Written by Gilad Bracha
December 2012 (updated June 2018)

This article provides a brief theoretical presentation of the evolution of mixins in Dart.

Mixin support by Dart release

Support for mixins changed in Dart 1.13 and Dart 2.1:

• Dart 1.12 or lower supports mixins that must extend Object, and must not call super().
• Dart 1.13 or greater supports mixins that can extend from classes other than Object, and can call super.method(). This support is only available by default in the Dart VM and in Analyzer behind a flag. More specifically, it is behind the --supermixin flag in the command-line analyzer. It is also available in the analysis server, behind a client-configurable option. Dart2js and dartdevc do not support super mixins.
• In Dart 2.1, mixins are expected to have fewer restrictions. For example, Flutter supports mixins calling super() and extending from a class other than Object, but the syntax is expected to change before appearing in all Dart SDKs. For details, see the mixin specification.

Basic concepts

If you are familiar with the academic literature on mixins you can probably skip this section. Otherwise, please do read it, as it defines important concepts and notation. Those wishing to delve deeply into the topic can start with this paper: Mixins in Strongtalk.

In a language supporting classes and inheritance, a class implicitly defines a mixin. The mixin is usually implicit—it is defined by the class body, and constitutes the delta between the class and its superclass. The class is in fact a mixin application—the result of applying its implicitly defined mixin to its superclass.

The term mixin application comes from a close analogy with function application. Mathematically, a mixin M can be seen as a function from superclass to subclass: feed M a superclass S, and a new subclass of S is returned. This is often written as M |> S in the research literature.

Based on the notion of function application, one can define function composition. The concept carries through to mixin composition; we define the composition of mixins M₁ and M₂, written M₁ * M₂, as: (M₁ * M₂) |> S = M₁ |> (M₂ |> S).

Functions are useful because they can be applied to different arguments. Likewise mixins. The mixin implicitly defined by a class is usually applied only once, to the superclass given in the class declaration. To allow mixins to be applied to different superclasses, we need to be able to either declare mixins independently of any particular superclass, or alternately, to extricate the implicit mixin of a class and reuse it outside its original declaration. That is what we propose to do below.

Syntax and semantics

Mixins are implicitly defined via ordinary class declarations. In principle, every class defines a mixin that can be extracted from it. However, in this proposal, a mixin may only be extracted from a class that has no declared constructors. This restriction avoids complications that arise due to the need to pass constructor parameters up the inheritance chain.

Example 1:

abstract class Collection<E> {
  Collection<E> newInstance();
  Collection<E> map((f) {
    var result = newInstance();
    forEach((E e) { result.add(f(e)); });
    return result;
  });
  void forEach(void f(E element)) {
    // ...
  }
  void add(E element) {
    // ...
  }
}

abstract class DOMElementList<E> = DOMList with Collection<E>;

abstract class DOMElementSet<E> = DOMSet with Collection<E>;

// ... 28 more variants

Here, Collection<E> is a normal class that is used to declare a mixin. Both the classes DOMElementList and DOMElementSet are mixin applications. They are defined by a special form of class declaration that gives them a name and declares them equal to an application of a mixin to a superclass, given via a with clause. The class is abstract because it does not implement the abstract method newInstance() declared in Collection.

In the above, DOMElementList is effectively Collection mixin |> DOMList, while DOMElementSet is Collection mixin |> DOMSet.

The benefit here is that the code in class Collection can be shared in multiple class hierarchies. We list two such hierarchies above—one rooted in DOMList and one rooted in DOMSet. One need not repeat/copy the code in Collection, and every change made to Collection will propagate to both hierarchies, greatly easing maintenance of the code. This particular example is loosely based on a real and very acute case in the Dart libraries.

The above examples illustrate one form of mixin application, where the mixin application specifies a mixin and a superclass to which it applies, and provides the application with a name.

