Mixin

In object-oriented programming languages, a mixin is a class that provides a certain functionality to be inherited by a subclass but is not ment to stand alone. Inheriting from a mixin is not a form of specialisation but is rather a means to collect functionality. A subclass may even choose to inherit most or all of it's functionality by inheriting from one or more mixins through multiple inheritance.

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, to define all attributes, methods and initialization at compile time.

Mixins were first used in Flavors, which was an approach to object-orientation used in Lisp Machine Lisp. The advantage of mixins is that they encourage code reuse and avoid well-known pathologies associated with multiple inheritance, but mixins introduce their own set of compromises.

Definition and implementation

In Simula, classes are defined in a block in which attributes, methods and class initialization are all defined together, and so 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 called "generic functions". These generic functions are functions which are defined in multiple cases by type dispatch.

Other than Flavors and CLOS, some languages that use mixins are:

• D
• Dylan
• Python
• Ruby
• Scala
• XOTcl (an object system for Tcl)

Commentary

Some of the functionality of mixins is provided by interfaces in popular languages like Java and C#, but since an interface only specifies what the class must support and cannot provide an implementation, they are only useful for providing polymorphism. Another class, providing an implementation and dependent with the interface, is useful 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.

See also

• abstract type
• trait (abstract type)

External links

• Wiki entry at Cunningham & Cunningham, Inc.
• 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.