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1 Abstract

Reactive is a simple foundation for programming reactive systems functionally. Like Fran/FRP, it has a notions of (reactive) behaviors and events. Like DataDriven, Reactive has an efficient, data-driven implementation. The main difference between Reactive and DataDriven are

  • Reactive provides and builds on "functional futures", which in turn build on Concurrent Haskell threads, while DataDriven builds on continuation-based computations; and
  • The algebras of events and reactive values (called events and sources in DataDriven) are purely functional. I couldn't figure out how to accomplish that in DataDriven.
  • Reactive manages (I hope) to get the efficiency of data-driven computation with a (sort-of) demand-driven architecture. For that reason, Reactive is garbage-collector-friendly, while DataDriven depends on weak references (because GC favors demand-driven computation.)
  • Reactive elegantly and efficiently caches values.
  • Reactive uses the term "reactive values" (
    Reactive
    ), where DataDriven uses "sources" (
    Source
    ).
The inspiration for Reactive was Mike Sperber's [Lula] implementation of FRP. Mike used blocking threads, which I had never considered for FRP before a conversation with him at ICFP 2007. While playing with the idea, I realized that I could give a very elegant and efficient solution to caching, which DataDriven doesn't do. (For an application
f <*> a
of a varying function to a varying argument, caching remembers the latest function to apply to a new argument and the latest argument to which to apply a new function.) As with DataDriven, Reactive provides instances for
Monoid
,
Functor
,
Applicative
, and
Monad
.

Besides this wiki page, here are more ways to find out about Reactive:

Please leave comments at the Talk page.

2 Modules

2.1 Data.Future

A "future" is a value that will become knowable only later. Primitive futures can be things like "the value of the next key you press", or "the value of LambdaPix stock at noon next Monday".

Composition is via standard type classes:
Functor
,
Applicative
,
Monad
, and
Monoid
.
  • Monoid
    :
    mempty
    is a future that never becomes knowable.
    a `mappend` b
    is whichever of
    a
    and
    b
    is knowable first.
  • Functor
    : apply a function to a future. The result is knowable when the given future is knowable.
  • Applicative
    :
    pure
    gives value knowable since the beginning of time.
    (<*>)
    applies a future function to a future argument. Result available when /both/ are available, i.e., it becomes knowable when the later of the two futures becomes knowable.
  • Monad:
    return
    is the same as
    pure
    (as always).
    (>>=)
    cascades futures.
    join
    resolves a future future value into a future value.
The current implementation is nondeterministic in
mappend
for futures that become knowable at the same time or nearly the same time. I want to make a deterministic implementation.

2.2 Data.SFuture

A target denotational semantics for Data.Future -- simple, precise, and deterministic, in terms of time/value pairs.

2.3 Data.Reactive

This module defines events and reactive values. An event is stream of future values in order of availability. A reactive value is a discretly time-varying value. These two types are closely linked: a reactive value is a current value and an event (the future values), while an event is simply a future reactive value.

newtype Event a = Event (Future (Reactive a))
data Reactive a = Reactive a (Event a)
This
Reactive
representation can be thought of a "reactive weak head normal form", to which arbitrary reactive expressions may be rewritten. The rewrite rules and their justification in terms of simple denotational semantics will be described in an upcoming paper. Although the basic
Reactive
type describes discretely-changing values, continuously-changing are defined simply by composing
Reactive
and a simple type functions of time (see below).
type Time = Double
type ReactiveB = Reactive :. Fun Time
Because the combination of
Reactive
and
Fun Time
is wrapped in a type composition, we get
Functor
and
Applicative
instances for free. The exact packaging of discrete vs continuous will probably change with more experience. Perhaps I'll fold
Fun Time a
into the
Reactive
type, making a dynamic rather than static distinction.

2.4 Data.Fun

This module defines a type of functions optimized for the constant case, together with instances of
Functor
,
Applicative
,
Monad
, and
Arrow
.