rfc:horizontalreuse

Request for Comments: Horizontal Reuse for PHP

This RFC discusses 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 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. In addition to that, this RFC introduces 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 (committed to trunk)

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 abort compilation with a fatal error since there have been conflicts. It will name the methods smallTalk() and bigTalk() as the reason of this conflict. A fatal error is issued since the conflict indicates that an incompatible change was made to the code that potentially breaks the expected behavior.

To solve the conflict, the developer can exactly define which of the colliding methods has to be used.

 <?php
 class Talker {
   use A, B {
     B::smallTalk insteadof A;
     A::bigTalk insteadof 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 {
     B::smallTalk insteadof A; 
     A::bigTalk insteadof 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 yet, 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.

Static Variables

The flattening property implies that all methods are independent from each other at their usage point, even so, they might origin from the same method/trait. Imagine you would like to implement a counter by using a static variable and use that at various places. Each of the counters should be independent.

  <?php
    trait Counter {
      public function inc() {
        static $c = 0;
        $c = $c + 1;
        echo "$c\n";
      }
    }
 
    class C1 {
      use Counter;
    }
 
    class C2 {
      use Counter;
    }
 
    $o = new C1(); $o->inc(); // echo 1
    $p = new C2(); $p->inc(); // echo 1
  ?>

Static Methods

Following the idea that traits are almost like compiler assisted copy'n'paste, all the features presented so far also apply to static methods. This is useful for instance to provide a reusable implementation for the singleton pattern:

  <?php
    trait Singleton {
      public static function getInstance() { ... }
    }
 
    class MySingleton extends SomeUnrelatedSuperClass {
      use Singleton;
    }
 
    MySingleton::getInstance();
  ?>

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 warning 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 make requirements explicit by 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 can not be used in this context. That means, a normal method can not be converted to a static method, because it would change the methods semantics and references to $this would break.

Handling of Properties/State

Traits do not provide any provisioning for handling state. They are meant to provide a light-weight mechanism for flexible code reuse, with the main goal being to avoid code duplication. Moreover, traits should not be confused with typical use cases of classes. When a strong coherence/coupling between methods and state is required, and invariants have to be maintained on the state, this is a good indication that a class is the right abstraction to implement that problem with.

However, every behavior needs state to operate on, otherwise it could be just a static functional helper method. Thus, trait code will either need to use accessors, which is favorite way to go since it provides full traits semantics, or they use properties, which is possible but rather a convenience feature.

Since state is a complex problem, and the knowledge about compatibility of state form different traits is only present in a concrete composition, proper state handling would need language features which are currently considered beyond the scope of what is necessary for PHP. (See Bergel et al)

Thus, the goal for a consistent language design is to raise awareness of the problem, promote the use of accessors, and break early in case the changes to a trait is potentially problematic for a class using it. This results in the following rules:

  1. Properties are considered incompatible if they differ in their definition. This means, they differ in the applied modifiers (static, public, protected, private) or their initial value.
  2. Incompatible properties result in a fatal error.
  3. In all other cases, i.e., when the definitions are identical, an E_STRICT notice is shown to raise awareness about the potentially problematic, and discouraged use of properties.
  4. For those checks, all properties are treated equal. Properties from the base class and the composing class have to be compatible with properties from traits as well as the properties between all traits have to be compatible.
  5. Non-coliding properties, and properties which are not considered incompatible behave exactly the same as if they would have been defined in the composing class.

This property handling was implemented in SVN revision 306476 and examples are given in the test cases.

Reflection

Status: The Reflection API is not completely adapted for traits yet.

The following functions are added to match the existing class- or interface-specific functions.

  • trait_exists() – returns true for traits, false for all kind of classes and interfaces
  • get_declared_traits() – returns the defined traits, no classes or interfaces

Grafts - Class Composition (not implemented)

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

  • stateless
  • can “break”
  • notation is DRY
  • flexible composition of behavior
  • flattenable, no runtime impact

Grafts

  • 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 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 proposing non-breakable Traits, which led finally to this proposal. The first thread on was started on the mailing list back in February 2008.

