Question

If I have a class and a module:

class Foo
end

module WhackyFoo
  def whacky
    puts 'shits whacky!'
  end
end

Why can I do this:

include WhackyFoo
Foo.new.whacky
# => shits whacky!

Can someone explain or point me to the Ruby docs where this usage is explained?

Answer 1

Because when you include module on the top level, it gets included into a special object main. Any method defined on main (including those included from module) become private instance methods of Object (and, therefore, all objects). That’s why it works.

If you include a module into a “normal” class (not top level main), it behaves as you would expect

module WhackyFoo
  def whacky
    puts 'shits whacky!'
  end
end

class Foo
end

class Bar
  include WhackyFoo
end


Bar.new.whacky # >> "shits whacky!"

Foo.new.whacky # ~> -:16:in `<main>': undefined method `whacky' for #<Foo:0x007fa92984d968> (NoMethodError)