From: Robert Dober <robert.dober@...>
Date: 2007-05-31T05:31:50+09:00
Subject: Re: #respond_to? not working for dynamically generated metho

On 5/30/07, James Edward Gray II <james@grayproductions.net> wrote:
> On May 30, 2007, at 8:34 AM, Robert Dober wrote:
>
> > On 5/30/07, James Edward Gray II <james@grayproductions.net> wrote:
> >> On May 30, 2007, at 7:53 AM, dblack@wobblini.net wrote:
> >>
> >> > You could of course shoehorn find_by_* into respond_to? for AR
> >> > objects, if you don't mind, essentially, writing method_missing
> >> twice
> >> > (once for real, once as a kind of pseudo-static twin).
> >>
> >> I don't really understand this stance.  My opinion is that providing
> >> a method_missing() implementation is a convenient way for a
> >> programmer to define a lot of dynamic methods.  This increases the
> >> messages an object responds to.
> >
> > I have always felt that dynamic method creation and dynamic message
> > interception are not quite the same beast. But I guess you have some
> > thoughts behind your claim of which I fail to grasp the concept. Would
> > you mind to elaborate?
>
> I'm not really sure what else to say.  I read the documentation for
> respond_to?():
>
> $ ri -T Object#respond_to?
> ----------------------------------------------------- Object#respond_to?
>       obj.respond_to?(symbol, include_private=false) => true or false
> ------------------------------------------------------------------------
>       Returns +true+> if _obj_ responds to the given method. Private
>       methods are included in the search only if the optional second
>       parameter evaluates to +true+.
>
> I know that ActiveRecord model classes will except find_by_*()
> methods, so the fact that respond_to?() returns false for them makes
> the above documentation lie.
James the documentation is not correct anyway, look at the wording,
nothing ever can reply to methods, only to messages. I liked a lot
what David has said about this.  >That does not seem good to me and
it's
> certainly possible to fix it.  Thus, I can only reason that it is a
> bug in ActiveRecord.
That is kind of a thing I have not considered in my reply, I
misunderstood that you were dreaming about a magical
Object#respond_to? which would interpret the semantics of
#method_missing generally, hence my surprise.
>
> > Is this really a good idea?
>
> Is it a good idea not to?  What exactly is the use case for
> respond_to?()?  I have only ever used it to find out if something I
> want is available.  If I can't do that reliably, it doesn't really
> help me much.
Hmm sure but I feel its need more in MetaProgramming, if you want to
fix ActiveRecord I cannot argue with you at all I do not know that
stuff, so I just bail out of the discussion.

However if you would like to spend some time in philosophy about
respond_to? I see a clearcut dilemma between the ideal behavior of
respond_to? and method_missing. See my example above but it is easy to
imagine that #method_missing is dispatching depending on an external
datasource.
How would you want #respond_to? to handle this?

I just feel that for practical reasons #respond_to? does a good job
already. This might
Robert

-- 
You see things; and you say Why?
But I dream things that never were; and I say Why not?
-- George Bernard Shaw