Table of Contents

Request for Comments: Traits for PHP (Superseded by Horizontal Reuse for PHP)

This RFC will discuss at first the motivation for Traits describing the rationals and presenting a short real world use case. The main part will describe the concept of Traits in detail using the syntax for Traits implemented in a patch which is part of this proposal. In the end, the URL of the patch and additional resources about Traits are given.

This RFC is outdated and replaced by RFC: Horizontal Reuse for PHP.

Introduction

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 newer versions of languages such as Perl 6, Squeak, Scala, Slate and Fortress. Traits have also been ported to Java and C#.

Why do we need Traits?

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++. Traits have been invented to avoid those problems, too. They enable designer 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.

Traits: A Mechanism for Fine-grained Reuse

A Trait is a unit of reuse much like 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.

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 some situations it would be possible to add a common base class, but in this case it is impossible, because the extended classes are not under our control, i.e., they are implemented in third party code or even in C, like it is the case here. 

 <?php
 class ezcReflectionMethod extends ReflectionMethod {
   /* ... */
   function getReturnType() { /*1*/ }
   function getReturnDescription() { /*2*/ }
   /* ... */
 }
 
 class ezcReflectionFunction extends ReflectionFunction {
   /* ... */
   function getReturnType() { /*1*/ }
   function getReturnDescription() { /*2*/ }
   /* ... */
 }
 ?>

With Traits it is possible to refactor this redundant code out. 

 <?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 on more advanced techniques and describe how the current implementation of Traits for PHP works.

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.

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

But now a problem will occur, if different Traits provide 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 notice 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;
     A::bigTalk instead B;
   }
 }
 ?>

This definition will result in the exclusion of smallTalk() from the Trait A and bigTalk() from Trait B. Therefore, the resulting class Talker would echo “b” for smallTalk() and “A” for bigTalk(). But simple exclusion of methods is not the best choice for all situations. 

 <?php
 class Talker {
   use A, B {
     B::smallTalk instead A; 
     A::bigTalk instead B;
     A::bigTalk as talk;
   }
 }
 ?>

Beside the exclusion an alias operation is available, too. This alias operation, notated like originalMethodName as additionalMethodName for arrays even has a similar semantics like the array notation. The definition A::bigTalk as talk lets the new name talk refer to the method body of bigTalk of the Trait B. The resulting Talker class will consist of following three methods:

Since the alias operation adds a new name to an existing method body, the bigTalk method still has to be excluded. Otherwise, PHP would print a notice that two methods from Traits have a conflict and are excluded. Aliasing 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 notice is generated and the conflicting methods are excluded.
  4. Specific methods can be excluded from a composition to handle conflicts.
  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.

Common Misconceptions

Aliasing vs. Renaming

The presented aliasing operation has not a semantic of renaming. Instead it does only provide a new name to be able to invoke the original method with this new name even if the original name was excluded. 

 <?php
 trait A {
   public function a() {
     echo 'a';
     $this->b();
   }
   public function b() {
     echo 'b';
   }
 }
 
 class Foo {
   use A {
     c => b
   }
 }
 
 $foo = new Foo();
 $foo->a();            //echos ab
 $foo->b();            //echos b
 $foo->c();            //echos b
 ?>

Since it is not renaming the original method b is still available and has not been influenced at all.

Furthermore, aliasing implies that the method body of an aliased method is not changed in any kind. From this it follows that a recursion available in the original method wont result in a recursion in the alias method: 

 <?php
 trait A {
   public function foo() {
     echo 'a';
     $this->foo();
   }
 }
 
 class MyA {
   use A { foo as bar }
   public function foo() {
     echo 'b';
     $this->foo();
   }
 }
 ?>

The result to a call on bar() would echo abbbbb... theoretically as infinity recursion on foo(), but not on bar(), since it is only executed once.

Proposal and Patch

This Request for Comments proposes a new language feature for PHP named Traits. Traits are a nice approach to enhance the capabilities to design conceptual consistent class hierarchies and avoid code duplication.

Patches:

Alternative Syntax Proposals

This section collects proposals for alternative Traits syntaxes.

Scala Synthax and practical example

This is inpired from http://www.scala-lang.org/node/117. 

trait User_Comments 
{
     function addComment($c)
     {
           $db = App::getDb();   
           $db->Execute("INSERT INTO user_comments (type, type_id, user_id, text)VALUES ($c->type, $c->type_id, $c->uid, $c->text)");
     }
 
     function getComments($filter)
     {
           return array();
     }
 
     function removeComment($id)
     {
           $db = App::getDb();   
           $db->Execute("DELETE FROM user_comments ... ");
     }
}
 
class App_Email with User_Comments {}
 
class App_Document with User_Comments  {
 
     function removeComment($id)
     {
           $db = App::getDb();   
           $db->Execute("DELETE FROM user_comments ... ");
 
           $db->Execute("UPDATE app_documents SET comments_count = ... ");
     }
}
// PHP notice : 'User_Comments::removeComment() definition skipped in App_Document'
 
