> One more idea I didn't see is to mark the variable as "invalid". A general "this variable doesn't have a value because the program did something bad", which c

> One more idea I didn't see is to mark the variable as "invalid". A general "this variable doesn't have a value because the program did something bad", which carries a full stack trace with itself. A simple way to handle exceptions like that (and I didn't see it mentioned among the answers) is to return 2-tuple from the function: result value and error information. The latter could be empty, but both could be present. Though this might be overcomplicating things, given that OP targets the language at novices.