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References: <99F37304-CFDF-4FBB-ABA2-25CFC079A685@lincoln.ox.ac.uk> <x3sliel335.fsf@nowhere.com>
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From: Simon Hay <simon.hay@lincoln.ox.ac.uk>
Subject: [sbp-interest]  Re: Quick reference?
Date: Wed, 27 Sep 2006 11:29:51 +0100
To: Adam Megacz <megacz@cs.berkeley.edu>
Envelope-To: sbp-interest@research.cs.berkeley.edu

When I try using

s = "safe" "(" [0-9]+ ")" /ws
ws = [\r\n ]**

to parse

safe(12)

it works fine:

expanding...
{1 2}

but if I try to parse

safe ( 12 )

or anything else with spaces, I get

Exception in thread "main" unexpected end of file at 1:6
      text: afe(12
expected:

         at edu.berkeley.sbp.ParseFailed.error(ParseFailed.java:218)
         at edu.berkeley.sbp.GSS$Phase.shift(GSS.java:269)
         at edu.berkeley.sbp.GSS$Phase.reset(GSS.java:90)
         at edu.berkeley.sbp.GSS$Phase.<init>(GSS.java:75)
         at edu.berkeley.sbp.Parser.parse(Parser.java:40)
         at edu.berkeley.sbp.chr.CharParser.parse(CharParser.java:16)
         at sjehtest.main(sjehtest.java:32)

Hmm...

The interspersing is a really neat idea - if only I could figure out  
what I'm doing wrong!

Simon

On 26 Sep 2006, at 22:39, Adam Megacz wrote:

>
> Simon Hay <simon.hay@lincoln.oxford.ac.uk> writes:
>> Is there a cheat-sheet/quick reference guide type thing for SBP's
>> metagrammar?
>
> Unfortunately no, although I really should write one.  The metagrammar
> (tests/meta.g) is a pretty concise reference of all the operators,
> although it doesn't say what they do.  Part of my reason for stalling
> is that I don't yet consider meta.g to be official/frozen yet (whereas
> the programmatic API is officially frozen).  However, I don't expect
> meta.g to change much.  See the end of this message for further
> explanation.
>
>> From looking through your examples, it seems like you can use / to
>> intersperse things - i.e. /ws means 'with any amount of whitespace
>> in between all these previous things',
>
> Yes, although "ws" needs to be defined as a production, ie
>
>   ws = [\r\n ]**
>
>> but when I try to actually use it like that it doesn't work.
>
> Hrm, in what way does it not work?
>
> The semantics of the "/" operator are this: it binds more weakly than
> almost any other operator except "|", so it acts on the largest
> possible subexpression to its left.  It then inserts the "spacer"
> production between each of the subexpression productions.  So
>
>    a b c /x
>
> Means
>
>    a x b x c
>
> However, note that
>
>    (a b c) /x
>
> Is just
>
>    (a b c)
>
> ... because the "/" operator doesn't do anything to single-element
> subexpressions, and parenthesis create a single element out of a
> sequence of elements.
>
> Also, note that "*/" is a single operator, as is "**/", "++/", and
> "+/".  These operators act like the repetition operator, but with a
> spacer "factored in" to the recursive production that is generated for
> repetition.  So, for example
>
>   (A=a+)  expands to something like   (A=B    B= a | a B)
>
> and
>
>   (A=a+/x)  expands to something like (A=B    B= a | a x B)
>
> Although these expansions aren't precisely correct because the
> "virtual production" that the repetition operator creates ("B" in the
> examples above) special magical abilities that let it hoist its output
> into a single parent with N children, rather than a parse tree of
> depth N where every node has only a single left child (which is what
> you would get with the "literal" expansion).
>
> The "special magical abilities" mentioned in the preceding paragraph
> are, in my opinion, a major "wart" on how the metagrammar works, and
> one of the reasons why I'm not yet ready to freeze it the way I have
> frozen all APIs in the edu.berkeley.sbp.* root package.
>
> Thanks again for your questions and comments!
>
>  - a
>
> -- 
> PGP/GPG: 5C9F F366 C9CF 2145 E770  B1B8 EFB1 462D A146 C380