diff --git a/imperative/src/impl/interpreter/option_manager.h b/imperative/src/impl/interpreter/option_manager.h
index 8100c15d..c140b1f4 100644
--- a/imperative/src/impl/interpreter/option_manager.h
+++ b/imperative/src/impl/interpreter/option_manager.h
@@ -25,6 +25,7 @@ public:
 #define DEF_OPTION(name, env_key, default_value, desc) \
     size_t name = (m_option_map[#name]=&name, get_option_from_env(env_key, default_value));
 
+    // Environ value would be read only when interpreter initializing.
     DEF_OPTION(async_level,             "MEGENGINE_INTERP_ASYNC_LEVEL",     2,
         "config whether raise error exactly when invoking op.\n"
         "level 2: both device and user side errors are async;\n"
diff --git a/imperative/src/include/megbrain/imperative/utils/to_string.h b/imperative/src/include/megbrain/imperative/utils/to_string.h
index 7a8dc9b1..92825db9 100644
--- a/imperative/src/include/megbrain/imperative/utils/to_string.h
+++ b/imperative/src/include/megbrain/imperative/utils/to_string.h
@@ -21,9 +21,13 @@
 
 namespace mgb::imperative {
 
+// Implement `ToStringTrait` for your printable class
+// note that it should be either implemented in this file
+// or in the same file with your class
 template <typename T>
 struct ToStringTrait;
 
+// Call `to_string` to print your value
 template <typename T>
 std::string to_string(const T& value) {
     return ToStringTrait<T>{}(value);
@@ -92,7 +96,6 @@ template <>
 struct ToStringTrait<TensorShape>{
     std::string operator()(TensorShape shape) const {
         if (shape.ndim > TensorShape::MAX_NDIM) {
-            printf("ndim: %d\n", (int)shape.ndim);
             return "[]";
         }
         mgb_assert(shape.ndim <= TensorShape::MAX_NDIM);