[SOLVED] Does haskell run all IO actions concurrently?

Does haskell run all IO actions concurrently?

I am trying the following code to check time differences between function calls:

module Main where

import Control.Concurrent
import Data.Time.Clock

printTimeDiff :: UTCTime -> IO ()
printTimeDiff t0 = do
  t <- getCurrentTime
  let tt = realToFrac $ diffUTCTime t t0
  print (tt :: Double)
  -- threadDelay 1
  printTimeDiff t

main :: IO ()
main = do
  t <- getCurrentTime
  printTimeDiff t

It prints zeros in 70%:

0.0
0.0
0.0
0.0
0.0
0.0
1.0e-3
1.0032e-3
9.994e-4
0.0
0.0
0.0
0.0
0.0
0.0
9.984e-4
0.0
0.0
0.0
0.0
0.0
0.0
0.0
9.993e-4
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
1.0007e-3
0.0
0.0
0.0
0.0

If I uncomment the line threadDelay 1 - it print real time differences.

Why is that?

Is it caused by thread manager running a few IO actions simultaneously? If so - how could I prevent it?

Or it relates to buffering used by print?

Solution

Probably getCurrentTime is accessing a clock that's too imprecise for the speeds your code is running at. Consider using one of the higher-resolution timers, using e.g. the clock package.