javacc - Avoid common prefixes without change lookahead -


i'm using javacc make specification recognize language. problem have javacc gives me warning because public common prefix of member() declaration. member() can has attributes() and/or method() must have @ least 1 method, order not matter.

the warning javacc gives me is:

choice conflict in (...)+ construct @ line 66, column 23. expansion nested within construct , expansion following construct have common prefixes, 1 of is: "public". consider using lookahead of 2 or more nested expansion.

the line 66 line of member(). need without change lookahead value.

here code:

void member() : {}     {         (attribute())* (method())+ (attribute() | method())*     }  void attribute() : {}     {         "private" type() <id> [":=" expr()]";"     }  void method() : {}     {         methodhead() methodbody()     }  void methodhead() : {}     {         ("public")? (<id> | type() | "void") <id> "(" parameter() ")"     }

thanks.

the problem regular expression

 (method())+ (attribute() | method())*

is ambiguous. let's abbreviate methods m , attributes a. if input mam, there no problem. (method())+ matches first m , (attribute() | method())* matches remaining am. if input mma, should divide be? either (method())+ matches m , (attribute() | method())* matches ma or (method())+ matches mm , (attribute() | method())* matches a. both parses possible. javacc doesn't know parse want, complains.

what can do:

  • nothing. ignore warning. default behaviour many methods possible recognized (method())+ , methods after first attribute recognized (attribute() | method())*.
  • suppress warning using lookahead. said don't want add lookahead, completeness, i'll mention change (method())+ (lookahead(1) method())+. won't change behaviour of parser, suppress warning.
  • rewrite grammar.

the offending line rewritten either

(attribute())* method() (attribute() | method())*

or as

(attribute())* (method())+ [attribute() (attribute() | method())*]

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