How to parse a parenthesized hierarchy root? - antlr

Question

I'm trying to parse values with ANTLR. Here's the relevant part of my grammar:
root : IDENTIFIER | SELF | literal | constructor | call | indexer;
hierarchy : root (SUB^ (IDENTIFIER | call | indexer))*;
factor : hierarchy ((MULT^ | DIV^ | MODULO^) hierarchy)*;
sum : factor ((PLUS^ | MINUS^) factor)*;
comparison : sum (comparison_operator^ sum)*;
value : comparison | '(' value ')';
I won't describe each token or rule since their name is quite explanatory of their role. This grammar works well and compiles, allowing me to parse, using value, things such as:
a.b[c(5).d[3] * e()] < e("f")
The only thing left for value recognition is to be able to have parenthesized hierarchy roots. For instance:
(a.b).c
(3 < d()).e
...
Naively, and without much expectations, I tried adding the following alternative to my root rule:
root : ... | '(' value ')';
This however breaks the value rule due to non-LL(*)ism:
rule value has non-LL(*) decision due to recursive rule invocations reachable
from alts 1,2. Resolve by left-factoring or using syntactic predicates or using
backtrack=true option.
Even after reading most of The Definitive ANTLR Reference, I still don't understand these errors. However, what I do understand is that, upon seeing a parenthesis opening, ANTLR cannot know if it's looking at the beginning of a parenthesized value, or at the beginning of a parenthesized root.
How can I clearly define the behavior of parenthesized hierarchy root?
Edit: As requested, the additional rules:
parameter : type IDENTIFIER -> ^(PARAMETER ^(type IDENTIFIER));
constructor : NEW type PAREN_OPEN (arguments+=value (SEPARATOR arguments+=value)*)? PAREN_CLOSE -> ^(CONSTRUCTOR type ^(ARGUMENTS $arguments*)?);
call : IDENTIFIER PAREN_OPEN (values+=value (SEPARATOR values+=value)*)? PAREN_CLOSE -> ^(CALL IDENTIFIER ^(ARGUMENTS $values*)?);
indexer : IDENTIFIER INDEX_START (values+=value (SEPARATOR values+=value)*)? INDEX_END -> ^(INDEXER IDENTIFIER ^(ARGUMENTS $values*));

Answer 1

Remove '(' value ')' from value and place it in root:
root : IDENTIFIER | SELF | literal | constructor | call | indexer | '(' value ')';
...
value : comparison;
Now (a.b).c will result in the following parse:
And (3 < d()).e in:
Of course, you'll probably want to omit the parenthesis from the AST:
root : IDENTIFIER | SELF | literal | constructor | call | indexer | '('! value ')'!;
Also, you don't need to append tokens in a List using += in your parser rules. The following:
call
: IDENTIFIER PAREN_OPEN (values+=value (SEPARATOR values+=value)*)? PAREN_CLOSE
-> ^(CALL IDENTIFIER ^(ARGUMENTS $values*)?)
;
can be rewritten into:
call
: IDENTIFIER PAREN_OPEN (value (SEPARATOR value)*)? PAREN_CLOSE
-> ^(CALL IDENTIFIER ^(ARGUMENTS value*)?)
;
EDIT
Your main problem is the fact that certain input can be parsed in two (or more!) ways. For example, the input (a) could be parsed by alternative 1 and 2 of your value rule:
value
: comparison // alternative 1
| '(' value ')' // alternative 2
;
Run through your parser rules: a comparison (alternative 1) can match (a) because it matches the root rule, which in its turn matches '(' value ')'. But that is also what alternative 2 matches! And there you have it: the parser "sees" for one input, two different
parses and reports about this ambiguity.