Added support for returning Inline in transformers (Issue #763)

lark/__init__.py

@@ -1,6 +1,6 @@
 from .utils import logger
 from .tree import Tree
-from .visitors import Transformer, Visitor, v_args, Discard, Transformer_NonRecursive
+from .visitors import Transformer, Visitor, v_args, Discard, Inline, Transformer_NonRecursive
 from .visitors import InlineTransformer, inline_args   # XXX Deprecated
 from .exceptions import (ParseError, LexError, GrammarError, UnexpectedToken,
                          UnexpectedInput, UnexpectedCharacters, LarkError)

lark/visitors.py

@@ -15,9 +15,16 @@ class Discard(Exception):
     """
     pass
 
-# Transformers
+class Inline:
+    """When returning an Inline instance in a transformer callback,
+    the node is inlined, and replaced by its children.
+    """
+    def __init__(self, *items):
+        self.items = items
 
 
+# Transformers
+
 class _Decoratable:
     "Provides support for decorating methods with @v_args"
 
@@ -113,11 +120,16 @@ class Transformer(_Decoratable):
         for c in children:
             try:
                 if isinstance(c, Tree):
-                    yield self._transform_tree(c)
+                    res = self._transform_tree(c)
                 elif self.__visit_tokens__ and isinstance(c, Token):
-                    yield self._call_userfunc_token(c)
+                    res = self._call_userfunc_token(c)
+                else:
+                    res = c
+                if type(res) is Inline:
+                    for i in res.items:
+                        yield i
                 else:
-                    yield c
+                    yield res
             except Discard:
                 pass
 

tests/test_trees.py

@@ -8,7 +8,7 @@ import functools
 
 from lark.tree import Tree
 from lark.lexer import Token
-from lark.visitors import Visitor, Visitor_Recursive, Transformer, Interpreter, visit_children_decor, v_args, Discard
+from lark.visitors import Visitor, Visitor_Recursive, Transformer, Interpreter, visit_children_decor, v_args, Discard, Inline
 
 
 class TestTrees(TestCase):
@@ -233,6 +233,24 @@ class TestTrees(TestCase):
         x = MyTransformer().transform( t )
         self.assertEqual(x, t2)
 
+    def test_inline(self):
+        class MyTransformer(Transformer):
+            def b(self, children):
+                return 'b'
+
+            def a(self, children):
+                return Inline('c', 'd')
+
+        t = Tree('root', [
+            Tree('b', []),
+            Tree('a', []),
+            Tree('b', []),
+        ])
+        t2 = Tree('root', ['b', 'c', 'd', 'b'])
+
+        x = MyTransformer().transform( t )
+        self.assertEqual(x, t2)
+
 
 if __name__ == '__main__':
     unittest.main()