BUGFIX: Solved an elusive bug in Earley parser, when empty rules repeat in the same column

lark/parsers/earley.py

@@ -98,7 +98,7 @@ class Column:
         for item in items:
 
             if item.is_complete:
-                # XXX TODO Potential bug: What happens if there's ambiguity in an empty rule?
+                # XXX Potential bug: What happens if there's ambiguity in an empty rule?
                 if item.rule.expansion and item in self.completed:
                     old_tree = self.completed[item].tree
                     if old_tree.data != 'ambig':

tests/test_parser.py

@@ -19,6 +19,7 @@ logging.basicConfig(level=logging.INFO)
 from lark.lark import Lark
 from lark.common import GrammarError, ParseError
 from lark.lexer import LexError
+from lark.tree import Tree
 
 __path__ = os.path.dirname(__file__)
 def _read(n, *args):
@@ -104,6 +105,21 @@ class TestEarley(unittest.TestCase):
         res = l.parse("aaa")
         self.assertEqual(res.children, ['aaa'])
 
+    def test_earley_repeating_empty(self):
+        # This was a sneaky bug!
+
+        grammar = """
+        !start: "a" empty empty "b"
+        empty: empty2
+        empty2:
+        """
+
+        parser = Lark(grammar, parser='earley', lexer=None)
+        res = parser.parse('ab')
+
+        empty_tree = Tree('empty', [Tree('empty2', [])])
+        self.assertSequenceEqual(res.children, ['a', empty_tree, empty_tree, 'b'])
+
 def _make_parser_test(LEXER, PARSER):
     def _Lark(grammar, **kwargs):
         return Lark(grammar, lexer=LEXER, parser=PARSER, **kwargs)