trait (computer programming)

In object-oriented programming, behavior is sometimes shared between classes which are not related to each other. For example, many unrelated classes may have methods to serialize objects to JSON. Historically, there have been several approaches to solve this without duplicating the code in every class needing the behavior. Other approaches include multiple inheritance and mixins, but these have drawbacks: the behavior of the code may unexpectedly change if the order in which the mixins are applied is altered, or if new methods are added to the parent classes or mixins.

Traits solve these problems by allowing classes to use the trait and get the desired behavior. If a class uses more than one trait, the order in which the traits are used does not matter. The methods provided by the traits have direct access to the data of the class.

Traits combine aspects of protocols (interfaces) and mixins. Like an interface, a trait defines one or more method signatures, of which implementing classes must provide implementations. Like a mixin, a trait provides additional behavior for the implementing class.

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 diamond problem of multiple inheritance. This is different from other composition methods in object-oriented programming, where conflicting names are automatically resolved by scoping rules.

Operations which can be performed with traits include:{{cite journal

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

|first2 = John |last2 = Reppy

|title = Statically typed traits

|url = http://newtraell.cs.uchicago.edu/files/tr_authentic/TR-2003-13.pdf

|archiveurl = https://web.archive.org/web/20040517161301/http://www.cs.uchicago.edu/files/tr_authentic/TR-2003-13.pdf

|archivedate = May 17, 2004

|url-status = live

|publisher = University of Chicago

|year = 2003

}}{{cite conference

|first1 = Kathleen |last1 = Fisher

|first2 = John |last2 = Reppy

|title = A typed calculus of traits

|conference = 11th Workshop on Foundations of Object-oriented Programming

|conference-url = http://www.cis.upenn.edu/~bcpierce/FOOL//FOOL11.html

|url = http://people.cs.uchicago.edu/~jhr/papers/2004/fool-traits.pdf

|publisher = University of Chicago

|year = 2004

}}

• symmetric sum: an operation that merges two disjoint traits to create a new trait
• override (or asymmetric sum): an operation that forms a new trait by adding methods to an existing trait, possibly overriding some of its methods
• alias: an operation that creates a new trait by adding a new name for an existing method
• exclusion: an operation that forms a new trait by removing a method from an existing trait. (Combining this with the alias operation yields a shallow rename operation).

If a method is excluded from a trait, that method must be provided by the class that consumes the trait, or by a parent class of that class. This is because the methods provided by the trait might call the excluded method.

Trait composition is commutative (i.e. given traits A and B, A + B is equivalent to B + A) and associative (i.e. given traits A, B, and C, (A + B) + C is equivalent to A + (B + C)).

While traits offer significant advantages over many alternatives, they do have their own limitations.

If a trait requires the consuming class to provide certain methods, the trait cannot know if those methods are 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 method signature, making it harder to guarantee that the required method is appropriate.

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.

Traits come originally from the programming language Self{{cite conference | first1=Gael | last1=Curry | first2=Larry | last2=Baer | first3=Daniel | last3=Lipkie | first4=Bruce | last4=Lee | title=Traits: An approach to multiple-inheritance subclassing | conference=SIGOA Conference on Office Information Systems | pages=1–9 | location=Philadelphia, Pennsylvania, USA | year=1982 | publisher=ACM Press | doi=10.1145/966873.806468 }} and are supported by the following programming languages:

