From: jeremy@... (Jeremy Henty)
Date: 2001-10-08T05:23:17+09:00
Subject: [ruby-talk:22211] Re: Why does Class undefine module_function?

Matz writes: 

>  ... Jeremy Henty <jeremy@chaos.org.uk> writes:
> 
> |/usr/local/src/ruby-1.6.5/eval.c: line 5961
> |
> |    rb_undef_method(rb_cClass, "module_function");
> |
> | ...  I
> |see no reason why a class should not invoke module_function just as a
> |module can.  What am I missing?
> 
> Remove that line, recompile, then try the following.
> 
>   class String
>     module_function :sub
>   end
>   String.sub(/a/, "b")
> 
> It would crash the interpreter.

So the reason is that instance methods of a class may depend on 
the receiver being of the right class (particularly if they are 
builtins or extensions that access the underlying C structures) 
whereas instance methods of a module are agnostic about the class 
of the receiver (because the module could be included into any 
class) and so can be invoked on any object without fear?  Including 
as a special case the module itself?  

Thanks for your help.  

Cheers, 

Jeremy Henty 
-- 
http://www.rubygarden.com/ruby?JeremyHenty

Ruby: programmer's best friend 
 -- John Robson