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Old page wikitext, before the edit (old_wikitext) | '{{about|the programming concept|the ice cream|Mix-in}}
In [[object-oriented programming language]]s, a '''mixin''' is a [[class (computer science)|class]] that provides a certain functionality to be inherited or just reused by a subclass, while not meant for [[Class_(computer_science)#Instantiation|instantiation]] (the generation of objects of that class). Mixins are synonymous with [[abstract type|abstract base classes]]. Inheriting from a mixin is not a form of specialization but is rather a means of collecting functionality. A class or object may "inherit" most or all of its functionality from one or more mixins, therefore mixins can be thought of as a mechanism of [[multiple inheritance]].
Mixins first appeared in the [[Symbolics]]' object-oriented [[Flavors (computer science)|Flavors]] system (developed by Howard Cannon), which was an approach to object-orientation used in [[Lisp Machine Lisp]]. The name was inspired by [[Steve's Ice Cream|Steve's Ice Cream Parlor]] in Somerville, Massachusetts:<ref>[http://www.linuxjournal.com/article/4540 Using Mix-ins with Python]</ref> The ice cream shop owner offered a basic flavor of ice cream (vanilla, chocolate, etc.) and blended in a combination of extra items (nuts, cookies, fudge, etc.) and called the item a "[[Mix-in]]", his own trademarked term at the time.<ref>[http://listserv.linguistlist.org/cgi-bin/wa?A2=ind0208a&L=ads-l&P=11751 Mix-Ins (Steve's ice cream, Boston, 1975)]</ref>
Mixins encourage [[code reuse]] and avoid well-known pathologies associated with multiple inheritance<ref>[http://books.google.com/books?id=sGvtaGy8SJ8C&pg=PA16&lpg=PA16&dq=pathologies+of+multiple+inheritance&source=bl&ots=sF3Ah-XZSq&sig=JG7VV5hrjm781yqT5iGHSnlLI0I&hl=en&ei=Obi-TuLMCebV0QHD9bnyBA&sa=X&oi=book_result&ct=result&resnum=2&ved=0CCMQ6AEwAQ#v=onepage&q&f=false ECOOP '96, Object-oriented Programming: 10th European Conference]</ref>. However, mixins introduce their own set of compromises.{{Citation needed|date=June 2010}}
A mixin can also be viewed as an [[Interface (object-oriented programming)|interface]] with implemented [[Method (computer science)|methods]]. When a class includes a mixin, the class implements the interface and includes, rather than inherits, all the mixin's attributes (fields, properties) and methods. They become part of the class during compilation. Mixins don't need to implement an interface. The advantage of implementing an interface is that instances of the class may be passed as parameters to methods requiring that interface.
A mixin can defer definition and binding of methods until [[Run time (program lifecycle phase)|runtime]], though attributes and instantiation parameters are still defined at [[compile time]]. This differs from the most widely-used approach, which originated in the programming language [[Simula]], of defining all attributes, methods and initialization at compile time.
==Definition and implementation==
In [[Simula]], classes are defined in a block in which attributes, methods and class initialization are all defined together; thus all the methods that can be invoked on a class are defined together, and the definition of the class is complete.
With mixins the class definition defines only the attributes and parameters associated with that class; methods are left to be defined elsewhere, as in [[Flavors (programming language)|Flavors]] and [[CLOS]], and are organized in "[[generic function]]s". These generic functions are functions which are defined in multiple cases (methods) by type dispatch and method combinations.
CLOS and Flavors allows mixin methods to add behavior to existing methods: :before and :after daemons, whoppers and wrappers in Flavors. CLOS added :around methods and the ability to call shadowed methods via CALL-NEXT-METHOD. So, for example, a stream-lock-mixin can add locking around existing methods of a stream class. In Flavors one would write a wrapper or a whopper and in CLOS one would use an :around method. Both CLOS and Flavors allow the computed reuse via method combinations. :before, :after and :around methods are a feature of the standard method combination. Other method combinations are provided. An example is the + method combination, where the results of all applicable methods of a generic function are added to compute the return value. This is used for example with the border-mixin for graphical objects. A graphical object may have a width generic function. The border-mixin would add a border around an object and has a method computing its width. A new class bordered-button which is both a graphical object and uses the border-mixin would compute its width by calling all applicable width methods - via the + method combination all return values are added and create the combined width of the object.
