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Day 5 – Namespaces

The namespace provides a way to organize different Clojure objects into logical groups which logical ones are commonly called the library and can be used in other namespaces.

ns

To define a namespace, use the ns macro.

(ns seven-days-of-clojure.example)

ns can take :require, :use, and :import keyword.

(ns seven-days-of-clojure.example
  (:require [clojure.java.io])
  (:use [clojure.data])
  (:import [java.util List Set]))

Typically, each ns references a specific file.

directory.file-name.clj
directory.subdirectory.file-name.clj
;; file-name.clj
(ns seven-days-of-clojure.file-name)

(defn function-1
  []
  "Hello from function-1")

(defn function-2
  []
  "Hello from function-2")

(defn function-3
  []
  "Hello from function-3")
;; ...

in-ns

in-ns allows you to switch to a specific namespace.

(in-ns 'test)
;; user=> #object[clojure.lang.Namespace 0x415156bf "test"]

(in-ns 'calculate)
;; user=> #object[clojure.lang.Namespace 0x1ecfcbc9 "calculate"]

(in-ns 'some-other)
;; user=> #object[clojure.lang.Namespace 0x660591fb "some-ohter"]

require

That you define in a namespace is not visible to other namespaces by default.

(ns seven-days-of-clojure.calculate)

(defn sum
  [a b]
    (+ a b))

(defn div
  [c d]
    (/ c d))

(defn mult
  [e f]
    (* e f))

Note that if you try to access definitions in another namespace, an exception will be thrown.

(sum 3 4)
;; user=> Unable to resolve symbol: sum in this context

To access other namespaces, use require.

(require 'seven-days-of-clojure.calculate)
;; user=> nill

(seven-days-of-clojure.calculate/sum 3 4)
;; user=> 7

To create a namespace alias, use the :as keyword.

(require '[seven-days-of-clojure.calculate :as calc])

(calc/mult 7 7)
;; user=> 49

(calc/div 4 2)
;; user=> 2

(calc/sum 3 5)
;; user=> 8

Load multiple namespaces.

(require '[seven-days-of-clojure.example :as eg]
         '[clojure.core :as cc])

Test your knowledge

  • Create a namespace with some functions and try to access them from another namespace.
    • Tip: use (in-ns' namespace-name) to switch namespace).
  • Load a namespace using an alias for a short name.
  • Load multiple namespaces and use at least one resource from each in the current namespace.
  • Build your imagination.

Day 6 - Tests

See more

License

This project is licensed under the MIT License.

License