CODEWITHSUNDEEP
antlrleft-recursion
Duy Duy
Duy Duy

Reputation: 621

Left-recursion in ANTLR

The stm and stmList gives me this error, it seems that ANTLR see it like a possible infinite recursion loop. How can I avoid it? The following sets of rules are mutually left-recursive [stmList]

stmList: stm stmList | ;
stm: ifStm | whStm;

ifStm: ifPart elifPart* elsePart?;
ifPart: IF LB exp RB CLB stmList CRB;
elifPart: ELIF LB exp RB CLB stmList CRB;
elsePart: ELSE CLB stmList CRB;

whStm: WHILE LB exp RB CLB stmList CRB;

LB: '(';
RB: ')';
CLB: '{';
CRB: '}';
WHILE: 'While';
IF: 'If';
ELIF: 'Elif';
ELSE: 'Else';

Upvotes: 1

Views: 103

Answers (1)

Mike Lischke
Mike Lischke

Reputation: 53317

This is probably because of the empty alt in stmList, though I also wonder why this error comes up. It doesn't seem right. However, I recommend not to use empty alts anyway, unless you guard the other alt(s) with a predicate and "call" the containing rule unconditionally. This can easily lead to problems when you forget that. Instead remove the empty alt and make the call optional:

stmList: stm stmList;
elsePart: ELSE CLB stmList? CRB;

Additionally, stmList looks pretty much like you would do such a definition in yacc, where no EBNF suffixes are possible. Instead just write:

stmList: stm+;

Upvotes: 2

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