-- 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
+#else
+import System.IO (hPutStrLn,stderr)
+#endif
+
+-- | 'putTraceMsg' function outputs the trace message from IO monad.
+-- Usually the output stream is 'System.IO.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__
+ hPutStrLn stderr msg
+#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