Re: [reactive] FRP, continuous time and concurrency

10 Jun 2009

      ...
..until further notice, just assume "broken".
Useful to know.  I shall postpone its use in critical projects.

BTW, s/joinMaybes/justE works quickly, in minimal CPU and memory.  Forgot
about that function.  Curious that the operational semantics have become so
odd though; I'm used to denotational ones being off -- late updates and so
forth -- but I haven't come across quite that level of resource consumption
in reactive code before.

Freddie

2009/6/10 Svein Ove Aas 
...
...
This is a silly example.  Console lines "b = x" update the value of b; "c
=
y" likewise; lines starting "a" cause the current value of a to be
2009/6/10 Freddie Manners :
printed.
...
module Main
   where
import FRP.Reactive
import FRP.Reactive.LegacyAdapters
import Data.List
import Control.Monad
import Control.Concurrent
import Control.Applicative
parseEvent :: String -> Event String -> Event Integer
parseEvent s = fmap read . joinMaybes . fmap (stripPrefix s)
main :: IO ()
main = do
      cl    <- makeClock
      (s,e) <- makeEvent cl
      forkIO . forever $ getLine >>= s
      let b = stepper 0 $ parseEvent "b =" e
      let c = stepper 0 $ parseEvent "c =" e
      let p = parseEvent "a" e
      let a = liftA2 (+) b c -- the only interesting line
adaptE . fmap print $ snapshot_ a p
So yes, this does use explicit concurrency because "feeding" the reactive
events (with getLine) and printing the answers must happen in different
threads.
Interestingly, this fairly simple program gobbles CPU and RAM on
reactive-0.11, as well as running with a bit of a lag.  Could joinMaybes
be
...
to blame?  I don't know how happy the Monad instance of Event is these
days.
"Fundamentally broken" about covers it.
Well, to be specific, joinE is broken, and looks hard to fix.
The Monoid instance for Event is also broken, but I think only when
all Events involved are finite.
Further, I was trying to fix it, but GHC is broken.
I'd also like to note that LegacyAdapters is broken. I've got a fix
for the broken bits, which happens to break everything else. Blocked
on another GHC bug, though.
..until further notice, just assume "broken".
--
Svein Ove Aas