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To: Stefan Marr <mail@stefan-marr.de>
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Date: Wed, 14 Oct 2009 21:26:03 +0200
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Subject: Re: [PHP-DEV] Request for Comments: Horizontal Reuse for PHP
From: mls@pooteeweet.org (Lukas Kahwe Smith)

Aloha,

So lets warm this up again.
HEAD is for development .. so lets get this into HEAD so that it will  
be part of the next bigger PHP release for sure!

regards,
Lukas

On 13.10.2008, at 21:12, Stefan Marr wrote:

> Hello,
>
> the last time the topic traits has been discussed is already a while  
> ago.
> At this time, there have been two very different opinions out there,  
> about
> how exactly this horizontal reuse should actually look like and no  
> common
> consensus was found. In the following RFC, I describe both  
> approaches in
> more detail.
> For neither of them is a work patch available, but this should not  
> hinder
> discussion. In this step, we should find an agreement on the  
> direction, in
> which PHP has to evolve.
>
> The part about Traits in this proposal, is equal to the former RFC.  
> The
> second
> Part introduces the new variation and is an alternative approach to  
> traits.
>
> Looking forward for your opinions,
>
> Kind Regards
> Stefan
>
>
>
> Request for Comments: Horizontal Reuse for PHP
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> :Version: 2.0alpha
> :HTML: http://www.stefan-marr.de/artikel/rfc-horizontal-reuse-for-php.html
> :TXT:  http://www.stefan-marr.de/rfc-horizontal-reuse-for-php.txt
> :Author: Stefan Marr <php.at.stefan-marr.de>
> :Previous RFC: http://www.stefan-marr.de/artikel/rfc-traits-for-php.html
> :Related RFC: http://wiki.php.net/rfc/nonbreakabletraits
>
> .. contents::
>
> This RFC will discuss two different approaches to reuse behavior
> independently
> from the class hierarchy i.e. in an horizontal manner. The main  
> motivation
> of both approaches is to provide a broader opportunity to model  
> classes and
> class hierarchies with a clear conceptual background and optimal  
> code reuse
> at
> the same time. Furthermore, the main distinction to other approaches  
> is the
> explicit handling of conflicts resulting from overlapping interfaces  
> in
> complex
> reuse scenarios. Both approaches would be valuable extensions to the  
> PHP
> language, but both of them have their own benefits and drawbacks.
> Thus, this RFC is meant to provide a base for further discussion to  
> be able
> to decide, which variant is the most PHP-like reuse mechanism.
>
> Why Do We Need Horizontal Reuse?
> ================================
>
> Code reuse is one of the main goals that object-oriented languages  
> try to
> achieve with inheritance. Unfortunately, single inheritance often  
> forces
> the developer to take a decision in favor for either code reuse *or*
> conceptual clean class hierarchies. To achieve code reuse, methods  
> have
> either to be duplicated or to be moved near the root of the class  
> hierarchy,
> but this hampers understandability and maintainability of code.
>
> To circumvent this problems multiple inheritance and Mixins have been
> invented.
> But both of them are complex and hard to understand. PHP5 has been
> explicitly
> designed with the clean and successful model of Java in mind:
> single inheritance, but multiple interfaces. This decision has been  
> taken
> to avoid the known problems of for example C++. The presented  
> approaches
> have
> been designed to avoid those problems and to enable designers to build
> conceptually clean class hierarchies without the need to consider  
> code reuse
> or complexity problems, but focusing on the real problem domain and
> maintainability instead.
>
> Limitations to Reuse in Single Inheritance Languages
> ----------------------------------------------------
>
> There are several issues with reuse in PHP. To achieve as much reuse  
> as
> possible you will probably move methods as high as possible in your
> inheritance
> hierarchy. At this point, there is a trade-off between conceptual
> consistency
> and reuse, because classes starts to have methods they do not need.  
> So, when
> it
> is decided that the conceptual consistency is more valuable because of
> understandability of the class model, code duplication is caused or  
> needs to
> be
> worked around by e.g. delegation, which is not always as nice as  
> method
> implemented in the class tree.
>
> Beside conceptual issues, there are problems with third-party code  
> you can
> not
> or might not want to modify. The following code illustrates the  
> current
> implementation of an extended version of the PHP reflection API which
> provides
> detailed access to doc comment blocks. ``ReflectionMethod`` and
> ``ReflectionFunction`` are classes from the reflection API and have  
> to be
> extended with exactly the same code. In this case it is impossible  
> to change
> the classes itself, because they are not under our control i.e. they  
> are
> implemented in C as part of the language.
