Ruby: Reopening a Class from within a Module

Why doesn't this work?

module XT
  puts Fixnum.class  # So we're sure to re-open the Fixnum class
  class Fixnum
    def hi
      puts "HI from module XT"
    end
  end
开发者_开发百科end

After requiring and loading the module, the hi method is still not added to Fixnum. It works if I remove the module wrapper.

---

As @Jeremy wrote, constants are namespaced by modules, and defining a class is really just assigning a class object to a constant. Basically,

class Fixnum; end

is (roughly) equivalent to

Fixnum = Class.new

(except for the fact that if Fixnum already exists, the former will reopen it, while the latter will overwrite it).

This means that if you do that inside of a module (or class, since class IS-A module), the constant Fixnum will be namespaced inside that module.

If you want to explicitly access a top-level constant, you can tell Ruby to start its lookup at the top-level in a very similar vein to how you tell Unix to start filesystem lookup at the top-level:

module XT
  class ::Fixnum; end
end

---

You are defining XT::Fixnum, not Fixnum.