I am writing a program in Haskell which repeatedly takes its most recent result and uses this to compute the next result. I want to be able to see the newest result in response to user input, so I tried something like this:
main = mainhelper 0
mainhelper count = do
count <- return (count + 1)
line <- getLine
if null line
then do mainhelper count
else do
putStrLn $ show count
return ()
I was hoping that getLine would return an empty line if the user hasn't entered anything, but this doesn't happen, instead the program does nothing until it receives user input. Is there a way around this?
One simple solution is to fork a thread for the complicated computation and communicate with the main UI thread via MVar. For example:
import Control.Exception
import Control.Monad
import Control.Concurrent
thinkReallyHard x = do
threadDelay 1000000 -- as a proxy for something that's actually difficult
evaluate (x+1)
main = do
v <- newMVar 0
forkIO (forever (modifyMVar_ v thinkReallyHard))
forever (getLine >> readMVar v >>= print)
You may wonder about the role of evaluate in thinkReallyHard. The subtlety there is that MVars are lazy -- they can contain thunks just as easily as computed values. In particular, this means it's easy to accidentally push all the pure computation from the forked thread into the thread that's reading and using the contents of the MVar. The call to evaluate simply forces the forked thread to finish the pure computation before writing to the MVar.
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