From: Dave Thomas <Dave@...>
Date: 2002-02-14T01:11:39+09:00
Subject: Re: qualms about respond_to? idiom

David Alan Black <dblack@candle.superlink.net> writes:

> All tags respond to tagStart? and tagEnd?  But not all events in my
> stream are tags.  So....
> 
>          if e.respond_to? :tagStart?
>            context.push(e.name) if e.tagStart?
>          end
> 
> is my way of testing, first, whether it's a tag (but without actually
> checking whether it's a tag), and, second, whether it's a start tag.

A polymorphic way of doing this is to move responsibility down to the
stream elements. Say each had a method handle_stack. Then your main
code could simply call handle_stack on _all_ tags:

   e.handle_stack(context)


start tags would have

   def handle_stack(stack)
      stack.push(name)
   end


end tags might have  (I'm guessing - I don't know what your app does
here)

   def handle_stack(stack)
      top = stack.pop
      raise "Havoc!" unless top == name
   end


and all other tags would have a null handle_stack

   def handle_stack(stack)
   end



Dave