superlib) to avoid inconvenient conflicts other developers will have to work around. If a library or application belongs to an organization or a group of projects, the [organization].[library|app].[group-of-functions] pattern is recommended. For example:

(ns clojurewerkz.welle.kv)


In addition, the ns macro takes a number of optional forms:

  • (:require ...)
  • (:import ...)
  • (:use ...)
  • (:refer-clojure ...)
  • (:gen-class ...)

These are just slightly more concise variants of clojure.core/import, clojure.core/require, et cetera.

The :require Helper Form

The :require helper form is for setting up access to other Clojure namespaces from your code. For example:

(ns megacorp.profitd.scheduling
  (:require clojure.set))

;; Now it is possible to do:
;; (clojure.set/difference #{1 2 3} #{3 4 5})

This will make sure the clojure.set namespace is loaded, compiled, and available as clojure.set (using its fully qualified name). It is possible (and common) to make a namespace available under an alias:

(ns megacorp.profitd.scheduling
  (:require [clojure.set :as cs]))

;; Now it is possible to do:
;; (cs/difference #{1 2 3} #{3 4 5})

One more example with two required namespaces:

(ns megacorp.profitd.scheduling
  (:require [clojure.set  :as cs]
            [clojure.walk :as walk]))

The :refer Option

To make functions in clojure.set available in the defined namespace via short names (i.e., their unqualified names, without the clojure.set or other prefix), you can tell Clojure compiler to refer to certain functions:

(ns megacorp.profitd.scheduling
  (:require [clojure.set :refer [difference intersection]]))

;; Now it is possible to do:
;; (difference #{1 2 3} #{3 4 5})

The :refer feature of the :require form is new in Clojure 1.4.

It is possible to refer to all functions in a namespace (usually not necessary):

(ns megacorp.profitd.scheduling
  (:require [clojure.set :refer :all]))

;; Now it is possible to do:
;; (difference #{1 2 3} #{3 4 5})

The :import Helper Form

The :import helper form is for setting up access to Java classes from your Clojure code. For example:

(ns megacorp.profitd.scheduling
  (:import java.util.concurrent.Executors))

This will make sure the java.util.concurrent.Executors class is imported and can be used by its short name, Executors. It is possible to import multiple classes:

(ns megacorp.profitd.scheduling
  (:import java.util.concurrent.Executors

If multiple imported classes are in the same namespace (like in the example above), it is possible to avoid some duplication by using an import list. The first element of an import list is the package and other elements are class names in that package:

(ns megacorp.profitd.scheduling
  (:import [java.util.concurrent Executors TimeUnit]

Even though import list is called a list, it can be any Clojure collection (typically vectors are used).

The Current Namespace

Under the hood, Clojure keeps current namespace a special var, *ns*. When vars are defined using the def special form, they are added to the current namespace.

The :refer-clojure Helper Form

Functions like clojure.core/get and macros like clojure.core/defn can be used without namespace qualification because they reside in the clojure.core namespace and Clojure compiler automatically refers all vars in it. Therefore, if your namespace defines a function with the same name (e.g. find), you will get a warning from the compiler, like this:

WARNING: find already refers to: #'clojure.core/find in namespace: megacorp.profitd.scheduling, being replaced by: #'megacorp.profitd.scheduling/find

This means that in the megacorp.profitd.scheduling namespace, find already refers to a value which happens to be clojure.core/find, but it is being replaced by a different value. Remember, Clojure is a very dynamic language and namespaces are basically maps, as far as the implementation goes. Most of the time, however, replacing vars like this is not intentional and Clojure compiler emits a warning.

To solve this problem, you can either rename your function, or else exclude certain clojure.core functions from being referred using the (:refer-clojure ...) form within the ns:

(ns megacorp.profitd.scheduling
  (:refer-clojure :exclude [find]))

(defn find
  "Finds a needle in the haystack."
  [^String haystack]
  (comment ...))

In this case, to use clojure.core/find, you will have to use its fully qualified name: clojure.core/find:

(ns megacorp.profitd.scheduling
  (:refer-clojure :exclude [find]))

(defn find
  "Finds a needle in the haystack."
  [^String haystack]
  (clojure.core/find haystack :needle))

The :use Helper Form

In Clojure versions before 1.4, there was no :refer support for the (:require ...) form. Instead, a separate form was used: (:use ...):

(ns megacorp.profitd.scheduling-test
  (:use clojure.test))

In the example above, all functions in clojure.test are made available in the current namespace. This practice (known as "naked use") works for clojure.test in test namespaces, but in general not a good idea. (:use ...) supports limiting functions that will be referred:

(ns megacorp.profitd.scheduling-test
  (:use clojure.test :only [deftest testing is]))

which is a pre-1.4 alternative of

(ns megacorp.profitd.scheduling-test
  (:require clojure.test :refer [deftest testing is]))

It is highly recommended to use (:require ...) (optionally with ... :refer [...]) on Clojure 1.4 and later releases. (:use ...) is a thing of the past and now that (:require ...) with :refer is capable of doing the same thing when you need it, it is a good idea to let (:use ...) go.

