I feel like the error handling is the trickiest part. I remember during my compilers course I fought YACC so hard to work well and provide meaningful error messages. Bison offered some relief at the time through custom extensions. Maybe the state of the art here is much better now. It was definitely fun to do as a project and it was depressing that my course didn’t cover error reporting in any way, likely because there’s no formal theory behind how to make it readable to humans (beyond a “wow - yours is the first to do gold error messages” in the project feedback, it wasn’t mentioned in class)