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

[Alex Storkey]

Thanks!

Now I'm getting the error:
[fatal] rule expression has non-LL(*) decision due to recursive rule
invocations reachable from alts 1,2. Resolve by left-factoring or using
syntactic predicates or using backtrack=true option.

Which I guess kinda makes sense, but how do I go about sorting it? Just to
reiterate, the rules I have are as follows:

expression
    :    term | ^(MINUS term);

term
    :    constant | variable | ^(operator expression expression);


On 8 August 2010 16:41, John B. Brodie <jbb at acm.org> wrote:

> 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
>
>
>
>


-- 
Alex Storkey <alex at storkey.co.uk>