From: "David A. Black" <dblack@...>
Date: 2010-08-23T08:15:16+09:00
Subject: Re: case and class

Hi --

On Sat, 21 Aug 2010, Ralph Shnelvar wrote:

> I can, reluctantly, see the use of === in an if statement.
>
> To me,
>  case x
>  when XXX then ...
>  end
>
> would be MUCH clearer as
>  case x.class
>  when XXX.arrayOfDecendants
>  end
>
> Of course, the code above is illegal ... but would have been clearer ... at least to me.

It doesn't suggest the same functionality, though. This:

   case obj
   when XXX

runs XXX === obj (as per Colin's explanation), and that examines whether
or not XXX is in the method look-up path of obj. This may or may not
have anything to do with obj's class -- for example:

   >> class C; end
   => nil
   >> module M; end
   => nil
   >> c = C.new.extend(M)
   => #<C:0x14cd14>
   >> case c
   >> when M; 1
   >> end
   => 1

If you want to know whether a given class has a certain ancestor, you
can do that too, but with a different technique:

   if C.ancestors.include?(B)

etc.

In general, the === mechanism is advantageous because it means you
always know what's going on in a case statement, and because it gives
you control over how your own objects behave in case statements.


David

-- 
David A. Black, Senior Developer, Cyrus Innovation Inc.

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