From: avi@...4.com (Avi Bryant)
Date: 2002-02-14T05:56:05+09:00
Subject: Re: qualms about respond_to? idiom

Received: from lethe.cardhealth.com ([143.98.10.68]) by cardhealth.com (Lotus SMTP MTA v4.6.7  (934.1 12-30-1999)) with SMTP id 88256B5F.0072E995; Wed, 13 Feb 2002 12:55:09 -0800
Received: (from uucp@localhost)
	by lethe.cardhealth.com (8.9.3 (PHNE_18546)/8.9.3) id MAA18250
	for <ken.lacrosse@cardinal.com>; Wed, 13 Feb 2002 12:55:41 -0800 (PST)
Received: from helium.ruby-lang.org(210.251.121.214) by lethe.cardhealth.com via csmap (V6.0)
	id srcAAAa18246; Wed, 13 Feb 02 12:55:41 -0800
Received: from helium.ruby-lang.org (localhost [127.0.0.1])
	by helium.ruby-lang.org (Postfix) with ESMTP
	id C421436F; Thu, 14 Feb 2002 05:54:56 +0900 (JST)
Received: from zip.local.thomases.com (pragdave210.august.net [216.87.136.210])
	by helium.ruby-lang.org (Postfix) with ESMTP id 4EC7A317
	for <ruby-talk@ruby-lang.org>; Thu, 14 Feb 2002 05:54:55 +0900 (JST)
Received: from zip (dave@localhost [127.0.0.1])
	by zip.local.thomases.com (8.11.2/8.11.2) with ESMTP id g1DKswW07107
	for <ruby-talk@ruby-lang.org>; Wed, 13 Feb 2002 14:54:58 -0600
Date: Thu, 14 Feb 2002 05:54:56 +0900
Posted: 13 Feb 2002 12:50:47 -0800
From: avi@beta4.com (Avi Bryant)
Reply-To: ruby-talk@ruby-lang.org
Subject: Re: qualms about respond_to? idiom
To: ruby-talk@ruby-lang.org (ruby-talk ML), undisclosed-recipients: ;
Message-Id: <6e869a6b.0202131250.52bc6da0@posting.google.com>
References: <Pine.LNX.4.30.0202131042320.32328-100000@candle.superlink.net>
X-ML-Name: ruby-talk
X-Mail-Count: 33792
X-MLServer: fml [fml 3.0pl#17]; post only (anyone can post)
X-ML-Info: If you have a question,
	please contact ruby-talk-admin@ruby-lang.org;
	<mailto:ruby-talk-admin@ruby-lang.org>
Path: news5.aus1.giganews.com!firehose2!nntp3.aus1.giganews.com!border1.nntp.aus1.giganews.com!nntp.giganews.com!hub1.nntpserver.com!headwall.stanford.edu!newsfeed.stanford.edu!postnews1.google.com!not-for-mail
Newsgroups: comp.lang.ruby
Organization: http://groups.google.com/
NNTP-Posting-Host: 24.68.1.127
X-Trace: posting.google.com 1013633448 21037 127.0.0.1 (13 Feb 2002 20:50:48 GMT)
X-Complaints-To: groups-abuse@google.com
NNTP-Posting-Date: 13 Feb 2002 20:50:48 GMT
X-rubymirror: yes
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 8bit
Precedence: bulk
Lines: 23

> 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.
> This is a case where I wish I knew a way to avoid the duplication,
> which feels really clunky to me.

Ok, how about:

class Object
  def try_sending(msg, *args)
     if respond_to? msg
       yield send(msg, *args)
     end
  end
end

e.try_sending(:tagStart?) {|s| context.push(e.name) if s}