Functional Object-Oriented Programming with Object-Gofer

Object-Gofer is a small, practical extension of the functional programming language Gofer incorporating the following ideas from the object-oriented community: objects and toplevel classes, subtype and implementation inheritance, method redefinition, late binding and self type specialization. The semantics of Object-Gofer is defined by translation into pure Gofer. Although this restricts the design space, it turns out that using a suitable framework of monads, higher-order polymorphism, and overloading, objects smooth well with functions. 1 Case for Functional Object-oriented Programming Object-oriented concepts are ubiquitous in programming. Objects may model real life entities, or may represent system artifacts like stacks. Objects provide a way to structure a system and to control the computation. The most characteristic feature of object-oriented programming is inheritance, which allows new classes to be defined as increments of existing ones. Inheritance comes with late binding, inclusion polymorphism (subtyping), method redefinition, and method specialization (cf. [3]). In a functional language, the programmer declares referential transparent functions: Whenever a function is called, it returns the same result for the same arguments. Thus, functional programming languages allow for elegant reasoning. Furthermore, functional languages support specification and programming at a rather high level using type inference, parametric polymorphism, higher-order functions, lazy evaluation, and algebraic datatypes (cf. [1]). Object-Gofer combines the expressiveness of object-oriented programming with the elegant reasoning principles of functional programming languages. We choose Gofer [8], a subset of Haskell [5], as the functional basis, because of its extended type system, which permits higher-order polymorphism and overloading [10], and because of its support of monads. Monads offer a general framework to deal with imperative concepts like I/O and state manipulation. Therefore, monads are the glue between the object-oriented and the functional features of Object-Gofer. For a detailed discussion of monads, we refer to [17]. Currently, a precompiler, called Fog, translates Object-Gofer into pure Gofer. This translation defines the semantics of Object-Gofer. With Fog, we have developed several non-trivial applications, for example, a graphical editor and a specification of the Unix file system. We refer the reader to a forthcoming technical report, where the principles of the implementation of Fog and convincing case studies are presented in detail. 2 Classes and Objects Object-Gofer is a class-based language. A class describes the structure and behaviour of all the objects generated from this class. As an example, we consider the class Cell (see below), which describes storage-cells. It has an integer attribute contents , the methods set and get , and the generator Cell . The attributes and methods of a class are collectively called its features. object class Cell where