From 24f86569da4dc67b0c96bf47e990851cdecedf72 Mon Sep 17 00:00:00 2001
From: Erez Shinan <erezshin+git@gmail.com>
Date: Thu, 9 Mar 2017 09:45:56 +0200
Subject: [PATCH] Fixed issue #3 (infinite recursion in grammar)

---
 lark/parsers/earley.py |  2 ++
 tests/test_parser.py   | 10 ++++++++++
 2 files changed, 12 insertions(+)

diff --git a/lark/parsers/earley.py b/lark/parsers/earley.py
index d1ec543c..49f9ed7c 100644
--- a/lark/parsers/earley.py
+++ b/lark/parsers/earley.py
@@ -105,6 +105,8 @@ def add(self, items):
                         new_tree = old_tree.copy()
                         new_tree.rule = old_tree.rule
                         old_tree.set('ambig', [new_tree])
+                    if item.tree.children[0] is old_tree:   # XXX a little hacky!
+                        raise ParseError("Infinite recursion in grammar!")
                     old_tree.children.append(item.tree)
                 else:
                     self.completed[item] = item
diff --git a/tests/test_parser.py b/tests/test_parser.py
index c4ef4bff..d3bf3947 100644
--- a/tests/test_parser.py
+++ b/tests/test_parser.py
@@ -39,6 +39,16 @@ def test_same_ast(self):
         l2 = g.parse('(a,b,c,*x)')
         assert l == l2, '%s != %s' % (l.pretty(), l2.pretty())
 
+    def test_infinite_recurse(self):
+        g = """start: a
+               a: a | "a"
+            """
+
+        self.assertRaises(GrammarError, Lark, g, parser='lalr')
+
+        l = Lark(g, parser='earley')
+        self.assertRaises(ParseError, l.parse, 'a')
+
 
 class TestEarley(unittest.TestCase):
     def test_anon_in_scanless(self):