Thanks for the link, Amon. As I suspected, it says you should travers the
ASF in such cases...
In a related question, is there any way for me to reject a rule during the
read() by listening for a prediction/completion events? A number of my
rules have conditions on their arguments that are known not to be
BNF-friendly. I know I can bail from those in a semantic action, but I
would like to reject before an exponential explosion in the number of
ambiguous parses. Does that make sense?
Thanks,
Mike
On Wednesday, August 8, 2018 at 12:01:24 AM UTC-7, amon wrote:
>
> Your grammar as written is ambiguous and therefore Marpa gives you all
> parses in an unspecified order – to see them, iterate over the value
> like
>
> while (my $ref = $recce->value) {
> print Dumper $$ref;
> }
>
> Marpa's ranks are a bit unintuitive, I previously ran into very
> similar problems. This lead to the Marpa::R2::Semantics::Rank
> document[1] being written (Thanks Jeffrey!). That document shows a
> related example. The solution seems to be to spell out the sequence
> rule explicitly:
>
> statements ::= xy rank => 1
> statements ::= x
> statements ::= y
> statements ::= statements xy rank => 1
> statements ::= statements x
> statements ::= statements y
>
> The docs emphasize: “The rank of a parse choice is the rank of the
> rule of its cause”, which suggests the problem is the intermediate
> statement rule. If I understand correctly, the "statements ::=
> statement+" sequence rule has no choices because it always gets a
> statement at each position (not a choice between x and xy). And the
> rank within statement does not matter because … I still don't
> understand this 100%.
>
> [1]:
> https://metacpan.org/pod/release/JKEGL/Marpa-R2-5.043_043/pod/Semantics/Rank.pod
>
>
> On Wed, 8 Aug 2018 at 07:33, Michael Spertus <[email protected]
> <javascript:>> wrote:
> >
> > This time with attachment :/
> >
> > On Wednesday, August 8, 2018 at 12:20:39 AM UTC-5, Michael Spertus
> wrote:
> >>
> >> Thanks for helping me to get to a (surprising) answer to my previous
> question. I was hoping you could help me with another. I want the following
> grammar to parse 'xy' as a single statement
> >>
> >>> statements ::= statement+
> >>> statement ::= xy | x | y
> >>> x ::= 'x'
> >>> y ::= 'y'
> >>> xy ::= 'x' 'y'
> >>>
> >>
> >> Unfortunately, it always parses as two statements, even if I use the
> attached "high_rule_only" code and rank the statement alternatives as
> >>
> >>> statement ::= xy rank => 1 | x | y
> >>
> >>
> >> I still get two statements. Is there a way I can do this with ordering,
> or do I need to do something like traverse the ASF? Note that I need to do
> this at the ::= level rather than with lexemes because in the actual
> grammar I care about x and y are complicated rules themselves.
> >>
> >> Thanks,
> >> Mike
> >>
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