Mixin

This article is about the programming concept. For the ice cream, seeMix-in.For the company, seeMixin Network.

Inobject-oriented programming languages,amixin(ormix-in)is aclassthat contains methods for use by other classes without having to be the parent class of those other classes. How those other classes gain access to the mixin's methods depends on the language. Mixins are sometimes described as being "included" rather than "inherited".

Mixins encouragecode reuseand can be used to avoid the inheritance ambiguity that multiple inheritance can cause(the "diamond problem"), or to work around lack of support for multiple inheritance in a language. A mixin can also be viewed as aninterfacewith implementedmethods.This pattern is an example of enforcing thedependency inversion principle.

History

Mixins first appeared inSymbolics's object-orientedFlavorssystem (developed by Howard Cannon), which was an approach to object-orientation used inLisp Machine Lisp.The name was inspired bySteve's Ice Cream Parlorin Somerville, Massachusetts:The owner of the ice cream shop 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.

Definition

Mixins are a language concept that allows a programmer to inject some code into aclass.Mixin programming is a style ofsoftware development,in which units of functionality are created in a class and then mixed in with other classes.

A mixin class acts as the parent class, containing the desired functionality. Asubclasscan then inherit or simply reuse this functionality, but not as a means of specialization. Typically, the mixin will export the desired functionality to achild class,without creating a rigid, single "is a" relationship. Here lies the important difference between the concepts of mixins andinheritance,in that the child class can still inherit all the features of the parent class, but, the semantics about the child "being a kind of" the parent need not be necessarily applied.

Advantages

1. It provides a mechanism formultiple inheritanceby allowing one class to use common functionality from multiple classes, but without the complex semantics of multiple inheritance.
2. Code reusability:Mixins are useful when a programmer wants to share functionality between different classes. Instead of repeating the same code over and over again, the common functionality can simply be grouped into a mixin and then included into each class that requires it.
3. Mixins allow inheritance and use of only the desired features from the parent class, not necessarily all of the features from the parent class.

Implementations

InSimula,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.

InFlavors,a mixin is a class from which another class can inherit slot definitions and methods. The mixin usually does not have direct instances. Since a Flavor can inherit from more than one other Flavor, it can inherit from one or more mixins. Note that the original Flavors did not use generic functions.

In New Flavors (a successor of Flavors) andCLOS,methods are organized in "generic functions".These generic functions are functions that are defined in multiple cases (methods) by class dispatch and method combinations.

CLOS and Flavors allow mixin methods to add behavior to existing methods::beforeand:afterdaemons, whoppers and wrappers in Flavors. CLOS added:aroundmethods and the ability to call shadowed methods viaCALL-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:aroundmethod. Both CLOS and Flavors allow the computed reuse via method combinations.:before,:afterand:aroundmethods are a feature of the standard method combination. Other method combinations are provided.

An example is the+method combination, where the resulting values of each of the applicable methods of a generic function are arithmetically added to compute the return value. This is used, for example, with the border-mixin for graphical objects. A graphical object may have a generic width function. The border-mixin would add a border around an object and has a method computing its width. A new classbordered-button(that is both a graphical object and uses thebordermixin) 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.

In an OOPSLA 90 paper,Gilad Bracha and William Cook reinterpret different inheritance mechanisms found in Smalltalk, Beta and CLOS as special forms of a mixin inheritance.

Programming languages that use mixins

Other than Flavors and CLOS (a part ofCommon Lisp), some languages that use mixins are:

• Ada(by extending an existing tagged record with arbitrary operations in a generic)
• C#(since C# 8.0, by means ofdefault methodsof interfaces)
• Cobra
• ColdFusion(Class based using includes and Object based by assigning methods from one object to another at runtime)
• Curl(with Curl RTE)
• D(called"template mixins";D also includes a"mixin"statement that compiles strings as code.)
• Dart
• Factor
• Groovy
• Go(by struct embedding)
• Java(since Java 8, by means ofdefault methodsof interfaces)
• JavaScript Delegation - Functions as Roles (Traits and Mixins)
• Kotlin
• Less
• Magik
• MATLAB
• OCaml
• Perl(throughrolesin the Moose extension of the Perl 5 object system)
• PHP's "traits"
• Python
• Racket(mixins documentation)
• Raku
• Ruby
• Rust
• Sass (A stylesheet language)
• Scala
• Smalltalk
• Swift
• SystemVerilog
• XOTcl/TclOO(object systems builtin toTcl)
• TypeScript(mixins documentation)
• Vala

Some languages 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. This is also possible withstatically typedlanguages, but it requires constructing a new object with the extended set of methods.

Other languages that do not support mixins can support them in a round-about way via other language constructs. For example,Visual Basic.NETand C# support the addition of extension methods on interfaces, meaning any class implementing an interface with extension methods defined will have the extension methods available as pseudo-members.

Examples

In Common Lisp

Common Lispprovides mixins in CLOS (Common Lisp Object System) similar to Flavors.

object-widthis a generic function with one argument that uses the+method combination. This combination determines that all applicable methods for a generic function will be called and the results will be added.

