Adding Pattern Matching to Existing Object-Oriented Languages

While object-oriented languages are designed for information hiding and data encapsulation, intensive data manipulation often calls for pattern matching, one of the main features of functional programming languages. Pattern matching provides a concise way to describe specific structures or conditions of objects so that programmers can clearly identify and easily access the corresponding objects. The need for both object information hiding and pattern matching over objects, which are seemingly conflicting features, shows up very well in the problem of language manipulation. In the compiler development of Fortress, a new object-oriented language with extensive support for functional programming, we originally implemented the Fortress type checker in Java. However, because Java does not provide pattern matching, we reimplemented the type checker in Scala. Compared to the pattern matching mechanism in Scala, Java’s visitor patterns are verbose, make it hard to capture the high-level specification of objects, and are nontrivial to maintain. In this paper, we introduce a new pattern-matching mechanism which can be added to existing object-oriented languages. We present the mechanism currently being implemented as an addition to the Fortress programming language. The mechanism provides a concise way of describing and using patterns in the presence of runtime manipulation of types using typecase and multimethods, unlike Scala which has type-erasure semantics. Even though it is premature to discuss the performance and scalability of the presented mechanism, we believe that the same optimization techniques used in Scala will apply to our mechanism.