Of course, you could have the 'interpretation' function be non-pure.
For example:
-- Library functions for a hypothetical FRP system
pollEvent :: IO [a] -> Event a
behavior :: a -> Event a -> Behavior a
accumB :: b -> (b -> a -> b) -> Event a -> Behavior b
accumE :: b -> (b -> a -> b) -> Event a -> Event b
union :: Event a -> Event a -> Event a
runFRP :: (a -> IO Bool) -> Behavior a -> IO ()
-- Event & Behavior become instances of Functor & Applicative
-- and now a hypothetical implementation
data Event a where
Event :: s -- initial state
-> (s -> IO ([a], s)) -- tick
-> Event a
data Behavior a = Behavior a (Event a)
pollEvent act = Event () $ \() -> do
xs <- act
return (xs, ())
behavior = Behavior
union (Event sL0 tickL) (Event sR0 tickR) = Event (sL0,sR0) tick where
tick (sL, sR) = do
(ls, sL') <- tickL sL
(rs, sR') <- tickR sR
return (ls ++ rs, (sL', sR'))
accumB b0 f e = Behavior b0 $ accumE b f e
accumE b0 f (Event s0 tickE) = Event (b0, s0) tick where
tick (b, s) = do
(as, s') <- tickE s
let bs = scanl f b as
return (bs, (last bs, s'))
-- Functor, Applicative instances are pretty easy and left as an exercise
runFRP tick (Behavior b0 (Event s0 e)) = runFRP' b0 s0 where
runFRP' b s = do
(bs, s') <- e s0
let val = last (b:bs)
k <- tick b
when k $ runFRP tick (Behavior
k <- tick b
-- sample application
keypress :: Event Char
keypress = pollEvent getCurrentPressedKeys where
getCurrentPressedKeys = undefined -- exercise for the reader
On Mon, Apr 25, 2011 at 5:28 PM, Edward Amsden
As far as I can tell, with classic FRP implementations (those which use behaviors as a first-class abstraction), the only way to create a behavior or event based on some external input (for instance keypresses or microphone input) is to do something with unsafePerformIO or unsafeInterleaveIO. A behavior is a value, which when evaluated at a specific time would have to either block its evaluation until input could be read, or check the input at that particular time.
Is there any other way of implementing external behaviors besides that?
-- Edward Amsden Student Computer Science Rochester Institute of Technology www.edwardamsden.com
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