`operation` seems questionable. IMO, the verbs seem stronger / more important than mere optional suggestions (particularly "open" vs. "edit" for files with read-only viewers), and only Windows supports them (so anyone requiring that feature might as well just use startfile() directly). By virtue of this function being cross-platform, we're kinda limited to just supporting the lowest common denominator.

Hobs, can you explain `gui`?

Also, does startfile() raise exceptions for either of the basic error conditions ("no such file" and "no associated application")? If not, I believe using the lower-level ShellExecute (http://msdn.microsoft.com/en-us/library/windows/desktop/bb762153%28v=vs.85%29.aspx ) or similar Windows API function would allow us to report such errors, as the other platform implementations currently do.