The :gen-class Helper Form

TBD: How to Contribute

Documentation and Metadata

Namespaces can have documentation strings. You can add one with the optional ns macro parameter:

(ns superlib.core
  "Core functionality of Superlib.

   Other parts of Superlib depend on functions and macros in this namespace."
  (:require [clojure.set :refer [union difference]]))

or metadata:

(ns ^{:doc "Core functionality of Superlib.
            Other parts of Superlib depend on functions and macros in this namespace."
      :author "Joe Smith"}
  (:require [clojure.set :refer [union difference]]))

Metadata can contain any additional keys such as :author which may be of use to various tools (such as Codox, Cadastre, or lein-clojuredocs).

How to Use Functions From Other Namespaces in the REPL

The ns macro is how you usually require functions from other namespaces. However, it is not very convenient in the REPL. For that case, the clojure.core/require function can be used directly:

;; Will be available as clojure.set, e.g. clojure.set/difference.
(require 'clojure.set)

;; Will be available as io, e.g. io/resource.
(require '[ :as io])

It takes a quoted libspec. The libspec is either a namespace name or a collection (typically a vector) of [name :as alias] or [name :refer [fns]]:

(require '[clojure.set :refer [difference]])

(difference #{1 2 3} #{3 4 5 6})  ; ⇒ #{1 2}

The :as and :refer options can be used together:

(require '[clojure.set :as cs :refer [difference]])

(difference #{1 2 3} #{3 4 5 6})  ; ⇒ #{1 2}
(cs/union #{1 2 3} #{3 4 5 6})    ; ⇒ #{1 2 3 4 5 6}

clojure.core/use does the same thing as clojure.core/require but with the :refer option (as discussed above). It is not generally recommended to use use with Clojure versions starting with 1.4. Use clojure.core/require with :refer instead.

Namespaces and Class Generation

TBD: How to Contribute

Namespaces and Code Compilation in Clojure

Clojure is a compiled language: code is compiled when it is loaded (usually with clojure.core/require).

A namespace can contain vars or be used purely to extend protocols, add multimethod implementations, or conditionally load other libraries (e.g. the most suitable JSON parser or key/value store implementation). In all cases, to trigger compilation, you need to require the namespace.

Private Vars

Vars (and, in turn, functions defined with defn) can be private. There are two equivalent ways to specify that a function is private: either via metadata or by using the defn- macro:

(ns megacorp.superlib)

;; Implementation

(def ^{:private true}
  source-name "supersource")

(defn- data-stream
  (comment ...))

Constant Vars

Vars can be constant. This is done by setting the :const metadata key to true. This will cause Clojure compiler to compile it as a constant:

(ns megacorp.epicgame)

;; Implementation

(def ^{:const true}
  default-score 100)

How to Look up and Invoke a Function by Name

It is possible to look up a function in particular namespace by-name with clojure.core/ns-resolve. This takes quoted names of the namespace and function. The returned value can be used just like any other function, for example, passed as an argument to a higher order function:

(ns-resolve 'clojure.set 'difference)  ; ⇒ #'clojure.set/difference

(let [f (ns-resolve 'clojure.set 'difference)]
   (f #{1 2 3} #{3 4 5 6}))  ; ⇒ #{1 2}

Compiler Exceptions

This section describes some common compilation errors.


This exception means that JVM could not load a class. It is either misspelled or not on the classpath. Potentially your project has unsatisfied dependency (some dependencies may be optional).


user=> (import java.uyil.concurrent.TimeUnit)
ClassNotFoundException java.uyil.concurrent.TimeUnit$ (

In the example above, java.uyil.concurrent.TimeUnit should have been java.util.concurrent.TimeUnit.

CompilerException java.lang.RuntimeException: No such var

This means that somewhere in the code a non-existent var is used. It may be a typo, an incorrect macro-generated var name or a similar issue. Example:

user=> ( "thought_leaders_quotes.csv")
CompilerException java.lang.RuntimeException: No such var:, compiling:(NO_SOURCE_PATH:1)

In the example above, should have been NO_SOURCE_PATH means that compilation was triggered from the REPL and not a Clojure source file.

Temporarily Overriding Vars in Namespaces

TBD: How to Contribute

Getting Information About and Programmatically Manipulating Namespaces

TBD: How to Contribute

Wrapping Up

Namespaces are basically maps (dictionaries) that map names to vars. In many cases, those vars store functions in them.

This implementation lets Clojure have many of its highly dynamic features at a very reasonable runtime overhead cost. For example, vars in namespaces can be temporarily altered for unit testing purposes.


Michael Klishin (original author)