Introduction to Functional Reactive Programming

Introduction toFunctionalReactive

Programming

Edward Amsden

Introduction

History of FRP

Classic andSignal-FunctionFRP

Implementing FRP

Applications ofFRP

Ongoing Work

Summary

Introduction to Functional ReactiveProgramming

Edward Amsden

PL WonksSeptember 28, 2012


Programming

Edward Amsden

Introduction

History of FRP


Implementing FRP

Applications ofFRP

Ongoing Work

Summary

Overview

Introduction

History of FRP

Classic and Signal-Function FRP

Implementing FRP

Applications of FRP

Ongoing Work

Summary


Programming

Edward Amsden

Introduction

History of FRP


Implementing FRP

Applications ofFRP

Ongoing Work

Summary

Introduction


Programming

Edward Amsden

Introduction

History of FRP


Implementing FRP

Applications ofFRP

Ongoing Work

Summary

Motivation

I Real programs have to deal with the real world.

I The real world is constrained by time.

I The real world produces unpredictable inputsto your program.

I Usual solution: Callbacks and event loops.

I Can we extend the benefits of functional programmingto handling these characteristics of the world?


Programming

Edward Amsden

Introduction

History of FRP


Implementing FRP

Applications ofFRP

Ongoing Work

Summary

Basic Abstractions

I FRP divides inputs into two basic classes:I Behaviors or signals: Functions of time.I Events: Temporal sequences of discrete values.

I An FRP language must include a means of altering orreplacing a program based on event occurrences.

I This is the basis of FRP’s reactivity.

I These abstractions may be reified in an FRP languageor may form the basis of other abstractions, but theymust be present.


Programming

Edward Amsden

Introduction

History of FRP


Implementing FRP

Applications ofFRP

Ongoing Work

Summary

History of FRP


Programming

Edward Amsden

Introduction

History of FRP


Implementing FRP

Applications ofFRP

Ongoing Work

Summary

People

I Conal ElliottI Previously at Microsoft Research.I Denotational semantics and implementations.

I Paul HudakI Yale University.I Applications of FRP to multimedia.I Euterpea.I Haskell School of Expression.

I Antony CourtneyI Student at Yale, graduated 2004.I Arrowized FRP.I Frappe (FRP in Java).


Programming

Edward Amsden

Introduction

History of FRP


Implementing FRP

Applications ofFRP

Ongoing Work

Summary

People

I Henrik NilssonI Previously at Yale University.I Collaborated on FRP with Hudak.I Now at University of Nottingham.I Created Yampa, an optimized implementation of AFRP.

I Neil SculthorpeI Student of Nilsson at Nottingham.I Now at University of Kansas.I Creator of N-Ary FRP.


Programming

Edward Amsden

Introduction

History of FRP


Implementing FRP

Applications ofFRP

Ongoing Work

Summary

“Functional Reactive Animation”

I Conal Elliott and Paul Hudak

I The original description of FRP.

I Proposed behaviors and events as abstractions.

I Evaluated a behavior carrying a pictureto form an animation.


Programming

Edward Amsden

Introduction

History of FRP


Implementing FRP

Applications ofFRP

Ongoing Work

Summary

“Genuinely Functional User Interfaces”

I Antony Courtney and Conal Elliott.

I Described a GUI toolkit based on “AFRP,”FRP formulated as an Arrow.

I Proposed “signal functions” as an FRP abstraction.

I Signal functions are reactive constructs with events andsignals as inputs and outputs.

I Signal functions are programs, and may be replacedin response to events.


Programming

Edward Amsden

Introduction

History of FRP


Implementing FRP

Applications ofFRP

Ongoing Work

Summary

“Push-pull Functional Reactive Programming”

I Conal Elliott

I Gave denotational semantics for FRP.

I Used to prove soundness of transformationto a normal form.

I Normal form lifts all reactivity to top level.I Evaluate with concurrent threads:

I Block waiting for next event to react to.I Spin evaluating current program with time steps.

I Advantage: no spinning sampling of events.

I Advantage: no sampling latency in reacting to events.

I Disadvantage: not a purely functional implementation.

I Disadvantage: Requires kludge to merge events.


Programming

Edward Amsden

Introduction

History of FRP


Implementing FRP

Applications ofFRP

Ongoing Work

Summary

“Dynamic Optimization for Functional ReactiveProgramming using Generalized Algebraic DataTypes”

I Henrik Nilsson

I Dynamically optimizing implementation of AFRP.

I Eliminate redundant composition introduced byswitching.


Programming

Edward Amsden

Introduction

History of FRP


Implementing FRP

Applications ofFRP

Ongoing Work

Summary

“Toward Safe and Efficient Functional ReactiveProgramming”

I Neil Sculthorpe(PhD Thesis, University of Nottingham)

I Embedded AFRP in Agda(a dependently-typed total functional language).

