-----------------------------------------------------------------------------
---
+-- |
-- Module : Debug.Trace
-- Copyright : (c) The University of Glasgow 2001
--- License : BSD-style (see the file libraries/core/LICENSE)
+-- License : BSD-style (see the file libraries/base/LICENSE)
--
-- Maintainer : libraries@haskell.org
-- Stability : provisional
-- Portability : portable
--
--- $Id: Trace.hs,v 1.2 2002/04/24 16:01:51 simonmar Exp $
---
--- The trace function.
+-- The 'trace' function.
--
-----------------------------------------------------------------------------
module Debug.Trace (
+ -- * Tracing
trace -- :: String -> a -> a
) where
#ifdef __GLASGOW_HASKELL__
{-# NOINLINE trace #-}
+{-|
+When called, 'trace' prints the string in its first argument to
+standard error, 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.
+-}
trace :: String -> a -> a
trace string expr = unsafePerformIO $ do
hPutStr stderr string