bison: one error causes additional but incorrect error

Question

In the class section of my code, if I write an error a few lines will be reported as errors when they should not. I put '| error' in certain locations where it's good/safe to recover from errors but I don't think it's using it. Maybe it's trying to resume mid expression someplace?

Is there a way I can force Bison to try to recover in designated locations? How does it work and what might I be doing wrong?

I put the error line next to the loop. Here is an example:

SomeRuleLoop:
    | Rule ',' SomeRuleLoop
Rule:
      A
    | B
    | C
    | Error
A:
      AA AAB AABC
    | AA AAB AABC Z
...

Here is an example of my rules. I see "funcBody error" in my console however the next line gets an error because of the first error. Even though each funcBodyRule is standalone.

funcBodyLoop:
    | funcBodyLoop funcBody

funcBody:
      funcBodyRule
    | error { printf("funcBody error"); $$=0; }
    | '#' EQ { printf("still in funcBody\n"); $$=0; }

I tried writing #== between the line with the first error and the line with the 2nd. I wrote this to check if the parser is still in the funcbody loop. ~~This doesnt give an error so it is~~. Nevermind i added a printf and the string isnt printed so maybe it isnt in the function loop anymore? how do i fix this?

It is going to be nigh on impossible to help much without some code to help us. But it needs to be close to minimal code, not several hundred lines.. — Jonathan Leffler
@Jonathan Leffler: Its where the loop is. I modified my question — user34537
You know that Yacc (Bison) is case sensitive, so the alternative with 'Error' is not the same as 'error' (which is 'built in' and has a specific meaning w.r.t error recovery). — Jonathan Leffler
@Jonathan Leffler: Ok noted. I checked and i dont use 'Error' anywhere, only 'error' — user34537
OK - good luck with resolving the issue. Conflicts can be exasperating. — Jonathan Leffler

Jonathan Leffler Jonathan Leffler · Accepted Answer · 2010-11-20T05:42:02

Yacc and Bison normally use left-recursive rules, and the rules shown are not left-recursive.

As shown, the first rule is equivalent to:

SomeRuleLoop:
        /* Nothing */
    |   Rule ',' SomeRuleLoop
    ;

This is a right-recursive rule which says that a 'SomeRuleLoop' is either an empty string of tokens or a 'Rule' followed by a comma and some more 'SomeRuleLoop'. Note that this means a 'SomeRuleLoop' ends with a comma, which is probably not what you had in mind.

The first rule should probably read:

SomeRuleLoop:
        Rule
    |   SomeRuleLoop ',' Rule
    ;

Note that allowing for empty alternatives is important - but adding them everywhere tends to make the grammar ambiguous (more shift/reduce conflicts)

You also need to use the token 'error' (all lower case) rather than 'Error' (mixed case) to indicate a point where error recovery can occur.

However, I'm not sure what the rest of your troubles are...

bison: one error causes additional but incorrect error

2 Answers