==Programming languages that use mixins==
Other than Flavors and CLOS (a part of [[Common Lisp]]), some languages that use mixins are:
*[[Ada (programming language)|Ada]] (by extending an existing tagged record with arbitrary operations in a generic)
*[[Ceylon Project|Ceylon]]
*[[C Sharp (programming language)|C#]] (with open source library re-mix <ref>re-mix: mixin library for .NET languages on codeplex: [[re-mix]]</ref>)
*[[ColdFusion]] (Class based using includes and Object based by assigning methods from one object to another at runtime)
*[[Curl (programming language)|Curl(with Curl RTE)]]
*[[D (programming language)|D]] (called [http://www.digitalmars.com/d/template-mixin.html "template mixins"])
*[[Factor programming language|Factor]]{{Citation needed|date=March 2009}}
*[[Fantom (programming language)|Fantom]]
*[[Groovy (programming language)|Groovy]]
*[[Ioke (programming language)|Ioke]]
*[[JavaFX Script]]
*[[JavaScript]]
*[[Magik (programming language)|Magik]]
*[[Object REXX]]
*[[OpenLaszlo]]
*[[Perl]]<ref>CPAN module for mixins:
[http://search.cpan.org/dist/mixin/lib/mixin.pm mixin].</ref>
*[[Perl 6]]
*[[PHP]]'s "traits"
*[[Python (programming language)|Python]]
*[[Racket_(programming_language)|Racket]] ([http://docs.racket-lang.org/guide/classes.html#(part._.Mixins) mixins documentation])
*[[Ruby (programming language)|Ruby]]
*[[Scala (programming language)|Scala]]
*[[Smalltalk]]
*[[Strongtalk]]
*[[Vala (programming language)|Vala]]
*[[Visual Dataflex]]
*[[XOTcl]]/[http://wiki.tcl.tk/18152 TclOO] (object systems for [[Tcl]])<ref>[http://media.wu-wien.ac.at/doc/tutorial.html#mixins Mixin classes in XOTcl]</ref>
*[[Self (programming language)|Self]]
Some languages like [[ECMAScript]] (commonly referred to as JavaScript) do not support mixins on the language level, but can easily mimic them by copying methods from one object to another at runtime, thereby "borrowing" the mixin's methods. Note that this is also possible with [[static typing|statically typed]] languages, but it requires constructing a new object with the extended set of methods.
==Example==
[[Common Lisp]] provides Mixins in CLOS (Common Lisp Object System) similar to Flavors.
object-width is a generic function with one argument and is using the + method combination. The + method combination determines that all applicable methods for a generic function will be called and the results will be added.
<source lang="lisp">
(defgeneric object-width (object)
(:method-combination +))
</source>
button is a class with one slot for the button text.
<source lang="lisp">
(defclass button ()
((text :initform "click me")))
</source>
There is a method for objects of class button that computes the width based on the length of the button text.
+ is the method qualifier for the method combination of the same name.
<source lang="lisp">
(defmethod object-width + ((object button))
(* 10 (length (slot-value object 'text))))
</source>
A border-mixin class. The naming is just a convention. No superclasses. No slots.
<source lang="lisp">
(defclass border-mixin () ())
</source>
There is a method computing the width of the border. Here it is just 4.
<source lang="lisp">
(defmethod object-width + ((object border-mixin))
4)
</source>
bordered-button is a class inheriting from both the border-mixin and button.
<source lang="lisp">
(defclass bordered-button (border-mixin button) ())
</source>
We can now compute the width of a button. Calling object-width computes 80.
The result is the result of the single applicable method: the method object-width for the class button.
<source lang="lisp">
? (object-width (make-instance 'button))
80
</source>
We can also compute the width of a bordered-button. Calling object-width computes 84.
