From: "David A. Black" <dblack@...>
Date: 2004-07-08T06:04:43+09:00
Subject: Re: Class#=== has interesting results

Hi --

On Thu, 8 Jul 2004, Charles Comstock wrote:

> On Thu, 8 Jul 2004, David A. Black wrote:
> 
> > Hi --
> >
> > On Thu, 8 Jul 2004, Charles Comstock wrote:
> >
> > > Mauricio Fern�ndez wrote:
> > >
> > > > On Wed, Jul 07, 2004 at 04:22:36PM +0900, Charles Comstock wrote:
> > > >
> > > >>Why does this happen?
> > > >>
> > > >>class A
> > > >>end
> > > >>
> > > >>A == A		#1 => true
> > > >>A === A		#2 => false
> > > >>
> > > >>But 2 doesn't make any sense to me, why doesn't an A match another A?
> > > >>Is this just something that slipped through or is there a specific
> > > >>reason for this?
> > > >
> > > >
> > > > A is not an instance of A; see the ri info for Module#===.
> > > >
> > >
> > > At what point does it cause trouble for it to also return true if the
> > > class matches?
> >
> > At the point where you call it :-) It can't do both; there would be no
> > way to interpret the result:
> >
> >   case s
> >     when String ...  # what do you do here? print it? instantiate it?
> >
> 
> Sorry perhaps I didn't word my question well.  I meant when does it cause a
> problem for a class x to match itself?  What code fails if A === A results in
> true?  Why would you ever want to match the descendents and instances of A but
> not match A itself.

It doesn't match the descendants of A, but rather instances of the
descendants of A:

  irb(main):001:0> class A; end; class B < A; end
  => nil
  irb(main):002:0> case A
  irb(main):003:1> when B: puts "yes" end
  => nil
  irb(main):004:0> case B
  irb(main):005:1> when A: puts "yes" end
  => nil
  irb(main):006:0> case B.new
  irb(main):007:1> when A: puts "yes" end
  yes

It's really the same test as #is_a?  The docs should probably add
an "of":

  Returns true if anObject is an instance of mod or of
  one of mod's descendents.


David

-- 
David A. Black
dblack@wobblini.net