> ::
>
> <?php
> class ezcReflectionMethod extends ReflectionMethod {
>  /* ... */
>  function getReturnType() { /*1*/ }
>  function getReturnDescription() { /*2*/ }
>  /* ... */
> }
>
> class ezcReflectionFunction extends ReflectionFunction {
>  /* ... */
>  function getReturnType() { /*1*/ }
>  function getReturnDescription() { /*2*/ }
>  /* ... */
> }
> ?>
>
> Thus, eventually, we end up with much duplicated code in both classes
> extending
> the original extension classes.
>
> Which Opportunities Does PHP Has?
> ---------------------------------
>
> For the sake of distinction and discussion, the two approaches
> are named differently. The idea of *Traits* for PHP has been already
> proposed
> in a former RFC_. This RFC introduces in addition the notion
> of *Grafts* (the term is borrowed from `agrarian cultivation`_).
>
> A *Trait* is an unit of behavioral reuse. It is very lightweight,  
> stateless
> and
> allows for a very flexible composition of behavior into classes.
> A *Graft* is a class composed into another class. It is very much like
> grafting is
> done in the agriculture. It allows for a full-fledged reuse of classes
> inside
> of other classes independent of the class hierarchy.
> The following proposal will introduce both flavors of horizontal  
> reuse and
> compares them in the context of PHP.
>
> Traits - Reuse of Behavior
> ==========================
>
> *Traits* is a mechanism for code reuse in single inheritance  
> languages such
> as PHP. A Trait is intended to reduce some limitations of single  
> inheritance
> by enabling a developer to reuse sets of methods freely in several
> independent
> classes living in different class hierarchies.
> The semantics of the combination of Traits and classes is defined in  
> a way,
> which reduces complexity and avoids the typical problems associated  
> with
> multiple inheritance and Mixins.
>
> They are recognized for their potential in supporting better  
> composition
> and reuse, hence their integration in languages such as Perl 6,  
> Squeak,
> Scala, Self, Slate and Fortress. Traits have also been ported to  
> Java and
> C#.
> In the following, the concepts behind Traits will be adapted for PHP  
> to
> propose
> two different approaches which resemble the main ideas.
>
> A Trait is similar to a class, but only intended to group  
> functionality in a
>
> fine-grained and consistent way. It is not possible to instantiate a  
> Trait
> on
> its own. It is an addition to traditional inheritance and enables  
> horizontal
> composition of behavior.
>
> In the introduction an example has been given illustrating reuse  
> limitations
> of single inheritance. With Traits it is possible to remove the  
> duplicated
> code
> without compromising conceptual consistency.
> ::
>
> <?php
> trait ezcReflectionReturnInfo {
>  function getReturnType() { /*1*/ }
>  function getReturnDescription() { /*2*/ }
> }
>
> class ezcReflectionMethod extends ReflectionMethod {
>  use ezcReflectionReturnInfo;
>  /* ... */
> }
>
> class ezcReflectionFunction extends ReflectionFunction {
>  use ezcReflectionReturnInfo;
>  /* ... */
> }
> ?>
>
> This is just a small example of what Traits are useful for.
> The next sections will discuss more advanced techniques and describe  
> how
> Traits are used in PHP.
>
> The Flattening Property
> -----------------------
>
> As already mentioned, multiple inheritance and Mixins are complex
> mechanisms.
> Traits are an alternative which have been designed to impose no
> additional semantics on classes. Traits are only entities of the  
> literal
> code
> written in your source files. There is no notion about Traits at  
> runtime.
> They are used to group methods and reuse code and are totally  
> flattened
> into the classes composed from them. It is almost like a language  
> supported
> and
> failsafe copy'n'paste mechanism to build classes.
>
> Even though, there is no runtime notion of Traits, since they are  
> part of
> the
> source code and thus, define the structure of the system, reflection  
> about
> Traits still is possible, but they do not influence the runtime  
> behavior of
> the
> system.
>
> Precedence Order
> """"""""""""""""
>
> Flattening is achieved by applying some simple rules on the  
> composition
> mechanism. Instead of implementing a fancy and awkward algorithm to  
> solve
> problems, the entire control about the composition is left in the  
> hand of
> the
> developer and fits nicely into the known inheritance model of PHP.
> The following examples illustrate the semantics of Traits and their  
> relation
> to methods defined in classes.
> ::
>
> <?php
> class Base {
>  public function sayHello() {
>    echo 'Hello ';
>  }
> }
>
> trait SayWorld {
>  public function sayHello() {
>    parent::sayHello();
>    echo 'World!';
>  }
> }
>
> class MyHelloWorld extends Base {
>  use SayWorld;
> }
>
> $o = new MyHelloWorld();
> $o->sayHello(); // echos Hello World!
