Re: lpeg could infer the validity of this grammar.

[Sam Putman <atmanistan@...>]

On Sun, Dec 14, 2014 at 1:50 PM, Sam Putman <atmanistan@gmail.com> wrote:

On Sun, Dec 14, 2014 at 12:56 PM, Dirk Laurie <dirk.laurie@gmail.com> wrote:

2014-12-14 21:39 GMT+02:00 Sam Putman <atmanistan@gmail.com>:

> expr = token + ( V"group" + V"factor" + V"term")
-- semicolon needed here
> group = P"(" * V"expr"^0 * P")";
> factor = #token * V"expr" * (P"*" + P"/") * V"expr";

> This program fails with "rule 'expr' may be left recursive".

If `expr` is not `token` or `group`, it tries `factor`.
The rule for `factor` invokes V"expr" before any
input has been consumed.

I do understand the cause for the failure to infer. The grammar is valid, but not valid lpeg, as lpeg stands.

To elaborate, I understand that #token returns the empty string, and it isn't always safe to call "expr" within "expr". When hand-writing recursive descent, it's nice when possible to do a few tokens of lookahead to ensure completion, then recur. This grammar attempts to mimic that idiom.