I Introduced signal vectors as a new abstractionto avoid embedding events in signals.


Programming

Edward Amsden

Introduction

History of FRP


Implementing FRP

Applications ofFRP

Ongoing Work

Summary

Upcoming: “Push-pull Signal FunctionFunctional Reactive Programming”

I Edward Amsden(M.S. Thesis, Rochester Institute of Technology)

I Using signal vectors to permit push-based evaluationof events in a signal-function FRP system.


Programming

Edward Amsden

Introduction

History of FRP


Implementing FRP

Applications ofFRP

Ongoing Work

Summary

Classic and Signal-Function FRP


Programming

Edward Amsden

Introduction

History of FRP


Implementing FRP

Applications ofFRP

Ongoing Work

Summary

Families of FRP Systems

I Classic FRPI FranI ReactiveI reactive-bananaI elmI Frappe

I Signal-Function FRPI FruitI AFRPI YampaI N-Ary FRPI time-flies


Programming

Edward Amsden

Introduction

History of FRP


Implementing FRP

Applications ofFRP

Ongoing Work

Summary

Classic FRP

I Events and behaviors are first-class abstractions.

I Primitive events and behaviors must be provided by thesystem.

I Programs are constructed by composing events andbehaviors and by lifting functions and values to simplebehaviors.


Programming

Edward Amsden

Introduction

History of FRP


Implementing FRP

Applications ofFRP

Ongoing Work

Summary

Signal-Function FRP

I Signal functions are first-class abstractions.

I An evaluation interface provides inputs to the systemwithout discrimination as to their origin.

I Programs are constructed by lifting functions to simplesignal functions and by composing signal functions.


Programming

Edward Amsden

Introduction

History of FRP


Implementing FRP

Applications ofFRP

Ongoing Work

Summary

Implementing FRP


Programming

Edward Amsden

Introduction

History of FRP


Implementing FRP

Applications ofFRP

Ongoing Work

Summary

Classic FRP

I Signals are implemented as functions from time to asample value and a new behavior.

I This permits a behavior to throw away history andavoid time-space leaks.

I Events are represented as lazy lists of time-value pairs.

I Some implementations use more complexrepresentations of improving lists.

I Either way, must have a representation of “Nooccurrence yet.”


Programming

Edward Amsden

Introduction

History of FRP


Implementing FRP

Applications ofFRP

Ongoing Work

Summary

Signal-Function FRP

I Signal functions are represented as one or morecontinuations.

I Continuations take signal sample update or eventoccurrence.

I Continuations produce output updates/occurrences andreplacement signal function.


Programming

Edward Amsden

Introduction

History of FRP


Implementing FRP

Applications ofFRP

Ongoing Work

Summary

Applications of FRP


Programming

Edward Amsden

Introduction

History of FRP


Implementing FRP

Applications ofFRP

Ongoing Work

Summary

Animations and Games

I Bouncing Balls etc. (Fran)

I Space Invaders (Yampa)

I Frag (Yampa)


Programming

Edward Amsden

Introduction

History of FRP


Implementing FRP

Applications ofFRP

Ongoing Work

Summary

Audio

I YampaSynth


Programming

Edward Amsden

Introduction

History of FRP


Implementing FRP

Applications ofFRP

Ongoing Work

Summary

Robotics

I FRob


Programming

Edward Amsden

Introduction

History of FRP


Implementing FRP

Applications ofFRP

Ongoing Work

Summary

Ongoing Work


Programming

Edward Amsden

Introduction

History of FRP


Implementing FRP

Applications ofFRP

Ongoing Work

Summary

Semantics

I Denotational semantics given for Classic FRP. (Elliott)

I Little work on formal semantics of signal-function FRP.

I Courtney and Elliott give informal semantics.

I Category-theoretic semantics for signal-function FRP?How does composition with switch work?


Programming

Edward Amsden

Introduction

History of FRP


Implementing FRP

Applications ofFRP

Ongoing Work

Summary

Performance

I Push-based event handling should yield performanceincrease.

I Dynamic optimizations (Yampa).

I Static optimizations (Causal Commutative Arrows).

I Staged code generation?

I Parallelism?


Programming

Edward Amsden

Introduction

History of FRP


Implementing FRP

Applications ofFRP

Ongoing Work

Summary

Summary


Programming

Edward Amsden

Introduction

History of FRP


Implementing FRP

Applications ofFRP

Ongoing Work

Summary

Summary

I Motivation: Reacting to real world

I Abstractions: Behaviors/signals and events.

I Implementation: Continuations.

I Applications: Anything reactive.


Programming

Edward Amsden

Introduction

History of FRP


Implementing FRP

Applications ofFRP

Ongoing Work

Summary

Questions?

Documents

Introduction to Functional Reactive Programming