View source for Functional Reactive Programming

Functional Reactive Programming (FRP) integrates time flow and compositional events into functional programming.  This provides an elegant way to express computation in domains such as interactive animations, robotics, computer vision, user interfaces, and simulation.


== Introduction ==
 
The original formulation of Functional Reactive Programming can be found in the ICFP 97 paper [http://conal.net/papers/icfp97/ Functional Reactive Animation] by Conal Elliott and Paul Hudak. It introduces the following ''semantic functions'':

* <b>at</b> : <i>Behavior<sub>α</sub> → Time → α</i>

* <b>occ</b> : <i>Event<sub>α</sub> → Time × α</i>

to provide a formal description of operations on values now commonly known as ''behaviors'' and ''events''. But what are they?

=== Behaviors ===

Traditionally a widget-based user interface is created by a series of imperative actions.  First an action is invoked to create an edit widget, then additional actions can be invoked to read its current content, set it to a specific value or to assign an event callback for when the content changes.  This is tedious and error-prone.

A better way to represent an edit widget's content is a time-varying value, called a ''behavior''.  The basic idea is that a time-varying value can be represented as a function of time:

<haskell>
newtype Behavior a =
    Behavior {
      at :: Time -> a
    }

myTodoList                :: Behavior Text
yesterday                 :: Time
myTodoList `at` yesterday :: Text
</haskell>

This is only a theoretical model, because a time-varying value can represent something impure like the content of an edit widget, the current value of a database entry as well as the system clock's current time.  Using this model the current content of an edit widget would be a regular first class value:

<haskell>
myEditWidget :: Behavior Text
</haskell>

In most frameworks there is an applicative interface for behaviors, such that you can combine them easily:

<haskell>
liftA2 (<>) myEdit1 myEdit2
</haskell>

The result is a time-varying value that represents the concatenation of <hask>myEdit1</hask> and <hask>myEdit2</hask>.  This could be the value of a third widget, a label, to display the concatenation.  The following is a hypothetical example:

<haskell>
do edit1 <- editWidget
   edit2 <- editWidget
   label <- label (liftA2 (<>) edit1 edit2)
   {- ... -}
</haskell>

Without behaviors you would have to write event callback ''actions'' for the edit widgets to ''update'' the label's content.  With behaviors you can express this relationship declaratively.

=== Events ===

While behaviours work well for describing properties which are continuous (e.g. the motion of an animated object), other properties are more ''discrete'' (e.g. what button on the mouse was clicked or the screen position where it happened). Events more easily accommodate information like this, by associating the data value of interest with a particular time:

<haskell>
newtype Event a =
    Event {
      occ :: (Time, a)
    }

mouseInWindowNow     :: Event Bool
occ mouseInWindowNow :: (Time, Bool)
</haskell>

The concept of an <i>event stream</i> (or simply <i>stream</i>), for representing a series of events, supports a basic model of interactivity:

<div style="border-left:1px solid lightgray; padding: 1em" alt="blockquote">
Suppose that we want to compute a function <i>F</i> on several arguments <i>A</i><sub>0</sub>, <i>A</i><sub>1</sub>, <i>A</i><sub>2</sub>,... appearing consecutively in time [as discrete events]. One can view this so-called <i>stream</i> (<i>A</i><sub>0</sub>, <i>A</i><sub>1</sub>, <i>A</i><sub>2</sub>,...) as a potentially infinite list <i>A</i> = (<i>A</i><sub>0</sub>:<i>A</i><sub>1</sub>:<i>A</i><sub>2</sub>:···:<i>A</i><sub>n</sub>:⊥) where ⊥ stands for “unspecified” and is interactively updated to <i>A</i><sub>n+1</sub>:⊥ each time the user has a new argument <i>A</i><sub>n+1</sub>. Then on this list <i>A</i>, the system applies the function <i>F</i><sup>*</sup> defined by
::<i>F</i><sup>*</sup> (<i>A</i>:<i>B</i>) = (<i>F</i> <i>A</i>):(<i>F</i><sup>*</sup> <i>B</i>)

obtaining
::<i>F</i><sup>*</sup> <i>A</i> = (<i>F</i> <i>A</i><sub>0</sub>:<i>F</i> <i>A</i><sub>1</sub>:<i>F</i> <i>A</i><sub>2</sub>:···:<i>F</i> <i>A</i><sub>n</sub>:<i>F</i><sup>*</sup> ⊥)

also appearing consecutively in time.

