    -----------------------------------------------------------------------------
    -- |
    -- Module      :  System.Exit
    -- Copyright   :  (c) The University of Glasgow 2001
    -- License     :  BSD-style (see the file libraries/base/LICENSE)
    --
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  provisional
    -- Portability :  portable
    --
    -- Exiting the program.
    --
    -----------------------------------------------------------------------------

    module System.Exit
        (
          ExitCode(ExitSuccess,ExitFailure)
        , exitWith      -- :: ExitCode -> IO a
        , exitFailure   -- :: IO a
        , exitSuccess   -- :: IO a
      ) where

    import Prelude

    #ifdef __GLASGOW_HASKELL__
    import GHC.IOBase
    #endif

    #ifdef __HUGS__
    import Hugs.Prelude (ExitCode(..))
    import Control.Exception.Base
    #endif

    #ifdef __NHC__
    import System
      ( ExitCode(..)
      , exitWith
      )
    #endif

    -- ---------------------------------------------------------------------------
    -- exitWith

    -- | Computation 'exitWith' @code@ throws 'ExitException' @code@.
    -- Normally this terminates the program, returning @code@ to the
    -- program's caller.  Before the program terminates, any open or
    -- semi-closed handles are first closed.
    --
    -- A program that fails in any other way is treated as if it had
    -- called 'exitFailure'.
    -- A program that terminates successfully without calling 'exitWith'
    -- explicitly is treated as it it had called 'exitWith' 'ExitSuccess'.
    --
    -- As an 'ExitException' is not an 'IOError', 'exitWith' bypasses
    -- the error handling in the 'IO' monad and cannot be intercepted by
    -- 'catch' from the "Prelude".  However it is an 'Exception', and can
    -- be caught using the functions of "Control.Exception".  This means
    -- that cleanup computations added with 'Control.Exception.bracket'
    -- (from "Control.Exception") are also executed properly on 'exitWith'.

    #ifndef __NHC__
    exitWith :: ExitCode -> IO a
    exitWith ExitSuccess = throwIO ExitSuccess
    exitWith code@(ExitFailure n)
      | n /= 0 = throwIO code
    #ifdef __GLASGOW_HASKELL__
      | otherwise = ioError (IOError Nothing InvalidArgument "exitWith" "ExitFailure 0" Nothing)
    #endif
    #endif  /* ! __NHC__ */

    -- | The computation 'exitFailure' is equivalent to
    -- 'exitWith' @(@'ExitFailure' /exitfail/@)@,
    -- where /exitfail/ is implementation-dependent.
    exitFailure :: IO a
    exitFailure = exitWith (ExitFailure 1)

    -- | The computation 'exitSuccess' is equivalent to
    -- 'exitWith' 'ExitSuccess', It terminates the program
    -- sucessfully.
    exitSuccess :: IO a
    exitSuccess = exitWith ExitSuccess