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diff --git a/libraries/base/GHC/Event/TimeOut.hs b/libraries/base/GHC/Event/TimeOut.hs
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+++ b/libraries/base/GHC/Event/TimeOut.hs
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+{-# LANGUAGE NoImplicitPrelude #-}
+-------------------------------------------------------------------------------
+-- |
+-- Module      :  GHC.Event.TimeOut
+-- Copyright   :  (c) Tamar Christina 2018
+-- License     :  BSD-style (see the file libraries/base/LICENSE)
+--
+-- Maintainer  :  libraries@haskell.org
+-- Stability   :  experimental
+-- Portability :  non-portable
+--
+-- Common Timer definitions shared between WinIO and RIO.
+--
+-------------------------------------------------------------------------------
+
+module GHC.Event.TimeOut where
+
+import GHC.IO
+import GHC.Base
+
+import qualified GHC.Event.PSQ as Q
+import GHC.Event.Unique (Unique)
+
+-- | A priority search queue, with timeouts as priorities.
+type TimeoutQueue = Q.PSQ TimeoutCallback
+
+-- |
+-- Warning: since the 'TimeoutCallback' is called from the I/O manager, it must
+-- not throw an exception or block for a long period of time.  In particular,
+-- be wary of 'Control.Exception.throwTo' and 'Control.Concurrent.killThread':
+-- if the target thread is making a foreign call, these functions will block
+-- until the call completes.
+type TimeoutCallback = IO ()
+
+-- | An edit to apply to a 'TimeoutQueue'.
+type TimeoutEdit = TimeoutQueue -> TimeoutQueue
+
+-- | A timeout registration cookie.
+newtype TimeoutKey = TK Unique
+    deriving (Eq, Ord)