Another angle on functional future values

An earlier post introduced functional future values, which are values that cannot be known until the future, but can be manipulated in the present. That post presented a simple denotational semantics of future values as time/value pairs. With a little care in the definition of Time (using the Max monoid), the instances of Functor, Applicative, Monad are all derived automatically.

A follow-up post gave an implementation of Future values via multi threading. Unfortunately, that implementation did not necessarily satisfy the semantics, because it allowed the nondeterminism of thread scheduling to leak through. Although the implementation is usually correct, I wasn’t satisfied.

After a while, I hit upon an idea that really tickled me. My original simple semantics could indeed serve as a correct and workable implementation if I used a subtler form of time that could reveal partial information. Implementing this subtler form of time turned out to be quite tricky, and was my original motivation for the unamb operator described in the paper Push-pull functional reactive programming and the post Functional concurrency with unambiguous choice.

It took me several days of doodling, pacing outside, and talking to myself before the idea for unamb broke through. Like many of my favorite ideas, it’s simple and obvious in retrospect: to remove the ambiguity of nondeterministic choice (as in the amb operator), restrict its use to values that are equal when non-bottom. Whenever we have two different methods of answering the same question (or possibly failing), we can use unamb to try them both. Failures (errors or non-termination) are no problem in this context. A more powerful variation on unamb is the least upper bound operator lub, as described in Merging partial values.

I’ve been having trouble with the unamb implementation. When two (compatible) computations race, the loser gets killed so as to free up cycles that are no longer needed. My first few implementations, however, did not recursively terminate other threads spawned in service of abandoned computations (from nested use of unamb). I raised this problem in Smarter termination for thread racing, which suggested some better definitions. In the course of several helpful reader comments, some problems with my definitions were addressed, particularly in regard to blocking and unblocking exceptions. None of these definitions so far has done the trick reliably, and now it looks like there is a bug in the GHC run-time system. I hope the bug (if there is one) will be fixed soon, because I’m seeing more & more how unamb and lub can make functional programming even more modular (just as laziness does, as explained by John Hughes in Why Functional Programming Matters).

I started playing with future values and unambiguous choice as a way to implement Reactive, a library for functional reactive programming (FRP). (See Reactive values from the future and Push-pull functional reactive programming.) Over the last few days, I’ve given some thought to ways to implement future values without unambiguous choice. This post describes one such alternative.

Edits:

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Simply efficient functional reactivity

I submitted a paper Simply efficient functional reactivity to ICFP 2008.

Abstract:

Functional reactive programming (FRP) has simple and powerful semantics, but has resisted efficient implementation. In particular, most past implementations have used demand-driven sampling, which accommodates FRP’s continuous time semantics and fits well with the nature of functional programming. Consequently, values are wastefully recomputed even when inputs don’t change, and reaction latency can be as high as the sampling period.

This paper presents a way to implement FRP that combines data- and demand-driven evaluation, in which values are recomputed only when necessary, and reactions are nearly instantaneous. The implementation is rooted in a new simple formulation of FRP and its semantics and so is easy to understand and reason about.

On the road to efficiency and simplicity, we’ll meet some old friends (monoids, functors, applicative functors, monads, morphisms, and improving values) and make some new friends (functional future values, reactive normal form, and concurrent “unambiguous choice”).

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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Future values via multi-threading

Future values

A previous post described future values (or simply “futures”), which are values depend on information from the future, e.g., from the real world. There I gave a simple denotational semantics for future values as time/value pairs. This post describes the multi-threaded implementation of futures in Reactive‘s Data.Reactive module.

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Future values

A future value (or simply “future”) is a value that might not be knowable until a later time, such as “the value of the next key you press”, or “the value of LambdaPix stock at noon next Monday” (both from the time you first read this sentence), or “how many tries it will take me to blow out all the candles on my next birthday cake”. Unlike an imperative computation, each future has a unique value — although you probably cannot yet know what that value is. I’ve implemented this notion of futures as part of a library Reactive.

Edits:

  • 2008-04-04: tweaked tag; removed first section heading.

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