Left recursive in ANTLR4 - recursion

Question

I have this below code :
grammar Hello;
expr : expr ('*'|'/') expr
| expr ('+'|'-') expr
| (expr '[' expr ']')
| ID
| INT
;
ID : ID_LETTER (ID_LETTER | DIGIT)* ; // INDENTIFIER
fragment ID_LETTER : [a-z] | [A-Z] | '_' ;
fragment DIGIT : [0-9] ;
INT : DIGIT+ ;
WS : [ \t\r\n]+ -> skip;
I want to build a index expression but i have an error :
mutually left-recursive
in the 4th line of my code
I had tried :
expr '[' expr ']'// without (
The error was gone, but when i put an input : a[2] to the rule expr, it has an error:
Hello::expr:1:0: mismatched input 'a[2]' expecting {BOOL, ID, INT, FLOAT, STRING}

Answer 1

I cannot reproduce the error you mention.
Given the grammar:
grammar Hello;
expr
: expr ('*'|'/') expr
| expr ('+'|'-') expr
| expr '[' expr ']'
| ID
| INT
;
ID : ID_LETTER (ID_LETTER | DIGIT)* ; // INDENTIFIER
fragment ID_LETTER : [a-z] | [A-Z] | '_' ;
fragment DIGIT : [0-9] ;
INT : DIGIT+ ;
WS : [ \t\r\n]+ -> skip;
And a driver class that parses both a[2] and a [ 2 ]:
import org.antlr.v4.runtime.*;
public class Main {
public static void main(String[] args) throws Exception {
String[] tests = {"a[2]", "a [ 2 ]"};
for (String test : tests) {
HelloLexer lexer = new HelloLexer(new ANTLRInputStream(test));
HelloParser parser = new HelloParser(new CommonTokenStream(lexer));
System.out.println(parser.expr().toStringTree(parser));
}
}
}
will print (expr (expr a) [ (expr 2) ]) twice.