Mixin

This article is about the programming concept. For the ice cream, see Mix-in.

In object-oriented programming languages, a mixin is a class that provides a certain functionality to be inherited by a subclass, while not meant for instantiation (the generating of objects of that class). Inheriting from a mixin is not a form of specialization but is rather a means of collecting functionality. A class may inherit most or all of its functionality from one or more mixins through multiple inheritance.

Mixins first appeared in the Symbolics' object-oriented 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 Parlor in Somerville, Massachusetts: 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.

Mixins encourage code reuse and avoid well-known pathologies associated with multiple inheritance. However, mixins introduce their own set of compromises.

A mixin can also be viewed as an interface with implemented methods. When a class includes a mixin, the class implements the interface and includes, rather than inherits, all the mixin's attributes and methods. They become part of the class during compilation. Interestingly enough, 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 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 and CLOS, and are organized in "generic functions". These generic functions are functions which are defined in multiple cases (methods) by type dispatch a 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 added 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:

• ColdFusion (Class based using includes and Object based by assigning methods from one object to another at runtime)
• Curl web-content language with Sumisho Surge RTE
• D (called "template mixins")
• Factor
• Fan
• JavaFX Script
• JavaScript
• Object REXX
• Perl
• PLT Scheme (mixins documentation)
• Python
• Ruby
• Scala
• Smalltalk
• Strongtalk
• XOTcl (an object system for Tcl)

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 not possible with statically typed languages, where an object's signature is fixed at compile time.

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.

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

button is a class with one slot for the button text.

(defclass button ()
  ((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.

(defmethod object-width + ((object button))
   (* 10 (length (slot-value object 'text))))

A border-mixin class. The naming is just a convention. No superclasses. No slots.

(defclass border-mixin () ())

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

(defmethod object-width + ((object border-mixin))
  4)

bordered-button is a class inheriting from both the border-mixin and button.

(defclass bordered-button (border-mixin button) ())

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.

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

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.

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

In the 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 clases 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.

In Python, the SocketServer module has both a UDPServer and TCPServer class that act as a server for UDP and TCP socket servers. Normally, all new connections are handled within the same process. Additionally, there are two mixin classes: ForkingMixIn and ThreadingMixIn. By extending TCPServer with the ThreadingMixIn like this

class ThreadingTCPServer(ThreadingMixIn, TCPServer):
  pass

the ThreadingMixIn class adds functionality to the TCP server such that each new connection creates a new thread. Alternatively, using the ForkingMixIn would cause the process to be 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 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.

See also

• abstract type
• trait (abstract type)
• Policy-based design

External links

• Wiki entry at Portland Pattern Repository
• Curl RIA platform from MIT, Curl Inc. and Sumisho
• Mix-in Classes chapter from Fundamentals of Object-Oriented Design book by Meilir Page-Jones.
• Mixins in ActionScript.
• Scala Overview: Mixin Class Composition - a step-by-step example in Scala
• 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.