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 Functional Reactive Animation by Conal Elliott and Paul Hudak.

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:

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

myName :: Behavior Text

myName `at` yesterday

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:

myEditWidget :: Behavior Text

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

liftA2 (<>) myEdit1 myEdit2

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

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

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

To do

Libraries

• DataDriven
• Elerea
• Fran (discontinued)
• Grapefruit
• Netwire
• Reactive
• reactive-banana
• Reflex
• Sodium
• wxFruit
• Yampa
•  [Dunai]
•  Rhine
• Hackage packages in the category FRP

A simple, practical comparison between FRP libraries is done by frp-zoo

Publications and talks

• ICFP 2014: Settable and Non-Interfering Signal Functions for FRP - Daniel Winograd-Cort (video)
• A Survey of Functional Reactive Programming
• Conal Elliott’s FRP-related publications
• Grapefruit-related publications and talks
• The Yale Haskell Group’s FRP-related publications
• The Essence of FRP (Conal Elliott -- July 22, 2015 -- video)
• A Sensible Intro to FRP (Tikhon Jelvis -- July 27, 2016 -- video)

Books

• Blackheath, Stephen; Jones, Antony. Functional Reactive Programming. Manning Publications (2015). p.245. ISBN 978-1-6334-3010-5

Blog posts

• frp-zoo; comparing many FRP implementations by reimplementing the same toy app in each.
• 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)
• FRP-related posts on Heinrich Apfelmus’ blog
• FRP-related posts on Conal Elliott’s blog
• FRP-related posts on Wolfgang Jeltsch’s blog
• Relative time FRP by Luke Palmer
• Demonstrating a Time Leak in Arrowized FRP by Edward Amsden
• FRP Systems discussion on reddit

People

• Heinrich Apfelmus
• Antony Courtney
• Conal Elliott
• Patai Gergely
• Andy Gill
• Liwen Huang
• Paul Hudak
• Wolfgang Jeltsch
• Henrik Nilsson
• Ivan Perez
• John Peterson
• Ertugrul Söylemez