$doc = new App_Document;
 
echo is_a($doc, 'App_Document');     // true
echo is_a($doc, 'User_Comments');    // false
echo is_with($doc, 'User_Comments'); // true

We deal with conflicting class definitions by simply ignoring them. 

trait Conflict_Comments 
{
     function removeComment($id)
     {
           return false;
     }
}
 
class App_Document_Conflict with User_Comments, Conflict_Comments  {
 
     function removeComment($id)
     {
           $db = App::getDb();   
           $db->Execute("DELETE FROM user_comments ... ");
 
           $db->Execute("UPDATE app_documents SET comments_count = ... ");
     }
}
// PHP notice : 'User_Comments::removeComment() definition skipped in App_Document_Conflict'
// PHP notice : 'Conflict_Comments::removeComment() definition skipped in App_Document_Conflict'
 
$doc = new App_Document_Conflict;
 
echo is_a($doc, 'App_Document_Conflict');// true
echo is_with($doc, 'User_Comments');     // true
echo is_with($doc, 'Conflict_Comments'); // true

Alternative Keywords for use

The keyword use is already reserved for the new namespace feature. Thus, alternative keywords has been proposed on the mailing list. The general idea remains the same, Trait usage is defined in the class body like this: 

 class Foo {        class Foo {
   exhibit Bar;        possess Bar;
 }                  }

Proposed keywords:

Alternatives for the ''instead'' keyword

There has been the proposal of the keyword over instead of the instead keyword: 

 use A, B, C, D {
     B::smallTalk over A, C, D; // to be read like: use B::smallTalk
                                // instead the implementations form A, C, D
 }

Alternatives for the Aliasing Notation

Aliasing is often misunderstood as renaming. May be some of the following notations will help: 

 [1] is keyword instead of the arrow 
 use Trait {
  bar is foo1;  //methodAlias is method
 }

Interpretation: is state something about bar, there is nothing stated about foo1.

Alternative keyword for the same notation meaning:

Or some explicit variations: 

 use Trait {
   alias bar as foo1;
   clone bar as foo1;
 }
 
 [3] with method as methodAlias
 use Trait {
   without foo3;
   with bar as foo1,
        boo as foo2;
 }

Think the proposal [3] reads very well, since, the keyword pair with/without expresses the opposed semantics. The Trait is partially modified in this composition by removing some thing (foo3) and adding something (bar, boo).

Rejected Features

Interfaces Propagation

Another important feature of PHP is the support of interfaces. A often used metaphor to describe Traits is *Traits are interfaces with implementation*. Traits can be utilized to provide the implementation for a specific interface and since an interface is a guarantee that some methods are available it fits in the concept of Traits which provides those methods very well.

To underpin this relationship, it is possible to declare that a Trait implements an interface like this: 

 <?php
 interface IHello {
   public function sayHello();
 }
 
 trait SayHello implements IHello {
   public function sayHello() {
     echo 'Hello World!';
   }
 }
 
 class MyHelloWorld {
   use SayHello;
 }
 
 $o = new MyHelloWorld();
 var_dump($o instanceof IHello);  // bool(true)

If a Trait implements an interface, this definition is propagated to the class using the Trait. Therefore, it is possible to provide implementations for an interface and reuse them in different classes.

Traits Use Definition in the Class Header

Instead of declaring the Trait composition in the class body, it could be defined in the class prologue like this: 

 <?php
 trait Hello {
   public function sayHello() {}
 }
 
 class MyHelloWorld extends BaseClass
   uses Hello (hello => sayHello, !sayHello) 
 {
   public function foo() {}
 }
 ?>

The drawback of this notation is the implied notation of Traits as some kind of a type changing construct. Since they do not influence the type as their major feature, this notion would be misleading. Furthermore, this notation seams to have readability problems. Complex compositions are not as clearly arranged as they are with the *In-Body* notation. A patch implementing this notation is available at: http://toolslave.net/snapshots/traits/traits-head-syntax.patch

Alternative Expression of Exclusion (Exclusion is rejected at all)

Some people do not like the notation with the exclamation mark. Possible alternative keywords for following notation style: 

 use Trait {          use Trait {
   not foo1, foo2;      without foo1, foo2;
 }                    }

Keywords:

More about Traits

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 2008-03-05 14:15:45
- added new style of traits composition and replaced the notion of an explicit exclude operator in favor for a very explicit conflict resolution

gron 2008-02-25 16:08:35
- fixed version number and some typos

gron 2008-02-23 18:57:21
- added an example to the aliasing vs renaming part illustrating the effect on recursion
- added a syntax proposal which expresses the opposing character of aliasing(add) and excluding(remove) very well

gron 2008-02-21 23:02:01
- added several new notation proposals
- added link to patch for PHP_5_3

gron 2008-02-20 18:47:17
- introduced explicit description of abstract methods to be used as requirements specification for traits (useful to access state)
- moved part about interface propagation to the section of rejected features
- added a section about common misconceptions i.e. aliasing is not renaming
- added various syntax proposals