Translator – Expression Type (Failed Design)

The first idea on how to implement expression type checking into the Translator failed. It's not necessary to go into the details of the design, but basically the idea was to set the expression type at the start of the expression. For assignments, the expression type would be the same as the variable(s) being assigned. For PRINT statements, any expression type would be allowed, but would be set based on the first operand. For INPUT PROMPT, the expression type would be string. However, certain expressions would trip this up. First consider this statement:

Z = A$ + B$ + C$

Currently this would report an “expected numeric expression” at the second plus. It should report the error at the A$. The idea was at the equal, the expression type would be set to numeric, then upon seeing A$, which is a string, an error would be reported. However, consider this valid statement:

Z = A$ + B$ < C$

The Translator can't set the expression type to numeric and then report the error at the A$ because the expression eventually becomes numeric at the less than.

The bottom line is that the Translator can't determine if the expression is an the correct type until the entire expression is translated, but, still needs to report at the first instance where the data type error occurs, not at the last operator processed. The goal now is to get the new assignment implementation working (needed first anyway), and then get the expression type handling implemented...