Re: lex/yacc "bugs"

I missed the original call-for-bugs. One that was discussed at great
length in this group a couple of years ago is related to error-recovery.
Many yaccs based on the original have this bug, which completely defeats
error-recovery using the ERROR token. Here is a test-file ("botch.y"):


%token a
%%
s : oseq
;
oseq : /* empty */
| oseq a
| oseq error
;


Do a "yacc -vd botch.y", and look at the y.output file. If you see a state
which can shift "error", but has a default reduction, you have a buggy
yacc.


EXAMPLE of buggy y.output:


        state 2
                s : oseq_ (1)
                oseq : oseq_a
                oseq : oseq_error


                error shift 4
                a shift 3
                . reduce 1


It seems that it can shift to state 4 on a (synthesized) error token, but
in fact it will do the default reduction (reduction 1) before the
error-token can be synthesized by the parser.


A properly working yacc would produce the following:


        state 2
                s : oseq_ (1)
                oseq : oseq_a
                oseq : oseq_error


                $end reduce 1
                error shift 4
                a shift 3
                . error


Notice that the default action is "error", not a reduction. The
error-action synthesizes the error-token, which can then be shifted.


If you have the original source, you can fix the file y3.c. Edit the line
which reads


if(temp1[1] > 0) lastred = 0;


Change it to say,


if( temp1[2] > 0 ) lastred = 0;


(The "2" stands for the error-token, the "1" for EOF.)
--

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