ANTLRWorks warning 'no start rule'

Question

please help a newbie to understand this warning in ANTLWorks:

[11:10:15] warning(138): BooleanExpr.g:0:1: grammar BooleanExpr: no start rule (no rule can obviously be followed by EOF)

This is how definition of my grammar looks like:

grammar BooleanExpr;
booleanExpr
    :   andExpr ('OR' andExpr)*;
andExpr :   notExpr (('AND' | ' ' ) notExpr)*;
notExpr :   kppExpr ('NOT' kppExpr);
kppExpr :   keywordExpr|phraseExpr|proximityExpr|'(' booleanExpr ')';
keywordExpr
    :CHAR+;
phraseExpr
    :   '"' keywordExpr '"';
proximityExpr
    :   keywordExpr|phraseExpr '~' INT;
CHAR    :   ('A'..'Z') | ('a'..'z');
INT :   '0'..'9'+;

Thanks a lot!

score 3 · Answer 1 · edited Aug 04 '15 at 09:38

3

Any grammar needs so-called start rule. Start rule is a rule that is not referenced by another rule. If your grammar does not have such rule, ANTLR generator will issue a warning:

no start rule (no rule can obviously be followed by EOF)

To avoid it, add a dummy start rule to your grammar:

start_rule: someOtherRule;

edited Aug 04 '15 at 09:38

Andreas Fester

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answered Dec 12 '14 at 11:39

Sanjiv

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Is "start_rule" supposed to be a magic hard-coded value? Adding that rule did not help me. – Basil Bourque Feb 01 '21 at 02:42

score 2 · Answer 2 · answered Jun 11 '13 at 14:56

2

Normally parsers are written to parse an entire input (from the beginning to the end of a file). Grammars used for this task generally include a rule like the following.

compilationUnit : someElement* EOF;

This rule says a compilation unit is a sequence of zero-or-more elements ending at the end of the file. If the EOF reference is omitted, you could have a rule like the following.

compilationUnit : someElement*;

The problem with this form lies in its error handling ability. If the input contains a syntax error, then rather than trying to correct and/or recover from it, the rule will simply return (an empty sequence is a valid interpretation of this compilationUnit rule, so that is preferred to a longer sequence containing an invalid element).

ANTLRWorks is informing you that the grammar does not contain a rule ending with an explicit EOF reference, which can be problematic if you are intending to parse complete files.

answered Jun 11 '13 at 14:56

Sam Harwell

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I do not plan to parse files, my intent here is validate boolean expressions that will be stored as a String objects. It doesn't make sense for me to append EOF in the end of the rule. How error handling could be supported in this case? – Ihor M. Jun 11 '13 at 16:25
If you plan you parse an entire string, then treat it like a file and add `EOF` to the end of the start rule. – Sam Harwell Jun 11 '13 at 18:22
Alright, should i append EOF symbol to the end of the String as well? – Ihor M. Jun 11 '13 at 19:50
@IhorM. No. `EOF` simply means the end of the input, which in your case is the end of the string. – Sam Harwell Jun 11 '13 at 22:26

ANTLRWorks warning 'no start rule'

2 Answers2