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diff --git a/XMonad/Util/SpawnOnce.hs b/XMonad/Util/SpawnOnce.hs
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+{-# LANGUAGE DeriveDataTypeable #-}
+
+-----------------------------------------------------------------------------
+-- |
+-- Module      :  XMonad.Util.ExtensibleState
+-- Copyright   :  (c) Spencer Janssen 2009
+-- License     :  BSD3-style (see LICENSE)
+--
+-- Maintainer  :  spencerjanssen@gmail.com
+-- Stability   :  unstable
+-- Portability :  not portable
+--
+-- A module for spawning a command once, and only once.  Useful to start
+-- status bars and make session settings inside startupHook.
+--
+-----------------------------------------------------------------------------
+
+module XMonad.Util.SpawnOnce (spawnOnce) where
+
+import XMonad
+import Data.Set as Set
+import XMonad.Util.ExtensibleState
+import Control.Monad
+
+data SpawnOnce = SpawnOnce { unspawnOnce :: (Set String) }
+    deriving (Read, Show, Typeable)
+
+instance ExtensionClass SpawnOnce where
+    initialValue = SpawnOnce $ Set.empty
+    extensionType = PersistentExtension
+
+-- | The first time 'spawnOnce' is executed on a particular command, that
+-- command is executed.  Subsequent invocations for a command do nothing.
+spawnOnce :: String -> X ()
+spawnOnce xs = do
+    b <- fmap (Set.member xs . unspawnOnce) $ getState
+    when (not b) $ do
+        spawn xs
+        modifyState (SpawnOnce . Set.insert xs . unspawnOnce)