The result is the sum of the results of the two applicable methods: the method object-width for the class button and the method object-width for the class border-mixin.
<source lang="lisp">
? (object-width (make-instance 'bordered-button))
84
</source>
In the [[Curl (programming language)|Curl web-content language]], multiple inheritance is used as classes with no instances may implement methods. Common mixins include all skinnable ControlUIs inheriting from SkinnableControlUI, user interface delegate objects that require dropdown menus inheriting from StandardBaseDropdownUI and such explicitly named mixin classes as FontGraphicMixin, FontVisualMixin and NumericAxisMixin-of class. Version 7.0 added library access so that mixins do not need to be in the same package or be public abstract. Curl constructors are factories which facilitates using multiple-inheritance without explicit declaration of either interfaces or mixins.{{Citation needed|date=May 2009}}
In [[Python (programming language)|Python]], the <code>SocketServer</code> module has both a <code>UDPServer</code> and <code>TCPServer</code> class that act as a server for [[User Datagram Protocol|UDP]] and [[Transmission Control Protocol|TCP]] socket servers.
Normally, all new connections are handled within the same process.
Additionally, there are two mixin classes: <code>ForkingMixIn</code> and <code>ThreadingMixIn</code>.
By extending <code>TCPServer</code> with the <code>ThreadingMixIn</code> like this
<source lang="python">
class ThreadingTCPServer(ThreadingMixIn, TCPServer):
pass
</source>
the <code>ThreadingMixIn</code> class adds functionality to the TCP server such that each new connection creates a new [[thread (computer science)|thread]].
Alternatively, using the <code>ForkingMixIn</code> would cause the process to be [[fork (operating system)|forked]] for each new connection.
Clearly, the functionality to create a new thread or fork a process is not terribly useful as a stand-alone class.
In this usage example, the mixins provide alternative underlying functionality without affecting the functionality as a socket server.
==Commentary==
Some of the functionality of mixins is provided by [[Interface (object-oriented_programming)|interfaces]] in popular languages like Java and C#. However, an interface only specifies what the class must support and cannot provide an implementation. Another class, providing an implementation and dependent with the interface, is needed for refactoring common behavior into a single place.
Interfaces combined with [[aspect-oriented programming]] can produce full fledged mixins in languages that support such features, such as C# or Java. Additionally, through the use of the [[marker interface pattern]], [[generic programming]], and extension methods, C# 3.0 has the ability to mimic mixins.<ref>[http://www.zorched.net/2008/01/03/implementing-mixins-with-c-extension-methods/ Implementing Mixins with C# Extension Methods]</ref><ref>[http://blogs.msdn.com/abhinaba/archive/2006/01/06/510034.aspx I know the answer (it's 42) : Mixins and C#]</ref><ref>[http://bloggingabout.net/blogs/erwyn/archive/2005/10/20/9925.aspx Mixins, generics and extension methods in C#]</ref>
==See also==
* [[Abstract type]]
* [[Trait (computer science)]]
* [[Policy-based design]]
==References==
<references />
== External links ==
* [http://c2.com/cgi/wiki?MixIn MixIn] at Portland Pattern Repository
* [http://www.macromedia.com/support/documentation/en/flex/1/mixin/index.html Mixins] in [[ActionScript]].
* [http://scala.epfl.ch/intro/mixin.html Scala Overview: Mixin Class Composition] - a step-by-step example in [[Scala programming language|Scala]]
* [http://www.dreamsongs.com/NewFiles/ECOOP.pdf The Common Lisp Object System: An Overview] by [[Richard P. Gabriel]] and Linda DeMichiel provides a good introduction to the motivation for defining classes by means of generic functions.