<tt>[https://repository.ubn.ru.nl/bitstream/handle/2066/17230/13246.pdf Functional Programming and Lambda Calculus], Henk Barendregt (page 325).</tt>
</div>

== Semantic functions? ==

From page 6 of [http://conal.net/papers/icfp97/icfp97.pdf the paper]:
<div style="border-left:1px solid lightgray; padding: 1em" alt="blockquote">
An early implementation of Fran represented behaviors as implied in the formal semantics:
<haskell>
data Behavior a = Behavior (Time -> a)
</haskell>
This representation, however, leads to a serious inefficiency [a “space-time” leak].
</div>

So instead of one canonical implementation of FRP, this tendency to "leak" has led to a variety of implementations, each with its own benefit-cost trade-offs.

== Libraries ==
* [[DataDriven]]
* [http://hackage.haskell.org/package/elerea Elerea]
* [http://conal.net/fran/ Fran] (discontinued)
* [[Grapefruit]]
* [[Netwire]]
* [[Reactive]]
* [[Reactive-banana|reactive-banana]]
* [https://reflex-frp.org/ Reflex]
* [http://hackage.haskell.org/package/sodium Sodium]
* [[WxFruit|wxFruit]]
* [[Yampa]]
* [[https://github.com/ivanperez-keera/dunai Dunai]]
* [[Rhine]]
* [http://hackage.haskell.org/packages/archive/pkg-list.html#cat:FRP Hackage packages in the category FRP]
A simple, practical comparison between FRP libraries is done by [https://github.com/gelisam/frp-zoo frp-zoo]

== Publications and talks ==

* [https://www.youtube.com/watch?v=zgNRM8tZguY ICFP 2014: Settable and Non-Interfering Signal Functions for FRP - Daniel Winograd-Cort] (video)
* [https://web.archive.org/web/20150217184805/https://www.cs.rit.edu/~eca7215/frp-independent-study/Survey.pdf A Survey of Functional Reactive Programming]
* [http://conal.net/papers/frp.html Conal Elliott’s FRP-related publications]
* [https://wolfgang.jeltsch.info/publications-and-talks Grapefruit-related publications and talks]
* [https://web.archive.org/web/20171012164802/http://haskell.cs.yale.edu/publications/#FunctionalReactiveProgramming The Yale Haskell Group’s FRP-related publications] (archived)
* [https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.11.6997&rep=rep1&type=pdf FROB: A Transformational Approach to the Design of Robot Software - Gregory D. Hager and John Peterson]
* [https://begriffs.com/posts/2015-07-22-essence-of-frp.html The Essence of FRP (Conal Elliott -- July 22, 2015 -- video)]
* [https://begriffs.com/posts/2016-07-27-tikhon-on-frp.html A Sensible Intro to FRP (Tikhon Jelvis -- July 27, 2016 -- video)]

== Books ==
* Blackheath, Stephen; Jones, Antony. [http://www.manning.com/blackheath Functional Reactive Programming]. Manning Publications (2015). p.245. ISBN 978-1-6334-3010-5

