Ruby Module可以访问它所需的类方法吗？

Question

I don't quite understand how this works:

module Perimeter
  def perimeter
    sides.inject(0) { |sum, side| sum + side }
  end
end

class Rectangle
  include Perimeter

  def initialize(length, breadth)
    @length = length
    @breadth = breadth
  end

  def sides
    [@length, @breadth, @length, @breadth]
  end
end

class Square
  include Perimeter

  def initialize(side)
    @side = side
  end

  def sides
    [@side, @side, @side, @side]
  end
end

When you call Rectangle.new(2, 3).perimeter it returns 10 as expected.

In this case, the module gets the arguments by calling the sides method from the class. How can a module have access to that method? Is it because of include keyword?

Answer 1

How can a module have access to that method?

That's exactly what modules do. Basically, you can define a bunch of methods in a module, then include it, and the end result is as if those methods were in your class directly.

If you want more technical, including a module injects it into your class' ancestors chain.

Rectangle.ancestors # => [Rectangle, Perimeter, Object, Kernel, BasicObject]

You might be familiar with Enumerable module. It contains many useful methods, like reduce , map , count and others. They're all implemented with method each and that's the one method that Enumerable module does not implement. It's the missing piece. Now, if you have a class (some kind of collection, perhaps. Genealogy tree or something) and it implements each , you can include Enumerable in that class and voilà, now you can map your collection. Same thing in your case with perimeter and sides .