Re: Problem with LPeg 0.12

[Roberto Ierusalimschy <roberto@...>]

> The attached module (a JSON parser) works fine with LPeg 0.10.2.
> After a upgrade to LPeg 0.12, an error occurs at load time :
>         $ lua -e "print(require'parser_peg'.parse'{}')"
>         lua: ./parser_peg.lua:61: empty loop in rule 'object'

The problem is related to a match-time capture (Cmt).  You have a
loop over a Cmt pattern. LPeg has no way to know whether such pattern
consumes any input. Older versions of LPeg were "optimistic" in those
cases, and blindly assumed that the pattern consumes something (and
therefore that the loop is valid). The new version is more strict and
refuses such loops.

The correction involves making the pattern associated with the Cmt
non empty. In your case, the problem seems to be this loop:

          ((V'value_separator'
          + Cmt(Cp() * -V'end_object', function (s, pos)
                                          error("',' expected at " .. pos)
                                       end))
         * V'member')^0

A simple solution is this:

          ((V'value_separator'
          + Cmt(Cp() * -V'end_object', function (s, pos)
                                          error("',' expected at " .. pos)
                                       end) * 1)     --<<<<<<<
         * V'member')^0

The "* 1" after the error does not affect the matching, but it tells
LPeg that the pattern cannot accept an empty string.

(You will have a similar problem in rule 'array', with a similar
solution.)

-- Roberto