Why not make a language with mixin-only inheritance? [duplicate]

Question

It seems that in all class-based or prototypal OOP languages, mixins are either an afterthought or a secondary feature. However, to me it looks like traditional inheritance is just a specific case of using a single mixin.

I am assuming this is because of historical reasons and familiarity, but maybe there is something else? Problems with sharing encapsulated state safely?

Answer 1

The seminal paper on mixins is Gilad Bracha's PhD thesis The Programming Language Jigsaw: Mixins, Modularity and Multiple Inheritance.

It has answers to all of your questions, like

• Why not make a language with mixin-only inheritance?

  Jigsaw is such a language, as is Newspeak, Bracha's current language

• However, to me it looks like traditional inheritance is just a specific case of using a single mixin.

Indeed, mixin inheritance subsumes all classical forms of single (class-based) inheritance (Simula-/Smalltalk-style inheritance as well as Beta-style prefixing), as well as multiple class-based inheritance with linearization (CLOS, C++, Python, …)

You may want to have a look at Newspeak. Don't be fooled by the fact that it has "classes". They are actually mixins. The Jigsaw paper describes how classes can be elevated to mixins, and that is exactly how it works in Newspeak.

Basically, a mixin is a class that doesn't know its superclass. Therefore, it can appear at multiple places in the inheritance tree. (Note that this is not multiple inheritance! It is, in fact, in some sense exactly the opposite or dual: in multiple inheritance, a class appears at a single place with multiple superclasses, in mixin inheritance, a class appears at multiple places with a single superclass.)

In Newspeak, OO is taken to the extreme: everything is a message send ("virtual method/function call" in Javanese/C++). Even superclass lookup. So, when you declare a class Sub as a subclass of Super, the superclass of Sub is not stored inside some pointer in the internal implementation of Sub, no, whenever you need the superclass (e.g. during message dispatch (virtual method/function lookup/dispatch in Javanese/C++)), the runtime interprets Super as the name of a method to call which returns a class. However, it is not known at runtime which class it returns. In fact, it might return a different class every time! Therefore, classes in Newspeak don't know their superclass, which is the/a definition of a mixin.

Note that in a prototype-based language, the idea of having a dedicated mixin makes no sense. The whole point of a prototype-based language is that there is only one kind of "things" (objects) as opposed to two kinds of "things" (objects and classes, objects and traits, objects and mixins, …). They can be used as a Design Pattern, of course. That's how they were originally invented, after all, in the Flavors object system for Lisp, which had classes with multiple inheritance, but the classes were often used in a peculiar way, which they called mixins (and from which the object system got its name).