[antlr-interest] Using previously matched parser rule in decision making

[Jim Idle]

>From anywhere in the parser:

java.util.List stack = getRuleInvocationStack(e, getParserName());

But this only works for Java and other targets that copy it (I think C# might do it). I don't do it in C because I prefer to take the view that the C stuff should be as close to the metal as it can be and the programmer will choose to add the overheads they need.

In the JavaFX front end, this stack is used to pin down errors a little more precisely - as it is open source you can download the code and look at AbstractGeneratedParserV4.java

Jim

> -----Original Message-----
> From: antlr-interest-bounces at antlr.org [mailto:antlr-interest-
> bounces at antlr.org] On Behalf Of Kieran Simpson
> Sent: Tuesday, March 09, 2010 1:58 AM
> To: Ron Burk
> Cc: antlr-interest at antlr.org
> Subject: Re: [antlr-interest] Using previously matched parser rule in
> decision making
> 
> I agree Ron.
> 
> Ron Burk wrote:
> > It is an interesting idea for a top-down parser generator
> > to just make the parsing stack of non-terminals available
> > to user actions. Whether that's easy or hard depends on
> > the details of how the tool generates parser code. But
> > certainly knowing the context you expect to be in is
> > arguably an advantage of top-down over bottom-up
> > parsing, so there's an argument to be made for making
> > that information available. As I struggle to think of
> > common/practical use for it, mainly error reporting or
> > recovery comes to mind. But, if the syntax made it
> > easy to ask things like "is X on the stack", I suppose
> > there are a variety of semantic checks that could be
> > made clearer and simpler than via flags and such. E.g.
> > checking that a 'break' keyword in C occurs within a
> > do/for/switch/while.
> >
> > I usually try to do things in one pass, so it may be more
> > interesting of an idea to me than to someone who intends
> > to build a syntax tree first before doing any actual work.
> >
> > Dinking with syntax:
> >
> > A: B
> > C: B
> > B:
> >     { if($Stack[A])... else if($Stack[C])... else assert(FALSE); }
> >
> > or maybe (also?)
> >
> >     { if($Stack[-1]==$NonTerm[A]) ...; else ...; }
> >
> > or
> >
> > LoopStmt: Do | For | Switch | While ;
> > ...
> > BreakStmt: 'break'
> >     { if(!$Stack[LoopStmt]) SynError("break is not inside
> > do/for/switch/while.\n"); }
> >
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