    -----------------------------------------------------------------------------
    -- |
    -- 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.IO
    import GHC.IO.Exception
    #endif

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

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

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

    -- | Computation 'exitWith' @code@ throws 'ExitCode' @code@.
    -- Normally this terminates the program, returning @code@ to the
    -- program's caller.
    --
    -- On program termination, the standard 'Handle's 'stdout' and
    -- 'stderr' are flushed automatically; any other buffered 'Handle's
    -- need to be flushed manually, otherwise the buffered data will be
    -- discarded.
    --
    -- 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 'ExitCode' 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 a 'SomeException', 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'.
    --
    -- Note: in GHC, 'exitWith' should be called from the main program
    -- thread in order to exit the process.  When called from another
    -- thread, 'exitWith' will throw an 'ExitException' as normal, but the
    -- exception will not cause the process itself to exit.
    --
    #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 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
    -- successfully.
    exitSuccess :: IO a
    exitSuccess = exitWith ExitSuccess