Hi all If I want to give warnings when doing something and I don't care too much about the order they appear in, can I use this? foo x = if success x then Just x else warn "Working out x went wrong" Nothing warn :: String -> a -> a warn s x = unsafePerformIO (hPutStrLn stderr s) `seq` x The hslibs docs say If the I/O computation wrapped in unsafePerformIO performs side effects, then the relative order in which those side effects take place (relative to the main I/O trunk, or other calls to unsafePerformIO) is indeterminate. but it's not entirely clear on whether or not I could end up with 2 warnings interspersed? And is it guaranteed that the warnings will be printed at some point? hugs and ghci only seem to print the first warning, but ISTR similar problems happen with threads and got the impression that they were in the wrong. Thanks Ian