Traits: A New Language Feature for PHP?

Please, feel free to interrupt me at any time.

Do you use: Multiple Inheritance? Mixins?

Traits A Language Feature for PHP? Stefan Marr PHP Unconference Hamburg 26-27 April 2008

Agenda Introduction to Traits Traits Applied Traits for PHP: Trait versus Graft Roundup and Conclusion

Which problem is to be solved? Single inheritance Simple but not expressive enough Leads to Code duplication Inappropriate hierarchies

Traits Set of methods Possibly composed of other Traits Composition is unordered Composition operations Sum Exclusion Aliasing Can be flattened away

TRAITS APPLIED Case Studies Illustrating Benefits of Traits

Use Case for Traits in a PHP Library ezcReflection enhances PHP Reflection API Adds XDoclet like annotations for PHP Enhances type hinting using PHPDoc type annotations Illustrates solution for code duplication problems

Refactoring ezcReflection with Traits original with Traits green parts are extracted to the Traits on the right-hand side, and will be removed from the classes

Case Study: java.io [6] Mature code base, in wide use java.io Library from Java 1.4.2 79 classes, 25000 lines of code Results 12 classes changed 30 duplicated methods removed 14 Traits introduces

Case Study: Smalltalk Collection Classes [10] Problem: code reuse vs. conceptual categorization Unnecessary inheritance Code duplication Methods to high in hierarchy Conceptual shortcomings Results for core collection classes 23 classes changed, 12% less code, 19.4% less methods

Case Study: Smalltalk Collection Classes [10] Lessons Learned Traits simplify refactoring Very fine-grained traits Tools are important Traits enable to defer design of class hierarchy Improvements Uniformity, understandability, reuse

Check for Limitations Code duplication Inappropriate hierarchies Diamond problem Fragile hierarchies Lack of composition control     

TRAITS FOR PHP Trait versus Graft

What is a Graft? Stateful, non-breakable entity of reuse Derived from the Traits notion Proposed with RFC: Non Breakable Traits for PHP Why Graft? From The Grafter's Handbook by R.J. Garner

A Comparison Traits Purpose, reuse of behavior Features Stateless Flattenable Explicit conflict resolution Highly composable Notion Language-base Copy and Paste Grafts Purpose, reuse of modules smaller then classes Features Stateful Hides complexity Conflict avoidiance Strong encapsulation Notion Delegation with self-impersonation

Traits Syntax and Semantics trait A { public function small() { echo 'a' ; } public function big() { echo 'A' ; } abstract public function doBFoo(); public function doAFoo() { echo 'AFoo uses BFoo: ' ; $this ->doBFoo(); } } trait B { public function s mall() { echo 'b' ; } public function big() { echo 'B' ; } public function doBFoo() { echo 'B-FOO' ; } } class Talker { use A, B { B::small instead A::small, A::big instead B::big, B::big as talk } } $talker = new Talker(); $talker ->small(); // b $talker ->big(); // A $talker ->talk(); // B $talker ->doAFoo(); // AFoo uses // BFoo: B-FOO $talker ->doBFoo(); // B-FOO

Grafts Syntax and Semantics graft Counter { private $cnt = 0 ; public function inc() { $this ->cnt++; } public function reset() { $this ->cnt = -1; $this ->inc(); } } graft DB { private $db; public function connect() { $this ->db = new FooDB( 'param' ); } public function reset() { $this ->db->flush(); $this ->db = null; } public function doFoo(){echo 'foo' ;} } class MyPage { include Counter { public incCnt() as inc(); public resetCnt() as reset(); } include DB { public connect(); public reset(); } public function inc() { /* next page */ } } $page = new MyPage(); $page ->connect(); $page ->incCnt(); //($Countercnt == 1) $page ->resetCnt(); // $Countercnt = 0; $page ->inc(); // goto next page $page ->doFoo(); // FATAL ERROR

ROUNDUP AND CONCLUSION Trait Support, Discussion, Resume

Environments Supporting Traits Squeak: In standard distribution since 3.9 [11] Trait named: #TDrawing uses: {} draw ˆself drawOn: World canvas in: bounds bounds self requirement Object subclass: #Circle instanceVariableNames: ’’ traits: { TDrawing } ...

