Yampa/reactimate

From HaskellWiki

reactimate :: IO a                          -- init
           -> (Bool -> IO (DTime, Maybe a)) -- input/sense
           -> (Bool -> b -> IO Bool)        -- output/actuate
           -> SF a b                        -- process/signal function
           -> IO ()

The Bool parameter of sense and actuate are unused if you look up the definition of reactimate so just ignore them (cf. the explanations below).

reactimate basically is an input-process-output loop and forms the interface between (pure) Yampa signal functions and the (potentially impure) external world. More specifically, a Yampa signal function of type SF a b is an abstract data type that transforms a signal of type Time -> a into a signal of type Time -> b (note that one does not have direct access to signals in Yampa but just to signal functions). The Time parameter here is assumed to model continuous time but to evaluate a signal function (or a signal for that matter) it is necessary to sample the signals at discrete points in time. This is exactly what reactimate does (among other things).

1 Further explanations

• The init action is rather self-explanatory; it executes an initial IO action (e.g. print a welcome message), which then yields an initial sample of type a for the signal function that is passed to reactimate as the last argument.
• The sense argument is then evaluated at False and should return an IO action yielding a pair that contains the time passed since the last sample and a new sample of type a (wrapped in a Maybe) for the signal function. If the second component of sense's return value is Nothing then the previous sample is used again.
• actuate is evaluated at True and the signal function's output of type b, obtained by processing the input sample previously provided by sense. actuate's job now is to process the output (e.g. render a collection of objects contained in it) in an IO action that yields a result of type Bool. If this result is True the processing loop stops (i.e. the IO action defined by reactimate returns ()).
• Finally, the last argument of reactimate is the signal function to be run (or "animated"). Keep in mind that the signal function may take pretty complex forms like a parallel switch embedded in a loop.

2 Example

To illustrate this, here's a simple example of a Hello World program but with some time dependence added. Its purpose is to print "Hello... wait for it..." to the console once and then wait for 2 seconds until it prints "World!" and then stops.

module Main where
 
import FRP.Animas -- Animas is a fork of Yampa
import Data.IORef
import Data.Time.Clock
 
sf :: SF () Bool -- The signal function to be run
sf = time >>> arr (\t -> if (t < 2) then False else True)
-- the time signal function ignores its input and returns the time
 
main :: IO ()
main = do
  t <- getCurrentTime -- Current UTC time
  timeRef <- newIORef t
  let init = putStrLn "Hello... wait for it..."
      sense = (\_ -> do
                 t' <- getCurrentTime
                 t <- readIORef timeRef
                 let dt = realToFrac (diffUTCTime t' t) -- Time difference in seconds
                 writeIORef timeRef t'
                 return (dt, Nothing)) -- we could equally well return (dt, Just ())
      actuate = (\_ x -> if x
                          then putStrLn "World!" >> return x
                          else return x)
  reactimate init sense actuate sf

Note that as soon as x in the definition of actuate becomes True (that is after 2 seconds), actuate returns True, hence reactimate returns () and the program stops. If we change the definition of actuate to always return False the line "World!" will be print out indefinitely.

3 Precision Issues

In the above example, we used the Data.Time.Clock module to measure time differences. One might be concerned about leap seconds and try to fall back to System.CPUTime. However, the precision of System.CPUTime is reduced by a factor of 10⁶ compared to Data.Time.Clock (1ms vs. 1ps). This precision is usually not sufficient even for simple real time applications (like the game Pong) because the integral and derivative signal functions provided by Yampa behave unpredictably. This can result in programs being highly system dependent, e.g. the ball in Pong moving significantly faster on faster hardware. In contrast to this, leap seconds are usually of no concern.