From: dblack@...
Date: 2006-11-05T03:34:40+09:00
Subject: Re: Redefining a class

Hi --

On Sun, 5 Nov 2006, Robert Dober wrote:

> On 11/4/06, ara.t.howard@noaa.gov <ara.t.howard@noaa.gov> wrote:
>> 
>> On Sat, 4 Nov 2006, Robert Dober wrote:
>> 
>> > Hi all
>> >
>> > Recently we had a thread where we talked about redefining a class.
>> > The usefuleness of that did not come into my mind before.
>> >
>> > Does somebody know a way to redefine a class (without the "already
>> > initialized constant <ClassName>" warning)?
>> > And if there is none, which I suspect, might it not be nice to have one?
>> 
>>
>>    c = Class.new{
>>      def initialize
>>        @x = 'foo'
>>      end
>>    }
>>    o = c.new
>>
>>    c = Class.new{
>>      def initialize
>>        @x = 'bar'
>>      end
>>    }
>>    o = c.new
>> 
>> ;-)
>> 
>> 
> Actually I am very disappointed that you cannot read my mind Ara ;) it would
> be quite antipols to use
> not constant classnames (wouldn't be there some warnings too BTW?)

Don't forget that several years ago, Matz deprecated POLS :-)
(Specifically, he said "Policy of least surprise is *my* least
surprise".  I think it was at RubyConf 2002.)

I wouldn't expect a warning just from assigning a class object to a
variable.

> class C...
>
> C=Class.new...
>
> works fine save for the warning I would like to get around
>
> Here go some ideas:
>
> (1) ruby does not insult you for constant redefinition if you assign a
> reference to Class (arrrghh bad dangerous)
> (2) allow an explicit syntax for redefining a class
>    (2.a)  class A < B  *always* redefines the class Pro: Some Gurus expect
> this behavior anyway :)

Not a good idea.  The inheritance should be a no-op if it already
exists, and an error if it's a different class.  The expression "A <
B" does not in any way imply that A should be undefined.  You'd end up
seeing:

   class C < Object

just to trigger a redefinition.  It's piggy-backing a behavior
(constant redefinition) on a completely unrelated construct.

> Cons: Quite implicit

And arbitrary.

>    (2.b)  new class A                                               Pro:
> Quite POLS and readable
> Cons: "new" overloaded maybe use "redefine class A"
> (3) some meta thingies
>   (3.a) const_set does not warn anymore
>   (3.b) Matz gives us a const_undefine for Christmas ;)
>
> Ok what you think?

Basically I think that if you find yourself in urgent need of defining
a new class using the same constant as one you've already defined, you
need to review your application design :-)  The warning is probably
enough of a slap on the wrist to remind you of this.

I do, however, think that (3.a) might be a good idea.  I'm not sure,
though.


David

-- 
                   David A. Black | dblack@wobblini.net
Author of "Ruby for Rails"   [1] | Ruby/Rails training & consultancy [3]
DABlog (DAB's Weblog)        [2] | Co-director, Ruby Central, Inc.   [4]
[1] http://www.manning.com/black | [3] http://www.rubypowerandlight.com
[2] http://dablog.rubypal.com    | [4] http://www.rubycentral.org