From: Mark Hubbart <discordantus@...>
Date: 2005-05-24T01:21:13+09:00
Subject: Re: Uniform vector class, inheriting from Array: How to make sure that methods return a Vector and not an Array?

On 5/23/05, Jacob Fugal <lukfugl@gmail.com> wrote:
> On 5/23/05, Mark Hubbart <discordantus@gmail.com> wrote:
> > Use of a "uniform" array would seem to preclude duck-typing. Even if
> > you use respond_to? instead of kind_of?, it wouldn't catch objects
> > that use method_missing to handle messages. For example:
> >
> > class Wrapper
> >   def initialize(obj)
> >     @obj = obj
> >   end
> >   def method_missing(*args, &block)
> >     @obj.__send__ *args, &block
> >   end
> > end
> >
> > If you use this class to wrap an object, code that relies entirely on
> > duck typing will still be able to use the object, even though it won't
> > respond_to? much of anything, and it won't be kind_of? the class you
> > want.
> 
> Well, on this point, isn't it recommended that whenever you override
> method_missing you should also override respond_to? as well so that
> they agree? I thought this was in the docs but can't find it now in
> either ri method_missing or ri respond_to. Despite that, I know I've
> heard it mentioned various times on this list and in general it seems
> like a Good Idea.

I'd never heard it mentioned on the list (that I remember), but it
seems to be mentioned in the style guide, on the rubygarden wiki. No
disrespect to those who posted it, but what you see on a wiki doesn't
always reflect the views of the entire community. (not that I'm the
authority on what this community thinks)

Grepping the standard library, I found 24 instances of method_missing
being overridden, and 0 instances of respond_to? being overridden:

mark@eMac% ruby -e'p ARGF.grep(/def (\S+\.)?method_missing/).size' **/*.rb
24
mark@eMac% ruby -e'p ARGF.grep(/def (\S+\.)?respond_to\?/).size' **/*.rb
0

So, even if the programmer follows that rule, the stdlib doesn't seem
to, so you can still get objects creeping in that will respond to a
method without raising an error, but don't respond_to? it.

cheers,
Mark