It turned out that we need split break as combination of what i denote cut and
stop
When we encounter cut in or we skip subsequent branches so A cut B | C has
semantics of A B | ~A C
A branch can be marked by stop and we exit loop if and only if we successfuly
took marked branch
We can rewrite ordinary loop A* as (A | stop)
This distinction does not make much sense for PEG but for my beast
It is nondeterministic topdown parser where we choose lexicographicaly smallest
derivation
with respect to took choices (I introduced break to add or in loop to catch
greedy matching)
Condition to make it linear-time is that we limit recursion to left,rigth and
inside expressions
marked as nested. We assume that expression marked as nested is suffix-free.
Semanticaly we want capture
being enclosed by pair tags like for example nested('(' expression ')' )
On Sun, Jan 02, 2011 at 11:57:17AM +0100, Roman Redziejowski wrote:
> This seems to be another way of writing the "repeat-until" operation.
>
> Apparently, version 4.0 of APG (ABNF Parser Generator) by Lowell D.
> Thomas
> had this operation in the form *A!B, meaning "repeat matching A until B is
> found."
> It was removed in version 5.0 as being equivalent to *(!B A)B, which is
> the same
> as indicated by Francisco (for some reason APG writes star before
> expressions).
> See [1]http://www.coasttocoastresearch.com/apg/docs/doc50, section
> 3.2.3.5.
>
> My argument for introducing "repeat-until" would be a very concise
> implementation
> (at least in the style I use in my "Mouse"), much shorter than that of
> *(!B A)B.
> Have been considering syntax such as A**B, but cannot make up my mind.
>
> Happy New Year!
> Roman
>
> On 2011-01-02 00:12, Francisco Mota wrote:
>
> It might be convenient sometimes, but I don't think it increases the
> power of PEGs.
> You can always rewrite an expression like "(E1 break | E2)*" as "(!E1
> E2)* E1?"
> I wonder if there are any grammars that can't be expressed without
> break?
> 2011/1/1 Ondřej Bílka <[2]nel...@seznam.cz>
>
> Hello
> as I started to LR/RR eliminator I noticed that break is handy
>
> It gave idea to introduce break statement to break iteration.
> For example C strings could be parsed as
> '"' ('"' break | '\"' | . )*
> whad do you think about it
> --
>
> SCSI's too wide.
>
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>
> Visible links
> 1. http://www.coasttocoastresearch.com/apg/docs/doc50
> 2. mailto:nel...@seznam.cz
> 3. mailto:PEG@lists.csail.mit.edu
> 4. https://lists.csail.mit.edu/mailman/listinfo/peg
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