Let’s talk about namespaces today. Below is a structure of the core.clj file that is located in /src/app/ directory. Function ns sets the namespace. We use directory-name.file-name for the name then :require to load external namespaces.

(ns app.core ; we use a file name dot directory name as a project namespace
  (:require ; loads the namespaces below
    [clojure.set :refer :all] ; imports all functions from clojure.set
    [clojure.string :as str] ; imports clojure.string with name "str"
    [clojure.set :refer [union]])) ; imports only "union" function from clojure.set

If we would like to get a current namespace in the REPL we use the function ns-name and pass a *ns*(keeps info about the current namespace) var to it.

(ns-name *ns*) ; => user

To switch to a specific namespace we use the in-ns function. It is important to remember to put a quote ' sign before the namespace name to stop the Clojure reader from trying to resolve the namespace’s value. We will receive a compiler exception error if we don’t quote the namespace.

(in-ns 'clojure.string) ; => #namespace[clojure.string]

When loading a namespace in the REPL we would use the require function.

(require '[clojure.set :as cset])

To see all functions available in a specific namespace we can use ns-publics function. It returns a map data structure so to get only names we use keys function.

(keys (ns-publics 'clojure.string)) ; => (capitalize reverse join trim ...)