[antlr-interest] RewriteEmptyStreamException when changing from quoted string to token?

[David Holroyd]

On Mon, Jan 14, 2008 at 05:40:10PM -0600, Jon Schewe wrote:
> David Holroyd wrote:
> > On Mon, Jan 14, 2008 at 02:26:26PM -0600, Jon Schewe wrote:
> >   
> >> I just got time to get back to this project, been stuck on a different 
> >> one.  I tried your suggestion of gathering everything up in a variable 
> >> and that didn't help.  Still the same error. 
> >>
> >> /**
> >> * @param negate if true, negate all expressions by multiplying by -1
> >> */   
> >> multExpr[boolean negate]
> >>    :   a+=atom (PRODUCT a+=atom)* -> {negate}? ^(PRODUCT ^(NUMBER DOUBLE["-1"]) $a)
> >>                                   ->           ^(PRODUCT $a)
> >>    ;
> >>
> >>    [junit]     Caused an ERROR
> >>    [junit] token PRODUCT
> >>    [junit] org.antlr.runtime.tree.RewriteEmptyStreamException: token PRODUCT
> >>     
> >
> > You can't fix just by writing PRODUCT* on the r.h.s., as a tree with
> > zero-or-more root nodes doesn't make sense.
> >
> > Also, do you really always want to create a tree with PRODUCT as the
> > root here, even if PRODUCT is not in the input?  I would have thought
> > that if no PRODUCT is present, you'd just want 'atom'?
> >
> >   
> Yes, I really want to always have PRODUCT, at least that's how my tree
> walker is setup right now.  It makes for a more regular tree.  Is the
> problem that antlr can't figure out which PRODUCT token to use?

Ah, sorry.

Yes, my understanding is that ANTLR sees that there are PRODUCT entries
on the left and right of the rewrite, and therefor tracks the PRODUCTs
seen while parsing in order to stick those actual entries in to the
rewritten tree.

I suspect RewriteEmptyStreamException is the result when the rewrite
tries to insert one of the captured PRODUCTs, but the parse didn't
capture one (because there was an 'atom' with no '*', I guess).


Here's another idea that I didn't test :)

  multExpr[boolean negate]
    :   a+=atom (a+=multHelp)* -> {negate}? ^(PRODUCT ^(NUMBER DOUBLE["-1"]) $a)
                               ->           ^(PRODUCT $a)
    ;

  multHelp
    :   PRODUCT atom -> atom


ta,
dave

-- 
http://david.holroyd.me.uk/