[[Category:Object-oriented programming languages]]
[[de:Mixin]]
[[es:Mixin]]
[[fr:Mixin]]
[[nl:Mixin]]
[[ja:Mixin]]
[[pl:Domieszka (programowanie obiektowe)]]
[[ru:Примесь (программирование)]]' |
New page wikitext, after the edit (new_wikitext) | '{{about|the programming concept|the ice cream|Mix-in}}
In [[object-oriented programming language]]s, a '''mixin''' is a [[class (computer science)|class]] that provides a certain functionality to be inherited or just reused by a subclass, while not meant for [[Class_(computer_science)#Instantiation|instantiation]] (the generation of objects of that class). Mixins are synonymous with [[abstract type|abstract base classes]]. Inheriting from a mixin is not a form of specialization but is rather a means of collecting functionality. A class or object may "inherit" most or all of its functionality from one or more mixins, therefore mixins can be thought of as a mechanism of [[multiple inheritance]].
Mixins first appeared in the [[Symbolics]]' object-oriented [[Flavors (computer science)|Flavors]] system (developed by Howard Cannon), which was an approach to object-orientation used in [[Lisp Machine Lisp]]. The name was inspired by [[Steve's Ice Cream|Steve's Ice Cream Parlor]] in Somerville, Massachusetts:<ref>[http://www.linuxjournal.com/article/4540 Using Mix-ins with Python]</ref> The ice cream shop owner offered a basic flavor of ice cream (vanilla, chocolate, etc.) and blended in a combination of extra items (nuts, cookies, fudge, etc.) and called the item a "[[Mix-in]]", his own trademarked term at the time.<ref>[http://listserv.linguistlist.org/cgi-bin/wa?A2=ind0208a&L=ads-l&P=11751 Mix-Ins (Steve's ice cream, Boston, 1975)]</ref>
Mixins encourage [[code reuse]] and avoid well-known pathologies associated with multiple inheritance<ref>[http://books.google.com/books?id=sGvtaGy8SJ8C&pg=PA16&lpg=PA16&dq=pathologies+of+multiple+inheritance&source=bl&ots=sF3Ah-XZSq&sig=JG7VV5hrjm781yqT5iGHSnlLI0I&hl=en&ei=Obi-TuLMCebV0QHD9bnyBA&sa=X&oi=book_result&ct=result&resnum=2&ved=0CCMQ6AEwAQ#v=onepage&q&f=false ECOOP '96, Object-oriented Programming: 10th European Conference]</ref>. However, mixins introduce their own set of compromises.{{Citation needed|date=June 2010}}
A mixin can also be viewed as an [[Interface (object-oriented programming)|interface]] with implemented [[Method (computer science)|methods]]. When a class includes a mixin, the class implements the interface and includes, rather than inherits, all the mixin's attributes (fields, properties) and methods. They become part of the class during compilation. Mixins don't need to implement an interface. The advantage of implementing an interface is that instances of the class may be passed as parameters to methods requiring that interface.
A mixin can defer definition and binding of methods until [[Run time (program lifecycle phase)|runtime]], though attributes and instantiation parameters are still defined at [[compile time]]. This differs from the most widely-used approach, which originated in the programming language [[Simula]], of defining all attributes, methods and initialization at compile time.
==Definition and implementation==
In [[Simula]], classes are defined in a block in which attributes, methods and class initialization are all defined together; thus all the methods that can be invoked on a class are defined together, and the definition of the class is complete.
With mixins the class definition defines only the attributes and parameters associated with that class; methods are left to be defined elsewhere, as in [[Flavors (programming language)|Flavors]] and [[CLOS]], and are organized in "[[generic function]]s". These generic functions are functions which are defined in multiple cases (methods) by type dispatch and method combinations.
CLOS and Flavors allows mixin methods to add behavior to existing methods: :before and :after daemons, whoppers and wrappers in Flavors. CLOS added :around methods and the ability to call shadowed methods via CALL-NEXT-METHOD. So, for example, a stream-lock-mixin can add locking around existing methods of a stream class. In Flavors one would write a wrapper or a whopper and in CLOS one would use an :around method. Both CLOS and Flavors allow the computed reuse via method combinations. :before, :after and :around methods are a feature of the standard method combination. Other method combinations are provided. An example is the + method combination, where the results of all applicable methods of a generic function are added to compute the return value. This is used for example with the border-mixin for graphical objects. A graphical object may have a width generic function. The border-mixin would add a border around an object and has a method computing its width. A new class bordered-button which is both a graphical object and uses the border-mixin would compute its width by calling all applicable width methods - via the + method combination all return values are added and create the combined width of the object.
