From: Yohanes Santoso <ysantoso@...>
Date: 2002-04-10T10:36:52+09:00
Subject: mix-in dependancy

I'm playing around with the idea of attaching modules to some class
at runtime.
Suppose I have this module:

module Sort
 def sort
   #depends on presence of self#<=>
   #do sorting
 end
end

that was selected at run time to be grafted into this object:

class A
  #May or may not have #<=>.
end

by the eval and include mechanisms:
eval "class A
        include Sort
      end"
  
What I want is for the including to fail if A does not have #<=>. Is
there a built-in mechanism to detect this or should I do this
manually:

module Sort
 #application module policy. 
 #require all application modules to have #prereq.
 def prereq 
   [:<=>]
 end
 def sort
   #depends on presence of self#<=>
   #do sorting
 end
end

class A
  #something
end

and the inclusion mechanism:
begin
  Sort.prereq.each {|x|
    A.send x
  }
  eval "class A; include Sort; end"
rescue
  raise "Unmet prerequisite."
end

YS.