> ?>
>
> As shown in the above code, an inherited method from a base class is
> overridden
> by the method inserted into ``MyHelloWorld`` from the ``SayWorld``  
> Trait.
> The behavior is the same for methods defined in the ``MyHelloWorld``  
> class.
> The precedence order is that methods from the current class override  
> Trait
> methods,
> which in return override methods from the base class.
> ::
>
> <?php
> trait HelloWorld {
>  public function sayHello() {
>    echo 'Hello World!';
>  }
> }
>
> class TheWorldIsNotEnough {
>  use HelloWorld;
>  public function sayHello() {
>    echo 'Hello Universe!';
>  }
> }
>
> $o = new TheWorldIsNotEnough();
> $o->sayHello(); // echos Hello Universe!
> ?>
>
> Multiple Traits Usage
> """""""""""""""""""""
>
> To keep things simple in the beginning, there has only one Trait  
> being used
> at
> a time, but obviously a class could use multiple Traits at the same  
> time.
> ::
>
> <?php
> trait Hello {
>  public function sayHello() {
>    echo 'Hello ';
>  }
> }
>
> trait World {
>  public function sayWorld() {
>    echo ' World';
>  }
> }
>
> class MyHelloWorld {
>  use Hello, World;
>  public function sayExclamationMark() {
>    echo '!';
>  }
> }
>
> $o = new MyHelloWorld();
> $o->sayHello();
> $o->sayWorld();
> $o->sayExclamationMark();
> // Results eventually in: Hello World!
>
> Conflict Resolution
> """""""""""""""""""
>
> Traits are already used in different programming languages and it  
> has shown
> that conflicts will occur, but they are the exception, not the rule.  
> In most
> systems under investigation, the mechanisms to resolve conflicts  
> have been
> used
> very infrequently, but also have proven to be a valuable mechanisms.  
> Since
> it
> increases the composition power of the developers.
> One example for a typical conflict are different Traits providing  
> methods
> with the
> same name.
> ::
>
> <?php
> trait A {
>  public function smallTalk() {
>    echo 'a';
>  }
>  public function bigTalk() {
>    echo 'A';
>  }
> }
>
> trait B {
>  public function smallTalk() {
>    echo 'b';
>  }
>  public function bigTalk() {
>    echo 'B';
>  }
> }
> ?>
>
> Both classes have to be used in a class named ``Talker``. Multiple
> inheritance
> and Mixins define an algorithm to resolve this conflict. Traits don't.
> Conflicts
> are not solved implicitly by any kind of precedence. Instead, to avoid
> implicit
> complexity, the developer has full control over class composition.
> ::
>
> <?php
> class Talker {
>  use A, B;
> }
> ?>
>
> In case of the above definition of ``Talker``, PHP will show a  
> waring that
> there
> have been conflicts and name the methods ``smallTalk()`` and  
> ``bigTalk()``
> as the reason of this conflict. Therefore, neither of the given
> implementations
> will be available in the class.
>
> Instead, the developer can exactly define which methods are used and  
> how the
> conflict is resolved.
> ::
>
> <?php
> class Talker {
>  use A, B {
>    B::smallTalk instead A::smallTalk,
>    A::bigTalk instead B::bigTalk
>  }
> }
> ?>
>
> This definition will result in leaving out ``smallTalk()`` from  
> Trait A
> and ``bigTalk()`` from Trait B. Therefore, the resulting class  
> Talker would
> echo ``'b'`` for ``smallTalk()`` and ``'A'`` for ``bigTalk().``
> But this simple form of exclusion of methods is not the best choice  
> for all
> situations.
> ::
>
> <?php
> class Talker {
>  use A, B {
>    B::smallTalk instead A::smallTalk,
>    A::bigTalk instead B::bigTalk,
>    B::bigTalk as talk
>  }
> }
> ?>
>
> Beside leaving out methods it is possible to introduce a new name  
> for a
> method
> from a Trait. This is done like ``originalMethodName as
> additionalMethodName``.
> In the example above, it has to be read as ``use B::bigTalk as talk  
> in class
> Talker``. This does not imply any renaming, instead ``talk`` is  
> introduced
> as
> an additional name for this method. Thus, recursion inside of  
> ``talk`` will
> still call a method with the name bigTalk.
> The resulting ``Talker`` class will consist of following three  
> methods:
>
> * ``bigTalk() { echo 'A'; }``
> * ``smallTalk() { echo 'b'; }``
> * ``talk() { echo 'B'; }``
>
> Since the new name is recognized as an additional method, the  
> ``bigTalk``
> method still has to be excluded. Otherwise, PHP would print
> a warning that two methods from Traits have a conflict and are  
> excluded.