==Programming languages that use mixins==
Other than Flavors and CLOS (a part of [[Common Lisp]]), some languages that use mixins are:
*[[Ada (programming language)|Ada]] (by extending an existing tagged record with arbitrary operations in a generic)
*[[Ceylon Project|Ceylon]]
*[[C Sharp (programming language)|C#]] (with open source library re-mix <ref>re-mix: mixin library for .NET languages on codeplex: [[re-mix]]</ref>)
*[[ColdFusion]] (Class based using includes and Object based by assigning methods from one object to another at runtime)
*[[Curl (programming language)|Curl(with Curl RTE)]]
*[[D (programming language)|D]] (called [http://www.digitalmars.com/d/template-mixin.html "template mixins"])
*[[Factor programming language|Factor]]{{Citation needed|date=March 2009}}
*[[Fantom (programming language)|Fantom]]
*[[Groovy (programming language)|Groovy]]
*[[Ioke (programming language)|Ioke]]
*[[JavaFX Script]]
*[[JavaScript]]
*[[Magik (programming language)|Magik]]
*[[MATLAB]] (the package "matlab.mixin" is accessible since MATLAB Version 7.12 (R2011a)[http://www.mathworks.com/help/techdoc/rn/bstrgub-1.html#bsvqsns-1].)
*[[Object REXX]]
*[[OpenLaszlo]]
*[[Perl]]<ref>CPAN module for mixins:
[http://search.cpan.org/dist/mixin/lib/mixin.pm mixin].</ref>
*[[Perl 6]]
*[[PHP]]'s "traits"
*[[Python (programming language)|Python]]
*[[Racket_(programming_language)|Racket]] ([http://docs.racket-lang.org/guide/classes.html#(part._.Mixins) mixins documentation])
*[[Ruby (programming language)|Ruby]]
*[[Scala (programming language)|Scala]]
*[[Smalltalk]]
*[[Strongtalk]]
*[[Vala (programming language)|Vala]]
*[[Visual Dataflex]]
*[[XOTcl]]/[http://wiki.tcl.tk/18152 TclOO] (object systems for [[Tcl]])<ref>[http://media.wu-wien.ac.at/doc/tutorial.html#mixins Mixin classes in XOTcl]</ref>
*[[Self (programming language)|Self]]
Some languages like [[ECMAScript]] (commonly referred to as JavaScript) do not support mixins on the language level, but can easily mimic them by copying methods from one object to another at runtime, thereby "borrowing" the mixin's methods. Note that this is also possible with [[static typing|statically typed]] languages, but it requires constructing a new object with the extended set of methods.
==Example==
[[Common Lisp]] provides Mixins in CLOS (Common Lisp Object System) similar to Flavors.
object-width is a generic function with one argument and is using the + method combination. The + method combination determines that all applicable methods for a generic function will be called and the results will be added.
<source lang="lisp">
(defgeneric object-width (object)
(:method-combination +))
</source>
button is a class with one slot for the button text.
<source lang="lisp">
(defclass button ()
((text :initform "click me")))
</source>
There is a method for objects of class button that computes the width based on the length of the button text.
+ is the method qualifier for the method combination of the same name.
<source lang="lisp">
(defmethod object-width + ((object button))
(* 10 (length (slot-value object 'text))))
</source>
A border-mixin class. The naming is just a convention. No superclasses. No slots.
<source lang="lisp">
(defclass border-mixin () ())
</source>
There is a method computing the width of the border. Here it is just 4.
<source lang="lisp">
(defmethod object-width + ((object border-mixin))
4)
</source>
bordered-button is a class inheriting from both the border-mixin and button.
<source lang="lisp">
(defclass bordered-button (border-mixin button) ())
</source>
We can now compute the width of a button. Calling object-width computes 80.
The result is the result of the single applicable method: the method object-width for the class button.
