I have been trying to use lemon with the 'error' keyword to enable error recovery, but I've run into something that might be a bug.
TLDR: Are reduction rules allowed to END with 'error'? If not, disregard the rest of this post.
Given a grammar like this:
```
start ::= top.
top ::= .
top ::= top error.
top ::= top topdef.
... a bunch of legal topdefs
```
When the generated parser encounters an error, the loop:
```
while( yypParser->yytos >= yypParser->yystack
&& (yyact = yy_find_reduce_action(
yypParser->yytos->stateno,
YYERRORSYMBOL)) > YY_MAX_SHIFTREDUCE
){
/* added for debug by me: */ if (yyact >= YY_MIN_REDUCE) printf("CONSIDERING %d: %s\n", yyact - YY_MIN_REDUCE, yyRuleName[yyact-YY_MIN_REDUCE]);
```
unwinds the stack until it gets an assertion failure due to popping the end of the stack:
```
Return. Stack=[top global export KW_DEF]
Input 'KW_DATA' in state 21
Syntax Error!
foo.wake:133:[5-8]: syntax error; found 'data', but was expecting one of:
if \ ( subscribe prim match
Popping KW_DEF
Popping export
CONSIDERING 41: export ::=
Popping global
CONSIDERING 2: top ::= top error
Popping top
CONSIDERING 1: top ::=
parser: parser.c:2096: void yy_pop_parser_stack(yyParser*): Assertion `pParser->yytos > pParser->yystack' failed.
Aborted
```
As you can see in the trace of 'yyact', it DID consider the rule `top ::= top error.`, but the number it had assigned to this rule was in the range of `[YY_MIN_REDUCE, YY_MAX_REDUCE)` instead of `[YY_MIN_SHIFTREDUCE, YY_MAX_SHIFTREDUCE)`. Thus, the loop popped past the point it could have shift-reduced. This led me to the work-around of not having 'error' as the last term in the error-handling rule, and that seems to have worked, except that now I cannot tolerate an error at the end of the input, which is quite unfortunate, as people often edit files at the bottom.
I've also noticed that when I try to create a minimal grammar reproducing this behavior, the bug(?) goes away and the reduction works. When I use the '-c' option, the bug(?) also goes away and the error reduction works.
I'm not sure where to post the full grammar that causes this issue. I did include it attached to the original email I sent.