Mixin
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In object-oriented programming languages, a mixin is an approach to implementing classes that differs from the most widely-used approach coming from Simula. Mixins were first used in Flavors, which was an approach to object-orientation used in LispMachine Lisp. The advantage of mixins is that they encourage code reuse and avoid well-known pathologies associated with multiple inheritance, but mixins are implicated in other design pathologies.
Definition and implementation
In Simula the classes are defined in a block in which attributes, methods and class initialisation 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 Flavours and CLOS, 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:
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 and not for refactoring common behavior into a single place.
External links
- http://c2.com/cgi/wiki?MixIn
- Mixins (http://www.macromedia.com/support/documentation/en/flex/1/mixin/index.html) in ActionScript.
- The Common Lisp Object System: An Overview (http://www.dreamsongs.com/NewFiles/ECOOP.pdf) by Richard P. Gabriel and Linda DeMichiel provides a good introduction to the motivation for defining classes by means of generic functions.