Okay, I was stupid, having ignored my own advice to always look at the sources. I stupidly assumed IO a was not a function-type. On retrospect, I can't imagine, how that came to me. I've looked it up now, and for ghc, I found newtype IO a = IO (State# RealWorld -> (# State# RealWorld, a #)) which is more or less what I've arrived at, eventually. For hugs, it's newtype IO a = IO ((IOError -> IOResult) -> (a -> IOResult) -> IOResult), which I find somewhat more contrived. So I apologize for all the nonsense that came from that and happily agree that getChar &c are well defined values of IO a. Whether we have putStrLn "hello" >> mzero === mzero depends on what raiseIO# exactly does, I've no idea. Again, sorry for all the fuss. Thanks to (in alphabetical order) Jorge, Jules and Ketil for helping me out of my error. Daniel