Instantaneous Transitions in Esterel

Esterel is an imperative synchronous programming language for the specification of deterministic concurrent reactive systems. While providing the usual control-flow constructs—sequences, loops, conditionals, and exceptions—its lack of a goto instruction makes the programming of arbitrary finite state machines awkward and hinders the design of source-to-source program transformations. We previously introduced to Esterel a non-instantaneous gotopause instruction, which prevents the synchronous execution of code before and code after the transition. Here, we tackle instantaneous transitions. Concurrency demands we assign scopes and priorities to gotos, so we extend Esterel’s exception handling mechanism to allow exception handlers in arbitrary locations. We advocate for and formalize the resulting language. We observe that instantaneous gotos complement but do not replace non-instantaneous gotopauses.