Contract System Metatheories a la Carte: A Transition-System View of Contracts

Over the last 15 years, researchers have studied a wide variety of important aspects of contract systems, ranging from internal consistency (complete monitoring and correct blame) to subtle details about the semantics of contracts combinators (dependency) to the difficulty of efficient checking (avoiding asymptotically bad redundant checking). Although each paper offers essential insights about contract systems, they also differ in inessential ways, making it hard to know how their metatheories combine. Even worse, the metatheories share tremendous tedium in their definitions and proofs, occupying researchers' time with no benefit. In this paper, we present the idea that higher-order contract systems can be viewed as transition systems and show that this perspective offers an important opportunity for reuse in their metatheories. We demonstrate the value of this perspective by proving representative properties from the literature, and by contributing a new proof establishing that elimination of redundant contract checks can eliminate asymptotic slowdowns. To confirm our claims and encourage the adoption of our ideas, we provide a mechanized development in Agda.