<source lang="lisp">
? (object-width (make-instance 'button))
80
</source>
We can also compute the width of a bordered-button. Calling object-width computes 84.
The result is the sum of the results of the two applicable methods: the method object-width for the class button and the method object-width for the class border-mixin.
<source lang="lisp">
? (object-width (make-instance 'bordered-button))
84
</source>
In the [[Curl (programming language)|Curl web-content language]], multiple inheritance is used as classes with no instances may implement methods. Common mixins include all skinnable ControlUIs inheriting from SkinnableControlUI, user interface delegate objects that require dropdown menus inheriting from StandardBaseDropdownUI and such explicitly named mixin classes as FontGraphicMixin, FontVisualMixin and NumericAxisMixin-of class. Version 7.0 added library access so that mixins do not need to be in the same package or be public abstract. Curl constructors are factories which facilitates using multiple-inheritance without explicit declaration of either interfaces or mixins.{{Citation needed|date=May 2009}}
In [[Python (programming language)|Python]], the <code>SocketServer</code> module has both a <code>UDPServer</code> and <code>TCPServer</code> class that act as a server for [[User Datagram Protocol|UDP]] and [[Transmission Control Protocol|TCP]] socket servers.
Normally, all new connections are handled within the same process.
Additionally, there are two mixin classes: <code>ForkingMixIn</code> and <code>ThreadingMixIn</code>.
By extending <code>TCPServer</code> with the <code>ThreadingMixIn</code> like this
<source lang="python">
class ThreadingTCPServer(ThreadingMixIn, TCPServer):
pass
</source>
the <code>ThreadingMixIn</code> class adds functionality to the TCP server such that each new connection creates a new [[thread (computer science)|thread]].
Alternatively, using the <code>ForkingMixIn</code> would cause the process to be [[fork (operating system)|forked]] for each new connection.
Clearly, the functionality to create a new thread or fork a process is not terribly useful as a stand-alone class.
In this usage example, the mixins provide alternative underlying functionality without affecting the functionality as a socket server.
==Commentary==
Some of the functionality of mixins is provided by [[Interface (object-oriented_programming)|interfaces]] in popular languages like Java and C#. However, an interface only specifies what the class must support and cannot provide an implementation. Another class, providing an implementation and dependent with the interface, is needed for refactoring common behavior into a single place.
Interfaces combined with [[aspect-oriented programming]] can produce full fledged mixins in languages that support such features, such as C# or Java. Additionally, through the use of the [[marker interface pattern]], [[generic programming]], and extension methods, C# 3.0 has the ability to mimic mixins.<ref>[http://www.zorched.net/2008/01/03/implementing-mixins-with-c-extension-methods/ Implementing Mixins with C# Extension Methods]</ref><ref>[http://blogs.msdn.com/abhinaba/archive/2006/01/06/510034.aspx I know the answer (it's 42) : Mixins and C#]</ref><ref>[http://bloggingabout.net/blogs/erwyn/archive/2005/10/20/9925.aspx Mixins, generics and extension methods in C#]</ref>
==See also==
* [[Abstract type]]
* [[Trait (computer science)]]
* [[Policy-based design]]
==References==
<references />
== External links ==
* [http://c2.com/cgi/wiki?MixIn MixIn] at Portland Pattern Repository
* [http://www.macromedia.com/support/documentation/en/flex/1/mixin/index.html Mixins] in [[ActionScript]].
* [http://scala.epfl.ch/intro/mixin.html Scala Overview: Mixin Class Composition] - a step-by-step example in [[Scala programming language|Scala]]
* [http://www.dreamsongs.com/NewFiles/ECOOP.pdf The Common Lisp Object System: An Overview] by [[Richard P. Gabriel]] and Linda DeMichiel provides a good introduction to the motivation for defining classes by means of generic functions.
[[Category:Object-oriented programming languages]]
[[de:Mixin]]
[[es:Mixin]]
[[fr:Mixin]]
[[nl:Mixin]]
[[ja:Mixin]]
[[pl:Domieszka (programowanie obiektowe)]]
[[ru:Примесь (программирование)]]' |