From: David Alan Black <dblack@...>
Date: 2002-02-14T20:15:22+09:00
Subject: Re: qualms about respond_to? idiom

Hi --

On Thu, 14 Feb 2002, Avi Bryant wrote:

> > 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}

I certainly like the general idea of something that does this in one
step, though I think it would be hard to handle iterators (since
you've already got a block).  Also, it doesn't let you branch on
non-finding of the method.  That's perhaps an insurmountable problem
with combining these steps (and not using exceptions): how do you
flag that a method wasn't found, as opposed to finding a method that
returns false or nil?


David

-- 
David Alan Black
home: dblack@candle.superlink.net
work: blackdav@shu.edu
Web:  http://pirate.shu.edu/~blackdav