• 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.{{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}}
• C#: Since version 8.0, C# has support for default interface methods,{{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}} which have some properties of traits.{{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}}
• C++: Used in Standard Template Library and the C++ Standard Library to support generic container classes{{cite web | url=http://www.sgi.com/tech/stl/iterator_traits.html | title=iterator_traits<Iterator> | website=Standard Template Library | publisher=SGI}}{{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}} and in the Boost TypeTraits library.{{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}}
• Curl: Abstract classes as mixins permit method implementations and thus constitute traits by another name.{{citation needed|date=January 2016}}
• Fortress{{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}}
• Groovy: Since version 2.3{{cite web | url=http://www.groovy-lang.org/objectorientation.html#_traits | title=Object Orientation: Traits | website=The Groovy Programming Language | access-date=January 23, 2016}}
• Haskell: In Haskell, Traits are known as Type classes.
• Haxe: Since version 2.4.0.{{Cite news|url=https://haxe.org/download/version/2.4.0/|title=Haxe 2.4.0 - Haxe - The Cross-platform Toolkit|work=Haxe - The Cross-platform Toolkit|access-date=2017-09-12}} Called Static Extension{{Cite news|url=https://haxe.org/manual/lf-static-extension.html|title=Manual - Haxe - The Cross-platform Toolkit|work=Haxe - The Cross-platform Toolkit|access-date=2017-09-12}} in the manual, it uses using keyword
• Java: Since version 8, Java has support for default methods,{{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}} which have some properties of traits.{{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 }}{{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 }}{{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}}{{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}}
• JavaScript: Traits can be implemented via functions and delegations{{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}} or through libraries that provide traits.{{cite web | url=https://traitsjs.github.io/traits.js-website/ | title=Traits.js: Traits for JavaScript | access-date=January 23, 2016}}{{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}}{{cite web | url=https://cocktailjs.github.io/ | title=CocktailJS | access-date=January 23, 2016}}
• Julia: Several packages implement traits, e.g.,{{cite web | url=https://github.com/mauro3/SimpleTraits.jl | title=SimpleTraits.jl | author=mauro3 | website=GitHub | access-date=March 23, 2017}}
• Kotlin: Traits have been called interfaces{{cite web | url=http://kotlinlang.org/docs/reference/interfaces.html | title=Interfaces | website=Kotlin Reference | publisher=JetBrains | access-date=January 23, 2016}} since M12.{{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}}
• Lasso{{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}}
• Mojo: Since version 0.6.0{{Cite web |title=Modular Docs - Mojo🔥 changelog |url=https://docs.modular.com/mojo/changelog.html#v0.6.0-2023-12-04 |access-date=2023-12-13 |website=docs.modular.com |language=en}}
• OCaml: Traits can be implemented using a variety of language features: module and module type inclusion, functors and functor types, class and class type inheritance, et cetera.
• Perl: Called roles, they are implemented in Perl libraries such as Moose, Role::Tiny and Role::Basic. Roles are part of the sister language Raku. {{cite web | url=http://www.modernperlbooks.com/mt/2009/04/the-why-of-perl-roles.html | title=The Why of Perl Roles | author=chromatic | date=April 30, 2009 | access-date=January 23, 2016}} With the acceptance of the Corinna OOP Proposal{{cite web | url=https://github.com/Ovid/Cor/blob/master/rfc/mvp.md | title=Corinna OOP Proposal | website=Corinna RFC | author=Curtis "Ovid" Poe | access-date=September 30, 2022}} Perl will have roles native to the language as part of a modern OOP system.
• PHP: Since version 5.4,{{cite web | url=http://www.php.net/manual/en/language.oop5.traits.php | title=Traits | website=PHP Documentation | publisher=The PHP Group | access-date=January 23, 2016}}{{cite web | last=Marr | first=Stefan | title=Request for Comments: Horizontal Reuse for PHP | url=https://wiki.php.net/rfc/horizontalreuse | website=PHP.net wiki | publisher=The PHP Group | date=January 9, 2011 | access-date=January 31, 2011}} PHP allows users to specify templates that provide the ability to "inherit" from more than one (trait-)class, as a pseudo multiple inheritance.
• Python: Via a third-party library,{{cite web | url=http://py3traits.readthedocs.org/ | title=py3traits Documentation | first=Teppo | last=Perä | access-date=January 23, 2016}}{{cite web | url=https://github.com/Debith/py2traits | title=py2traits | first=Teppo | last=Perä | website=GitHub | access-date=January 23, 2016| date=2015-03-25 }} or via higher-order mixin classes{{cite web | url=http://stupid-python-tricks.blogspot.com/2015/04/computed-properties-and-higher-order.html | title = Higher Order Mixin Classes | archive-date = 2016-10-09 | archive-url = https://web.archive.org/web/20161009062141/http://stupid-python-tricks.blogspot.com/2015/04/computed-properties-and-higher-order.html }}
• Racket: Supports traits as a library and uses macros, structures, and first-class classes to implement them.{{cite web | url=http://docs.racket-lang.org/reference/trait.html | title=Traits | website=The Racket Reference | access-date=January 23, 2016}}
• Ruby: Module mixins can be used to implement traits.{{cite web | url=http://ruby-naseby.blogspot.com/2008/11/traits-in-ruby.html | title=Traits in Ruby | website=Ruby Naseby | author=David Naseby | date=February 14, 2004 | access-date=January 23, 2016}}
• Rust{{cite web | url=https://doc.rust-lang.org/book/ch10-02-traits.html | title=Traits | website=The Rust Programming Language | access-date=September 30, 2019}}
• Scala{{cite web | url=http://www.scala-lang.org/node/126 | title=Traits | website=A Tour of Scala | publisher=École polytechnique fédérale de Lausanne | access-date=January 23, 2016}}{{cite web | url=http://www.ibm.com/developerworks/java/library/j-scala04298/ | title=The busy Java developer's guide to Scala: Of traits and behaviors | first=Ted | last=Neward | website=IBM developerWorks | publisher=IBM | date=April 29, 2008 | access-date=January 23, 2016}} trait is builtin supported with the key word trait.
• Smalltalk: Traits are implemented in two dialects of Smalltalk, Squeak and Pharo.{{cite web | url=http://pharo.gemtalksystems.com/book/LanguageAndLibraries/Traits/ | title=Traits in 10 minutes | website=Pharo: The CollaborActive Book | access-date=January 23, 2016}}
• Swift: Traits can be implemented with protocol extensions.{{cite web | url=https://machinethink.net/blog/mixins-and-traits-in-swift-2.0/ | title=Mixins and Traits in Swift 2.0 | first=Matthijs | last=Hollemans | date=July 22, 2015 | access-date=January 23, 2016}}