> The introduction of a new name is not renaming and references in  
> methods to
> a
> given method name aren't changed either. On the first look this may  
> sound
> strange, but it provides the opportunity to build Traits and even
> hierarchies
> of Traits which fit together very well.
>
> Traits Composed from Traits
> """""""""""""""""""""""""""
>
> Not explicitly mentioned jet, but implied by the flattening property  
> is the
> composition of Traits from Traits.
> Since Traits are fully flattened away at compile time it is possible  
> to use
> Traits to compose Traits without any additional impact on the  
> semantics.
> The following code illustrates this::
>
> <?php
> trait Hello {
>  public function sayHello() {
>    echo 'Hello ';
>  }
> }
>
> trait World {
>  public function sayWorld() {
>    echo 'World!';
>  }
> }
>
> trait HelloWorld {
>  use Hello, World;
> }
>
> class MyHelloWorld {
>  use HelloWorld;
> }
>
> $o = new MyHelloWorld();
> $o->sayHello();
> $o->sayWorld();
> // Results eventually in: Hello World!
> ?>
>
> Traits itself can take part in arbitrary compositions, but Traits  
> are not
> part
> of the inheritance tree i.e., it is not possible to inherit from a  
> Trait to
> avoid confusion and misuse of Traits.
>
> Express Requirements by Abstract Methods
> """"""""""""""""""""""""""""""""""""""""
>
> Since Traits do not contain any state/properties, there is a need to
> describe
> the requirements a Trait will rely on. In PHP it would be possible to
> utilize
> the dynamic language features, but it is a common practice to give  
> this
> requirements explicitly.
> This is possible with abstract methods like it is used for abstract  
> classes.
> ::
>
> <?php
> trait Hello {
>  public function sayHelloWorld() {
>    echo 'Hello'.$this->getWorld();
>  }
>  abstract public function getWorld();
> }
>
> class MyHelloWorld {
>  private $world;
>  use Hello;
>  public function getWorld() {
>    return $this->world;
>  }
>  public function setWorld($val) {
>    $this->world = $val;
>  }
> }
> ?>
>
> The usage of abstract methods allows to state not always obvious  
> relation
> ships
> and requirements explicitly. It is favored over the implicit usage  
> of the
> dynamic method resolution and property creation in the context of  
> complex
> projects for the sake of readability.
>
> Traits Semantics Summarized
> ---------------------------
>
> 1. Traits do not add runtime semantics, they only take part in the  
> process
> of
>  building a class.
> 2. Traits integrate into the precedence order of method overriding.
> 3. To avoid complexity, conflicts between Trait methods have to be  
> solved
>  explicitly. Otherwise a waring is generated and the conflicting  
> methods
>  are excluded.
> 4. In combinations with conflicts, developers have to chose  
> explicitly which
>  method has to be used, methods not chosen will be excluded from the
>  composition.
> 5. Aliases can be defined for methods to enable reuse of conflicting
> methods.
> 6. Traits can be composed from Traits.
> 7. Traits can state requirements explicitly by the use of abstract  
> methods.
>
> As a result of this semantics, at runtime, classes build using  
> Traits are
> not distinguishable
> from classes not using Traits but traditional code duplication  
> instead.
> Semantics of ``parent`` and ``$this`` hasn't changed, too. Used in a  
> Trait
> method, they behave exactly the same as if the method has been  
> defined in
> the
> class directly.
>
> Visibility
> ----------
>
> Visibility modifiers have not been discussed so far. Since Traits  
> are meant
> as
> units of reuse, modifiers should be changeable easily in the context  
> of a
> composed class. Therefore, the aliasing operation is able to change  
> the
> visibility modifier of a method, too.
> ::
>
> <?php
> trait HelloWorld {
>  public function sayHello() {
>    echo 'Hello World!';
>  }
> }
>
> class MyClass1 {
>  use HelloWorld { sayHello as protected }
> }
>
> class MyClass2 {
>  use HelloWorld { doHelloWorld as private sayHello }
> }
> ?>
>
> The final modifier is supported, too. The static modifier is not  
> supported,
> because it would change the methods semantics and references to `` 
> $this``
> would break.
>
> Grafts - Class Composition
> ==========================
>
> A *Graft* is a class composed into another class to reuse it avoiding
> inheritance and without an explicit need for delegation. The most  
> important
> difference to a Trait is the possibility to define state in addition  
> to
> behavior inside the reused entity. Furthermore, the grafting  
> approach is not
>
> about reuse methods in a manner focused on flexibility, but instead  
> it is
> about reusing small encapsulated units of behavior and state to build
> classes
> from them. Thus, it could be viewed as a form of private multiple
> inheritance
> which avoids conflicts by keeping everything private to the Graft per
> default.
