Re: GNU Guile PEG-parser

[Krister Svanlund <svanlund@student.chalmers.se>]

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Hi, thanks for a quick response!

I've actually found no PEG library that has a string syntax for the
equivalent of ignore. I'm guessing most people are satisfied with just
specifying another nonterminal and matching that one. Probably because it
is seen as less ugly than extending on the formal definition of PEG but I
really think we could get a cleaner PEG definition of our parser if we
where able to ignore text that wasn't needed or gets in the way while using
string-patterns.

It's actually exactly Python I'm thinking about, we are currently doing a
preprocessor that will put #{ and #} before and after each block but I was
hoping that there exists a cleaner solution using the power of PEG instead
of basic string manipulation. If you could help in any way shape or form
that would be greatly appreciated, even just suggesting on what parts of
PEG internals to look at would be really useful.

I hope you or the guile-devel list can be of help.

Yors,
Krister Svanlund

On Wed, Feb 8, 2012 at 1:47 AM, Noah Lavine <noah.b.lavine@gmail.com> wrote:

> Hello,
>
> Thanks for emailing! I suppose I am the one to talk to, since I was
> the last one to work on it.
>
> I didn't make the PEG parsing syntax, but I would guess the reason
> there isn't a string syntax for ignore is that there's no conventional
> way to write it, but there is for the other PEG elements. It would be
> easy to add one if it was useful, but we'd want to make sure our
> syntax agreed with other PEG libraries, so people wouldn't be confused
> later.
>
> For blank-space indented blocks, do you mean you want to group
> together lines with the same indentation, like Python syntax? If you
> know what the indentation will be at the beginning of each line, you
> can do something like this:
>
> (* (and "\t" <match-line> "\n")),
>
> where you replace "\t" with whatever indentation you want.
>
> However, what you probably want to do is look at the indentation in
> the first line and then group it with every following line that has
> the same indentation. I'm not sure if it's possible, but it would
> probably be ugly. If you tell me what you're trying to do, though, I
> can help you write your own parser to handle it. You can even write
> some of your parser yourself and use PEGs for the rest, if you're
> willing to use PEG internals.
>
> Can you tell me more about what you're trying to do? I am happy to
> help now, but I will be more helpful if I know more.
>
> I'm going to CC the guile-devel mailing list because of the issue with
> the string syntax.
>
> Noah
>
> On Tue, Feb 7, 2012 at 10:03 AM, Krister Svanlund
> <krister.svanlund@gmail.com> wrote:
> > Hi,
> > I'm currently involved in a project that plans on using the PEG module
> for
> > Guile for parsing and I've understod that you are the one to talk to
> about
> > it. I'm mostly just curious how come there isn't an equivalent to ignore
> in
> > string-patterns and if this would be complex to add?
> >
> > I'm also curious if there is any way to deal with blank-space indented
> > blocks in PEG.
> >
> > Yours
> > Krister Svanlund
>

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