Bugfix in tools.nearley: Added support for null keyword (Issue #342)

lark/tools/nearley.py

@@ -20,12 +20,13 @@ nearley_grammar = r"""
 
     ?expr: item [":" /[+*?]/]
 
-    ?item: rule|string|regexp
+    ?item: rule|string|regexp|null
          | "(" expansions ")"
 
     rule: NAME
     string: STRING
     regexp: REGEXP
+    null: "null"
     JS: /{%.*?%}/s
     js: JS?
 
@@ -83,6 +84,9 @@ class NearleyToLark(InlineTransformer):
     def regexp(self, r):
         return '/%s/' % r
 
+    def null(self):
+        return ''
+
     def string(self, s):
         return self._extra_rule(s)
 

tests/test_nearley/test_nearley.py

@@ -83,6 +83,15 @@ class TestNearley(unittest.TestCase):
         parse = d['parse']
         parse(u'"')
 
+    def test_null(self):
+        grammar = r'main -> "a" | null'
+        code = create_code_for_nearley_grammar(grammar, 'main', BUILTIN_PATH, './')
+        d = {}
+        exec (code, d)
+        parse = d['parse']
+        parse('a')
+        parse('')
+
     def test_utf8_2(self):
         fn = os.path.join(TEST_PATH, 'grammars/unicode.ne')
         nearley_tool_main(fn, 'x', NEARLEY_PATH)