On 05/04/2009, at 2:10 AM, Emmanuel Onzon wrote:
>
> The problem goes away if non-terminals have unique priorities,
> in which case the priority name can just be used in lieu of the non-
> terminal.
>
> Interesting idea.
> For the extensions of the grammar done in the felix code,
> it is possible to do it without any change to dypgen this way:
>
> When the user defines a new rule like:
>
> term: <=term "+" <term
>
> Felix would add the rule:
>
> "expr",
> [Non_ter ("expr",Priority_Lesseq "term");
> Ter "Plus";
> Non_ter ("expr",Priority_Less "term")],
> "term",
> []
>
> The user only writes the priorities.
> This means Felix would have to guess which non terminal
> is implicitly used, which shouldn't be difficult if each priority
> is used for only one distinct non terminal.
Well computers can't "guess" .. so we need some rules and
ways to break out.
I think already you can add new statements: that would be
quite common, the other common case is expressions.
Of course, other extensions would have to be done long hand,
and/or provide a way to associate a priority with a non-terminal.
============
There is also the *inverse* of this problem already.
Felix has something like:
statement:
declaration
definition
directive
Now this looks harmless and for an LR parser it is. But it is quite
different
to just listing all the statements instead of grouping them for a GLR
parser!
This is because of the way merging is done: you can only merge for a
given
nonterminal. Felix use some kind of aging concept to chose between
conflicting
productions for the same non-terminal. However the resolution is not
transitive,
in other words it matters a lot which productions are definitions and
which
are declarations if you make a tree structure like above!
I didn't expect this, but I got caught out by it: with the
classification scheme
above GLR first resolves conflicts between all declarations, then all
definitions, then all directives, and only then does it try to resolve a
conflict between a single remaining declaration, definition, or
directive.
Because Felix uses aging .. in other words, order of writing,
a definition always beats a declaration, even if the actual declaration
was parsed by a brand new production.
At one stage, I passed the age up, so this wouldn't happen, but that
doesn't work either ***
So actually the Felix grammar needs a way to group things with
"psuedo-non-terminals" which are basically macros (not
real non-terminals).
*** I can't remember exactly why this failed, but it has something
to do with recursion. Recursion is evil ;->
--
john skaller
[email protected]
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