FRP through value streams and monadic splines.

Varying is a FRP library aimed at providing a simple way to describe values that change over a domain. It allows monadic, applicative and arrow notation and has convenience functions for tweening. Great for animation.

varying

This library provides automaton based value streams and sequencing useful for functional reactive programming (FRP) and locally stateful programming (LSP).

Getting started

module Main where

import Control.Varying
import Control.Applicative
import Control.Concurrent (forkIO, killThread)
import Data.Functor.Identity
import Data.Time.Clock

-- | A simple 2d point type.
data Point = Point { px :: Float
                   , py :: Float
                   } deriving (Show, Eq)

newtype Delta = Delta { unDelta :: Float }

-- An exponential tween back and forth from 0 to 50 over 1 seconds that
-- loops forever. This spline takes float values of delta time as input,
-- outputs the current x value at every step.
tweenx :: Monad m => TweenT Float Float m Float
tweenx = do
    -- Tween from 0 to 50 over 1 second
    tween_ easeOutExpo 0 50 1
    -- Chain another tween back to the starting position
    tween_ easeOutExpo 50 0 1
    -- Loop forever
    tweenx

-- An exponential tween back and forth from 0 to 50 over 1 seconds that never
-- ends.
tweeny :: Monad m => TweenT Float Float m Float
tweeny = do
    tween_ easeOutExpo 50 0 1
    tween_ easeOutExpo 0 50 1
    tweeny

-- Our time signal counts input delta time samples.
time :: Monad m => VarT m Delta Float
time = var unDelta

-- | Our Point value that varies over time continuously in x and y.
backAndForth :: Monad m => VarT m Delta Point
backAndForth =
    -- Turn our splines into continuous output streams. We must provide
    -- a starting value since splines are not guaranteed to be defined at
    -- their edges.
    let x = tweenStream tweenx 0
        y = tweenStream tweeny 0
    in
    -- Construct a varying Point that takes time as an input.
    (Point <$> x <*> y)
        -- Stream in a time signal using the 'plug left' combinator.
        -- We could similarly use the 'plug right' (~>) function
        -- and put the time signal before the construction above. This is needed
        -- because the tween streams take time as an input.
        <~ time

main :: IO ()
main = do
    putStrLn "An example of value streams using the varying library."
    putStrLn "Enter a newline to continue, and then a newline to quit"
    _ <- getLine

    t   <- getCurrentTime
    tId <- forkIO $ loop backAndForth t

    _ <- getLine
    killThread tId

loop :: Var Delta Point -> UTCTime -> IO ()
loop v t = do
  t1 <- getCurrentTime
  -- Here we'll run in the Identity monad using a time delta provided by
  -- getCurrentTime and diffUTCTime.
  let dt = realToFrac $ diffUTCTime t1 t
      Identity (Point x y, vNext) = runVarT v $ Delta dt
      xStr = replicate (round x) ' ' ++ "x" ++ replicate (50 - round x) ' '
      yStr = replicate (round y) ' ' ++ "y" ++ replicate (50 - round y) ' '
      str  = zipWith f xStr yStr
      f 'x' 'y' = '|'
      f 'y' 'x' = '|'
      f a ' ' = a
      f ' ' b = b
      f _ _ = ' '
  putStrLn str
  loop vNext t1

Publications

The concept of VarT that this library is built on is isomorphic to Monadic Stream Functions as defined in "Functional Reactive Programming, Refactored" (mirror).

The isomorphism is

toMSF :: Functor m => VarT m a b -> MSF m a b
toMSF = MSF . (fmap . fmap . fmap $ toMSF) . runVarT

toVarT :: Functor m => MSF m a b -> VarT m a b
toVarT = VarT . (fmap . fmap . fmap $ toVarT) . unMSF