== Blog posts ==

* [https://github.com/gelisam/frp-zoo frp-zoo]; comparing many FRP implementations by reimplementing the same toy app in each.
* [http://blog.reactiveprogramming.org/ Functional Reactive Programming, a better way to build interactive applications] (about the Sodium FRP Library currently for C#, C++, Haskell and Java and more to come)
* [http://apfelmus.nfshost.com/blog.html#functional-reactive-programming-frp FRP-related posts on Heinrich Apfelmus’ blog]
* [http://conal.net/blog/tag/frp FRP-related posts on Conal Elliott’s blog]
* [http://jeltsch.wordpress.com/tag/frp/ FRP-related posts on Wolfgang Jeltsch’s blog]
* [http://web.archive.org/web/20210420065305/https://lukepalmer.wordpress.com/2008/11/28/relative-time-frp/ Relative time FRP] by Luke Palmer (archived)
* [http://blog.edwardamsden.com/2011/03/demonstrating-time-leak-in-arrowized.html Demonstrating a Time Leak in Arrowized FRP] by Edward Amsden
* [https://www.reddit.com/r/haskell/comments/3fr5ij/frp_systems_discussion/ FRP Systems discussion] on reddit
* [https://t0yv0.blogspot.com/2014/07/the-fatal-attraction-of-frp.html The fatal attraction of FRP] by  Anton Tayanovskyy (about the WebSharper UI.Next framework for single-page JavaScript applications)

== People ==
* [http://apfelmus.nfshost.com/ Heinrich Apfelmus]
* [http://www.facebook.com/antony.courtney Antony Courtney]
* [http://conal.net/ Conal Elliott]
* [http://sgate.emt.bme.hu/patai/ Patai Gergely]
* [http://www.ittc.ku.edu/csdl/fpg/Users/AndyGill Andy Gill]
* Liwen Huang
* Paul Hudak
* [https://wolfgang.jeltsch.info/ Wolfgang Jeltsch]
* [http://www.cs.nott.ac.uk/~nhn/ Henrik Nilsson]
* [http://github.com/ivanperez-keera Ivan Perez]
* [http://mcis.western.edu/~jpeterson/ John Peterson]
* Ertugrul Söylemez

== History ==

=== A possible predecessor? ===

In his thesis [https://core.ac.uk/download/9835633.pdf Functional Real-Time Programming: The Language Ruth And Its Semantics], some parts of the semantics Dave Harrison gives for ''Ruth'' bears a curious resemblance to those for FRP:

* <small>(page 48)</small>
:{|
|<div style="border-left:1px solid lightgray; padding: 1em" alt="blockquote">
A channel is an infinite stream of timestamped data values, or messages, each message denoting an event in the system. [...]
</div>
<sup> </sup>
<haskell>
type Channel a = [Event a]
data Event a   = At Time a
</haskell>
|}

* <small>(page 61)</small>
:{|
|<div style="border-left:1px solid lightgray; padding: 1em" alt="blockquote">
In the semantics given in Chapter 5 every <i>Ruth</i> program is supplied with a tree of time values (or clock) as suggested in [https://academic.oup.com/comjnl/article-pdf/31/3/243/1157325/310243.pdf this paper] and each <i>Ruth</i> process is given a different sub-tree of the clock [...]
</div>
<sup> </sup>
<haskell>
type Program = Clock -> ...
type Process a = Clock -> a

type Clock = Tree Time
left  :: Clock -> Clock
right :: Clock -> Clock
</haskell>
<sup> </sup>
<div style="border-left:1px solid lightgray; padding: 1em" alt="blockquote">
A clock tree is composed of a node holding a non-negative integer denoting the current time and two sub-trees containing the times of future events. As the tree is (lazily) evaluated each of the nodes is instantiated with the value of system time <i>at the time at which the node is instantiated</i>, thus giving programs reference to the current time. [...]
</div>
<sup> </sup>
<haskell>
type Time   = Integer  -- must be zero or larger 
data Tree a = Node { contents :: a,
                     left     :: Tree a,
                     right    :: Tree a }

currentTime :: Clock -> Time
currentTime = contents
</haskell>
|}

Regarding Harrison's semantics:
<div style="border-left:1px solid lightgray; padding: 1em" alt="blockquote"><small>(page 53)</small>

The semantics of <i>Ruth</i> given in Chapter 5 assume a normal order evaluation strategy such as is provided by the technique of lazy evaluation [...]. Consequently <code>whererec</code> can be used to define infinite data structures.
</div>

[[Category:FRP|*]]