namespaces

Introduction to namespaces

A namespace is a dynamic mapping from symbols to vars and classes.

Simple commands in the REPL

(ns foo.bar) switches to namespace foo.bar.

(use 'foo.baz) compiles and loads foo.baz namespace into the current namespace.

(require '[foo.baz :as fb]) compiles and loads and binds the prefix fb to foo.baz namespace.

*ns* references the current namespace.

(def s (...)) interns a var in the current namespace that can be referenced by the symbol s.

(ns-unmap *ns* 's) removes the var that the symbol s references from the current namespace.

Query functions on namespaces

ns-interns returns a map of symbols to vars that the given ns defines.

ns-publics like ns-interns, but only public mappings.

ns-refers returns a map of symbol to vars that the given ns refers to.

ns-imports returns a map of symbol to class names that the given ns refers to.

ns-map returns a map of all namespace mappings of the given ns.

dir (from clojure.repl) prints sorted list of publics vars of given ns.

Exercise 1

• Define some symbols in the current namespace.
• List all symbols that the current namespace defines.
• Remove one mapping.

Exercise 2

Create a map of name to function var for all functions with a suffix like "-page".

(defn s-page
  []
  "S")


(nsfns "-page" *ns*)
;=> { "s" #'user/s-page }

The ns form

An example

(ns foo.bar "documentation"
  (:require [clojure.tools.logging :as log]      ; introduce ns alias
            [clojure.test :refer :all]           ; use all public vars
			[clojure.string :refer [blank?]]     ; use only selected vars
			[:refer-clojure :exclude [map]])     ; exclude clojure default vars
  (:import [java.utils ArrayList HashMap]))

Exercise 3

Take the solution of exercise 2 and add it to the webapp. Add another function render and extend the webapp application handler app so that a URL /pages/:page is generically dispatched to a function named -page.

For example, if you have two rendering functions defined

(defn s-page [] "S") 

(defn t-page [] "T")

and an application handler app

(defroutes app
  (GET "/page/:page" [page] (render page)))

then in a REPL you can enter

(app {:uri "/page/s" :request-method :get})
;=> {:status 200, :headers {"Content-Type" "text/html; charset=utf-8"}, :body "S"}

This means the function render executes the renderer function that corresponds to the URL.

Refresh using org.clojure/tools.namespace

Include in your project.clj [org.clojure/tools.namespace "0.2.4"] as dependency.

(use '[clojure.tools.namespace.repl :only (refresh)]) imports the refresh function.

(refresh) destroys and reloads all namespaces that have changed since the last refresh.

Exercise 4

• Create two files, foo.clj and bar.clj, each declaring a corresponding ns.
• Define one function per file, the function foofn calls a the function barfn.
• Start a REPL, (use 'foo), execute foofn.
• Change barfn in bar.clj without reloading the namespace bar.
• Use the clojure.tools.namespace.repl ns as shown above and call (refresh).
• Execute foofn again.

License

Copyright © 2013 F.Riemenschneider

Distributed under the Eclipse Public License, the same as Clojure.