CSC448: Type Checking: Adding Null [15/18]

What if we add null and allow it to be either an integer or a string:

exp ::= <INT>
      | <STRING>
      | exp "+" exp
      | exp "-" exp
      | "length" "(" exp ")"
      | "inttostring" "(" exp ")"
      | <ID>
      | "null"

        

Top-down vs bottom-up type checking.