[antlr-interest] ANTLR 3.1.2 bug in tree grammars? - RewriteEmptyStreamException

["Paul Bouché (NSN)"]

Hi,

I just upgraded to ANTLR v3.1.2 from 3.0. What worked before does not 
work anymore.... :-( I looked through the changes list, but could not 
find anything related.

We first create an AST and then parse it with a tree grammar. The syntax 
is a list key = value pairs, where a structure can be built using curly 
brackets. A structure is again a list of key-value pairs, i.e.
struct = { a = 3, b = 4 }

also
struct = { }

was allowed which created an empty sub structure.

This is broken now. I get the exception:
Exception in thread "main" 
org.antlr.runtime.tree.RewriteEmptyStreamException: rule properties
    at 
org.antlr.runtime.tree.RewriteRuleElementStream._next(RewriteRuleElementStream.java:158)
    at 
org.antlr.runtime.tree.RewriteRuleElementStream.nextTree(RewriteRuleElementStream.java:145)
    ...

I have no clue whatsoever what this means. Well obviously somehow the 
parser tries to rewrite an empty stream. Well before 3.1.2, in 3.0 this 
worked fine without errors. Now it does not work. Something must have 
changed or is it a bug?

The grammar for parsing the above syntax is basically as follows:
start
        :
        propertynode EOF
        ;
propertynode
        :
        properties        -> ^( TYPE PROPERTYNODE ) properties
        ;
properties
        :
        ( property ( ',' property )* )? ','?        -> property*
        ;
property
        :
        name        '='        value        -> ^( PROPERTY name cast? 
attributes? value )
        ;      
value
        :
        ( plain        | '{' propertynode '}'        )        -> plain? 
list? propertynode?
        ;

As you can see rule value calls propertynode which allows substructures. 
If the rule properties does match an empty obviously a stream is not 
instanciated as it was before in 3.0 hence the exception. Can you tell 
me how I need to modify the grammar to make it work in 3.1.2?

Thanks a bunch!
Paul

-- 
Paul Bouché
Voice: +49 30 590080-1284
 
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