>
> Grafts can be used in an similar way as Traits can be used. Thus, the
> example
> from the introduction can be optimized with Grafts as well.
> ::
>
> <?php
> class ezcReflectionReturnInfo {
>  function getReturnType() { /*1*/ }
>  function getReturnDescription() { /*2*/ }
> }
>
> class ezcReflectionMethod extends ReflectionMethod {
>  use ezcReflectionReturnInfo {
>    public getReturnType();
>    public getReturnDescription();
>  }
>  /* ... */
> }
>
> class ezcReflectionFunction extends ReflectionFunction {
>  use ezcReflectionReturnInfo {
>    public getReturnType();
>    public getReturnDescription();
>  }
>  /* ... */
> }
> ?>
>
> Since, everything is local to the Graft per default, the methods  
> need to be
> enumerated to be accessible in the class and its interface.
>
> Grafting Classes
> ----------------
>
> A Graft is a class itself and can be instantiated at will. Compared  
> to a
> normal
> class, there are no restrictions, it can use state and it can use
> everything else allowed for a class i.e. it is a normal class.
>
> The following class is an example for a simple counter::
>
> <?php
> class Counter {
>  private $cnt = 0;
>  public function inc() {
>    $this->cnt++;
>  }
>  public function reset() {
>    $this->cnt = -1;
>    $this->inc();
>  }
> }
> ?>
>
> The ``reset()`` is defined a bit strange, because it uses the  
> ``inc()``
> function to set the counter value to zero. This is to illustrate the
> semantics
> of a Graft inside another class. This counter can be used totally on  
> its
> own.
> Thus, ``$c = new Counter(); $c->inc();`` is valid code and will work  
> as
> expected.
>
> Another example class might be a database helper class to initialize  
> and
> reset
> a connection::
>
> <?php
> class DB {
>  private $db;
>  public function connect() {
>    $this->db = new FooDB('param');
>  }
>  public function reset() {
>    $this->db->flush();
>    unset($this->db);
>  }
>  public function doFoo(){echo 'foo';}
> }
>
> Nothing special in this class. The ``connect()`` function uses some  
> Foo
> database abstraction layer to initialize the connection and  
> ``reset()`` will
> issue a flush operation on the database object and unsets it  
> afterwards.
>
> Compose A Class From Classes
> """"""""""""""""""""""""""""
>
> Inheritance is one of the most misunderstood relationships in
> object-oriented
> programming. It is often abused to achieve code reuse, but  
> originally it was
> meant to describe the relationship between classes from a conceptual  
> view.
> Thus, apples and oranges might be characterized as subclasses of a  
> class
> fruit
> and could be subclassed themselves into classes for specific variants.
>
> In a web application where you for instance would like to build a  
> page with
> a
> language allowing multiple inheritance you could start to model a  
> ``MyPage``
> class as subclass of ``Counter`` and ``DB``, since you like to  
> derive the
> behavior of both of them. But from the conceptual view it is not  
> clear why
> MyPage is a counter or a database. Both relationships are implied by  
> the
> ``instanceof``
> operation. Technically, this might have its values, but still it  
> sounds
> strange.
>
> Thus, in single inheritance languages, *delegation* i.e.  
> *forwarding* is
> used
> instead. The ``MyPage`` class provides the necessary interfaces for
> ``Counter``
> and ``DB`` but instead of implementing it itself, it forwards the  
> method
> call
> to another object::
>
> <?php
> class MyPage {
>  private $cnt;
>  private $db;
>  /* ... */
>  public function inc() { $cnt->inc(); }
>  public connect() { $db->connect(); }
>  /* ... */
> }
> ?>
>
> This approach is very common and has its merits. Unfortunately, it  
> requires
> explicit code to implement forwarding and object injection or  
> creation which
> is
> tedious. In complex cases this might even cause a broken  
> encapsulation,
> since
> it might be necessary to forward data to another object which should  
> be
> private
> to the calling object.
>
> Grafts are designed to overcome this situation and provide similar
> opportunities for reuse like Traits do. The following code  
> demonstrates how
> Grafts can be used to compose the ``MyPage`` class from the two other
> classes::
>
> <?php
> class MyPage {
>  use Counter {
>    public incCnt() from inc();
>    public resetCnt() from reset();
>  }
>  use DB {
>    public connect();
>    public reset();
>  }
>  public function inc() {
>    /* next page */
>  }
> }
> ?>
>
> The example above shows ``MyPage`` using classes ``Counter`` and  
> ``DB``, to
> graft their functionality into it. Since, all methods from a grafted  
> class
> are
> hidden from the grafting class by default, the methods to be used in  
> the
> grafting class or from another class need to be named explicitly.
