Smoothly Navigating between Functional Reactive Programming and Actors

We formally define an elegant multi-paradigm unification of Functional Reactive Programming, Actor Systems, and Object-Oriented Programming. This enables an intuitive form of declarative programming, harvesting the power of concurrency while maintaining safety. We use object and reference capabilities to highlight and tame imperative features: reference capabilities track aliasing and mutability, and object capabilities track I/O. Formally, our type system limits the scope, impact and interactions of impure code. - Scope: Expressions whose input is pure will behave deterministically. - Impact: Data-races and synchronisation issues are avoided. The only way for an actor to behave nondeterministically, is by mutating its state based on message delivery order. - Interactions: Signals provide a functional boundary between imperative and functional code, preventing impure code from invalidating functional assumptions.