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Programming language: Haskell
License: MIT License
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README
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