Mixin

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,[6] Gilad Bracha and William Cook reinterpret different inheritance mechanisms found in Smalltalk, Beta and CLOS as special forms of a mixin inheritance.

Java 8 introduces a new feature in the form of default methods for interfaces.[14] 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 adds a possibility of applying the concept in a mixin sort of a way.

Interfaces combined with aspect-oriented programming can also 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. With C# 3.0 came the introduction of Extension Methods[2] and they 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.[15][16][17]

ECMAScript (in most cases implemented as JavaScript) does not need to mimic object composition by stepwise copying fields from one object to another. It natively[18] supports Trait and Mixin[19][20] based object composition via function objects that implement additional behavior and then are delegated via call or apply to objects that are in need of such new functionality.

trait Singer{
  def sing { println(" singing … ") }
  //more methods
}

class Birds extends Singer

class Person
class Actor extends Person with Singer
class Actor extends Singer with Performer

class Person{
  def tell {  println (" Human ") }
  //more methods
}

val singingPerson = new Person with Singer
singingPerson.sing

protocol ErrorDisplayable {
    func error(message:String)
}

extension ErrorDisplayable {
    func error(message:String) {
        // Do what it needs to show an error
        //...
        print(message)
    }
}

struct NetworkManager : ErrorDisplayable{
    func onError() {
        error("Please check your internet Connection.")
    }
}