A first-class approach to genericity

Abstract

This paper describes how to add first-class generic typesincluding mixins-to strongly-typed OO languages with nominal subtyping such as Java and C#. A generic type system is "first-class" if generic types can appear in any context where conventional types can appear. In this context, a mixin is simply a generic class that extends one of its type parameters, e.g., a class C<T> that extends T. Although mixins of this form are widely used in C++ (via templates), they are clumsy and error-prone because C++ treats mixins as macros, forcing each mixin instantiation to be separately compiled and type-checked. The abstraction embodied in a mixin is never separately analyzed.Our formulation of mixins using first-class genericity accommodates sound local (class-by-class) type checking. A mixin can be fully type-checked given symbol tables for each of the classes that it directly references-the same context in which Java performs incremental class compilation. To our knowledge, no previous formal analysis of first-class genericity in languages with nominal type systems has been conducted, which is surprising because nominal subtyping has become predominant in mainstream object-oriented programming languages.What makes our treatment of first-class genericity particularly interesting and important is the fact that it can be added to the existing Java language without any change to the underlying Java Virtual Machine. Moreover, the extension is backward compatible with legacy Java source and class files. Although our discussion of a practical implementation strategy focuses on Java, the same implementation techniques could be applied to other object-oriented languages such as C# or Eiffel that support incremental compilation, dynamic class loading, and nominal subtyping. *

Magda: A New Language for Modularity

“…In the case of methods with the same name occurring in different mixins (like the aforementioned SetBufferSize), mixins permit explicit direct control of the order in which those methods will override each other, however, they might not allow the resulting class to have all variants. Nevertheless, there are two proposals offering this feature: MixGen [3] and MixedJava [24]. In the MixGen language, the class obtained from applying both CompressedStream and BufferedStream mixins will keep both variants, while giving access to one of them at each moment, depending on the static type of the variable referring to the object.…”

“…Notably, there are languages offering constructs and mechanisms to deal with conflicts: Delphi [2], C [28], Eiffel [41], MixedJava [24], MixGen [3] 5 , McJava [32,31] are some examples. An analysis of the main features of these proposals can be found in [37].…”

“…In C++, for example, in the case of multiple inheritance it is necessary to know if there are common superclassess, in oder to choose one of the semantics of inheritance (private, public or virtual). In the cases of MixGen [3] and MixedJava [24], a class can have many distinct implementations of one method. Then, to call a method declared in a specific superclass or mixin, one has to cast the type of an object to that specific type.…”

“…A mixin is a class parameterized over a superclass, introduced to model some forms of multiple inheritance and improve code modularization and reusability [43,17,15,8,24,14,7,3,45,16]. There are usually two operations defined on mixins: (i ) application, by which a mixin is applied to a class to obtain a fully-fledged subclass (the class argument plays the role of the parent); (ii ) composition, which makes a more specialized mixin by composing two existing ones.…”

Shakeins: Nonintrusive Aspects for Middleware Frameworks

“…They allow programmers to carry out such a change without rewriting it in each inheriting class. In languages such as MODULA-π [16] the repeating change to the base class can be captured in a mixin M , which, given a class c, generates a class M c such that M c inherits from c. In languages with first-class genericity, mixins can be emulated by writing e.g., template<typename c> class M: public c { ... } in C++, or class M<c> extends c in NEXTGEN [17].…”