> With this approach, conflicts are avoided upfront. In case of  
> methods with
> the
> same name in a graft and another graft or the class itself, it is  
> possible
> to
> make a method available by another name. In the given example the  
> method
> ``reset()`` is made available by the name ``resetCnt()``. This alias  
> does
> not
> influence the inner working of the ``Counter`` class in anyway. Thus,
> recursion
> inside of ``Counter::reset()`` would still work like expected and  
> the call
> to
> ``incCnt()`` results in an invocation of ``Counter::inc()``. This  
> would be
> not
> true for Traits as explained earlier.
>
> With this attributes of Grafts in mind, the following example shows  
> the
> results
> of the execution of methods in the context of ``MyPage``::
>
> <?php
> $page = new MyPage();
> $page->connect();
> $page->incCnt();   // Counter::$cnt == 1
> $page->resetCnt(); // Counter::$cnt == 0
> $page->inc();      // goto next page
> $page->doFoo();    // FATAL ERROR
> ?>
>
> The call to ``connect()`` is forwarded as expected to  
> ``DB::connect()``,
> ``incCnt()`` results in a call to ``Counter::inc()`` as already  
> mentioned,
> the
> call to ``inc()`` will invoke the ``MyPage::inc()`` method defined  
> directly
> in
> the class. The call to ``doFoo()`` results in an error, since the  
> method
> defined in ``DB`` has not been made available in ``MyPage``  
> explicitly.
>
> Interaction of Grafts with Grafted Classes
> """"""""""""""""""""""""""""""""""""""""""
>
> In class-based languages like PHP, the notion of abstract classes was
> introduced, to be able to define a unit of reuse in a partial manner  
> and to
> be able to refine it in a more concrete context.
>
> This notion can be used combined with grafting as well. Thus, abstract
> methods
> can be fulfilled by a graft or from the grafting class.
>
> The following example shows how a class can be used by grafting to  
> fulfill a
> required abstract method::
>
> <?php
> abstract class Base {
>  abstract public function foo();
> }
>
> class MyGraft {
>  public function foo() { echo 'foo'; }
> }
>
> class MyGraftedClass extends Base {
>  use MyGraft {
>    public foo();
>  }
> }
>
> $o = new MyGraftedClass();
> $o->foo(); // echos 'foo'
> ?>
>
> In addition, this notion can also be utilized to provide methods to  
> a graft,
> which states a requirement by an abstract class. This case is shown  
> in the
> code below::
>
> <?php
> abstract class MyGraft {
>  abstract public function foo();
>  public function bar() {
>    $this->foo();
>  }
> }
>
> class MyGraftedClass {
>  public function foo() { echo 'foo'; }
>  use MyGraft {
>    public bar();
>  }
> }
>
> $o = new MyGraftedClass();
> $o->bar(); // echos 'foo'
> ?>
>
> For properties and methods not explicitly required, it is not  
> possible to
> provide something similar, since this would lead to ambiguity. The  
> reasons
> are described in the discussion section.
>
> Grafting of the Same Class Multiple Times
> """""""""""""""""""""""""""""""""""""""""
>
> Since, different grafts are separated from each other, it is  
> possible to
> graft
> the same class multiple times into one class::
>
> <?php
> class MyGraft {
>  public function foo() { echo 'foo'; }
> }
> class MyGraftedClass {
>  use MyGraft {
>    public foo();
>  }
>  use MyGraft {
>    public bar() from foo();
>  }
> }
> $o = new MyGraftedClass();
> $o->foo(); // echos 'foo'
> $o->bar(); // echos 'foo'
> ?>
>
> This might not be useful for all classes, but could be used to graft  
> data
> structures like lists or maps into a class which are used for multiple
> purposes.
>
> Restrictions and Initialization of Grafts
> """""""""""""""""""""""""""""""""""""""""
>
> Actually an instantiated graft is nothing else but an object. Since  
> grafts
> are
> full-fledged classes themselves, they could use typical constructors  
> to
> initialize resources or prepare their inner state for further usage.  
> In the
> current form of this proposal, the notion of initializing grafts is  
> not
> supported, thus, only classes with a standard constructor without  
> arguments
> can
> be used to be grafted into another class.
>
> Another restriction is that classes can not graft themselves and more
> generally, grafting is not allowed to cause recursion in any way.  
> Since, it
> is
> not meant to be evaluated lazily, it would case an infinite  
> recursion which
> needs to be prevented.
>
> Implementation Idea
> """""""""""""""""""
>
> Eventually, the implementation seems only to be feasible by  
> maintaining the
> notion of separate objects for classes grafted into another class.  
