Mixin

Key takeaways

Mixin belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Mixin to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Mixin from memory before moving on to harder problems.

Reference excerpt

In object-oriented programming languages, a mixin (or mix-in) is a class that 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 encourage code reuse and 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 an interface with implemented methods. This pattern is an example of enforcing the dependency inversion principle.

History Mixins first appeared in Symbolics's 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 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 a class. Mixin programming is a style of software 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. A subclass can then inherit or simply reuse this functionality, but not as a means of specialization. Typically, the mixin will export the desired functionality to a child class, without creating a rigid, single "is a" relationship. Here lies the important difference between the concepts of mixins and inheritance, 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 It provides a mechanism for multiple inheritance by allowing one class to use common functionality from multiple classes, but without the complex semantics of multiple inheritance. 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. 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 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. In Flavors, 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) and CLOS, 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: :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 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 class bordered-button (that 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. 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 of Common Lisp), some languages that use mixins are:

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Mixin

Start with the simplest possible case. Write down what Mixin claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Mixin before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Mixin ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Mixin

In research: Mixin appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Mixin in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Mixin is common in secondary-school and first-year university syllabi. It links to neighbouring topics Object-oriented programming, Programming language concepts, Software design patterns, so understanding it makes those chapters shorter.
In everyday life: Look for Mixin outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Mixin in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Mixin means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Mixin out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Mixin in simple terms?

In object-oriented programming languages, a mixin (or mix-in) is a class that 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.

Why does Mixin matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Mixin?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Mixin.