From: "Brian F. Feldman" <green@...>
Date: 2000-09-26T12:31:33+09:00
Subject: [ruby-talk:5123] Re: Types and ===

> 
> ----- Original Message -----
> From: Yasushi Shoji <yashi@yashi.com>
> To: ruby-talk ML <ruby-talk@netlab.co.jp>
> Sent: Monday, September 25, 2000 7:25 PM
> Subject: [ruby-talk:5120] Re: Types and ===
> 
> 
> > '===' does not imply '==' at all.  '===' is defined in each class the
> > way to make writing case statement easier.  most classes use '==' for
> > '===', but some class, such as class Class and class Range uses
> > other.is_a?(self) and self.include?(other), respectively.
> >
> 
> Yes... I guess one root of my difficulty is that I have chosen to understand
> Ruby's case statement in terms of the switch statement in C/C++ or the
> case statement in Pascal (among others).
> 
> But now I think that Ruby's case statement is not really like any other
> multiway branch I have seen in any other language.

Right!  That's because Ruby is purely OO =)

> > > Actually I have trouble remembering what === means --
> > > whether to say x === y or y === x, which are of course
> > > not the same.
> >
> > so, i guess you don't have to remember what '===' at all ;)
> >
> > I think the moment you need to remember about '===' is when you write
> > your own class.  you can define '===' in your own class to control
> > case statement.
> 
> Yes, I guess that is true... but I do like to have a thorough understanding.
> 
> But sometimes I struggle with the case statement because of the lack of
> commutativity or symmetry. (Probably there are good reasons === is not
> symmetrical. I have not thought about it much.)

Well, it can't be symmetrical if operators are done as real OO (which, 
except for '=' and ',', they are -- just methods).

> For example, if I want to ask, "Which pattern does this string match?" I can
> easily do something like:
> 
> case mystring
>   when /pat1/
>      stmt1
>   when /pat2/
>       stmt
>   ...
> end
> 
> But to ask the question, "Which of these strings matches this pattern?" I
> cannot
> do this:
> 
> case /pattern/
>   when str1
>       stmt1
>   when str2
>       stmt2
>   ...
> end

So extend Ruby :)  Like this:

class String
        private
        OldEEE = method :===
        public
        def ===(other)
                if other.is_a? Regexp
                        return other === self
                end
                return OldEEE.call other
        end
end

> Hal

--
 Brian Fundakowski Feldman           \  FreeBSD: The Power to Serve!  /
 green@FreeBSD.org                    `------------------------------'