From: Peter <Peter.Vanbroekhoven@...>
Date: 2005-05-17T22:01:05+09:00
Subject: Re: Extending an Instance only Once

> I have a method that requires that the arguments passed to it have been 
> in-mixed with a module. Currently, my code looks like this:
>
> def my_method( the_obj )
>    the_obj.extend( MyModule ) unless the_obj.inherits_from?( MyModule )
> end

Ruby already checks this:

irb(main):001:0> module M ; end
=> nil
irb(main):002:0> class A ; end
=> nil
irb(main):003:0> class B < A; end
=> nil
irb(main):004:0> class A ; include M ; end
=> A
irb(main):005:0> class B ; include M ; end
=> B
irb(main):006:0> B.ancestors
=> [B, A, M, Object, Kernel]

Note that there is a single M in there. If the includes happen in the 
reverse order, Ruby's check fails and there are two M's in the list:

irb(main):001:0> module M ; end
=> nil
irb(main):002:0> class A ; end
=> nil
irb(main):003:0> class B < A; end
=> nil
irb(main):004:0> class B ; include M ; end
=> B
irb(main):005:0> class A ; include M ; end
=> A
irb(main):006:0> B.ancestors
=> [B, M, A, M, Object, Kernel]

This does not proof the check is actually done, but in the source you can 
see that check is actually there without doubt.

The same goes for Object#extend because it is implemented as an include in 
the singleton class (or selfclass as some people prefer :) which obeys the 
same rules as includes in regular classes. So no need to check this 
yourself!

Peter