Fix #3257: document that exitWith in a forkIO'd thread does not exit the process

[ghc-base.git] / System / Exit.hs

diff --git a/System/Exit.hs b/System/Exit.hs
index 1dab14f..1e74c1a 100644 (file)
--- a/System/Exit.hs
+++ b/System/Exit.hs
@@ -13,10 +13,11 @@
 -----------------------------------------------------------------------------
 
 module System.Exit
-    ( 
+    (
       ExitCode(ExitSuccess,ExitFailure)
     , exitWith      -- :: ExitCode -> IO a
     , exitFailure   -- :: IO a
+    , exitSuccess   -- :: IO a
   ) where
 
 import Prelude
@@ -26,8 +27,8 @@ import GHC.IOBase
 #endif
 
 #ifdef __HUGS__
-import Hugs.Prelude
-import Hugs.Exception
+import Hugs.Prelude (ExitCode(..))
+import Control.Exception.Base
 #endif
 
 #ifdef __NHC__
@@ -56,14 +57,19 @@ import System
 -- 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 (ExitException ExitSuccess)
+exitWith ExitSuccess = throwIO ExitSuccess
 exitWith code@(ExitFailure n)
-  | n /= 0 = throwIO (ExitException code)
+  | n /= 0 = throwIO code
 #ifdef __GLASGOW_HASKELL__
-  | otherwise = ioError (IOError Nothing InvalidArgument "exitWith" "ExitFailure 0" Nothing)
+  | otherwise = ioError (IOError Nothing InvalidArgument "exitWith" "ExitFailure 0" Nothing Nothing)
 #endif
 #endif  /* ! __NHC__ */
 
@@ -72,3 +78,9 @@ exitWith code@(ExitFailure n)
 -- 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