Added VisitError for transformers

lark/exceptions.py

@@ -86,4 +86,6 @@ class UnexpectedToken(ParseError, UnexpectedInput):
 
         super(UnexpectedToken, self).__init__(message)
 
+class VisitError(Exception):
+    pass
 ###}

lark/utils.py

@@ -57,12 +57,16 @@ from functools import wraps, partial
 from contextlib import contextmanager
 
 Str = type(u'')
+try:
+    classtype = types.ClassType # Python2
+except AttributeError:
+    classtype = type    # Python3
 
 def smart_decorator(f, create_decorator):
     if isinstance(f, types.FunctionType):
         return wraps(f)(create_decorator(f, True))
 
-    elif isinstance(f, (type, types.BuiltinFunctionType)):
+    elif isinstance(f, (classtype, type, types.BuiltinFunctionType)):
         return wraps(f)(create_decorator(f, False))
 
     elif isinstance(f, types.MethodType):

lark/visitors.py

@@ -2,6 +2,7 @@ from functools import wraps
 
 from .utils import smart_decorator
 from .tree import Tree
+from .exceptions import VisitError, GrammarError
 
 ###{standalone
 from inspect import getmembers, getmro
@@ -28,16 +29,21 @@ class Transformer:
         except AttributeError:
             return self.__default__(tree.data, children, tree.meta)
         else:
-            if getattr(f, 'meta', False):
-                return f(children, tree.meta)
-            elif getattr(f, 'inline', False):
-                return f(*children)
-            elif getattr(f, 'whole_tree', False):
-                if new_children is not None:
-                    raise NotImplementedError("Doesn't work with the base Transformer class")
-                return f(tree)
-            else:
-                return f(children)
+            try:
+                if getattr(f, 'meta', False):
+                    return f(children, tree.meta)
+                elif getattr(f, 'inline', False):
+                    return f(*children)
+                elif getattr(f, 'whole_tree', False):
+                    if new_children is not None:
+                        raise NotImplementedError("Doesn't work with the base Transformer class")
+                    return f(tree)
+                else:
+                    return f(children)
+            except GrammarError:
+                raise
+            except Exception as e:
+                raise VisitError('Error trying to process rule "%s":\n\n%s' % (tree.data, e))
 
     def _transform_children(self, children):
         for c in children: