bison sift/reduce conflict

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I have these rules :

Q_ID : Q_ID '.' Id
|Id;
classdecl : modifier CLASS Id IS END
|modifier CLASS Id INHERITS Q_ID IS END
|modifier CLASS Id IMPLEMENTS Q_ID_LIST IS END
|modifier CLASS Id INHERITS Q_ID IMPLEMENTS Q_ID IS END <<<<
|modifier CLASS Id IS feature_closer END
|modifier CLASS Id INHERITS Q_ID IS feature_closer END
|modifier CLASS Id IMPLEMENTS Q_ID_LIST IS feature_closer END
|modifier CLASS Id INHERITS Q_ID IMPLEMENTS Q_ID_LIST IS feature_closer END

| CLASS Id IS END
| CLASS Id INHERITS Q_ID IS END
| CLASS Id IMPLEMENTS Q_ID_LIST IS END
| CLASS Id INHERITS Q_ID IMPLEMENTS Q_ID IS END <<<<
| CLASS Id IS feature_closer END
| CLASS Id INHERITS Q_ID IS feature_closer END
| CLASS Id IMPLEMENTS Q_ID_LIST IS feature_closer END
| CLASS Id INHERITS Q_ID IMPLEMENTS Q_ID_LIST IS feature_closer END;

the parser stops on Q_ID . IS END
how can I solve this conflict

Posted 30-Nov-10 23:54pm

fresh_girl

Updated 1-Dec-10 0:23am

v4

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Comments

CPallini 1-Dec-10 6:11am

It looks like an incomplete grammar to me...

fresh_girl 1-Dec-10 6:15am

sure it is .. but this is the rule connected to the conflict
should I post it all ??

Manfred Rudolf Bihy 3-Dec-10 20:03pm

Hey fresh, got any news on this?

fresh_girl 4-Dec-10 3:18am

yup , just solved it
thanks alot for the help :)
God bless u

Manfred Rudolf Bihy 4-Dec-10 5:59am

Care to elaborate what you did to make it work?

fresh_girl 4-Dec-10 11:05am

sure ^^
I added :
%nonassoc low
%nonassoc IS
then used it here :
Q_ID_LIST : Q_ID_LIST COMMA Q_ID
| Q_ID %prec low
what do u think ??

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Manfred Rudolf Bihy · Accepted Answer · 2010-12-01T23:18:00

Hi Fresh,

eventhough it's been some time since fooling around with Lex & Yacc (aka bison) I'll give this a shot. For reference reasons I've added line numbers to your grammar:

 1 Q_ID : Q_ID '.' Id |Id;
 2 classdecl : modifier CLASS Id IS END
 3 |modifier CLASS Id INHERITS Q_ID IS END
 4 |modifier CLASS Id IMPLEMENTS Q_ID_LIST IS END
 5 |modifier CLASS Id INHERITS Q_ID IMPLEMENTS Q_ID IS END <<<<
 6 |modifier CLASS Id IS feature_closer END
 7 |modifier CLASS Id INHERITS Q_ID IS feature_closer END
 8 |modifier CLASS Id IMPLEMENTS Q_ID_LIST IS feature_closer END
 9 |modifier CLASS Id INHERITS Q_ID IMPLEMENTS Q_ID_LIST IS feature_closer END
10
11 | CLASS Id IS END
12 | CLASS Id INHERITS Q_ID IS END
13 | CLASS Id IMPLEMENTS Q_ID_LIST IS END
14 | CLASS Id INHERITS Q_ID IMPLEMENTS Q_ID IS END <<<<
15 | CLASS Id IS feature_closer END
16 | CLASS Id INHERITS Q_ID IS feature_closer END
17 | CLASS Id IMPLEMENTS Q_ID_LIST IS feature_closer END
18 | CLASS Id INHERITS Q_ID IMPLEMENTS Q_ID_LIST IS feature_closer END;

The shift/reduce conflicts are probably based on the rules in lines 3 and 5 as well as lines 12 and 14. Most probable cause for that lies in the rule IMPLEMENTS which isn't listed. BTW is Id a terminal symbol or a rule?

If you're sure you're doing things right you can instruct bison to resolve this issue by forcing either resolve or shift to occur.
The best way to go for sure is to make your grammar conflict free, if it's possible to do so.
This one was a real time saver for me way back when:

O'Reilly, Lex & Yacc

Cheers

Manfred