Environments Supporting Traits Scala: static-typed language running on JVM [7] trait TDrawing { def draw() : Any = drawOn(World.canvas(), bounds) def bounds() : Any } class Circle extends TDrawing { ... }

Environments Supporting Traits Perl6: Flavor of Traits with states called Roles [12] role TDrawing { method draw() { return drawOn(World.canvas(), bounds); } } class Circle does TDrawing { ... }

Environments Supporting Traits Approaches for plain Java Using AspectJ [2] Utilizing an Eclipse Plugin [8]

Environments Supporting Traits C#: Prototype Implementation for Rotor [9] trait TDrawing { public Object draw() { return drawOn(World.canvas(), this .bounds); } requires { public Object bounds(); } } class Circle { uses { TDrawing } ... }

Resume Traits are a appropriate language construct to: Achive conceptual consistent class hierarchies Avoid code duplication Enhance class compositon capabilites Available for different languages Two different flavors proposed for PHP

Request for Comments Traits for PHP Version: 1.5 Date: 2008-03-06 Author: Stefan Marr Wiki: http://wiki.php.net/rfc/traits reST: http://www.stefan-marr.de/rfc-traits-for-php.txt HTML: http://www.stefan-marr.de/artikel/rfc-traits-for-php.html Non Breakable Traits for PHP Version: 1.0 Date: 2008-02-29 Author: Joshua Thompson Wiki: http://wiki.php.net/rfc/nonbreakabletraits

Basic Literature Main Article about Traits [3] S. DUCASSE, O. NIERSTRASZ, N. SCHÄRLI, R. WUYTS, AND A. P. BLACK, Traits: A Mechanism for Fine-Grained Reuse , ACM Trans. Program. Lang. Syst., 28 (2006), pp. 331–388. Traits Extensions [1] A. BERGEL, S. DUCASSE, O. NIERSTRASZ, AND R. WUYTS, Stateful Traits and their Formalization , Journal of Computer Languages, Systems and Structures, 34 (2007), pp. 83–108. [4] S. DUCASSE, R. WUYTS, A. BERGEL, AND O. NIERSTRASZ, User-Changeable Visibility: Resolving Unanticipated Name Clashes in Traits , SIGPLAN Not., 42 (2007), pp. 171–190.

Literature [1] A. BERGEL, S. DUCASSE, O. NIERSTRASZ, AND R. WUYTS, Stateful Traits and their Formalization , Journal of Computer Languages, Systems and Structures, 34 (2007), pp. 83–108. [2] S. DENIER, Traits Programming with AspectJ , RSTI - L'objet, 11 (2005), pp. 69–86. [3] S. DUCASSE, O. NIERSTRASZ, N. SCHÄRLI, R. WUYTS, AND A. P. BLACK, Traits: A Mechanism for Fine-Grained Reuse , ACM Trans. Program. Lang. Syst., 28 (2006), pp. 331–388. [4] S. DUCASSE, R. WUYTS, A. BERGEL, AND O. NIERSTRASZ, User-Changeable Visibility: Resolving Unanticipated Name Clashes in Traits , SIGPLAN Not., 42 (2007), pp. 171–190. [5] S. MARR AND F. MENGE, ezcReflection , SVN Repository, eZ Components, January 2008. http://svn.ez.no/svn/ezcomponents/experimental/Reflection. [6] E. R. MURPHY-HILL, P. J. QUITSLUND, AND A. P. BLACK, Removing Duplication from java.io: a Case Study using Traits , in OOPSLA '05: Companion to the 20th annual ACM SIGPLAN conference on Object-oriented programming, systems, languages, and applications, New York, NY, USA, 2005, ACM, pp. 282–291.

Literature [7] PROGRAMMING METHODS LABORATORY, Traits for Scala , Programming Language, Ecole Polytechnique Fédérale de Lausanne, 2006. http://www.scala-lang.org/intro/traits.html. [8] P. J. QUITSLUND, E. R. MURPHY-HILL, AND A. P. BLACK, Supporting Java traits in Eclipse , in eclipse '04: Proceedings of the 2004 OOPSLA workshop on eclipse technology eXchange, New York, NY, USA, 2004, ACM, pp. 37–41. [9] S. REICHHART, Traits in C# . Video of Talk at Microsoft Research, September 2005. [10] N. SCHÄRLI, Traits — Composing Classes from Behavioral Building Blocks , PhD thesis, University of Berne, Feb. 2005. [11] SOFTWARE COMPOSITION GROUP, Traits for Squeak , Smalltalk VM, University of Berne, December 2006. http://www.iam.unibe.ch/ scg/Research/Traits/index.html. [12] L. WALL, Apocalypse 12: Class Composition with Roles , tech. report, The Perl Foundation, 2004. http://www.perl.com/pub/a/2004/04/16/a12.html?page=11.

Any questions? Discussion What du you think? Traits vs. Grafts

TRAITS ENHANCED User-Changeable Visibility and Stateful Traits

User-Changeable Visibility Add flexibility to conflict handling Introduces “Trait private” methods Methods visibility changeable on composition

Stateless Traits Problem: stateless Traits are incomplete, not self-contained units of reuse Required state accessors bloat the interfaces Encapsulation is violated by public accessors Reuse, composition is hampered State access introduces fragility

Stateful Traits Solution requirements: Preserver faltening property Minimal and general approach Language independed Solution State is private to the Traits scope Client can request access under new private name Client can merge state

Traits: A New Language Feature for PHP?

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