empty rule in ocamlyacc - ocaml

Question

I have the following lexer rules:
let ws = [' ' '\t' '\n']+
...
| ws {Printf.printf "%s" (Lexing.lexeme lexbuf); WS(Lexing.lexeme lexbuf)}
And the following parser rules:
%token <string> WORD WS
cs : LSQRB wsornon choices wsornon RSQRB {$2}
;
wsornon : /* nothing */
| WS {$1}
;
choices : choice {$1}
| choices choice {$2}
;
choice : CHOICE LCURLYB mainbody RCURLYB {$3}
;
I basically want to get wsornon to match with whitespace or nothing. But cs gives syntax errors for the case without whitespace (which corresponds to the empty rule).
Am I missing something?

Answer 1

Even if you parse the empty stream, you should have a production rule:
wsornon:
| { something for nothing }
| WS { something for whitespace }
Note that menhir has an OPTION parametrized rule that is just fine for this kind of things, so that you don't have to write another rule for that. In fact OPTION(foo) return a production of type bar option if rule foo returns something of type bar, while you're going to ignore them anyway, so that's a bit of a different situation.
If you want to ignore whitespace, why don't you drop it altogether at the lexer step? Is it useful somewhere else in your grammar? I'd rather hack the lexer a bit to have some whitespace token just after some tokens where I know they're important than have them pollute my whole grammar. Of course, menhir allows to define parametrized rules that could help with that (example below untested):
ws(rule):
| LIST(WS) result = rule LIST(WS) { result }