Bridging Functional and Object-Oriented Programming

Abstract

Proponents of the functional programming paradigm contend that higher-order functions combined with (parametric) polymorphism result in much more reusable code. Object-oriented (OO) programming has independently followed a parallel approach to reusability. Objects can be used instead of higher-order functions and subtype polymorphism partially substitutes parametric polymorphism. In this paper, we draw strong analogies between the object-oriented and functional programming paradigms. We show that several common OO design patterns (Visitor, Virtual Proxy, Command, Observer, and more) are closely related to functional programming patterns. Additionally, we show how better support for functional programming in OO languages can result in improvements for many design patterns (Command, Virtual Proxy, Builder, Abstract Factory, and more). The context for demonstrating our ideas is the FC++ library. FC++ adds to C++ many of the capabilities of modern functional programming languages, without any modiÞcation to the base language. Compared to other attempts to program functionally in C++, FC++ offers much richer support for polymorphic functions (allowing functions that take polymorphic functions as arguments and/ or return them as results).