VisitError now allows explicit access to previous exception (Issue #314)

lark/exceptions.py

@@ -87,5 +87,10 @@ class UnexpectedToken(ParseError, UnexpectedInput):
         super(UnexpectedToken, self).__init__(message)
 
 class VisitError(Exception):
-    pass
+    def __init__(self, tree, orig_exc):
+        self.tree = tree
+        self.orig_exc = orig_exc
+
+        message = 'Error trying to process rule "%s":\n\n%s' % (tree.data, orig_exc)
+        super(VisitError, self).__init__(message)
 ###}

lark/visitors.py

@@ -43,7 +43,7 @@ class Transformer:
             except GrammarError:
                 raise
             except Exception as e:
-                raise VisitError('Error trying to process rule "%s":\n\n%s' % (tree.data, e))
+                raise VisitError(tree, e)
 
     def _transform_children(self, children):
         for c in children: