[Haskell-cafe] First call to putStr takes far more time than subsequent calls

Ben Gunton ben.gunton at gmail.com
Thu Jul 9 20:17:50 UTC 2015

This is a simplified version of an issue I'm running into when timing some

In this example, the first putStr inside the replicateM_ takes far longer
than subsequent putStrs. If I add in a putStr at the top (the commented
line), each iteration takes the same. (Though I presume I just shifted the
long execution to the putStr on top).

The same thing happens on the first iterations of a function when I need to
evaluate some state, or deepseq some value. putStr was just a simpler
version that still showed the same issue.

Relevant code: http://lpaste.net/136250

What's going on here?


import Control.Monadimport Data.Time.Clock
main :: IO ()main = do
    -- putStr ""
    replicateM_ 5 $ do
        t1 <- getCurrentTime
        putStr ""
        t2 <- getCurrentTime
        print . round $ 1000 * 1000 * diffUTCTime t2 t1
Sample output:
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