First-class rules and generic traversal

Abstract

In this paper we present a functional language supporting first-class rules and generic traversal. This is achieved by generalizing the pattern matching constructs of standard functional languages. The case construct that ties rules together and prevents their reuse, is replaced by separate, firstclass, pattern matching rules and a choice combinator that deals with pattern match failure. Generic traversal is achieved through application pattern matching in which a constructor application is generically divided into a prefix and a suffix, thus giving generic access to the subterms of a constructor term. Many highly generic term traversals can be defined in a type-safe way using this feature. These features support a direct and natural encoding of program transformation rules and strategies. The generalization of pattern matching subsumes several proposals for extensions of pattern matching such as views, guarded patterns, and transformational patterns.