[NODE] GetRef helper (apache#31)

relay/include/relay/node.h

@@ -862,6 +862,26 @@ class FnValueNode : public ValueNode {
 
 RELAY_DEFINE_VALUE(FnValue, FnValueNode);
 
+
+/*!
+ * \brief Get a reference type from a Node ptr type
+ *
+ *  It is always important to get a reference type
+ *  if we want to return a value as reference or keep
+ *  the node alive beyond the scope of the function.
+ *
+ * \param ptr The node pointer
+ * \tparam RefType The reference type
+ * \tparam NodeType The node type
+ * \return The corresponding RefType
+ */
+template<typename RefType, typename NodeType>
+RefType GetRef(const NodeType* ptr) {
+  static_assert(std::is_same<typename RefType::ContainerType, NodeType>::value,
+                "Can only cast to the ref of same container type");
+  return RefType(const_cast<NodeType*>(ptr)->shared_from_this());
+}
+
 }  // namespace relay
 }  // namespace nnvm
 

relay/src/relay/evaluator.cc

@@ -21,6 +21,8 @@ Evaluator::Evaluator() : env() {}
 Value Evaluator::Eval(const Expr& expr) { return this->operator()(expr); }
 
 Value Evaluator::VisitExpr_(const LocalIdNode* local_node) {
+  // will error: GetRef<GlobalId>(local_node);
+  LocalId local_id = GetRef<LocalId>(local_node);
   // We should instead compile this to a stack machine with statically resolved
   // offsets.
   // LocalId local = LocalId(local_node->GetNodeRef().node_);