diff --git a/compiler/erg_common/error.rs b/compiler/erg_common/error.rs
index 5af2c41f..55198aec 100644
--- a/compiler/erg_common/error.rs
+++ b/compiler/erg_common/error.rs
@@ -397,10 +397,56 @@ pub struct SubMessage {
 }
 
 impl SubMessage {
+    ///
+    /// Used when the msg or hint si empty.
+    /// `msg` is Vec\<String\> instead of Option\<String\> because it can be used when there are multiple `msg`s as well as multiple lines.
+    /// # Example
+    /// ```
+    /// let msg = SubMessage::ambiguous_new(loc, vec![], None); // this code same as only_loc()
+    ///
+    /// let hint = Some("hint message here".to_string())
+    /// let msg = SubMessage::ambiguous_new(loc, vec![], hint);
+    /// /* example
+    ///    -------
+    ///          `- hint message here
+    /// */
+    ///
+    /// let hint = Some("hint here".to_string())
+    /// let first = StyledString::new("1th message", Color::Red, None);
+    /// let second = StyledString::new("2th message", Color::White, None);
+    ///      :
+    /// let nth = StyledString::new("nth message", Color::Green, None);
+    /// let msg = SubMessage::ambiguous_new(
+    ///     loc,
+    ///     vec![
+    ///         first.to_string(),
+    ///         second.to_string(),
+    ///         ...,
+    ///         nth.to_string(),
+    ///     ],
+    ///     hint);
+    /// /* example
+    ///    -------
+    ///          :- 1th message
+    ///          :- 2th message
+    ///                :
+    ///          :- nth message
+    ///          `- hint here
+    /// */
+    ///
+    /// ```
+    ///
     pub fn ambiguous_new(loc: Location, msg: Vec<String>, hint: Option<String>) -> Self {
         Self { loc, msg, hint }
     }
 
+    ///
+    /// Used when only Location is fixed.
+    /// In this case, error position is just modified
+    /// # Example
+    /// ```
+    /// let sub_msg = SubMessage::only_loc(loc);
+    /// ```
     pub fn only_loc(loc: Location) -> Self {
         Self {
             loc,
diff --git a/compiler/erg_common/style.rs b/compiler/erg_common/style.rs
index 5e686bf2..d67bb1a8 100644
--- a/compiler/erg_common/style.rs
+++ b/compiler/erg_common/style.rs
@@ -313,6 +313,14 @@ pub struct StyledString {
 }
 
 impl StyledString {
+    ///
+    /// # Example
+    /// ```
+    /// let s = String::from("Hello, world");
+    /// StyledString::new(s, None, None);
+    /// let s = "Hello, world";
+    /// StyledString::new(s, None, None);
+    /// ```
     pub fn new<'a, S: Into<Cow<'a, str>>>(
         s: S,
         color: Option<Color>,
@@ -464,7 +472,18 @@ impl StyledStrings {
         }
     }
 
-    pub fn is_same_color(&self, color: Color) -> bool {
+    ///
+    /// Determine if all strings in Vec are empty
+    /// Returns False if any string is present.
+    ///
+    pub fn is_empty(&self) -> bool {
+        self.texts
+            .iter()
+            .inspect(|x| println!("{:#?}", x))
+            .all(|s| s.is_empty())
+    }
+
+    fn is_same_color(&self, color: Color) -> bool {
         if let Some(text) = self.texts.last() {
             return text.color == Some(color);
         }