Functional reactive programming with propagators

Functional reactive programming (FRP) is a declarative programming paradigm that is most commonly used in interactive applications where imperative, event-driven, callback-laden code quickly becomes overwhelming and difficult to reason about. The reduction in cognitive overhead comes at a price, however. Popular reactive systems are limited to one-way data flow (a directed acyclic graph) which limits the types of problems these systems can solve elegantly. Fortunately, a way to remove this limitation has been known for over 15 years! Alexey Radul's 2009 PhD thesis "Propagation Networks: A Flexible and Expressive Substrate for Computation" tells us how. In this talk, I'll use Guile Scheme to demonstrate how an FRP system built on the propagator model allows for cyclic dependencies without user-visible glitches whilst keeping implementation complexity low.