Request for Comments: Horizontal Reuse for PHP

Request for Comments: Horizontal Reuse for PHP

Version: 2.0
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
Patch:http://www.stefan-marr.de/archives/20-New-Traits-Patch-Ready-for-Testing.html

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 conceptually clean class hierarchies. To achieve code reuse, methods have to be either duplicated or moved near the root of the class hierarchy, but this hampers understandability and maintainability of code.

To circumvent these 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 a high reusability 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 start to have methods they do not need. So, when it is decided that 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 methods implemented in the class tree.

Besides 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, we end up with much duplicated code in both classes extending the original extension classes.

Which Opportunities Does PHP Have?

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 
HelloWorld;
  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 
AB;
}
?>

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::smallTalk instead A;
    
A::bigTalk instead B;
  }
}
?>

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::smallTalk instead A;
    
A::bigTalk instead B;
    
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 
HelloWorld;
}

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 completely 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 unset 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 damaging the implied relationships expressed by inheritance. From a conceptual view apples and oranges can be characterized as subclasses of a class fruit since an apple is a fruit and the same is true for oranges. In application development this simple is_a is often not respected to achieve code reuse.

For example, in a web application where you 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 from a conceptual point of view the simple is_a does not hold. Instead a consists_of relation ship sounds much more appropriated.

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 have 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 not be 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 
MyGraftingClass extends Base {
  use 
MyGraft {
    public 
foo();
  }
}

$o = new MyGraftingClass();
$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 to be feasible only 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 avoid complex modifications in method lookup and the Zend Engine in general.

Methods propagated to the grafting class will be generated like usual methods implemented manually. 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 sophisticated 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
  • can "break"
  • notation is DRY
  • flexible composition of behavior
  • flattenable, no runtime impact
  • complete classes
  • robust encapsulation
  • some overhead in notation
  • convenient composition of classes
  • language supported 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 Trait methods 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 can 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 reliably 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 their nature of being classes, but this comes at the cost of flexibility. Grafts can not be combined with each other. 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. Compared to Traits, there is overhead in memory consumption for the objects and runtime cost for forwarding method calls.

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, have 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 grafting 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 grafting 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 
ABC{
    
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

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.

Changelog

gron 2008-11-02 17:06:25 - fixed some language issues pointed out by FM (thanks)

gron 2008-10-13 20:11:36 initial version for discussion