From: Charles Comstock <cc1@...>
Date: 2004-05-31T07:13:39+09:00
Subject: Re: Visitor Pattern in Ruby


On Sat, 29 May 2004, Christoph wrote:

> Charles Comstock wrote:
> 
> >module Visitable
> >  def accept(visitor,&block)
> >    for klass in self.class.ancestors do
> >      break if (v = visitor.methods.include?("visit_#{klass}"))
> >    end
> >    if v
> >      visitor.__send__("visit_#{klass}",self,&block)
> >    else
> >      visitor.default_visit(self,&block)
> >    end
> >  end
> >  
> >
> The following "included" redefinition is superfluous since any
> child of a "Visitable" class  is already "Visitable" - for example
> 
> ---
> module Visitable
>   def accept(visitor,&block)
>     for klass in self.class.ancestors do
>       break if (v = visitor.methods.include?("visit_#{klass}"))
>     end
>     if v
>       visitor.__send__("visit_#{klass}",self,&block)
>     else
>       visitor.default_visit(self,&block)
>     end
>   end
> end
> 
> class A
>     include Visitable
> end
> 
> class B < A
> end
> 
> p A < Visitable # true
> p B < Visitable # true
> ---
> 
> >  def Visitable.included(kls)
> >    kls.module_eval <<-"end_eval"
> >    def self.inherited(child)
> >      child.module_eval "include Visitable"
> >    end
> >    end_eval
> >  end
> >end
> >
> >  
> >
> /Christoph
> 
> 
> 

Your probably correct, originally the way I had to do it required a module eval 
for the whole thing and so I needed that to force it to propegate down the 
inheritance chain.

Charlie