    {-# LANGUAGE DeriveDataTypeable #-}

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  XMonad.Util.SpawnOnce
    -- 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 qualified XMonad.Util.ExtensibleState as XS
    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 <- XS.gets (Set.member xs . unspawnOnce)
        when (not b) $ do
            spawn xs
            XS.modify (SpawnOnce . Set.insert xs . unspawnOnce)