Minimal timeout library for Haskell, depending only on base.

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Timeout.hs

-- | A note: This is pretty much strictly worse than the functionality
-- offered by <hackage.haskell.org/package/async> . Use this library if
-- you cannot be bothered to depend on async.
module Timeout where

import Control.Concurrent
import Control.Exception

data TimeoutException = TimeoutException Int
instance Show TimeoutException where
  show (TimeoutException seconds) = "timed out after " <> show seconds <> " seconds."
instance Exception TimeoutException

-- | Runs the given IO action in a separate thread, and kills that thread
-- after the specified number of seconds.
--
-- If the action could not finish in the given time, a TimeoutException is thrown.
-- If the action threw some other exception, that exception is re-thrown.
timeout :: {-| seconds -} Int -> {-| action -} IO a -> IO a
timeout maxtime inner = do
  var <- newEmptyMVar
  worker <- forkFinally (inner >>= evaluate) (putMVar var)
  killer <- forkIO (threadDelay (1000000 * maxtime) >> throwTo worker (TimeoutException maxtime))
  takeMVar var >>= either throwIO pure

-- | Convenience wrapper around timeout for evaluating pure computations.
timeout' :: {-| second -} Int -> {-| value -} a -> IO a
timeout' maxtime val = timeout maxtime (pure val)