Blending continuity into reactive values

Reactive normal form

Reactive values from the future

I’ve gotten interested lately in revisiting functional reactive programming (FRP). I was never entirely satisfied with the semantics or implementation in my original Fran formulation or in its successors. Over the last year, I’ve enjoyed getting more interface and functionality from standard type classes, and I’ve realized that quite a lot of FRP could be both packaged and implemented by leveraging those classes. This post describes that packaging, and in particular shows how the Monad interface makes some operations very easy to define. I suspect that monadic functional reactivity is a very powerful structuring tool with lovely applications.

At ICFP 07, I had a conversation with Mike Sperber about FRP and about his FRP-based Lula system for stage lighting. Mike used blocking threads in Lula, which I had never considered for FRP. While playing with the idea, I realized that I could give a very elegant and efficient solution to caching, unlike my previous FRP implementations, including the recent DataDriven library. From there, I stumbled on the idea of reactive normal form and separating out an aspect of reactivity into the notion of future values. A third new (to me) idea is to factor out continuity from reactivity, so that reactive behaviors arise by composing orthogonal notions of reactive values and non-reactive functions of continuous time.

Two previous posts presented future values, first describing interface and semantics and then a multi-threaded implementation. This post builds a simple foundation for FRP on top of future values, as part of a library Reactive.

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