Precedence ambiguity in ANTLR 4 - antlr4

Question

I have a huge ANTLR grammar, and I am facing problem with a small piece of it. Grammar has two rules expr and sets as defined below:
expr:
id
|(PLUS|MINUS|MULTIPLY|AND|NEGATION)expr
| expr (MULTIPLY |DIVIDE| MODULO)
| expr (PLUS | MINUS) expr
;
set:
EMPTY
| MULTIPLY set
| set PLUS set
| UNION '(' set (COMMA set)* ')'
| INTER '(' set (COMMA set)* ')'
| expr
;
The problem here is that for a set of form *s1 + *s2 should be reduced as following:
set -> set PLUS set
and then each set in RHS should reduce to:
set -> MULTIPLY set
set -> expr
term -> id
But instead they are reducing as:
set -> MULTIPLY set
set -> expr
expr -> expr PLUS expr
Because of whichset of forn *s1 +*s2 is parsed as *(s1 + *s2) instead of (*s1) + (*s2).
One of the rules of set, reduces it to expr. There are many other similar rules in grammar which reduces to expr. The problem is occurring here becuase some of the rules in set and expr are similar. But because some rules are different, I cannot merge them together.
In set even though the precedence of rule MULTIPLY set is higher than set PLUS set, set is reduced by MUTIPLY set rule.
Is there a way to fix this issue?
EDIT:
Adding a working example :
Grammar:
grammar T;
expr
: ID
| ( PLUS | MINUS | MULTIPLY | AND | NEGATION ) expr
| expr ( MULTIPLY | DIVIDE | MODULO )
| expr ( PLUS | MINUS ) expr
;
set:
EMPTY
| MULTIPLY set
| set PLUS set
| UNION '(' set (COMMA set)* ')'
| INTER '(' set (COMMA set)* ')'
| expr
;
ID : [a-zA-Z] [a-zA-Z0-9]*;
PLUS : '+';
MINUS : '-';
MULTIPLY : '*';
AND : '&&';
NEGATION : '!';
DIVIDE : '/';
MODULO : '%';
COMMA : ',';
EMPTY: '\\empty';
UNION: '\\union';
INTER: '\\inter';
SPACES : [ \t\r\n] -> skip;
Code to execute it:
TLexer lexer = new TLexer(new ANTLRInputStream("*s1 + *s2"));
TParser parser = new TParser(new CommonTokenStream(lexer));
RuleContext tree = parser.set();
tree.inspect(parser);
Output it generated:
set
/ \
* set
|
expr
/ | \
/ | \
expr + expr
| / \
s1 * expr
|
s2

Answer 1

I can't reproduce this.
Given the grammar:
grammar T;
expr
: ID
| ( PLUS | MINUS | MULTIPLY | AND | NEGATION ) expr
| expr ( MULTIPLY | DIVIDE | MODULO )
| expr ( PLUS | MINUS ) expr
;
ID : [a-zA-Z] [a-zA-Z0-9]*;
PLUS : '+';
MINUS : '-';
MULTIPLY : '*';
AND : '&&';
NEGATION : '!';
DIVIDE : '/';
MODULO : '%';
SPACES : [ \t\r\n] -> skip;
your input *s1 + *s2 will be parsed as:
expr
/ | \
/ | \
expr + expr
/ \ / \
* expr * expr
| |
s1 s2
Or, in plain code:
TLexer lexer = new TLexer(new ANTLRInputStream("*s1 + *s2"));
TParser parser = new TParser(new CommonTokenStream(lexer));
System.out.println(parser.expr().toStringTree(parser));
will print:
(expr (expr * (expr s1)) + (expr * (expr s2)))