Haskell: be careful when measuring elapsed time

Measure.hs

-- | >>> measure (return $ sum [1..100000000])
-- (5000000050000000,1.892e-6)
measure :: IO a -> IO (a, Double)
measure io = do
  start <- getMonotonicTimeNSec
  a <- io
  end <- getMonotonicTimeNSec
  return (a, fromIntegral (end - start) / 1_000_000_000)

-- | >>> measure (return $ sum [1..100000000])
-- (5000000050000000,1.329306812)
measure' :: NFData a => IO a -> IO (a, Double)
measure' io = do
  start <- getMonotonicTimeNSec
  a <- io
  end <- a `deepseq` getMonotonicTimeNSec
  return (a, fromIntegral (end - start) / 1_000_000_000)