Add support for empty graphs in the subject and object positions

src/N3Parser.js

@@ -662,13 +662,35 @@ export default class N3Parser {
 
     const graph = this._graph;
 
+    let closingEmptyGraph = false;
+
     // Store the last quad of the formula
-    if (this._subject !== null)
-      this._emit(this._subject, this._predicate, this._object, graph);
+
+    if (this._subject !== null) {
+      // Catch the empty graph being closed when parsing N3.
+      // In this case, we emit the empty graph as the value "true"^^xsd:boolean
+      // The predicate value is set to 'null' in the _readSubject function,
+      // after which the graph is closed directly when processing an empty graph.
+      if (!this._predicate) {
+        closingEmptyGraph = true;
+      }
+      else {
+        this._emit(this._subject, this._predicate, this._object, graph);
+      }
+    }
 
     // Restore the parent context containing this formula
     this._restoreContext('formula', token);
 
+    if (closingEmptyGraph) {
+      const xsdBoolean = 'http://www.w3.org/2001/XMLSchema#boolean';
+      if (this._subject && this._subject.equals(graph))
+        this._subject = this._literal('true', this._namedNode(xsdBoolean));
+
+      if (this._object && this._object.equals(graph))
+        this._object = this._literal('true', this._namedNode(xsdBoolean));
+    }
+
     // If the formula was in a list context, continue reading the list
     if (this._contextStack.length > 0 && this._contextStack[this._contextStack.length - 1].type === 'list') {
       return this._readListItem(token, graph);

test/N3Parser-test.js

@@ -1763,7 +1763,7 @@ describe('Parser', () => {
       return isImpliedBy ? [to, 'http://www.w3.org/2000/10/swap/log#isImpliedBy', from] : [from, 'http://www.w3.org/2000/10/swap/log#implies', to];
     }
 
-    describe(`A Parser instance for the N3 format with rdfStar support disabled and with ${isImpliedBy ? 'enabled' : 'disabled'}`, () => {
+    describe(`A Parser instance for the N3 format with rdfStar support disabled and with isImpliedBy ${isImpliedBy ? 'enabled' : 'disabled'}`, () => {
       function parser() { return new Parser({ baseIRI: BASE_IRI, format: 'N3', rdfStar: false, isImpliedBy }); }
 
       describe('should parse a single triple',
@@ -2337,6 +2337,28 @@ describe('Parser', () => {
       it('should not parse RDF-star in the object position',
       shouldNotParse(parser, '<a> <b> <<<a> <b> <c>>>.',
         'Unexpected RDF-star syntax on line 1.'));
+
+      describe('should parse the empty graph as an rdf:value of "true"^^xsd:boolean in the object position',
+        shouldParse(parser, '<a> <b> {}.',
+            ['a', 'b', '"true"^^http://www.w3.org/2001/XMLSchema#boolean']
+        ));
+
+      describe('should parse the empty graph as an rdf:value of "true"^^xsd:boolean in the subject position',
+        shouldParse(parser, '{} <b> <c>.',
+            ['"true"^^http://www.w3.org/2001/XMLSchema#boolean', 'b', 'c']
+        ));
+
+      describe('should parse the empty graph in the object position as an rdf:value of "true"^^xsd:boolean while retaining the encompassing graph',
+        shouldParse(parser, '<a> <b> { <x> <y> {} }.',
+          ['a', 'b', '_:b0'],
+          ['x', 'y', '"true"^^http://www.w3.org/2001/XMLSchema#boolean', '_:b0']
+      ));
+
+      describe('should parse the empty graph in the subject position as an rdf:value of "true"^^xsd:boolean while retaining the encompassing graph',
+        shouldParse(parser, '<a> <b> { {} <y> <z> }.',
+          ['a', 'b', '_:b0'],
+          ['"true"^^http://www.w3.org/2001/XMLSchema#boolean', 'y', 'z', '_:b0']
+      ));
     });
   }