Adds the time to every line.

chrono-legionnaire.hs

#!/usr/bin/env stack
-- stack --resolver ghc-8.2.1 script --package time
import Data.Time
import Data.IORef
import Text.Printf

main :: IO ()
main = do
  start <- getCurrentTime
  before <- newIORef start
  contents <- getContents
  mapM_ (decorate start before) (lines contents)

decorate :: UTCTime -> IORef UTCTime -> String -> IO ()
decorate start before line = do
  now <- getCurrentTime
  let elapsed = diffUTCTime now start
  step <- atomicModifyIORef before (getStep now)
  printf "%s %f %f %s\n" (iso8601 now) (toDouble elapsed) (toDouble step) line

getStep :: UTCTime -> UTCTime -> (UTCTime, NominalDiffTime)
getStep now before = (now, diffUTCTime now before)

iso8601 :: UTCTime -> String
iso8601 = formatTime defaultTimeLocale "%Y-%m-%dT%H:%M:%S.%q%z"

toDouble :: NominalDiffTime -> Double
toDouble = realToFrac

example-output.txt

$ echo -e "hello\nworld" | ./chrono-legionnaire.hs
2017-10-05T14:30:11.137545000000+0000 0.00024 0.00024 hello
2017-10-05T14:30:11.137874000000+0000 0.000569 0.000329 world