- From: Dave Pawson <dave.pawson@gmail.com>
- Date: Thu, 16 Dec 2021 12:12:09 +0000
- To: Norm Tovey-Walsh <norm@saxonica.com>
- Cc: ixml <public-ixml@w3.org>

Working through your expansions Norm... On Thu, 16 Dec 2021 at 11:23, Norm Tovey-Walsh <norm@saxonica.com> wrote: > That took me a while to get my head around too. I think it’s just part > of the definition of repeat0 and repeat1. Note that these are different: > > a*"#" > > and > > a*,"#" > > The former can be distinguished as “factor * sep” and has the semantics > that the spec describes for repeat0. The latter is “a*” followed by “#”. I'm good with the latter, taking the comma as ending any repetition. > > Later, “hints for implementors” observes that > > a*"#" > > can be rewritten. For example: > > X: a*"#" . > > can be rewritten as > > X: Y . > -Y: a+"#" ; . which smells to my way of thinking? That *insists* on at least one 'a' which seems incorrect? > > I find “the alternative that matches nothing” in the grammar quite hard > to read. I almost wish we had a special symbol for it, like ∅. Then we’d > have: > > X: Y . > -Y: Z ; ∅ . > -Z: a, A . > -A: "#", a, A ; ∅ . > > Unless I’ve got something wrong along the way, of course. Nothing does seem to clarify that point! > > All of this has turned out to be an interesting challenge for me to > implement because I don’t think the PEP parser can be made to accept > “nothing” as an alternative. Instead, I think (I think!) I’ve modified > it so that it understands an optional terminal or nonterminal. So I’ll > end up rewriting it something like this: > > X: Y . > -Y: Z? . > -Z: a, A . > -A: ("#", a, A)? . Is this expansion on X: a*"#" . ? In which case I've the same objection as above? > > Where the optionality is addressed in the parser when candidate edges > are selected. > > But now we’re into about four levels of manual rewriting so I make no > claims of correctness! regards -- Dave Pawson XSLT XSL-FO FAQ. Docbook FAQ.

Received on Thursday, 16 December 2021 12:12:33 UTC