Re: Not getting parse error

[Hans Aberg]

At 10:53 -0700 2002/02/04, Rich Yonts wrote:
>I was expecting that the invalid token (.) would cause an error since it
>doesn't fit in with my grammar at all.
...
>Let me give you the whole grammar:
>
>%token AND CONDITIONALOP FIELD LITERAL NOT OR
>
>%left OR
>%left AND
>
>%%
>
>Query :
>    Conditional ',' FieldList {
>          return(0);
>      } |
>    FieldList {
>          return(0);
>      }

You wrote before:
>Reading a token: Next token is 259 (FIELD)
>Shifting token 259 (FIELD), Entering state 22
>Reducing via rule 9 (line 44), FieldList ',' FIELD
>state stack now 0
>Entering state 6
>Reading a token: Next token is 46 ($undefined.)
>Reducing via rule 2 (line 26), FieldList  -> Query
and both I and Akim asked to see the rest.

-- It helps if you supply the things asked for. But:

As I said, LALR(1) may apply some additional reductions, but no more
shifts, before an error is issued. There is always one token to shift, the
implicit endmarker called "$" in yytname[] in the .tab.c file, unless one
aborts the parser.

Now, the "return 0" you have is the same as "abort with a success value".
So mu guess is that when the Bison generated parser reduces the rule Query
-> FieldList it aborts, exactly as you have written. -- If you would have
submitted the information requested, one would have been able to verify
this, but now you should be able to do it yourself.

Change the grammar to
Query:
    Conditional ',' FieldList {}
  | FieldList {}
...
to see if it helps.

>FieldList :
>    FIELD ',' FieldList {} |
>    FIELD {}

Also note that as the Bison generated parser reduces to the right, it is
better to write
FieldList:
    FieldList ',' FIELD {}
  | FIELD {}
as the parse stack then does not built up. See the Bison manual for more
about this.

  Hans Aberg