rfc:horizontalreuse

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rfc:horizontalreuse [2010/06/09 07:23]
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rfc:horizontalreuse [2017/09/22 13:28] (current)
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 ====== Request for Comments: Horizontal Reuse for PHP ====== ====== Request for Comments: Horizontal Reuse for PHP ======
-  * Version: 2.0 +  * Version: 2.1.1 
-  * Date: 2008-10-12+  * Date: 2008-10-12, last update 2011-01-09
   * Author: Stefan Marr <​php.at.stefan-marr.de>​   * Author: Stefan Marr <​php.at.stefan-marr.de>​
-  * Status: ​Committed to trunk (traits only)+  * Status: ​Implemented in PHP 5.4 (traits only)
   * First Patch: http://​www.stefan-marr.de/​2009/​09/​traits-patch-updated-backported-and-available-on-github/​   * First Patch: http://​www.stefan-marr.de/​2009/​09/​traits-patch-updated-backported-and-available-on-github/​
   * First Published at: http://​www.stefan-marr.de/​artikel/​rfc-horizontal-reuse-for-php.html   * First Published at: http://​www.stefan-marr.de/​artikel/​rfc-horizontal-reuse-for-php.html
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 </​code>​ </​code>​
    
-In case of the above definition of ''​Talker'',​ PHP will show warning that there +In case of the above definition of ''​Talker'',​ PHP will abort compilation with fatal error since there 
-have been conflicts ​and name the methods ''​smallTalk()''​ and ''​bigTalk()''​  +have been conflicts. It will name the methods ''​smallTalk()''​ and ''​bigTalk()''​  
-as the reason of this conflict. ​Therefore, neither of the given implementations +as the reason of this conflict.  
-will be available in the class.+A fatal error is issued since the conflict indicates that an incompatible change was made to the code that potentially breaks ​the expected behavior.
  
-Instead, the developer can exactly define which methods ​are used and how the +To solve the conflict, the developer can exactly define which of the colliding ​methods ​has to be used.
-conflict is resolved.+
  
 <code php> <code php>
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 ==== Traits Composed from Traits ==== ==== Traits Composed from Traits ====
  
-Not explicitly mentioned ​jet, but implied by the flattening property is the+Not explicitly mentioned ​yet, but implied by the flattening property is the
 composition of Traits from Traits. composition of Traits from Traits.
 Since Traits are fully flattened away at compile time it is possible to use Since Traits are fully flattened away at compile time it is possible to use
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 of the inheritance tree i.e., it is not possible to inherit from a Trait to of the inheritance tree i.e., it is not possible to inherit from a Trait to
 avoid confusion and misuse of Traits. avoid confusion and misuse of Traits.
- 
 ==== Express Requirements by Abstract Methods ==== ==== Express Requirements by Abstract Methods ====
  
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 dynamic method resolution and property creation in the context of complex dynamic method resolution and property creation in the context of complex
 projects for the sake of readability. 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.
 +
 +<code php>
 +  <?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
 +  ?>
 +</​code>​
 +
 +==== 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:
 +
 +<code php>
 +  <?php
 +    trait Singleton {
 +      public static function getInstance() { ... }
 +    }
 +
 +    class MySingleton extends SomeUnrelatedSuperClass {
 +      use Singleton;
 +    }
 +
 +    MySingleton::​getInstance();​
 +  ?>
 +</​code>​
 +
 ===== Traits Semantics Summarized ===== ===== Traits Semantics Summarized =====
  
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 ===== Visibility ===== ===== Visibility =====
- 
  
 Visibility modifiers have not been discussed so far. Since Traits are meant as Visibility modifiers have not been discussed so far. Since Traits are meant as
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 </​code>​ </​code>​
  
-The final modifier is supported, too. The static modifier ​is not supported, +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 ''​$this'' ​would break. 
-because it would change the methods semantics and references to ''​$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 [[http://​scg.unibe.ch/​archive/​papers/​Berg07eStatefulTraits.pdf|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 itThis results in the 
 +following rules: 
 + 
 +  - 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. 
 +  - Incompatible properties result in a fatal error. 
 +  - 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. 
 +  - 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. 
 +  - 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 [[http://​svn.php.net/​viewvc?​view=revision&​revision=306476|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) ====== ====== Grafts - Class Composition (not implemented) ======
  
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 Renaming would imply that it would not be possible to use this or similar techniques with Traits. 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. 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.
 +
 +<code php>
 +<?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;​
 +}
 +
 +</​code>​
 +
 +Note: brought up in the discussion of http://​marc.info/?​l=php-internals&​m=129188077704898
  
 ===== Grafts ===== ===== Grafts =====
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 ====== Changelog ====== ====== 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 2008-10-13: initial version for discussion
rfc/horizontalreuse.1276068190.txt.gz · Last modified: 2017/09/22 13:28 (external edit)