1 files changed, 65 insertions, 0 deletions
diff --git a/third_party/python/fluent.syntax/fluent/syntax/visitor.py b/third_party/python/fluent.syntax/fluent/syntax/visitor.py
new file mode 100644
index 0000000000..0df9f5963e
--- /dev/null
+++ b/third_party/python/fluent.syntax/fluent/syntax/visitor.py
@@ -0,0 +1,65 @@
+from typing import Any, List
+from .ast import BaseNode, Node
+
+
+class Visitor:
+    '''Read-only visitor pattern.
+
+    Subclass this to gather information from an AST.
+    To generally define which nodes not to descend in to, overload
+    `generic_visit`.
+    To handle specific node types, add methods like `visit_Pattern`.
+    If you want to still descend into the children of the node, call
+    `generic_visit` of the superclass.
+    '''
+
+    def visit(self, node: Any) -> None:
+        if isinstance(node, list):
+            for child in node:
+                self.visit(child)
+            return
+        if not isinstance(node, BaseNode):
+            return
+        nodename = type(node).__name__
+        visit = getattr(self, f'visit_{nodename}', self.generic_visit)
+        visit(node)
+
+    def generic_visit(self, node: BaseNode) -> None:
+        for propvalue in vars(node).values():
+            self.visit(propvalue)
+
+
+class Transformer(Visitor):
+    '''In-place AST Transformer pattern.
+
+    Subclass this to create an in-place modified variant
+    of the given AST.
+    If you need to keep the original AST around, pass
+    a `node.clone()` to the transformer.
+    '''
+
+    def visit(self, node: Any) -> Any:
+        if not isinstance(node, BaseNode):
+            return node
+
+        nodename = type(node).__name__
+        visit = getattr(self, f'visit_{nodename}', self.generic_visit)
+        return visit(node)
+
+    def generic_visit(self, node: Node) -> Node:  # type: ignore
+        for propname, propvalue in vars(node).items():
+            if isinstance(propvalue, list):
+                new_vals: List[Any] = []
+                for child in propvalue:
+                    new_val = self.visit(child)
+                    if new_val is not None:
+                        new_vals.append(new_val)
+                # in-place manipulation
+                propvalue[:] = new_vals
+            elif isinstance(propvalue, BaseNode):
+                new_val = self.visit(propvalue)
+                if new_val is None:
+                    delattr(node, propname)
+                else:
+                    setattr(node, propname, new_val)
+        return node