[project @ 2002-05-10 16:18:28 by simonmar]
[packages/base.git] / Debug / Trace.hs
1 -----------------------------------------------------------------------------
2 -- |
3 -- Module : Debug.Trace
4 -- Copyright : (c) The University of Glasgow 2001
5 -- License : BSD-style (see the file libraries/base/LICENSE)
6 --
7 -- Maintainer : libraries@haskell.org
8 -- Stability : provisional
9 -- Portability : portable
10 --
11 -- The 'trace' function.
12 --
13 -----------------------------------------------------------------------------
14
15 module Debug.Trace (
16 -- * Tracing
17 trace -- :: String -> a -> a
18 ) where
19
20 import Prelude
21 import System.IO.Unsafe
22 import System.IO
23
24 #ifdef __GLASGOW_HASKELL__
25 import GHC.IOBase
26 import GHC.Handle
27 #endif
28
29 #ifdef __GLASGOW_HASKELL__
30 {-# NOINLINE trace #-}
31 {-|
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.
38 -}
39 trace :: String -> a -> a
40 trace string expr = unsafePerformIO $ do
41 hPutStr stderr string
42 hPutChar stderr '\n'
43 fd <- withHandle_ "trace" stderr $ (return.haFD)
44 postTraceHook fd
45 return expr
46
47 foreign import ccall "PostTraceHook" postTraceHook :: Int -> IO ()
48 #endif