> Thus, for
> each graft in a class, a normal object is instantiated. This  
> approach will
> ease
> the implementation and will reduce modification in method lookup and  
> the
> Zend
> Engine in general.
>
> Methods propagated to the grafting class will be generated like usual
> methods
> implemented by hand. Thus, they will contain the opcodes to forward  
> the
> method
> call to the grafted object. In reverse, it would be the same for  
> methods
> introduced by abstract methods satisfied by the grafting class. Those
> methods
> will call the method provided by the grafting class.
>
> The most visible problem in this approach will be the handling of  
> object
> identity of grafted objects. Imagine the following code::
>
> <?php
> class MyGraft {
>  public function returnThis() {
>    return $this;
>  }
> }
> class MyGraftedClass {
>  use MyGraft {
>    public returnThis();
>  }
> }
> $o = new MyGraftedClass();
> var_dump($o->returnThis());
> ?>
>
> The result returned should be definitely equal to ``$o`` i.e. ``$o ===
> $o->returnThis()``. The reason for this requirement is to preserve
> encapsulation and hide implementation details from all clients of
> ``MyGraftedClass``. To achieve this property some kind of context  
> dependent
> treatment of the ``$this`` lookup has to be implemented or some kind  
> of data
> flow
> analysis will have to be done. Neither of them seem to be easy to  
> achieve
> with
> respect to the fact, that a grafted class cold hold its own `` 
> $this`` value
> in
> some property, for whatever reason.
>
> The other way around would be to open up the notion of grafts and add
> mechanisms to inject instances of grafts into a class while object
> construction. By the way, this would allow more fancy dependency  
> injection
> and gives a reason to think about grafts not only as implementation
> details, but an interface related i.e. *client visible*  
> characteristic.
> But, this notion is currently not part of this proposal.
>
> Grafts Semantics Summarized
> ---------------------------
>
> 1. A Graft is a normal class composed into another class.
> 2. The encapsulation of grafts is preserved completely.
> 3. All methods available as part of the grafting class have to be  
> enumerated
>  explicitly and can have a different name than the method in the  
> grafted
>  class, without breaking inner coherence.
> 4. Grafts can interact with grafting classes via abstract methods.
> 5. Methods introduced by a Graft can be used to fulfill abstract  
> methods.
> 6. Classes can be grafted into a class multiple times.
> 7. Classes to be used as Grafts are not allowed to use constructors  
> with
>  arguments.
>
> Traits vs. Grafts
> =================
>
> This section gives a basic overview about the differences of both  
> concepts
> and
> discusses benefits and drawbacks.
>
> *Traits* are entities of behavioral reuse. Thus, they provide a  
> lightweight
> way
> to compose methods into classes. They are highly composable  
> themselves, not
> hindered by a strong notion of encapsulation and abandon state for  
> the sake
> of
> simplicity.
>
> *Grafts* are classes composed into other classes to achieve reuse of
> full-fledged classes. It introduces a stronger notion of composition  
> of
> classes
> than typical delegation/forwarding does for OOP languages.
>
> +------------------------+-----------------------------+
> |   Traits               |   Grafts                    |
> +========================+=============================+
> | - stateless            | - complete classes          |
> | - can "break"          | - robust encapsulation      |
> | - notation is DRY      | - some overhead in notation |
> | - flexible composition | - convenient composition    |
> |   of behavior          |   of classes                |
> | - flattenable,         | - language supported        |
> |   no runtime impact    |   delegation/forwarding     |
> +------------------------+-----------------------------+
>
> This proposal does only suggest Traits without state to avoid the
> introduction
> of any notion of Traits at runtime. The key idea of Traits is that  
> they
> provide
> method implementations which are flattened into a class. Thus, all  
> state is
> defined in the class and Traits method have to rely on PHPs dynamic  
> nature
> or
> have to use abstract method definitions to require getters and  
> setters.
> Since Grafts are full-fledged classes, they have state and handle it  
> as
> expected for distinct objects.
>
> One point of criticism from the community on the notion of Traits is  
> that
> the
> methods provided by Traits are not reliable connected. This leads to  
> missing
> features like recursion for methods introduced by aliases. From the
> perspective
> of composition, this is desired, since it provides additional  
> opportunities
> for
> combination of behavior. Traits are not meant to provide  
> encapsulation on
> the
> level of classes, but flexibility.
> Grafts do avoid this problem by its nature of being classes, but  
> this comes
> for
> the cost of flexibility. Grafts can not combined with another. They  
> are
> completely separated entities which only can be combined by abstract
> methods.
