Clojure Namespaces

Namespaces

Namespaces help us modularize the code in Clojure. They help us divide things like functions or pieces of data into namespaces. We use namespaces to define public and private functions/members

Namespaces allow us to declare Vars, keywords and Java type names in a particular scope, which is not possible by using function arguments or let

Vars

Vars are things that are created when we use def or defn inside the namespace. Vars is an object that represent a named value and are stored in the namespace.

Keywords

Keywords are lightweight strings that evaluate themselves. Keywords can also be namespace qualified

 ;; In the namespace "foo.bar"
 :x        ; Keyword with no namespace
 ::x       ; Keyword with "foo.bar" namespace
 :baz/x    ; Keyword with "baz" namespace

REPL namespace

REPL always starts in user namespace. We can use in-ns to switch to a namespace. in-ns will create a namespace if it doesn’t exist.

Namespace Operations

Load - require find source in classpath and evaluates it

      ;; plain require to load another namespace
      (require 'clojure.string)
      (println (clojure.string/upper-case "Rob"))
      ;; ROB

Alias - can be used for have shorter aliases

      ;; use :as to create an alias
      (require '[clojure.set :as set-alias])
      (println (set-alias/union #{1 2 3} #{2 3 4 7}))
      ;; #{7 1 4 3 2}
        
      (require '[clojure.java.io :as io]
               '[clojure.string :as stringy])
      (println (stringy/index-of "Rob" "b"))
      (println (io/file "file-name"))
      ;; #object[java.io.File 0x66f57048 file-name]

Refer - use copy symbol binding from another namespace into current namespace

      (use `clojure.string)
      (reverse hello)

      (use `[clojure.string :only (join)])
      (join "," ["1", "2", "3"])

Import - make Java class available in current namespace so that they can be used without package prefix
```
      (import (java.io FileReader File))
        
      (FileReader. (File. "readme.md"))
```

`ns`

(ns Intro) creates namespace and then do REQUIREs and IMPORTs. It refers all of clojure.core and imports all of java.lang Clojure converts Dots in namespace to / and Hyphens to underscores

Use require with ns

    ;; use require with ns
    (ns my.clojure.project
        (:require [clojure.string :as stringy]))
        

    (println (stringy/upper-case "rob"))

Use use with ns

    (ns my.clojure.project
      (:use [clojure.string :only (upper-case)]))
    
    (println (upper-case "rob"))

Use import with ns

    (ns my.clojure.project
      (:import (java.io File)))
    
    (println (File. "new-file"))

Private Vars

Private Vars can be declared by adding ^:private metadata to a definition. defn- is a shortcut for defn ^:private. Private prevents accidental refer with use

`the-ns`

To get access to namespace we use the-ns

    (the-ns `my.clojure.project)