`forever`: How to forward information to next iteration? - loops

Question

There is a thread waiting for new input in a queue to safe it to the file system. It also creates backup copies. The sscce looks like this:
import Control.Concurrent
import Control.Concurrent.STM
import Control.Monad
import Data.Time.Clock.POSIX
main :: IO ()
main = do
contentQueue <- atomically $ newTQueue
_ <- forkIO $ saveThreadFunc contentQueue
forever $ do
line <- getLine
atomically $ writeTQueue contentQueue line
saveThreadFunc :: TQueue String -> IO ()
saveThreadFunc queue = forever $ do
newLine <- atomically $ readTQueue queue
now <- round `fmap` getPOSIXTime :: IO Int
writeFile "content.txt" newLine
-- todo: Backup no more than once every 86400 seconds (24 hours).
backupContent now newLine
backupContent :: Int -> String -> IO ()
backupContent t = writeFile $ "content.backup." ++ show t
Now it would be great if the backup would not be written more than once every 24 hours. In imperative programming I would probably use a mutable int lastBackupTime inside the "forever loop" in saveThreadFunc. How can the same effect be achieved in Haskell?

Answer 1

How about Control.Monad.Loops.iterateM_? This is slightly neater as it avoids explict recursion.
iterateM_ :: Monad m => (a -> m a) -> a -> m b
saveThreadFunc :: TQueue String -> Int -> IO ()
saveThreadFunc queue = iterateM_ $ \lastBackupTime -> do
newLine <- atomically $ readTQueue queue
now <- round `fmap` getPOSIXTime :: IO Int
writeFile "content.txt" newLine
let makeNewBackup = now >= lastBackupTime + 86400
when makeNewBackup (backupContent now newLine)
return (if makeNewBackup then now else lastBackupTime)

Answer 2

Replace forever with explicit recursion.
foo :: Int -> IO ()
foo n = do
use n
foo (n+1)
Of course, you can use any type for your state instead of Int.
Otherwise, if you really want the mutable state:
foo :: IO ()
foo = do
r <- newIORef (0 :: Int)
forever $ do
n <- readIORef r
use n
writeIORef r (n+1)
Unless you really need mutability for some other reason, I'd not recommend the second option.
Adapting the above idea to the concrete code:
saveThreadFunc :: Int -> TQueue String -> IO ()
saveThreadFunc lastBackupTime queue = do
newLine <- atomically $ readTQueue queue
now <- round `fmap` getPOSIXTime :: IO Int
writeFile "content.txt" newLine
let makeNewBackup = now >= lastBackupTime + 86400
if makeNewBackup then do
backupContent now newLine
saveThreadFunc now queue
else
saveThreadFunc lastBackupTime queue

Answer 3

The usual way to add state to a monad is using StateT from Control.Monad.Trans.State.Strict in the transformers package (part of the Haskell Platform). In this case, you would change the type of saveThreadFunc:
saveThreadFunc :: TQueue String -> StateT Int IO ()
You'd have to Control.Monad.Trans.lift the actual IO things to StateT Int IO, and then in the end evalStateT to turn the whole thing into IO a.
This approach is perhaps more modular than the the iterateM_ one Tom Ellis suggests (although that's something of a matter of taste), and will generally be optimized better than the IORef version chi suggests you avoid.