1 -----------------------------------------------------------------------------
3 -- Module : Debug.Trace
4 -- Copyright : (c) The University of Glasgow 2001
5 -- License : BSD-style (see the file libraries/base/LICENSE)
7 -- Maintainer : libraries@haskell.org
8 -- Stability : provisional
9 -- Portability : portable
11 -- The 'trace' function.
13 -----------------------------------------------------------------------------
17 trace -- :: String -> a -> a
21 import System.IO.Unsafe
24 #ifdef __GLASGOW_HASKELL__
29 #ifdef __GLASGOW_HASKELL__
30 {-# NOINLINE trace #-}
32 When called, 'trace' prints the string in its first argument to
33 standard error, before returning the second argument as its result.
34 The 'trace' function is not referentially transparent, and should only
35 be used for debugging, or for monitoring execution. Some
36 implementations of 'trace' may decorate the string that\'s output to
37 indicate that you\'re tracing.
39 trace :: String -> a -> a
40 trace string expr = unsafePerformIO $ do
43 fd <- withHandle_ "trace" stderr $ (return.haFD)
47 foreign import ccall "PostTraceHook" postTraceHook :: Int -> IO ()