Modules and mixins
Define a module
The keyword module groups methods and constants under one name. Unlike a class, a module cannot be instantiated on its own. Classes later borrow the grouped behavior through include or extend.
module Loud
def shout(text)
puts text.upcase
end
end
puts Loud.class
Module
Include a mixin
Writing include inside a class mixes the module methods in as instance methods. Every instance then answers to the shared behavior. The class body stays short while the module carries the reusable part.
module Loud
def shout(text)
puts text.upcase
end
end
class User
include Loud
end
User.new.shout("hello")
HELLO
Extend with class methods
Writing extend inside a class mixes the module methods in as class methods instead. This placement suits finders, builders, and other helpers that belong to the class itself. Instances remain untouched by the mixed behavior.
module Finder
def find(id)
puts "found #{id}"
end
end
class User
extend Finder
end
User.find(7)
found 7
Modules as namespaces
Modules also fence off names: nesting classes and constants inside keeps each name scoped. A Cart defined inside Shop is addressed as Shop::Cart and never clashes with a Cart from another library.
module Shop
class Cart
def size
0
end
end
end
puts Shop::Cart.new.size
0
Next: Classes and objects
Article author: Arthur Isaev