>
> The notion of Traits avoids any impact on the runtime. For Grafts  
> this is
> not
> feasible. They need to maintain strong encapsulation which is  
> achieved in an
> consistent way only by preserving the notion of internal objects.  
> Thus,
> there
> is overhead in memory consumption for the objects and runtime cost for
> forwarding method calls compared to Traits.
>
> Another point where Grafts might be improved (see syntax proposals)  
> is the
> syntax for using a class. To avoid conflicts methods are private to  
> a graft
> per
> default and thus, need to be enumerated explicitly. Compared to  
> Traits this
> is
> not DRY and implies a little overhead in code, but obviously less  
> overhead
> than
> typical forwarding cases. With respect to Traits, research has shown  
> that
> conflicts occur only rarely and explicit handling of it is much less
> overhead
> and brings more power than trying to solve it implicitly.
>
> Discussions
> ===========
>
> For the original Traits proposal a lot discussion has already taken  
> place.
> The RFC_
> summarizes the most important ones. Eventually, it resulted in an  
> additional
> RFC-NBT_ proposing non-breakable Traits, which led finally to this  
> proposal.
> The first thread on was started on the `mailing list`
> (http://news.php.net/php.internals/35562) back in February 2008.
>
> Grafts
> ------
>
> PHP as a dynamic language would give the opportunity to work with this
> grafts functionality more flexible, even without abstract method
> definitions,
> but this would be very unintuitive since the graft does not know it  
> is using
>
> methods or properties from the grafted class. On the one hand this  
> would
> even
> break the notion of encapsulation and on the other hand, it would  
> cause
> trouble
> at least for properties because here it might be that the different  
> order of
> method
> execution results in unintended different results. Imagine something  
> simple
> like::
>
> <?php
> class MyGraft {
>  public function get() { return $this->value; }
>  public function set($value) { $this->value = $value; }
> }
> ?>
>
> Grafted into a class it would depend on the behavior of the grafted  
> class,
> whether there is a property named value and thus the encapsulation is
> broken.
> Thus, it would be ambiguous to allow a forwarding from within a  
> grafted
> class
> to its grafting class for non-defined properties and methods.
>
> Alternative Syntax Proposals
> ----------------------------
>
> Different keywords and alternative syntaxes for traits have been  
> already
> discussed on the `mailing list` and are documented more detailed in  
> the
> original RFC_.
>
> Some important proposals and new additional proposals are listed  
> below for
> traits as well as grafts.
>
> Traits
> """"""
>
> Assignment instead of ``instead``:
>
> Keywords are a rare resource in any language. Thus, new keywords  
> should be
> introduced carefully. To avoid the ``instead`` keyword, a notion of
> assignment
> was proposed. On the first look, it seems even to avoid an  
> impression of
> renaming::
>
> <?php
> class Talker {
>  use A, B, C, D {
>    smallTalk = A::smallTalk; // this says that if
>                              // B, C or D implement smallTalk,
>                              // it is ignored
>    talk = A::bigTalk;
>  }
> }
> ?>
>
> Grafts
> """"""
>
> Grafts use wildcard:
>
> The proposed Grafts syntax is not DRY and all methods names to be made
> available need to be enumerated. To avoid this enumeration, a syntax  
> with a
> wildcard was proposed::
>
> <?php
> class MyGraftingClass {
>  use MyGraftClass *;
> }
> ?>
>
> Links and Literature
> ====================
>
> .. _RFC: http://www.stefan-marr.de/artikel/rfc-traits-for-php.html
> .. _RFC-NBT: http://wiki.php.net/rfc/nonbreakabletraits
> .. _agrarian cultivation:
> http://www.robinsonlibrary.com/agriculture/plant/propagation/grafting.htm
> .. _mailing list: http://marc.info/?l=php-internals&m=120336491008719
>
> As already mentioned, Traits is not a totally new concept, but the  
> semantics
> used in this proposal has been fully defined at first in 2003. For
> scientific
> information and papers about Traits
> http://www.iam.unibe.ch/~scg/Research/Traits/
> is a good starting point. Since it isn't a purely academic concepts,  
> there
> are
> already languages supporting Traits out there. Squeak, Perl6, Scala,  
> Slate,
> Fortress and even for C#/Rotor implementation are available.
>
> A detailed technical report has been published at
> http://www.iam.unibe.ch/~scg/Archive/Papers/Duca06bTOPLASTraits.pdf
> It explains Traits and gives some formal proves about the soundness of
> Traits, too.
>
> Last but not least, in this PhD thesis
> http://www.iam.unibe.ch/~scg/Archive/PhD/schaerli-phd.pdf
> two case studies have been publish illustrating the benefits Traits  
> are
> providing.
>
>
> -- 
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>

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