diff --git a/compiler/erg_compiler/build_hir.rs b/compiler/erg_compiler/build_hir.rs
index 0434f100..6a298e87 100644
--- a/compiler/erg_compiler/build_hir.rs
+++ b/compiler/erg_compiler/build_hir.rs
@@ -10,7 +10,6 @@ use crate::artifact::{CompleteArtifact, IncompleteArtifact};
 use crate::context::{Context, ContextProvider};
 use crate::effectcheck::SideEffectChecker;
 use crate::error::{CompileError, CompileErrors};
-use crate::hir::HIR;
 use crate::lower::ASTLowerer;
 use crate::mod_cache::SharedModuleCache;
 use crate::ownercheck::OwnershipChecker;
@@ -58,16 +57,18 @@ impl Runnable for HIRBuilder {
     fn exec(&mut self) -> Result<i32, Self::Errs> {
         let mut builder = ASTBuilder::new(self.cfg().copy());
         let ast = builder.build(self.input().read())?;
-        let hir = self.check(ast, "exec").map_err(|arti| arti.errors)?;
-        println!("{hir}");
+        let artifact = self.check(ast, "exec").map_err(|arti| arti.errors)?;
+        artifact.warns.fmt_all_stderr();
+        println!("{}", artifact.hir);
         Ok(0)
     }
 
     fn eval(&mut self, src: String) -> Result<String, Self::Errs> {
         let mut builder = ASTBuilder::new(self.cfg().copy());
         let ast = builder.build(src)?;
-        let hir = self.check(ast, "eval").map_err(|arti| arti.errors)?;
-        Ok(hir.to_string())
+        let artifact = self.check(ast, "eval").map_err(|arti| arti.errors)?;
+        artifact.warns.fmt_all_stderr();
+        Ok(artifact.hir.to_string())
     }
 }
 
@@ -98,19 +99,19 @@ impl HIRBuilder {
         }
     }
 
-    pub fn check(&mut self, ast: AST, mode: &str) -> Result<HIR, IncompleteArtifact> {
+    pub fn check(&mut self, ast: AST, mode: &str) -> Result<CompleteArtifact, IncompleteArtifact> {
         let artifact = self.lowerer.lower(ast, mode)?;
         if self.cfg().verbose >= 2 {
             artifact.warns.fmt_all_stderr();
         }
         let effect_checker = SideEffectChecker::new(self.cfg().clone());
         let hir = effect_checker.check(artifact.hir).map_err(|(hir, errs)| {
-            IncompleteArtifact::new(Some(hir), errs, CompileErrors::empty())
+            IncompleteArtifact::new(Some(hir), errs, artifact.warns.clone())
         })?;
         let hir = self.ownership_checker.check(hir).map_err(|(hir, errs)| {
-            IncompleteArtifact::new(Some(hir), errs, CompileErrors::empty())
+            IncompleteArtifact::new(Some(hir), errs, artifact.warns.clone())
         })?;
-        Ok(hir)
+        Ok(CompleteArtifact::new(hir, artifact.warns))
     }
 
     pub fn build(
@@ -122,8 +123,7 @@ impl HIRBuilder {
         let ast = ast_builder.build(src).map_err(|errs| {
             IncompleteArtifact::new(None, CompileErrors::from(errs), CompileErrors::empty())
         })?;
-        let hir = self.check(ast, mode)?;
-        Ok(CompleteArtifact::new(hir, CompileErrors::empty()))
+        self.check(ast, mode)
     }
 
     pub fn pop_mod_ctx(&mut self) -> Context {
diff --git a/compiler/erg_compiler/error.rs b/compiler/erg_compiler/error.rs
index 199e917e..fbfab343 100644
--- a/compiler/erg_compiler/error.rs
+++ b/compiler/erg_compiler/error.rs
@@ -18,7 +18,7 @@ use erg_parser::error::{ParserRunnerError, ParserRunnerErrors};
 
 use crate::context::Context;
 use crate::hir::{Expr, Identifier, