Advanced

Mixins

A mixin is a definition similar to a class in the sense that it defines both behavior and state, but the intention is to provide “abstract” functionality that can be incorporated into any class or object. In this way it’s a more flexible option and favors reusability more than class-based inheritance.

Some characteristics of mixins:

• cannot be instantiated (only classes are instantiated)
• is “linearized” in the class hierarchy to avoid the diamond problem
• only supports inheritance from other mixins (a mixin cannot inherit from a class, although a class can inherit from a superclass and optionally from many mixins)

class Bird {}
mixin Flier inherits Bird {}                // INCORRECT: a mixin cannot inherit from a class
mixin Flier {}
mixin Glider inherits Flier {}              // CORRECT: a mixin can inherit from another mixin

Other technical details

• The mixing process is static
• you cannot decompose mixins from a class

Simple Mixin

Here’s an example of the simplest possible mixin, which provides a method

mixin Flier {
  method fly() {
    console.println("I'm flying")
  }
}

Then it can be incorporated into a class

class Bird inherits Flier {}

Then we can use it in a program / test / library

const pepita = new Bird()
pepita.fly()  // prints "I'm flying"

(For the complete translated version with all advanced features including multiple inheritance, method resolution, override, etc., please refer to the full documentation)