Hi, Recently, I've been annoyed how Win32-package throws user errors, ie String, on failures. The System.Win32-being rather low level, it often makes sense for caller to catch some specific errors. I don't really understand how, or if, I could extend System.IO.Error. unix- package uses it via Foreign.C.Error, which makes me wonder if the underlying functionality in there should be extended. Is the only or best choice to go for Dynamic exceptions? That introduces a subtle difference for (already existing) programs that rely on exceptions to be presentable via Show and default handler. Best regards, --Esa Ilari Vuokko