Re: Error reporting in LR parsers

>From article <1989Aug8.131112.1081@esegue.uucp>, by heirich@cs.ucsd.edu (Alan Heirich):
> This posting describes modifications to
> DECUS yacc to permit automatic diagnostic generation.
...
>
> The changes are nearly all in the routine "output". This routine writes out
> the parser description to the description file. You will want to modify it
> to write out five pieces of information:
>
> (1) a set of strings containing token names
> (2) a set of strings containing nonterminal names
> (3) a set of states containing items sets
> (4) a set of states containing lookahead sets
> (5) a set of states containing goto sets
>


All this info is in the y.output file from standard yacc. There's no need to
(ahem) hack yacc. If there's a demand, and I can find the time, I'll package
up and post some a nawk (new awk) scripts and so forth that use the y.output
file to generate a procedural, rather than table-based, parser. They could
be modified easily enough to write the above info in any format you might
want.


But be warned, however you obtain this info, you still have to calculate the
legal-token-sets dynamically. There is not enough info in the LALR(1)
item-sets to calculate them from the state-number alone. You have to keep up
with the default-reduction states as they are popped. I hope I have said
this often enough and loudly enough now.


I did the scripts partly as an exercise in learning nawk, partly to get a
faster parser, but mostly to aid in debugging compilers. It's impossible to
pick through the coded tables in a debugger and make any kind of sense, but
it is easy to single step through code that looks like the following, which
is cut and pasted from a compiler I'm in the process of writing:


switch(state) {
...
    case 3:switch(lookahead){
        /*
          * file : $$2 _ declarations
          * declaration_list : _ (28)
          */
          case EOF: YREDUCE(28,0,NT_declaration_list);
          case error: YSHIFT(7);
          case CONST: YREDUCE(28,0,NT_declaration_list);
          case INSERT: YREDUCE(28,0,NT_declaration_list);
          case FUNCTION: YREDUCE(28,0,NT_declaration_list);
          case LABEL: YREDUCE(28,0,NT_declaration_list);
          case PROCEDURE: YREDUCE(28,0,NT_declaration_list);
          case TYPE: YREDUCE(28,0,NT_declaration_list);
          case VAR: YREDUCE(28,0,NT_declaration_list);
          case WITH: YREDUCE(28,0,NT_declaration_list);
          default: YERROR();
    };


The scripts "wrote" the above code from inspection of the y.output
file.
[From djones@megatest.com (Dave Jones)]

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