Implemented a new visitor class (Interpreter) that works top-down (PR #130)

It emulates antlr's visitor behavior for a dynamic evaluation order of subtrees

lark/tree.py

@@ -174,6 +174,30 @@ class Visitor_NoRecurse(Visitor):
         return tree
 
 
+from functools import wraps
+def visit_children_decor(func):
+    @wraps(func)
+    def inner(cls, tree):
+        values = cls.visit_children(tree)
+        return func(cls, values)
+    return inner
+
+class Interpreter(object):
+
+    def visit(self, tree):
+        return getattr(self, tree.data)(tree)
+
+    def visit_children(self, tree):
+        return [self.visit(child) if isinstance(child, Tree) else child
+                for child in tree.children]
+
+    def __getattr__(self, name):
+        return self.__default__
+
+    def __default__(self, tree):
+        self.visit_children(tree)
+
+
 class Transformer_NoRecurse(Transformer):
     def transform(self, tree):
         subtrees = list(tree.iter_subtrees())

tests/test_trees.py

@@ -5,7 +5,7 @@ from unittest import TestCase
 import copy
 import pickle
 
-from lark.tree import Tree
+from lark.tree import Tree, Interpreter, visit_children_decor
 
 
 class TestTrees(TestCase):
@@ -21,6 +21,37 @@ class TestTrees(TestCase):
         assert pickle.loads(data) == s
 
 
+    def test_interp(self):
+        t = Tree('a', [Tree('b', []), Tree('c', []), 'd'])
+
+        class Interp1(Interpreter):
+            def a(self, tree):
+                return self.visit_children(tree) + ['e']
+
+            def b(self, tree):
+                return 'B'
+
+            def c(self, tree):
+                return 'C'
+
+        self.assertEqual(Interp1().visit(t), list('BCde'))
+
+        class Interp2(Interpreter):
+            @visit_children_decor
+            def a(self, values):
+                return values + ['e']
+
+            def b(self, tree):
+                return 'B'
+
+            def c(self, tree):
+                return 'C'
+
+        self.assertEqual(Interp2().visit(t), list('BCde'))
+
+
+
+
 if __name__ == '__main__':
     unittest.main()