Trait (computer programming): Difference between revisions

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{{Short description|ConceptSet usedof inmethods object-orientedthat programmingextend the functionality of a class}}

{{Technical|date=March 2012}}

{{more citations needed|date=November 2022}}

In [[computer programming]], a '''trait''' is a concept used in [[Programmingprogramming language|programming languageslanguage]] whichconcept that represents a set of [[Method (computer programming)|methods]] that can be used to extend the functionality of a [[Class (computer science)|class]].<ref name="schaerli-ecoop-2003">{{cite journalbook | first1=Nathanael | last1=Schärli | first2=Stéphane | last2=Ducasse | first3=Oscar | last3=Nierstrasz | author-link3=Oscar Nierstrasz | first4=Andrew P. | last4=Black | chapter-url=http://scg.unibe.ch/archive/papers/Scha03aTraits.pdf | titlechapter=Traits: Composable Units of Behaviour | journaltitle=Proceedings of the European Conference on Object-Oriented Programming (ECOOP). | series=Lecture Notes in Computer Science | volume=2743 | year=2003 | pages=248–274 |publisher=Springer |doi=10.1007/978-3-540-45070-2_12 |isbn=978-3-540-45070-2 |citeseerx=10.1.1.1011.8}}</ref><ref>{{cite journal | first1=Stéphane | last1=Ducasse | first2=Oscar | last2=Nierstrasz | first3=Nathanael | last3=Schärli | first4=Roel | last4=Wuyts | first5=Andrew P. | last5=Black | title=Traits: A mechanism for fine-grained reuse. | journal= ACM Transactions on Programming Languages and Systems | volume=28 | issue=2 | pages=331–388 | date=March 2006 | doi=10.1145/1119479.1119483 |citeseerx=10.1.1.64.2480| s2cid=16434119 }}</ref>

==Rationale==

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In case of a [[naming collision]] between methods provided by different traits, the programmer must explicitly disambiguate which one of those methods will be used in the class; thus manually solving the ''[[Multiple inheritance#The diamond problem|diamond problem]]'' of [[multiple inheritance]]. This is different from other composition methods in object-oriented programming, where conflicting names are automatically resolved by [[Scope (computer science)|scoping rules]].

Operations which can be performed with traits include:<ref>{{cite journalreport

|first1 = Kathleen |last1 = Fisher | author1-link = Kathleen Fisher

|first2 = John |last2 = Reppy

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=== Required methods ===

If a trait requires the consuming class to provide certain methods, the trait cannot know if those methods are [[Semantic_equivalence|semantically equivalent]] to the trait's needs. For some dynamic languages, such as Perl, the required method can only be identified by a method name, not a full [[Type signature|method signature]], making it harder to guarantee that the required method is appropriate.

=== Excluding methods ===

If a method is excluded from a trait, that method becomes a 'required' method for the trait because the trait's other methods might call it.

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* [[AmbientTalk]]: Combines the properties of Self traits (object-based multiple inheritance) and [[Smalltalk]]'s [[Squeak]] traits (requiring explicit composition of traits by the programmer). It builds on the research on ''stateful'' and ''freezable'' traits to enable state within traits, which was not allowed in the first definitions.<ref>{{cite conference | url=http://soft.vub.ac.be/Publications/2009/vub-prog-tr-09-04.pdf | title=Adding State and Visibility Control to Traits Using Lexical Nesting | conference=European Conference on Object-Oriented Programming (ECOOP 2009) | first1=Tom | last1=Van Cutsem | first2=Alexandre | last2=Bergel | first3=Stéphane | last3=Ducasse | first4=Wolfgang | last4=De Meuter | pages=220–243 | publisher=Springer-Verlag | isbn=978-3-642-03012-3 | date=2009 | series=Lecture Notes in Computer Science | volume=5653 |doi=10.1007/978-3-642-03013-0_11 |citeseerx=10.1.1.372.1265}}</ref>

*[[C Sharp (programming language)|C#]]: Since version 8.0, C# has support for ''default interface methods'',<ref>{{cite web | url=https://docs.microsoft.com/en-gb/dotnet/csharp/whats-new/csharp-8#default-interface-methods | title=Default interface methods | website=What's new in C# 8.0 | publisher=Microsoft | access-date=November 29, 2019}}</ref> which have some properties of traits.<ref>{{cite web | url=https://www.talkingdotnet.com/default-implementations-in-interfaces-in-c-sharp-8/ | title=Interfaces in C# 8.0 gets a makeover | website=Default Implementation in Interfaces in C# 8.0 | date=9 September 2019 | publisher=Talking Dotnet | access-date=November 29, 2019}}</ref>

* [[C++]]: Used in [[Standard Template Library]] and the [[C++ standardStandard libraryLibrary]] to support generic [[Container (abstract data structuretype)|container classes]]<ref>{{cite web | url=http://www.sgi.com/tech/stl/iterator_traits.html | title=iterator_traits&lt;Iterator&gt; | website=Standard Template Library | publisher=SGI}}</ref><ref>{{cite magazine | url=http://www.cantrip.org/traits.html | title=Traits: a new and useful template technique | first=Nathan C. | last=Myers | magazine=C++ Report | date=June 1995 | access-date=January 23, 2016}}</ref> and in the [[Boost C++ Libraries|Boost]] TypeTraits library.<ref>{{cite web | url=http://www.boost.org/more/generic_programming.html#traits | title=Generic Programming Techniques: Traits | website=Boost C++ Libraries | first=David | last=Abrahams | access-date=January 23, 2016}}</ref>

* [[Curl (programming language)|Curl]]: Abstract classes as mixins permit method implementations and thus constitute traits by another name.{{citation needed|date=January 2016}}

