Question

What are the differences between this:

module Outer
  module Inner
    class Foo
    end
  end
end

and this:

module Outer::Inner
  class Foo
  end
end

I know the latter example won’t work if Outer was not defined earlier, but there are some other differences with constant scope and I could find their description on SO or in documentation (including Programming Ruby book)

Answer 1

there is at least one difference – constant lookup, check this code:

module A
  CONST = 1

  module B
    CONST = 2

    module C
      def self.const
        CONST
      end
    end
  end
end

module X
  module Y
    CONST = 2
  end
end

module X
  CONST = 1

  module Y::Z
    def self.const
      CONST
    end
  end
end

puts A::B::C.const # => 2, CONST value is resolved to A::B::CONST
puts X::Y::Z.const # => 1, CONST value is resolved to X::CONST