From: ts <decoux@...>
Date: 2001-12-07T00:59:19+09:00
Subject: [ruby-talk:27705] Re: Emulating multimethods

>>>>> "C" == Christopher S Kush <csk@indra.com> writes:

C> I'm familiar with State and Visitor patterns, but I really want
C> multi-methods.

 There are problems on multi-methods. Just an example with this case
   
   class A < Array
   end
   
   class B
      def b(A a)
         puts "A"
         next_method
      end
   
      def b(Array a)
         puts "Array"
      end
   end
   
   B.new.b(A.new) 

 If you expect the output

  A
  Array

 like it must be done with multi-methods then you have introduced a new
 rule in ruby. This rule say that to find the right call, ruby choose
 the "best" call by comparing the class of the arguments with the classes
 given in the prototype.

 Unfortunately you have a similar case already in ruby and this is this
 case 

   class A < Array
   end
   
   a = Array.new
   b = A.new
   
   p ((a + b).class)

 ruby is in a similar situation and if it use the rule given by the
 multi-methods, the result must be A.

 But ruby has made another choice, and actually the result is Array.

 These 2 choices are valids (and incompatible) but the choice made by ruby
 (base class) is  *easier* to understand rather than the *complex* choice
 made by  multi-methods.

 Trying to introduce multi-methods in ruby is like trying to introduce a
 new rule which conflict with an (valid and easy to understand) existent
 rule  


Guy Decoux