From: Pit Capitain <pit@...>
Date: 2002-02-14T02:14:58+09:00
Subject: Re: qualms about respond_to? idiom

On 14 Feb 2002, at 0:57, David Alan Black wrote on usage of respond_to?

> 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.

I don't know what you'd like to improve. In your code the further 
processing depends on the result of the method call. Would you 
like something like this better:

  class Object
    def send_if_respond_to( else_result, method, *args, &block )
      if respond_to? method
        send( method, *args, &block )
      else
        else_result
      end
    end
  end

  p "abc".send_if_respond_to( 5, :size ) # => 3
  p :abc.send_if_respond_to( 5, :size )  # => 5

or in your case

  if e.send_if_respond_to( false, :tagStart? )
    context.push(e.name)
  end

I don't know if something like this feels better. If we had an 
Objective-C or Smalltalk like syntax it could be written as

  e.send_if_respond_to: :tagStart? else: false

Regards,
Pit