reactimate :: IO a -- init -> (Bool -> IO (DTime, Maybe a)) -- input/sense -> (Bool -> b -> IO Bool) -- output/actuate -> SF a b -- process/signal function -> IO ()
Bool parameter of
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
initaction is rather self-explanatory; it executes an initial IO action (e.g. print a welcome message), which then yields an initial sample of type
afor the signal function that is passed to
reactimateas the last argument.
senseargument is then evaluated at
Falseand 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
Nothingthen the previous sample is used again.
actuateis evaluated at
Trueand the signal function's output of type
b, obtained by processing the input sample previously provided by
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
Truethe processing loop stops (i.e. the IO action defined by
- Finally, the last argument of
reactimateis 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.
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.Yampa import Data.IORef import Data.Time.Clock.POSIX 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 <- getPOSIXTime -- Current POSIX time in seconds timeRef <- newIORef t let init = putStrLn "Hello... wait for it..." sense = (\_ -> do t' <- getPOSIXTime t <- readIORef timeRef let dt = realToFrac (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
True (that is after 2 seconds),
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.POSIX module to measure time differences (alternatively
Data.Time.Clock can be used, which internally again uses POSIX but with the disadvantage of having leap seconds). One should not use
System.CPUTime because its precision is hardware dependent and can be very low. E.g. on a Core i7-2720QM, the precision is reduced by a factor of 1010 compared to
Data.Time.Clock (10ms vs. 1ps). This hardware dependence and potentially low precision make
System.CPUTime unusable even for simple real time applications (like the game Pong) because the
derivative signal functions provided by Yampa behave unpredictably. This results in programs being highly system dependent; e.g. the ball in Pong moving significantly faster on faster hardware or even moving through a paddle because a collision is missed.