[antlr-interest] Building a tree grammar expression to recognize arithmetic expressions

[John B. Brodie]

Greetings!

the ^ meta-operator in the suffix position is a tree construction
operation and is NOT valid for tree recognition. you should be getting a
warning similar to:

warning(149): treeTest.g:0:0: rewrite syntax or operator with no output
option; setting output=AST

or if you have already set the output=AST in your options{} block then
this warning is masked. change your rule to:

expression : term | ^(MINUS term) ;

On Sun, 2010-08-08 at 15:54 +0100, Alex Storkey wrote:
> Okay, sorry to keep posting but I've managed to get the antlrworks debugger
> working on my tree grammar and I have discovered the problem.
> 
> It seems like instead of the rule *expression* consuming the MINUS symbol,
> the minus symbol is ignore by the expression rule and consumed by the
> *term*rule, specifically this part:
> ^(operator expression expression)
> even when I try it on a really simple AST like (- 1) - obviously there is
> only one expression here, so the wrong rule is being invoked. What's up with
> that?
> 
> 
> On 8 August 2010 10:11, Alex Storkey <alex at storkey.co.uk> wrote:
> 
> >
> >
> > On 7 August 2010 19:19, Junkman <j at junkwallah.org> wrote:
> >
> >> Hi Alex,
> >>
> >> Alex Storkey wrote:
> >> > Hi, it's my first time posting in a mailing list like this so go easy on
> >> me
> >> > if I'm breaking some etiquette or anything :)
> >> >
> >> > I'm trying to construct an expression in my tree grammar to recognize an
> >> AST
> >> > of simple mathematical expressions like 1+(-(a-b)) in tree format of (+
> >> 1 (-
> >> > (- a b))) that is generated by my parser grammar.
> >> >
> >> > I've tried a couple of different approaches and I can't figure out where
> >> I'm
> >> > going wrong. Could someone explain what's wrong with the following two
> >> > expressions:
> >> > expression
> >> >     :    (MINUS^)? term;
> >>
> >> If I understand you correctly, you are asking about writing tree parser
> >> grammar.
> >>
> >> Does Antlr even compile the grammar (i.e., generate a tree parser) with
> >> the above rule?  I think the rule must be of the form of rewrite rules.
> >>
> >> J
> >>
> > Yes, that sounds about right. I have my lexer and parser grammar set up to
> > generate an AST and I'm trying to write a tree grammar to read the AST and
> > print some information about it, but I'm struggling to construct a tree to
> > interpret my AST.
> >
> > All of my grammars compile fine - is there a reason they shouldn't?
> >
> > --
> > Alex Storkey <alex at storkey.co.uk>
> >

hope this helps...
   -jbb