Question

So Module can be used in Ruby to provide namespacing in addition to mixins, as so:

module SomeNamespace
  class Animal

  end
end

animal = SomeNamespace::Animal.new

But I’ve also seen the following used:

module SomeNamespace
end

class SomeNamespace::Animal

end

animal = SomeNamespace::Animal.new

My question is how they’re different (if they are) and which is more idiomatic Ruby?

Answer 1

The difference lies in nesting.

In the example below, you can see that the former method using class Foo, can get the outer scope’s constant variables BAR_A without errors.

Meanwhile, class Baz will bomb with an error of uninitialized constant A::B::Baz::BAR_A. As it doesn’t bring in A::* implicitly, only A::B::*explicitly.

module A
  BAR_A = 'Bar A!'
  module B
    BAR_B = 'Bar B!'
      class Foo
        p BAR_A
        p BAR_B
      end
  end
end

class A::B::Baz
  p BAR_A
  p BAR_B
end

Both behaviors have their place. There’s no real consensus in the community in my opinion as to which is the One True Ruby Way ™. I personally use the former, most of the time.