Write stdin to a file and rewrite the file after a timeout

rewrite-after-timeout.hs

-- See http://jakob.keramik-schoettl.de/blog/
-- usage: rewrite-after-timeout SECONDS FILENAME

import Prelude
import System.IO
import System.Timeout
import System.Environment
import Data.Time
import Data.Maybe
import Control.Monad

main :: IO ()
main = do
    pause <- read . head <$> getArgs :: IO Integer
    filename <- head . tail <$> getArgs :: IO String
    start <- getCurrentTime
    getLineAndAppendOrReplace pause filename start

getLineAndAppendOrReplace :: Integer -> FilePath -> UTCTime -> IO ()
getLineAndAppendOrReplace pause filename lastAppend = do
    maybeLine <- timeout 100000 getLine -- timeout in microseconds
    time <- case maybeLine of
              Just line -> appendLine pause filename lastAppend line
              Nothing   -> pure lastAppend
    getLineAndAppendOrReplace pause filename time

appendLine :: Integer -> FilePath -> UTCTime -> String -> IO UTCTime
appendLine pause filename lastAppend line = do
    now <- getCurrentTime
    let mode = if diffUTCTime now lastAppend > realToFrac pause
        then WriteMode
        else AppendMode
    file <- openFile filename mode
    hPutStrLn file line
    hClose file
    pure now