On C# 8.0, it is possible to define an implementation as a member of an interface.

using System;

namespace CSharp8NewFeatures;

interface ILogger

{

// Traditional interface methods

void Log(string message);

void LogError(Exception exception);

// Default interface method

void LogWarning(string message)

{

Console.WriteLine(message);

}

}

class Logger : ILogger

{

public void Log(string message)

{

Console.WriteLine(message);

}

public void LogError(Exception exception)

{

Console.WriteLine(exception.ToString());

}

}

class Program

{

static void Main(string[] args)

{

ILogger logger = new Logger();

logger.LogWarning("Some warning message");

}

}

This example uses a trait to enhance other classes:

// The template

trait TSingleton

{

private static $_instance = null;

private function __construct() {} // Must have private default constructor and be aware not to open it in the class

public static function getInstance()

{

if (null === self::$_instance) {

self::$_instance = new self();

}

return self::$_instance;

}

}

class FrontController

{

use TSingleton;

}

// Can also be used in already extended classes

class WebSite extends SomeClass

{

use TSingleton;

}

This allows simulating aspects of multiple inheritance:

trait TBounding

{

public $x, $y, $width, $height;

}

trait TMoveable

{

public function moveTo($x, $y)

{

// …

}

}

trait TResizeable

{

public function resize($newWidth, $newHeight)

{

// …

}

}

class Rectangle

{

use TBounding, TMoveable, TResizeable;

public function fillColor($color)

{

// …

}

}

A trait in Rust declares a set of methods that a type must implement.{{Cite web | url=http://gradebot.org/doc/ipur/trait.html | title=Traits - Introduction to Programming Using Rust}} Rust compilers require traits to be explicated, which ensures the safety of generics in Rust.

// type T must have the "Ord" trait

// so that ">" and "<" operations can be done

fn max(a: &[T]) -> Option<&T> {

let mut result = a.first()?;

for n in a {

if *n > *result {

result = &n;

}

}

Some(result)

}

To simplify tedious and repeated implementation of traits like Debug and Ord, the derive macro can be used to request compilers to generate certain implementations automatically.{{Cite web | url=https://doc.rust-lang.org/book/first-edition/traits.html | title=Traits - the Rust Programming Language}} Derivable traits include: Clone, Copy, Debug, Default, PartialEq, Eq, PartialOrd, Ord and Hash.

• Extension method
• Interface (object-oriented programming)
• Parametric polymorphism
• UFCS

{{Reflist|30em}}

• {{cite web | url=http://scg.unibe.ch/research/traits | title=Traits: Composable Units of Behavior | website=Software Composition Group | publisher=University of Bern}}

Category:C++

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