(defgenericobject-width(object)
(:method-combination+))

buttonis a class with one slot for the button text.

(defclassbutton()
((text:initform"click me")))

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.

(defmethodobject-width+((objectbutton))
(*10(length(slot-valueobject'text))))

Aborder-mixinclass. The naming is just a convention. There are no superclasses, and no slots.

(defclassborder-mixin()())

There is a method computing the width of the border. Here it is just 4.

(defmethodobject-width+((objectborder-mixin))
4)

bordered-buttonis a class inheriting from bothborder-mixinandbutton.

(defclassbordered-button(border-mixinbutton)())

We can now compute the width of a button. Callingobject-widthcomputes 80. The result is the result of the single applicable method: the methodobject-widthfor the classbutton.

?(object-width(make-instance'button))
80

We can also compute the width of abordered-button.Callingobject-widthcomputes 84. The result is the sum of the results of the two applicable methods: the methodobject-widthfor the classbuttonand the methodobject-widthfor the classborder-mixin.

?(object-width(make-instance'bordered-button))
84

In Python

InPython,an example of the mixin concept is found in theSocketServermodule,which has both aUDPServerclass and aTCPServerclass. They act as servers forUDPandTCPsocket servers, respectively. Additionally, there are two mixin classes:ForkingMixInandThreadingMixIn.Normally, all new connections are handled within the same process. By extendingTCPServerwith theThreadingMixInas follows:

classThreadingTCPServer(ThreadingMixIn,TCPServer):
pass

theThreadingMixInclass adds functionality to the TCP server such that each new connection creates a newthread.Using the same method, aThreadingUDPServercan be created without having to duplicate the code inThreadingMixIn.Alternatively, using theForkingMixInwould cause the process to beforkedfor 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.

In Ruby

Most of the Ruby world is based around mixins viaModules.The concept of mixins is implemented in Ruby by the keywordincludeto which we pass the name of the module asparameter.

Example:

classStudent
includeComparable# The class Student inherits the Comparable module using the 'include' keyword
attr_accessor:name,:score

definitialize(name,score)
@name=name
@score=score
end

# Including the Comparable module requires the implementing class to define the <=> comparison operator
# Here's the comparison operator. We compare 2 student instances based on their scores.

def<=>(other)
@score<=>other.score
end

# Here's the good bit - I get access to <, <=, >,>= and other methods of the Comparable Interface for free.
end

s1=Student.new("Peter",100)
s2=Student.new("Jason",90)

s1>s2#true
s1<=s2#false

In JavaScript

TheObject-Literal andextendApproach

It is technically possible to add behavior to an object by binding functions to keys in the object. However, this lack of separation between state and behavior has drawbacks:

1. It intermingles properties of the model domain with that of implementation domain.
2. No sharing of common behavior. Metaobjects solve this problem by separating the domain specific properties of objects from their behaviour specific properties.

An extend function is used to mix the behavior in:

'use strict';

constHalfling=function(fName,lName){
this.firstName=fName;
this.lastName=lName;
};

constmixin={
fullName(){
returnthis.firstName+' '+this.lastName;
},
rename(first,last){
this.firstName=first;
this.lastName=last;
returnthis;
}
};

// An extend function
constextend=(obj,mixin)=>{
Object.keys(mixin).forEach(key=>obj[key]=mixin[key]);
returnobj;
};

constsam=newHalfling('Sam','Loawry');
constfrodo=newHalfling('Freeda','Baggs');

// Mixin the other methods
extend(Halfling.prototype,mixin);

console.log(sam.fullName());// Sam Loawry
console.log(frodo.fullName());// Freeda Baggs

sam.rename('Samwise','Gamgee');
frodo.rename('Frodo','Baggins');

console.log(sam.fullName());// Samwise Gamgee
console.log(frodo.fullName());// Frodo Baggins

Mixin with using Object.assign()

'use strict';

// Creating an object
constobj1={
name:'Marcus Aurelius',
city:'Rome',
born:'121-04-26'
};

// Mixin 1
constmix1={
toString(){
return`${this.name}was born in${this.city}in${this.born}`;
},
age(){
constyear=newDate().getFullYear();
constborn=newDate(this.born).getFullYear();
returnyear-born;
}
};
// Mixin 2
constmix2={
toString(){
return`${this.name}-${this.city}-${this.born}`;
}
};

// Adding the methods from mixins to the object using Object.assign()
Object.assign(obj1,mix1,mix2);

console.log(obj1.toString());// Marcus Aurelius - Rome - 121-04-26
console.log(`His age is${obj1.age()}as of today`);// His age is 1897 as of today

The pure function and delegation basedFlight-Mixin Approach

Even though the firstly described approach is mostly widespread the next one is closer to what JavaScript's language core fundamentally offers -Delegation.

Two function object based patterns already do the trick without the need of a third party's implementation ofextend.

