It's definitely pretty leaky (just prompting the user for a filename already breaks it; you either leak the rules for machine strings to the user or you pray to

It's definitely pretty leaky (just prompting the user for a filename already breaks it; you either leak the rules for machine strings to the user or you pray to god they don't enter something too weird). However the problem with interaction is really only one-way, since machine strings can be safely promoted to human strings. Unfortunately even concatenating unicode strings is not necessarily straightforward; for example A+B where A has a right-to-left embedding in it but you want B to display left-to-right you need to surround it with left-to-right embedding and pop direction (this actually solves any issues I can think of off the top of my head but no one does this). The really bad problems involve parsing user-provided non-programmer-oriented text (e.g. markdown). I really don't know if there's a robust way to do that.