What's the benefit of having a scope-resolution operator within the language?

Question

For example: puts FileManagement::CSV.Reader.new

Why not: puts FileManagement.CSV.Reader.new?

Why this differentiation? What's the benefit of having the additional ::-operator?

Answer 1

:: is for namespaces and . is for designating a receiver in a method call.

If you have both, a constant and a class method with the same name within the same namespace, e.g.:

module Foo
  Bar = 123

  def self.Bar
    456
  end
end

then the scope resolution operator resolves to the constant whereas the dot resolves to the class method:

Foo::Bar
#=> 123

Foo.Bar
#=> 456

This is also true if the constant refers to another class or module which was defined under Foo.

If you only had Foo.Bar there’d be no way to get the Bar constant.

In addition, you can target the top-level namespace by using the scope resolution operator as a prefix:

Bar = 123

module Foo
  Bar = 456

  def self.bars
    [::Bar, Bar]
  end
end

Foo.bars
#=> [123, 456]

In the above, .Bar would raise a SyntaxError.

Finally, :: can be used to define constants within a namespace:

module Foo
end

module Foo::Bar
end

Foo::Bar::Baz = 123

module Foo.Bar or Foo.Bar.Baz = 123 both wouldn't work.