[antlr-interest] Syntactic anti-predicates

[Gerald B. Rosenberg]

Won't this do the trick?

table:
    LEFT_BRACE PIPE ws? table_format? NL
    ( ( ws? PIPE RIGHT_BRACE )=> ws? PIPE RIGHT_BRACE | table_line )+
;

At 05:45 AM 2/11/2008, Steve Bennett wrote:
>On 2/10/08, Gavin Lambert <antlr at mirality.co.nz> wrote:
> > Why not just use:
> >
> > ruleA
> >    :  (X Y Z) => ruleXYZ
> >    |  ruleNotXYZ
> >    ;
>
>Ok, I've now got a good, non-contrived example to show the problem:
>
>table:
>    LEFT_BRACE PIPE ws? table_format? NL
>    table_line*
>    ws? PIPE RIGHT_BRACE;
>
>table_line:
>    ws?
>    (
>    (PIPE PLUS) => table_caption
>    | (PIPE HYPHEN) => table_row_separator
>    | (EXCLAMATION) => table_header_cells
>    | (PIPE) => table_data_cells
>    );
>
>----
>Here, the problem is that table_line is inside a looping construct.
>Each time it loops, there can be whitespace, then a |} (end of table),
>or one of the other forms: |-, |+, |text.
>
>By the time the syntactic predicate inside fails the match, it's too
>late. I could left factor table_line:
>
>table_line:
>    (
>    (ws? PIPE PLUS) => ws? table_caption
>    | (ws? PIPE HYPHEN) => ws? table_row_separator
>    | (ws? EXCLAMATION) => ws? table_header_cells
>    | (ws? PIPE) => ws? table_data_cells
>    );
>
>But that's inelegant. My ideal solution would have this line:
>
>    table_line*
>
>replaced with something like:
>
>    (~(ws? PIPE RIGHT_BRACE) => table_line)*
>
>The only way to get around it at the moment that I can see is to
>either left factor as above, or use a semantic predicate - which I'm
>not really sure how to use with an arbitrary length rule like ws.
>
>Oh, or I can turn the loop inside out:
>
>table:
>    LEFT_BRACE PIPE ws? table_format? NL
>    ws?
>    (table_line ws?)*
>    PIPE RIGHT_BRACE;
>
>This does actually work. It's slightly disconcerting having the
>whitespace grouped with the previous line, but it's less bad than the
>other alternatives I think.
>
>Steve

----
Gerald B. Rosenberg, Esq.
NewTechLaw
260 Sheridan Ave., Suite 208
Palo Alto, CA  94306-2009

650.325.2100  (office)  /  650.703.1724  (cell)
650.325.2107  (facsimile)

www.newtechlaw.com


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