From: Robert Dober <robert.dober@...>
Date: 2006-11-05T03:58:42+09:00
Subject: Re: Redefining a class

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On 11/4/06, dblack@wobblini.net <dblack@wobblini.net> wrote:
>
> 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.)


And you think we ought let him get away with that ;)

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 :-)


Honestly I do not know, probably yes, because it did never even occur to me.
OTH you can already redefine
classes as Ara showed above, only that one will use a different idiom.
I can also redefine classes by hiding classes inside Modules e.g. than one
does not know if it is done on purpose or not.
Imagine the semantic power of
" redefine class ::String "
although that kind of code might disgust you (it disgusts me!) it tells us
that the author knows what he does, quite a feature, no?

 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.


Thanks that's more than I expected ;)

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
>
>
Robert

-- 
The reasonable man adapts himself to the world; the unreasonable one
persists in trying to adapt the world to himself. Therefore all progress
depends on the unreasonable man.

- George Bernard Shaw

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