I use printf-style debugging a lot, so I am always adding and removing applications of trace. There is the Debug.Trace.traceIO function that makes this easy to do in the IO monad (it just applies hPutStrLn stderr), but is that specialisation to IO unnecessary?

I find myself always using this utility function:

traceM :: (Monad m) => String -> m ()

traceM message = trace message $ return ()

Which can be used to implement traceIO.

traceIO :: String -> IO ()

traceIO = traceM

That way I can traceM in whatever monad I am in. It is easier to add and remove an application of trace, because I do not have to wrap it around something, and I do not have to remember the return. Of course, you are still subject to however your monad chooses to order the execution of these traces, but that is what people expect when using trace.

Would traceM be a wanted addition to Debug.Trace?

Regards,

Chris Seaton