Handling C ambiguity

c-ambiguity.txt

Modeling C Ambiguity.

How can we model the AST for the following C expression?

(b)-d

It’s confusing because, if b is a type, then the above is a unary negation, and if b is a value, then it is a binary subtraction.

The solution is to ignore the meaning of “-“, and treat it a bit more literally and call it a “hyphen” node in the AST.

struct Hyphen {
    Node *left; // ‘left’ can be NULL
    Node *right;
);

This Hyphen node can represent both meanings of the above expression. The ‘left’ field is optional in order to accomodate all unary uses too.

Once, the AST is built, it’s straight-forward to write a function to test whether a node is a type or a value (by using the symbol table). We can use this function to resolve the meaning of the Hyphen node.