Re: x + (y) + z
Frank,
>> Your grammar contained a single %merge. I thought at
>> least two are required?
>
>All the involved (top-level) rules must have a `%merge'. In the
>original example, both happened to be the same rule.
Thanks for a great example. It looks like the implementation of
%merge is different from that of %dprec (big mistaken assumption
on my part). After reading the source I still don't understand why
yyparse handles them differently.
My expression parsing problems are now solved.
>The second example works and shows all four trees (after fixing a
>few precedences in the grammar) with another `%merge' -- in the
>final grammar you might need some more.
This is a great example (the attached version tries to be visually
simpler). It ought to be included in the bison documentation to
show how the details of %merge differ from those of %dprec.
Or perhaps %dprec should be made as general as %merge.
%glr-parser
%token ID
%{
#include
#include
static const char *input = "(x)+(y)+z";
#define YYSTYPE char *
static void yyerror (const char *s)
{
printf ("%s\n", s);
}
static YYSTYPE stmtMerge (YYSTYPE a, YYSTYPE b)
{
char *s;
asprintf (&s, "\n{ %s %s }\n", a, b);
return s;
}
static int yylex ();
%}
%%
main: add_expr { puts ($1); };
add_expr: mult_expr %merge
| add_expr '+' mult_expr %merge
{ asprintf (&$$, "[b+ %s %s]", $1, $3); }
| add_expr '-' mult_expr { asprintf (&$$, "[b- %s %s]", $1, $3); }
;
mult_expr: factor
| mult_expr '*' factor { asprintf (&$$, "[b* %s %s]", $1, $3); }
| mult_expr '/' factor { asprintf (&$$, "[b/ %s %s]", $1, $3); }
;
factor: '(' add_expr ')' { asprintf (&$$, "(%s)", $2); }
| '(' ID ')' factor{ asprintf (&$$, "<%s>%s", $2, $4); }
| '+' factor { asprintf (&$$, "u+%s", $2); }
| '-' factor { asprintf (&$$, "u-%s", $2); }
| ID
;
%%
static int yylex ()
{
char c = *input ? *input++ : 0;
asprintf (&yylval, "%c", c);
return isalnum (c) ? ID : c;
}
int main ()
{
printf ("%i\n", yyparse ());
}
derek
--
Derek M Jones tel: +44 (0) 1252 520 667
Knowledge Software Ltd mailto:[EMAIL PROTECTED]
Applications Standards Conformance Testing http://www.knosof.co.uk
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Re: x + (y) + z
Derek M Jones wrote:
> >> Your grammar contained a single %merge. I thought at
> >> least two are required?
> >
> >All the involved (top-level) rules must have a `%merge'. In the
> >original example, both happened to be the same rule.
>
> Thanks for a great example. It looks like the implementation of
> %merge is different from that of %dprec (big mistaken assumption
> on my part). After reading the source I still don't understand why
> yyparse handles them differently.
Is it really different? `%dprec' obviously needs two different
precedences, therefore two different rules.
`%merge' in contrast actually requires the same merge function in
all affected places.
So I can imagine yyparse actually handles them the same way -- until
the point where the dprecs are compared (for inequality) or the
merge functions are compared (for equality).
So it really seems there are cases that `%dprec' can't handle.
Attached is the simplest one I could come up with. It's obviously
ambiguous, and even adding `%dprec' in every single rule does not
resolve it.
AFAICS, that's because the conflict is between
v -> e e -> ('a' 'a') 'a'
and
v -> e e -> 'a' ('a' 'a')
I.e., at the time the ambiguity is deteced the choice is between two
applications of the same rule (v -> e e) which cannot have different
`%dprec'. Whereas at the time the choice is between the two e rules,
which have different `%dprec', the parser doesn't know yet that
there will be an ambiguity. So it might indeed be useful to somehow
look at the sub-productions when deciding dynamic precedences ...
BTW, I'm interested in this case as well since it might occur in one
of my grammars. `%merge' probably isn't an (easy) option for me
because of semantic actions. Of course, I know that rearranging the
grammar will help, and if it's the only way, I guess I'll have to do
it, but it would be nice if there's some way without rearranging.
> My expression parsing problems are now solved.
> >The second example works and shows all four trees (after fixing a
> >few precedences in the grammar) with another `%merge' -- in the
> >final grammar you might need some more.
>
> This is a great example (the attached version tries to be visually
> simpler). It ought to be included in the bison documentation to
> show how the details of %merge differ from those of %dprec.
I have no objections, and I have already signed a copyright
assignment for bison, so no problem from my side. I don't know if
your changes are considered non-trivial enough to require an
assignment as well, and I don't know if any bison maintainers are
reading this here. Otherwise you might want to post it to bug-bison,
possibly already as a diff to the texinfo file with accompanying
notes in the text.
Frank
--
Frank Heckenbach, [EMAIL PROTECTED]
http://fjf.gnu.de/
GnuPG and PGP keys: http://fjf.gnu.de/plan (7977168E)
dprec-problem.y
Description: Binary data
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