inSeminar Methods and Applications of Functional Programming

Johannes Kepler University, Linz SS2012

by Gerold Meisinger

Functional reactive programming

Background

FH Hagenberg Digital Media

FRP diploma thesis Gamedev startup

”Modern Alchemists”

Types of programs

Transformative

Interactive

Reactive

What is FRP?

New paradigm Time-varying values

Behavior a :: Time → a Hybrid modeling

Continous-time Discrete events

React to input

Paradigms (by Van Roy)

Similarities

Paradigms Dataflow

Common Lisp/Cells Temporal logic Control theory ...

Languages Esterel Lustre Lucid Signal Synchrone Simulink Modelica ...

Dataflow programming

FRP implementations

Classic FRP e.g. Fran, FrTime

Push-pull FRP e.g. Reactive

Arrowized FRP e.g. Yampa, Elera

Yampa examples

Robotics GUIs Games Music synthesis Computer vision Networking ...

Classic game programming

interface IUpdateablevoid update( float elapsedTime )

class GameObject : IUpdateableVector3 positionVector3 velocityvoid update( float elapsedTime ) position += elapsedTime * velocity

Yampa semantics

Signal a :: Time → a Only used internally

SF a b :: Signal a → Signal b SF … signal function

data Event t = NoEvent | Event t

Yampa design goals

Continous-time & discrete events Denotative semantics Dynamic structures Higher-orderness Avoid time and space leaks ⇒ Arrow typeclass Synchronous (vs concurrent)

Transformative

f :: a → b

Interactive

Reactive

Temporal

Loop >

Loop > Processing (black-box)

Loop > Processing

Loop > Processing >

Loop > Processing > SF (BB)

Loop > Processing > SF

Loop > Processing > SF

Loop > Processing > SF

Loop > Processing > SF

Loop > Processing > SF Mover

Loop > Processing > Mover

Loop > Processing > Mover

Loop > Processing >

Loop > Processing

Loop > Processing

Loop > Processing Player

Example game: Fran

Haskell typeclasses (by Yorgey)

Arrow class

class Arrow a wherearr :: (b → c) → a b c(<<<) :: a b c → a c d → a b dfirst :: a b c → a (b, d) (c, d)

+ lots of arrow lawsderive combinators: second, (***), (&&&)

Arrow combinators

Derived Arrow combinators

second :: (Arrow a) a b c → a (d, b) (d, c)⇒

second f = arr swap >>> first f >>> arr swapwhere swap (a, b) = (b, a)

(***) :: (Arrow a) a b c → a b' c' → a (b, b') (c, c')⇒

f ⋆⋆⋆ g = first f >>> second g

(&&&) :: (Arrow a) a b c → a b c' → a b (c, c')⇒

f &&& g = arr (λx → (x, x)) >>> (f *** g)

ArrowCircuit

class Arrow a ArrowCircuit a where⇒

loop :: a (b, d) (c, d) → a b cinit :: b → a b b

+ even more laws

Arrow syntax

myArrow x y = proc (a, b) → do c1 ← arrow1 −< a c2 ← arrow3 <<< arrow2 −< b c ← arrow4 −< (c1, c2) rec d ← arrow5 −< (c, b, d) returnA (c, d)

Signal function primitives

Events

Yampa switch

switch :: SF in (out, Event t) → (t → SF in out) → SF in out

Yampa parallel switch

pSwitch :: Functor col => (forall sf. (in → col sf → col (ext, sf))) → col (SF ext out) → SF (in, col out) (Event mng) → (col (SF ext out) → mng → SF in (col out)) → SF in (col out)

Yampa switches

immediate delayedonce switch dSwitchrecurring rSwitch drSwitchparallel broadcasted pSwitchB

rpSwitchBdpSwitchBdrpSwitchB

parallel routed pSwitchrpSwitch

dpSwitchdrpSwitch

continuation kSwitch dkSwitch

Processing

reactimate :: IO (DTime, a) –- input → (b → IO ()) –- output → SF a b → IO ()

Game-Engine

Ongoing research

Semantics "Garbage collecting the semantics of FRP" "Why classic FRP does not fit interactive behavior"

Performance Causal Commutative Arrows (CCA) Concurrency Arrow tuples vs. signal combining

Yampa alternatives

ELM Web runtime and IDE => converts to JavaScripthttp://elm-lang.org

Reactive-Banana Many exampleshttp://apfelmus.nfshost.com

The End

http://lambdor.net