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References: <416155e11003121750o4b1f8e11j4c9124fdaa34a22d@mail.gmail.com>
	 <e666c3891003121811i297d5531r89b7f6d7c0bdcf3e@mail.gmail.com>
Date: Fri, 12 Mar 2010 20:28:20 -0600
Message-ID: <416155e11003121828i41e11coe8a681da055baf48@mail.gmail.com>
To: Etienne Kneuss <colder@php.net>
Cc: internals@lists.php.net
Content-Type: multipart/alternative; boundary=0016e6d285b8bd52ac0481a56654
Subject: Re: [PHP-DEV] RFC - "class underloading" -or- "ancestor overloading"
From: christrahey@gmail.com (Chris Trahey)

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The old class is still there, think of it as if the inserted (overloading)
class extends the base (overloaded) class and any classes the were extending
the base now extend the inserted class. So as far as the runtime, it's
run-of-the-meill inheritance. Methods that are not re-implimented in the
inserted class are called in the original class, etc.

It could be implemented either at the time a class is loaded (when we see
'overloads' keyword) or perhaps in a function call:
overload_class('Library_Class', 'My_LibClass_Overload');

As for conflicts where multiple overloads are attempted, they could be in
sequence, such that you'd end up having an inheritance chain like this:

called_class <-- second_overload <-- first_overload <-- Library_Class <--
etc.


On Fri, Mar 12, 2010 at 8:11 PM, Etienne Kneuss <colder@php.net> wrote:

> Hi,
>
> On Sat, Mar 13, 2010 at 2:50 AM, Chris Trahey <christrahey@gmail.com>
> wrote:
> > Perhaps a new concept in class-based OO programming, I'm not sure.
> >
> > Depending on your perspective you could call it ancestor overloading (or
> > upstream overloading) or class underloading.
> >
> >
> > We are increasingly developing with the aid of frameworks & libraries. In
> > fact, this idea came from my current project using the Zend Framework.
> >
> > These libraries, while greatly extensible, are also fairly
> self-extending.
> > That is, they include many classes that extend many classes, which is
> great.
> >
> > As consumers of these libraries, we can extend the classes and consume
> the
> > API however we please, but there is one sticking point.
> >
> > We cannot change classes that many other classes extend without extending
> or
> > changing each child class and then making sure that our code uses the new
> > class.
> >
> >
> > For a concrete example, I was working with the Zend_Form_Element
> subclasses,
> > and I realized that I wanted to change some of the default behavior (in
> > Zend_Form_Element).
> >
> > - at this point I will assume the reader understands why I wouldn't want
> to
> > just start changing the Zend library files -
> >
> > There are many subclasses of Zend_Form_Element. If you want to change the
> > default behavior for all of them, you have 3 choices currently:
> >
> > 1. Directly edit the Zend_Form_Element file in the library, -bad for
> updates
> > & other projects that use the library
> >
> > 2. subclass Zend_Form_Element and change declaration of the descendants
> to
> > extend new class - same problems
> >
> > 3. extend each child class and implement those subclasses in your app
> code
> > -very tedious and TONS of repeated code, breaks consistency of API for
> > developers.
> >
> >
> > There could be a better way, if we could insert a class into the family
> > tree.
> >
> > And that's the heart of this idea, so I'll repeat it:
> >
> > * insert a class into the family tree *
> >
> >
> > Image we do it using an alternative keyword to "extends", such as
> > "overloads".
> >
> >
> > Example:
> >
> >
> > class Library_Class { }
> >
> > class Library_Subclass extends Library_Class {}
> >
> > and then:
> >
> > class My_LibClass_Overload overloads Library_Class{}
> >
> >
> > Now new instances of Library_Subclass actually extend
> My_LibClass_Overload,
> > which "extends" Library_Class. The developer would then code
> > My_LibClass_Overload as if it were declared like this:
> >
> > class Library_Class {}
> >
> > class My_LibClass_Overload extends Library_Class {}
> >
> > class Library_Subclass extends My_LibClass_Overload {}
> >
> >
> > But indeed the declaration of Library_Subclass would *not* have to
> change.
> >
> >
> > This way developers could "extend" default functionality and have
> *existing*
> > library classes pick up the new functionality without redeclaring
> anything
> > in the library.
> >
> > Downstream classes would still override any methods that they redeclare.
> If
> > you wanted to have end-point classes in the library have different
> behavior,
> > you would overload them instead, such as
> >
> > class My_LibSubclass_Overload overloads Lib_Subclass {}
> >
> >
> > The benefit is that the application code can still consume "standard"
> > classes, such as Library_Subclass and not need to know or care about the
> > extended functionality.
> >
> >
> > Going back to my concrete example, my code could then still use
> > Zend_Form_Element_Text, but benefit from the modifications I added,
> without
> > me having to touch the library code.
> >
> >
> > I hope I've explained clearly what this could look like. I'm a younger
> > developer, so forgive me if I'm rough on the terminology -perhaps
> > overload/underload is not the best word for this functionality. Also, I'm
> > not sure if there are other class-based OO languages that allow this kind
> of
> > behavior... Prototypal languages perhaps, as is the case with javascript
> and
> > the Obj.prototype which (combined with anonymous functions) allows you to
> > extend the "base" functionality of other objects that "extend" it.
>
> Even though it might look appealing from a framework user perspective,
> it looks fishy from a language design perspective. It sounds like
> you're trying to fix a framework design lack by a language trick.
>
> For the fishy part: what happens to the old class? what about static
> method calls on that old class? What if two classes overwrites the
> same class? Basically it would mean there is no way to know at compile
> time which class new Foo; is supposed to instantiate.
>
> >
> >
> > Thank you for your comments and thoughts!
> >
> >
> > Chris Trahey
> >
> > Web Applications Developer
> >
> > Database Administrator
> >
> > CSISD [Technology]
> >
> >
> > footnote: I sent this message from a different address and it did not
> show
> > up. I tested sending to internals-help@lists.php.net and did not get a
> > response -so I assume there is an outgoing issue on my other server's
> side.
> > Forgive me if this message shows up again.
> >
>
>
>
> --
> Etienne Kneuss
> http://www.colder.ch
>

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