From: dblack@...
Date: 2002-08-13T12:43:20+09:00
Subject: Re: class === class often false

Hi --

On Tue, 13 Aug 2002, Hal E. Fulton wrote:

> > A (whimsical) further test:
> >
> >   irb(main):036:0> class Class; include Enumerable; end
> >   Class
> >   irb(main):037:0> Enumerable === Array
> >   true
> >
> > So ancestors/is_a?/Module#=== are related but can vary independently.
>
> Yes, this is where my brain itches again.

I feel some culpability, given what a cop-out that concluding sentence
is :-)  But we're in this together....  I definitely want to clear my
own mental cobwebs in this area.

OK, after some further tinkering, here's what I think:

1.  "Mod === obj" and "obj.is_a? Mod" are in effect the same test.

2. Module#ancestors can be confusing because it's not
Object#ancestors.  I mean, it's right that it isn't, but one finds
oneself expecting "abc".ancestors to be String, Enumerable....  (At
least, one does if one is me, or one is I, or one == I, or I === one.)
But only modules and classes have ancestors, and all ancestors are
modules or classes.  This is a case where, even though they are
regular objects, modules and classes get to be different.  (I repeat:
it makes complete sense that there's no Object#ancestors.  The problem
is getting myself to grasp it.)

3. However, every object does respond to #type.  In every case --
whether the RHS is a module or not -- "Mod === obj" tests whether
obj's type is among Mod's ancestors.

4. All of this means that this:

   Enumerable === String

which perhaps looks like it's asking whether strings are Enumerable,
is in fact testing whether the object String is of a type which counts
Enumerable among its ancestors.  Class (String's type) does not do
that, so this test is false.

Jotting down a possibly helpful train of thought:

Are Array objects Enumerable?  Let's test one:

   Enumerable === []       # true

Are Class objects Enumerable?  Let's test one:

   Enumerable === String   # false

I'll leave the Kernel/Object/Module/Class circularity for another time
:-)


David

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