A Clojure namespace organizes names used to access functions, vars, and classes. It is an instance of clojure.lang.Namespace with two tables: one for symbol mappings and one for aliases. section 16.0
The two tables serve different purposes:
- The mapping table associates symbols with objects such as vars or classes. def and intern add mappings; ns-map displays them. section 16.0
- The alias table associates short symbols with other namespace names. :as or alias adds an alias; ns-aliases displays aliases. section 16.0
Names in clojure.core are available without an explicit reference. Other standard-library namespaces include clojure.string; a qualified name such as clojure.string/upper-case may be usable directly, while some namespaces must first be loaded with require. section 1.4
(def user-ns-mapping (ns-map 'user)) ; #1
(def lookup-var ('+ user-ns-mapping)) ; #2
(lookup-var 1 1) ; #3
;; 2
The listing reads the user namespace’s mappings with ns-map, retrieves the var mapped to +, and calls it with 1 and 1 to produce 2. section 16.0
A namespace is not necessarily a loaded library. A library is a source file named for its namespace declaration: require loads it and its transitive dependencies, while create-ns can create a namespace without a library. Created namespaces are held in a global repository for the running process. section 16.0 section 16.0
