[il-antlr-interest: 32968] [antlr-interest] Interesting parser grammar issue

[Bill Andersen]

(Using 3.3)

Check out this partial grammar for SPARQL 1.1 
(http://www.w3.org/TR/sparql11-query/)

// [2]
query
        : prologue 
          ( select_query
          | construct_query
          | describe_query
          | ask_query
          )
          bindings_clause 
        ;
        
// [4]
prologue
        : prologue_decl*
        ; 
        
// [4.1]
prologue_decl
  : prefix_decl
  | base_decl
  ;
        
// [5]
base_decl
        : BASE IRI_REF
        ;
        
// [6]
prefix_decl
        : PREFIX PNAME_NS IRI_REF
        ;

The above version works - at least for SELECT queries.  The original production 
[4] that I replaced with [4] and [4.1] looked like

// [4]
prologue
        : ( base_decl | prefix_decl )*
        ;

The grammar with this production will fail to parse SPARQL queries like the 
following, with the error "NoViableAltException: line 1:0 no viable alternative 
at input 'PREFIX'"

PREFIX  data:  <http://example.org/foaf/>
PREFIX  foaf:  <http://xmlns.com/foaf/0.1/>
PREFIX  rdfs:  <http://www.w3.org/2000/01/rdf-schema#>

SELECT ?mbox ?nick ?ppd
FROM NAMED <http://example.org/foaf/aliceFoaf>
FROM NAMED <http://example.org/foaf/bobFoaf>
WHERE
{
  GRAPH data:aliceFoaf
  {
    ?alice foaf:mbox <alice> ;
           foaf:knows ?whom .
    ?whom  foaf:mbox ?mbox ;
           rdfs:seeAlso ?ppd .
    ?ppd  a foaf:PersonalProfileDocument .
  } .
  GRAPH ?ppd
  {
      ?w foaf:mbox ?mbox ;
         foaf:nick ?nick
  }
}

So, why should the new [4] + [4.1] combination work where the original [4] 
would not?



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