From: Paul Brannan <paul@...>
Date: 2001-12-20T03:50:47+09:00
Subject: [ruby-talk:28977] overriding methods: (almost) a replacement for alias_method

I think almost all of us will agree that it's pretty ugly to do:

  class Foo
    alias_method :old_foo, :foo
    def foo
      # do stuff
      old_foo()
    end
  end

Well, I was playing around with define_method() last night, and I came
up with this idea: when I include a module, an anonymous base class is
created from the included module.  So if I could create a temporary
module with one method, have that method be an alias for the method I
want to override, and include it in the current class, then I could call
the original function with a simple call to super():

  class Foo
    def foo
      puts "foo!"
    end

    tmp_module = Module.new
    tmp_module.__send__(:define_method, :foo, instance_method(:foo))
    include tmp_module

    def foo
      puts "foo 2!"
      super()
    end
  end
  
  Foo.new.foo

This produces:
  
  foo 2!
  foo!

The problem is that this doesn't work quite the way I would expect.  If
I try to override the new foo(), such as with:

  class Foo
    def foo
      puts "foo!"
    end

    tmp_module = Module.new
    tmp_module.__send__(:define_method, :foo, instance_method(:foo))
    include tmp_module

    def foo
      puts "foo 2!"
      super()
    end

    tmp_module = Module.new
    tmp_module.__send__(:define_method, :foo, instance_method(:foo))
    include tmp_module

    def foo
      puts "foo 3!"
      super()
    end
  end

Then I get:
  
  foo 3!
  foo 2!
  ...
  foo2.rb:11:in `foo': stack level too deep (SystemStackError)
        from foo2.rb:11:in `foo'
        from foo2.rb:12:in `foo'
        from foo2.rb:12:in `foo'
        from foo2.rb:12:in `foo'
        from foo2.rb:12:in `foo'
        from foo2.rb:12:in `foo'
        from foo2.rb:12:in `foo'
        from foo2.rb:12:in `foo'
         ... 6360 levels...
        from foo2.rb:12:in `foo'
        from foo2.rb:21:in `foo'
        from foo2.rb:21:in `foo'
        from foo2.rb:25

So it seems that "foo 3" makes a call to super(), which calls "foo 2".
Then "foo 2" makes a call to super(), which also calls "foo 2"....  This
sort of makes sense, because "foo 2" was defined in class Foo, and class
Foo's base class is the anonymous class in which "foo 2" is defined.

I feel really horrible being so close to a solution to this problem
(finding an alternative for alias_method), but not being quite there.
Does anyone have some ideas for how this idea could be extended or
enhanced to achieve the desired result?

Paul