Simple threadsafe stopwatch

stopwatch.hs

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-- |
-- Module : Database.EventStore.Internal.Stopwatch
-- Copyright : (C) 2017 Yorick Laupa
-- License : (see the file LICENSE)
--
-- Maintainer : Yorick Laupa <yo.eight@gmail.com>
-- Stability : provisional
-- Portability : non-portable
--
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module Database.EventStore.Internal.Stopwatch
  ( Stopwatch
  , newStopwatch
  , stopwatchElapsed
  ) where

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import ClassyPrelude
import Data.Time

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data Internal =
  Internal { _lastTime :: !UTCTime
           , _acc      :: !NominalDiffTime
           }

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initInternal :: UTCTime -> Internal
initInternal now = Internal now 0

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update :: UTCTime -> Internal -> Internal
update now (Internal before acc) = Internal now acc'
  where
    acc' = acc + diffUTCTime now before

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newtype Stopwatch = Stopwatch (MVar Internal)

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newStopwatch :: IO Stopwatch
newStopwatch = fmap Stopwatch . newMVar . initInternal =<< getCurrentTime

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stopwatchElapsed :: Stopwatch -> IO NominalDiffTime
stopwatchElapsed (Stopwatch var) =
  modifyMVar var $ \prev -> do
    now <- getCurrentTime
    let next = update now prev
    return (next, _acc next)