The design of a class mechanism for Moby

Abstract

Typical class-based languages, such as C++ and JAVA, provide complex class mechanisms but only weak module systems. In fact, classes in these languages incorporate many of the features found in richer module mechanisms. In this paper, we describe an alternative approach to designing a language that has both classes and modules. In our design, we rely on a rich ML-style module system to provide features such as visibility control and parameterization, while providing a minimal class mechanism that includes only those features needed to support inheritance. Programmers can then use the combination of modules and classes to implement the full range of class-based features and idioms. Our approach has the advantage that it provides a full-featured module system (useful in its own right), while keeping the class mechanism quite simple.We have incorporated this design in MOBY, which is an ML-style language that supports class-based object-oriented programming. In this paper, we describe our design via a series of simple examples, show how various class-based features and idioms are realized in MOBY, compare our design with others, and sketch its formal semantics.

Modular typechecking for hierarchically extensible datatypes and functions

“…Each class declares a record type of its instance variables, using the of clause. Superclass instance variables are inherited: the representation type of a class C is the representation type (recursively) of its direct superclass (if any) Each class declaration also implicitly declares a constructor, similar to constructor declarations in OCaml [Rémy and Vouillon 1998] and XMOC [Fisher and Reppy 2000], a core language for Moby [Fisher and Reppy 1999]. For example, the CListSet constructor expects arguments es of type int list and c of type int, initializes inherited instance variables via the call ListSet(es) to the superclass constructor, and initializes the new count instance variable to c. In general, the arguments to the superclass constructor call and the instance-variable initializers may be arbitrary expressions.…”

Inheritance-Based Subtyping

“…MOBY is a language that combines an ML-style module system with classes and objects [FR99a]. Object types are related by structural subtyping, which is extended to other types in the standard way.…”

“…From the programmer's perspective, a more natural approach is to make the classes themselves serve the rôle of types when this connection is needed. In this section and the next, we describe an extension to MOBY [FR99a] that has class types and inheritancebased subtyping. We describe two alternative approaches to typing this extension.…”

“…In this paper, we explore how inheritance-based subtyping relations are useful by examining the common object-oriented idiom of friend functions. Through a series of examples in Section 2, we investigate how one might implement this idiom in MOBY [FR99a], a language with only structural subtyping. We selected friend functions for our case study because they are the canonical example of a class permitting outside code to access its internal representation, which is the forté of inheritance-based subtyping.…”

“…This appendix provides a brief introduction to some of MOBY's features to help the reader understand the examples in the paper. A more detailed description of MOBY object-oriented features can be found in [FR99a]. This code defines a module M that is constrained by a signature with two specifications: the class Hi and the value hi.…”