Hi, An earlier version of this question is cross-posted at StackOverflow http://stackoverflow.com/questions/26437770/timing-inaccuracy-in-haskell-thr... . What should I use for precise (and eventually, concurrency-compatible) timing in Haskell? I want to make rhythms. I tried the following to produce a repeating rhythm in which one note is twice as long as the other two. (That rhythm is encoded by the list [1,1,2].) import Control.Concurrent import Text.Printf import Control.Monad main = mapM_ note (cycle [1,1,2]) beat = round (10^6 / 4) -- measured in microseconds note :: Int -> IO () note n = do threadDelay $ beat * n printf "\BEL\n" When I run it the long note is three times as long as the others, not twice. If I speed it up, by changing the number 4 to a 10, the rhythm is destroyed completely: the notes all have the same length. threadDelay is guaranteed http://hackage.haskell.org/package/base-4.7.0.1/docs/Control-Concurrent.html... to wait at least as long, but potentially longer than, what the caller specifies. Another potential problem could be buffering in printf. (Eventually I intend to replace the printf statement with OSC output to a sound generator.) Thanks, Jeff