Translator – Expression Type (Procedure)

To show how keeping the first operand will aid it reporting errors at the correct token, consider these examples again:

Z% = A$ + B$ + C$
Z% = A$ + B$ > C$

The first statement is processed in this sequence:

1. The Z% token is appended to the output and pushed to the done stack.
2. Since the mode is Command, the = token is interpreted as an assignment, so an AssignInt command is pushed to the command stack, the Z% token is popped from the done stack and the mode is set to EqualAssigment.
3. The A$ token is appended to the output and pushed to the done stack.
4. The first + is pushed to the hold stack, and being an operator, the mode is changed to Expression (further equal tokens will be interpreted as an equality operator).
5. The B$ is appended to the output next and pushed to the done stack.
6. When the second + is received, it empties the first + from the hold stack (being the same or higher precedence).
7. The first + pops the A$ and B$ from the done stack, and a +$ is appended to the output. The first operand, A$, does not contain a first operand (it's not an operator), so the +$ is pushed to the done stack with A$ as it's first operand.
8. The second + is pushed to the hold stack.
9. The C$ is appended to the output.
10. The end of statement empties the second + from the hold stack.
11. The second + pops the +$(A$) and the C$ from the done stack, and a +$ is appended to the output. The first operand, the +$(A$) has a first operand, A$, so the second +$ is pushed to the done stack with the A$ as it's first operand.
12. The assign command handler will be called since there is an assignment on the command stack, which will pop the value being assigned, the second +$, and will see that it is the wrong data type (an integer is expected), so an “expected numeric expression” is reported. But instead of pointing to the second +$, it's first operand, the A$ token, is returned.

For the second example, the >$ that would be on top of the done stack when the assign command handler is called. It's data type is an integer, which is correct, so no error occurs. However, sub-expressions in parentheses need additional handling...