'use strict';

// Implementation
constEnumerableFirstLast=(function(){// function based module pattern.
constfirst=function(){
returnthis[0];
},
last=function(){
returnthis[this.length-1];
};
returnfunction(){// function based Flight-Mixin mechanics...
this.first=first;//... referring to...
this.last=last;//... shared code.
};
}());

// Application - explicit delegation:
// applying [first] and [last] enumerable behavior onto [Array]'s [prototype].
EnumerableFirstLast.call(Array.prototype);

// Now you can do:
consta=[1,2,3];
a.first();// 1
a.last();// 3

In other languages

In theCurlweb-content language, multiple inheritance is used as classes with no instances may implement methods. Common mixins include all skinnableControlUIs inheriting fromSkinnableControlUI,user interface delegate objects that require dropdown menus inheriting from StandardBaseDropdownUI and such explicitly named mixin classes asFontGraphicMixin,FontVisualMixinandNumericAxisMixin-ofclass. 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 that facilitates using multiple-inheritance without explicit declaration of either interfaces or mixins.

Interfaces and traits

Java 8 introduces a new feature in the form of default methods for interfaces.Basically it allows a method to be defined in an interface with application in the scenario when a new method is to be added to an interface after the interface class programming setup is done. To add a new function to the interface means to implement the method at every class which uses the interface. Default methods help in this case where they can be introduced to an interface any time and have an implemented structure which is then used by the associated classes. Hence default methods add the ability to applying the mixin concept in Java.

Interfaces combined withaspect-oriented programmingcan also produce full-fledged mixins in languages that support such features, such as C# or Java. Additionally, through the use of themarker interface pattern,generic programming,and extension methods, C# 3.0 has the ability to mimic mixins. With Dart 2.7 and C# 3.0 came the introduction of extension methods which can be applied, not only to classes, but also to interfaces. Extension Methods provide additional functionality on an existing class without modifying the class. It then becomes possible to create a static helper class for specific functionality that defines the extension methods. Because the classes implement the interface (even if the actual interface doesn’t contain any methods or properties to implement) it will pick up all the extension methods also.C# 8.0 adds the feature of default interface methods.

ECMAScript(in most cases implemented as JavaScript) does not need to mimic object composition by step-wise copying fields from one object to another. It nativelysupportsTraitand mixinbased object composition via function objects that implement additional behavior and then are delegated viacallorapplyto objects that are in need of such new functionality.

In Scala

Scala has a rich type system and Traits are a part of it which helps implement mixin behaviour. As their name reveals, Traits are usually used to represent a distinct feature or aspect that is normally orthogonal to the responsibility of a concrete type or at least of a certain instance. For example, the ability to sing is modeled as such an orthogonal feature: it could be applied to Birds, Persons, etc.

traitSinger{
defsing{println("singing…")}
//more methods
}

classBirdextendsSinger

Here, Bird has mixed in all methods of the trait into its own definition as if class Bird had defined method sing() on its own.

Asextendsis also used to inherit from a super class, in case of a traitextendsis used if no super class is inherited and only for mixin in the first trait. All following traits are mixed in using keywordwith.

classPerson
classActorextendsPersonwithSinger
classActorextendsSingerwithPerformer

Scala allows mi xing in a trait (creating ananonymous type) when creating a new instance of a class. In the case of a Person class instance, not all instances can sing. This feature comes use then:

classPerson{
deftell{println("Human")}
//more methods
}

valsingingPerson=newPersonwithSinger
singingPerson.sing

In Rust

Rust makes extensive use of mixins viatraits.Traits, like in Scala, allow users to implement behaviours for a defined type. They are also used forgenericsanddynamic dispatch,allowing types implementing a trait to be used interchangeably statically or dynamically at runtime.

// Allows for types to "speak"
traitSpeak{
fnspeak();

// Rust allows implementors to define default implementations for functions defined in traits
fngreet(){
println!("Hi!")
}
}

structDog;

implSpeakforDog{
fnspeak(){
println!("Woof woof");
}
}

structRobot;

implSpeakforRobot{
fnspeak(){
println!("Beep beep boop boop");
}

// Here we override the definition of Speak::greet for Robot
fngreet(){
println!("Robot says howdy!")
}
}

In Swift

Mixin can be achieved in Swift by using a language feature called Default implementation in Protocol Extension.

protocolErrorDisplayable{
funcerror(message:String)
}

extensionErrorDisplayable{
funcerror(message:String){
// Do what it needs to show an error
//...
print(message)
}
}

structNetworkManager:ErrorDisplayable{
funconError(){
error("Please check your internet Connection.")
}
}

See also

• Abstract type
• Decorator pattern
• Policy-based design
• Trait,a similar structure that does not require linear composition

External links

• MixInat Portland Pattern Repository
• MixinsinActionScript
• The Common Lisp Object System: An OverviewbyRichard P. Gabrieland Linda DeMichiel provides a good introduction to the motivation for defining classes by means of generic functions.