Why do parentheses create a left-recursion?-CodePudding

The following grammar works fine:

grammar DBParser;
statement: expr EOF;
expr: expr ' ' expr | Atom;
Atom: [a-z]  | [0-9]  ;

However, neither of the following do:

grammar DBParser;
statement: expr EOF;
expr: (expr ' ' expr) | Atom;
Atom: [a-z]  | [0-9]  ;

grammar DBParser;
statement: expr EOF;
expr: (expr ' ' expr | Atom);
Atom: [a-z]  | [0-9]  ;

Why does antlr4 raise an error when adding in parentheticals, does that somehow change the meaning of the production that is being parsed?

CodePudding user response：

Parentheses create a subrule, and subrules are handled internally by treating them as though they were new productions (in effect anonymous, which is why the mutual recursion error message only lists one non-terminal).

In these particular examples, the subrule is pointless; the parentheses could simply be removed without altering the grammar. But apparently Antlr doesn't attempt to decide which subrules are actually serving a purpose. (I suppose it could, but I wonder if it's a common enough usage to make justify the additional code complexity. But it's certainly not up to me to decide.)