Renaming

One of the main issues of the Traits proposal has always been the fact, that aliasing does not mean renaming. The reason for this is the dynamic nature of PHP. The fact that it is possible to use meta-programming features everywhere in the code means that names as present in the source-code have to be equal to the names at run time.

Consider the following example:

<?php
class TestClass {
    function foo() { echo "func: foo\n"; }
    function bar() { echo "func: bar\n"; }
 
    function test() {
        $funcName = "foo";
        $this->$funcName();
 
        $funcName = "bar";
        $this->$funcName();
    }
}
 
$o = new TestClass;
$o->test();

Renaming would imply that it would not be possible to use this or similar techniques with Traits. At least, it would not be stable. It would be possible to hack around it, but these hacks would be brittle, and lead to additional maintenance work.

Requiring Composing Class to Implement Interface

Traits are able to express required methods by using abstract method declarations. An abstract method can be satisfied in varios ways, for instance by implementing it in the composing class or by bringing it in from another Trait.

However, for traits that require complex interfaces to be satisfied, this approach is tedious and fragile, since it requires the developer to state all used methods explicitly.

Another solution is that a Trait expresses a requirement for the composing class to implement a certain interface. This is not entirely identical to using abstract methods, however. First, it imposes a requirement on interface level and thus will have the same fragility with respect to interface changes as all other clients of an interface. On the other hand, it avoids duplications of abstract method definitions and makes the interface the main entity of responsibility as for normal client-interface uses in current code.

<?php
// IteratorUser works with $this using the Iterator interface
trait IteratorUser require Iterator {
    function foo() {
        $next = $this->next();
        ...
    }
}
 
// composed into a class that needs to implement the interface
class Example implements Iterator {
  use IteratorUser;
}

Note: brought up in the discussion of http://marc.info/?l=php-internals&m=129188077704898

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 insteadof:

Keywords are a rare resource in any language. Thus, new keywords should be introduced carefully. To avoid the insteadof keyword, a notion of assignment was proposed. On the first look, it seams 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 *;
 }
 ?>

Scala Synthax and practical example

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

Use case #1 (user comments on a given object)

A trait would prevent duplicating this code (~roughly similar) in 9 classes (existing PHP framework).

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 ... ");
     }
}

Using the trait:

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

Use case #2 (bitmask flags)

A trait would prevent duplicating this code (~roughly similar) in 21 classes (existing PHP framework).

trait Bitmask_Flags {
 
	private $_flags;
 
	private function _hasFlag($flag, $yesNo)
	{
		if(is_null($yesNo))
			return (($this->_flags & $flag) == $flag);
 
		if($yesNo)
			$this->_flags |= $flag;
		else
			$this->_flags &= ~$flag;
 
		if((int)$flag === 0)
			$this->_flags = 0;
	}
}

Using the trait:

class Sports_Game with Bitmask_Flags {
 
	function isOvertime($yesNo = NULL)
	{
		return $this->_hasFlag(self::FLAG_OVERTIME, $yesNo);
	}
 
	function isShootout($yesNo = NULL)
	{
		return $this->_hasFlag(self::FLAG_SHOOTOUT, $yesNo);
	}
        //...
}
 
class User with Bitmask_Flags {
 
	function isAdmin($yesNo = NULL)
	{
		return $this->_hasFlag(self::FLAG_ADMIN, $yesNo);
	}
 
	function isActive($yesNo = NULL)
	{
		return $this->_hasFlag(self::FLAG_ACTIVE, $yesNo);
	}
        //...
}

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

2010-11-18: Added clarification on static modifier in the context of aliasing/visibility changes, and added section on static methods with an example suggested by Simas Toleikis

2008-10-13: initial version for discussion

rfc/horizontalreuse.txt · Last modified: 2013/10/29 18:24 by nikic