[project @ 2005-01-29 16:10:27 by wolfgang]

[ghc-base.git] / Debug / Trace.hs

diff --git a/Debug/Trace.hs b/Debug/Trace.hs
index 6f1ce82..5bc7128 100644 (file)
--- a/Debug/Trace.hs
+++ b/Debug/Trace.hs
@@ -8,32 +8,50 @@
 -- Stability   :  provisional
 -- Portability :  portable
 --
--- The trace function.
+-- The 'trace' function.
 --
 -----------------------------------------------------------------------------
 
 module Debug.Trace (
-       trace -- :: String -> a -> a
+       -- * Tracing
+       putTraceMsg,      -- :: String -> IO ()
+       trace             -- :: String -> a -> a
   ) where
 
 import Prelude
 import System.IO.Unsafe
-import System.IO
 
 #ifdef __GLASGOW_HASKELL__
-import GHC.IOBase
-import GHC.Handle
+import Foreign.C.String
+#endif
+
+-- | 'putTraceMsg' function outputs the trace message from IO monad.
+-- Usually the output stream is 'stderr' but if the function is called
+-- from Windows GUI application then the output will be directed to the Windows
+-- debug console.
+putTraceMsg :: String -> IO ()
+putTraceMsg msg = do
+#ifndef __GLASGOW_HASKELL__
+    hPutStr handle msg
+    hPutChar handle '\n'
+#else
+    withCString "%s\n" $ \cfmt ->
+     withCString msg  $ \cmsg ->
+      debugBelch cfmt cmsg
+
+foreign import ccall unsafe debugBelch :: CString -> CString -> IO ()
 #endif
 
-#ifdef __GLASGOW_HASKELL__
 {-# NOINLINE trace #-}
+{-|
+When called, 'trace' outputs the string in its first argument, before 
+returning the second argument as its result. The 'trace' function is not 
+referentially transparent, and should only be used for debugging, or for 
+monitoring execution. Some implementations of 'trace' may decorate the string 
+that\'s output to indicate that you\'re tracing. The function is implemented on
+top of 'putTraceMsg'.
+-}
 trace :: String -> a -> a
 trace string expr = unsafePerformIO $ do
-    hPutStr stderr string
-    hPutChar stderr '\n'
-    fd <- withHandle_ "trace" stderr $ (return.haFD)
-    postTraceHook fd
+    putTraceMsg string
     return expr
-
-foreign import ccall "PostTraceHook" postTraceHook :: Int -> IO ()
-#endif