* [[Fortress (programming language)|Fortress]]<ref>{{cite web | url=http://stephane.ducasse.free.fr/Teaching/CoursAnnecy/0506-Master/ForPresentations/Fortress-PLDITutorialSlides9Jun2006.pdf | title=Fortress Programming Language Tutorial | first1=Guy | last1=Steele | first2=Jan-Willem | last2=Maessen | publisher=Sun Microsystems | date=June 11, 2006 | access-date=January 23, 2016}}</ref>

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* [[Java (programming language)|Java]]: Since version 8, Java has support for ''default methods'',<ref>{{cite web | url=https://docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html | title=Default Methods | website=The Java Tutorials | publisher=Oracle | access-date=January 23, 2016}}</ref> which have some properties of traits.<ref>{{cite journal | url=https://hal.inria.fr/inria-00432538/ | title= FeatherTrait: A Modest Extension of Featherweight Java | first1=Luigi | last1=Liquori | first2=Arnaud | last2=Spiwack | journal= ACM Transactions on Programming Languages and Systems | date=2008 | volume= 30 | issue= 2 | pages= 11:1 |doi=10.1145/1330017.1330022| s2cid= 17231803 | doi-access=free }}</ref><ref>{{cite journal | url=https://hal.inria.fr/inria-00432540/ | title= Extending FeatherTrait Java with Interfaces | first1=Luigi | last1=Liquori | first2=Arnaud | last2=Spiwack | journal= Theoretical Computer Science | date=2008 | volume= 398 | issue= 1–3 | pages= 243–260 |doi=10.1016/j.tcs.2008.01.051| s2cid= 12923128 | doi-access=free }}</ref><ref>{{cite conference | url=https://hal.inria.fr/hal-01026531/en | title=Trait-oriented Programming in Java 8 | first1=Viviana | last1=Bono | first2=Enrico | last2=Mensa | first3=Marco | last3=Naddeo | conference=International Conference on Principles and Practices of Programming on the Java Platform: virtual machines, languages, and tools (PPPJ ’14) | date=September 2014 | conference-url=http://pppj2014.pk.edu.pl/ |pages=181–6 |doi=10.1145/2647508.2647520 |citeseerx=10.1.1.902.161}}</ref><ref>{{cite web | title=Definition of the Trait Pattern in Java | url=http://ageofjava.com/2016/02/definition-of-trait-pattern-in-java.html | archive-url=https://web.archive.org/web/20160804005218/http://ageofjava.com/2016/02/definition-of-trait-pattern-in-java.html | url-status=dead | archive-date=August 4, 2016 | website=Age of Java | date=February 3, 2016 | access-date=February 3, 2016 | first=Emil | last=Forslund}}</ref>

* [[JavaScript]]: Traits can be implemented via functions and delegations<ref>{{cite web | url=http://peterseliger.blogspot.com/2014/04/the-many-talents-of-javascript.html | title=The Many Talents of JavaScript | first=Peter | last=Seliger | date=April 11, 2014 | access-date=January 23, 2015}}</ref> or through libraries that provide traits.<ref>{{cite web | url=https://traitsjs.github.io/traits.js-website/ | title=Traits.js: Traits for JavaScript | access-date=January 23, 2016}}</ref><ref>{{cite journal | url=http://soft.vub.ac.be/Publications/2012/vub-soft-tr-12-19.pdf | title=Robust Trait Composition for Javascript | first1=Tom | last1=Van Cutsem | first2=Mark S. | last2=Miller | journal=Science of Computer Programming| year=2012 | access-date=January 23, 2016}}</ref><ref>{{cite web | url=https://cocktailjs.github.io/ | title=CocktailJS | access-date=January 23, 2016}}</ref>

* [[Julia (programming language)|Julia]]: Several packages implement traits, e.g.,<ref>{{cite web | url=https://github.com/mauro3/SimpleTraits.jl | title=SimpleTraits.jl | author=mauro3Mauro Werder | website=[[GitHub]] | access-date=March 23, 2017}}</ref>

* [[Kotlin (programming language)|Kotlin]]: Traits have been called ''interfaces''<ref>{{cite web | url=http://kotlinlang.org/docs/reference/interfaces.html | title=Interfaces | website=Kotlin Reference | publisher=JetBrains | access-date=January 23, 2016}}</ref> since M12.<ref>{{cite web | url=http://blog.jetbrains.com/kotlin/2015/05/kotlin-m12-is-out/ | title=Kotlin M12 is out! | first=Andrey | last=Breslav | website=Kotlin Blog | publisher=JetBrains | date=May 29, 2015 | access-date=January 23, 2016}}</ref>

* [[Lasso (programming language)|Lasso]]<ref>{{cite web | url=http://lassoguide.com/language/traits.html | title=Traits | website=Lasso Language Guide | publisher=LassoSoft | date=January 6, 2014 | access-date=January 23, 2016}}</ref>

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=== Rust ===

A trait in Rust declares a set of methods that a type must implement.<ref>{{Cite web | archive-url=https://web.archive.org/web/20230529022632/http://gradebot.org/doc/ipur/trait.html|archive-date=2023-05-29|url-status=dead|url=http://gradebot.org/doc/ipur/trait.html | title=Traits - Introduction to Programming Using Rust}}</ref> Rust compilers require traits to be explicated, which ensures the safety of [[Generic programming|generics]] in Rust.

<syntaxhighlight lang="rust">

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==See also==

* [[Extension method]]

* [[Interface (object-oriented programming)]]

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[[Category:Programming language topics]]

[[Category:Type theory]]

[[Category:Articles with example C Sharp code]]

[[Category:Articles with example PHP code]]

[[Category:Articles with example Rust code]]