From: Dave Thomas <Dave@...>
Date: 2002-02-14T02:23:04+09:00
Subject: Re: qualms about respond_to? idiom

Paul Brannan <paul@atdesk.com> writes:

> On Thu, Feb 14, 2002 at 01:11:39AM +0900, Dave Thomas wrote:
> > start tags would have
> > 
> >    def handle_stack(stack)
> >       stack.push(name)
> >    end
> 
> Does this really belong in the stream object itself?

Probably not, but I wanted to keep it simple.

If this were a big deal, I'd probably arrange to wrap the stream
objects in objects that handled things like the stack manipulation,
delegating the normal stream object calls.

However, there's also a bit of a smell to the original:

  if e.responds_to? :tagStart
    do_somthing if e.tagStart
  end

What are we doing here? All tags have a method that says "yes I'm a
start tag" or "no I'm not a start tag", and we're using that method to
determine if something is a tag. We then _call_ that method to see it
it's a start tag.

So the next step would be to say "only start tags respond to
:tagStart", and the code becomes

  do_somthing if e.responds_to? :tagStart

But then that smells: we have a method that we never call, simply so
we can use it as a flag. Hmm, let's have a mixin called StartTag that
anything that is a start tag includes

  module StartTag; end

Then we have

  do_somthing if e.kind_of? StartTag


Does that communicate the intent?


Dave