My proposal is to replace the hooks:

void PreTraceHook(long fd); void PostTraceHook(long fd); void ErrorHdrHook(long fd);

with the general hooks:

void TraceHook(char *msg) { fputs(msg, stderr); }

void ErrorHook(char *msg) { fprintf(stderr, "Fail: %s", msg); }

I am waiting forward to your comments and proposals. If you accept the idea, I will be happy to implement it.

Let's take these seperately: (1) ErrorHdrHook: yes. Most of the contents of writeErrString__ (in libraries/base/cbits/writeError.c) can be replaced by a hook, which looks like a good idea. This affects the top-level exception handler for a program and the top-level exception handler for concurrent threads. Perhaps on Windows, for a program that is compiled with -mwindows(?) we should link in a version of ErrorHook that pops up a MessageBox instead of using stderr (and fix all the other places that the RTS relies on having stdout/stderr...). (2) TraceHook: I'm not so sure about this. In my opinion, it's fine for trace to be defined as outputing to stderr: trace s e = unsafePerformIO (hPutStr stderr s >> return e) but an application should use its own version of trace if needs be. (eg. in GHC we should use our own version of trace which might use the debug console on Windows). IMO we should get rid of PreTraceHook and PostTraceHook. The difference between this and the ErrorHook is that the programmer has the option to call a different trace, but the programmer has no control over what's used for the top-level exception handler in the program, so it makes sense to allow the use of a hook there. trace shouldn't be used by library code. Cheers, Simon