Comment by amluto

> UTF-8 was designed such that invalid data can be detected and handled, without affecting the meaning of valid subsequences in the same string.

But there is not a canonical response to invalid data. So literally every operation that might need to make a choice of what to do when presented what invalid data should either (a) accept a parameter asking what to do on error and potentially fail or (b) take a parameter type that forces errors to be handled in advance.