Classes and mixins

Abstract

While class-based object-oriented programming languages provide a flexible mechanism for re-using and managing related pieces of code, they typically lack linguistic facilities for specifying a uniform extension of many classes with one set of fields and methods. As a result, programmers are unable to express certain abstractions over classes.In this paper we develop a model of class-to-class functions that we refer to as mixins. A mixin function maps a class to an extended class by adding or overriding fields and methods. Programming with mixins is similar to programming with single inheritance classes, but mixins more directly encourage programming to interfaces.The paper develops these ideas within the context of Java. The results are 1. an intuitive model of an essential Java subset;2. an extension that explains and models mie and 3. type soundness theorems for these languages. Organizing Programs with Functions and ClassesObject-oriented programming languages offer classes, inheritance, and overriding to parameterize over program pieces for management purposes and re-use. Functional programming languages provide various flavors of functional abstractions for the same purpose. The latter model was developed from a well-known, highly developed mathematical theory. The former grew in response to the need to manage large programs and to re-use as many components as possible.Each form of parameterization is useful for certain situations. With higher-order functions, a programmer can easily define many functions that share a similar core but differ in a few details.As many language designers and programmers readily acknowledge, however, the functional approach to parameterization is best used in situations with a relatively small number of parameters. When a function must *Pmi=ion to mnke digihlhrd copies of all or part ofthis mateGal for PemolIal or dilssroonl use is granted without fee provided Ihal tile apie are not made or distributed for profit or commercial advmhge, the copyG&t SOli% ihe title of the publication and its date appear, ;md notice is given ihat copyright is by permission of the ACM, Inc. To copy &en&e, 10 republish, 10 post on servem or to redistribute to I&, requires specific pemkion .mdlor fee. POPL 98 San Diego CA USA CW+ht 1998 ACM O-89791-979-3/98/01..$3.50 171 consume a large number of arguments, the approach quickly beconies unwieldy, especially if many of the arguments are the same for most of the function's u~e.s.~ Class systems provide a simple and flexible mechanism for managing collections of highly parameterized program pieces. Using class extension {inheritance) and overriding, a programmer derives a new class by specifying only the elements that change in the derived class.Nevertheless, a pure class-based approach suffers from a lack of abstractions that specify uniform extensions and modifications of classes. For example, the construction of a programming environment may require many kinds of text editor frames, including frames that can contain multiple text buffers and frames t...

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