How to tell if an identifier is being assigned or referenced? (FLEX/BISON)

Question

So, I'm writing a language using flex/bison and I'm having difficulty with implementing identifiers, specifically when it comes to knowing when you're looking at an assignment or a reference, for example:

1) A = 1+2

2) B + C (where B and C have already been assigned values)

Example one I can work out by returning an ID token from flex to bison, and just following a grammar that recognizes that 1+2 is an integer expression, putting A into the symbol table, and setting its value.

examples two and three are more difficult for me because: after going through my lexer, what's being returned in ex.2 to bison is "ID PLUS ID" -> I have a grammar that recognizes arithmetic expressions for numerical values, like INT PLUS INT (which would produce an INT), or DOUBLE MINUS INT (which would produce a DOUBLE). if I have "ID PLUS ID", how do I know what type the return value is?

Here's the best idea that I've come up with so far: When tokenizing, every time an ID comes up, I search for its value and type in the symbol table and switch out the ID token with its respective information; for example: while tokenizing, I come across B, which has a regex that matches it as being an ID. I look in my symbol table and see that it has a value of 51.2 and is a DOUBLE. So instead of returning ID, with a value of B to bison, I'm returning DOUBLE with a value of 51.2

I have two different solutions that contradict each other. Here's why: if I want to assign a value to an ID, I would say to my compiler A = 5. In this situation, if I'm using my previously described solution, What I'm going to get after everything is tokenized might be, INT ASGN INT, or STRING ASGN INT, etc... So, in this case, I would use the former solution, as opposed to the latter.

My question would be: what kind of logical device do I use to help my compiler know which solution to use?

NOTE: I didn't think it necessary to post source code to describe my conundrum, but I will if anyone could use it effectively as a reference to help me understand their input on this topic.

Thank you.

Answer 1

If possible redesign your language so that the situation is unambiguous. This is why even Javascript has var.

Otherwise you're going to need to disambiguate via semantic rules, for example that the first use of an identifier is its declaration. I don't see what the problem is with your case (2): just generate the appropriate code. If B and C haven't been used yet, a value-reading use like this should be illegal, but that involves you in control flow analysis if taken to the Nth degree of accuracy, so you might prefer to assume initial values of zero.

In any case you can see that it's fundamentally a language design problem rather than a coding problem.

Answer 2

The usual way is to have a yacc/bison rule like:

expr: ID { $$ = lookupId($1); }

where the the lookupId function looks up a symbol in the symbol table and returns its type and value (or type and storage location if you're writing a compiler rather than a strict interpreter). Then, your other expr rules don't need to care whether their operands come from constants or symbols or other expressions:

expr: expr '+' expr { $$ = DoAddition($1, $3); }

The function DoAddition takes the types and values (or locations) for its two operands and either adds them, producing a result, or produces code to do the addition at run time.