In an alternative form, mixin applications appear in the with clause of a class declaration as a comma-separated list of identifiers. All the identifiers must denote classes. In this form, multiple mixins are composed and applied to the superclass named in the extends clause, producing an anonymous superclass. Taking the same examples again, we would have:

class DOMElementList<E> extends DOMList with Collection<E> {
   DOMElementList<E> newInstance() => new DOMElementList<E>();
}

class DOMElementSet<E> extends DOMSet with Collection<E> {
  DOMElementSet<E> newInstance() => new DOMElementSet<E>();
}

Here, DOMElementList is not the application Collection mixin |> DOMList. Instead, it is a new class whose superclass is such an application. The situation with respect to DOMElementSet is analogous. Note that in each case, the abstract method newInstance() is overridden with an implementation, so these classes can be instantiated directly.

Consider what happens if DOMList has a non-trivial constructor:

class DOMElementList<E> extends DOMList with Collection<E> {
  DOMElementList<E> newInstance() => new DOMElementList<E>(0);
  DOMElementList(size): super(size);
}

Each mixin has its own constructor called independently, and so does the superclass. Since a mixin constructor cannot be declared, the call to it can be elided in the syntax; in the underlying implementation, the call can always be placed at the start of the initialization list.

The constructor would set the values for any fields and for the generic type parameters.

This rule ensures that these examples run smoothly and also generalize cleanly once one lifts the restriction on constructors.

The second form is a convenient sugar that allows multiple mixins to be mixed into a class without the need to introduce multiple intermediate declarations. For example:

class Person {
  String name;
  Person(this.name);
}

class Maestro extends Person with Musical, Aggressive, Demented {
  Maestro(name):super(name);
}

Here, the superclass is the mixin application:

Demented mixin |> Aggressive mixin |> Musical mixin |> Person

We assume that only Person has a constructor with arguments. Hence Musical mixin |> Person inherits Person’s constructors, and so on until the actual superclass of Maestro, which is formed by a series of mixin applications.

In reality in this example we’d expect that Demented, Aggressive, and Musical actually have interesting properties that are likely to require state.

Details

We now discuss a few issues in more detail:

• Privacy
• Statics
• Types

Privacy

A mixin application may well be declared outside the library that declared the original class. This does not have any effect on who can access members of a mixin application instance. Access to members is determined based on the library where they were originally declared, exactly as with ordinary inheritance. This follows from the semantics of mixin application, which are determined by the semantics of inheritance in the underlying language.

Statics

Can one use the statics of the original class via the mixin application or not?

Again, the answer (No) follows from the semantics of inheritance. Statics are not inherited in Dart.

Types

What is the type of a mixin application instance? In general, it is a subtype of its superclass, and also a subtype of the type denoted by the mixin name itself, that is, the type of the original class.

The original class has its own superclass. To ensure that a particular mixin application is compatible with the original class being mixed in, Dart places extra requirements on classes that use with clauses.

If a class A is defined using a with clause that applies a mixin M where M was derived from a class K, then A must support the direct superinterfaces of K.

class S {
  twice(int x) => 2 * x;
}

abstract class I {
   twice(x);
}

abstract class J {
   thrice(x);
}
class K extends S implements I, J {
  int thrice(x) => 3* x;
}

class B {
  twice(x) => x + x;
}
class A = B with K;

In particular, A must support the implicit interface of the superclass S of K. This ensures that A is indeed a subtype of M, even though its superclass chain is different. In our example above, K needs to implement twice() to meet the requirements of I and must also implement thrice() in order to satisfy the requirements imposed by J. K meets these requirements because it defines thrice() directly, and inherits an implementation of twice() from S.

Now when we define A, we get the implementation of thrice() from K’s mixin. However, the mixin won’t provide us with an implementation of twice(). Fortunately, B does have such an implementation, so overall A does satisfy the requirements of I, J as well as S.

In contrast, given class D:

class D {
   double(x) => x+x;
}

class E = D with K;

we will get a warning, because class E does not have a twice method, and so does not conform to either I or S and so cannot be used where a K is expected.

Generics

If a class has type parameters, its mixin necessarily has identical type parameters.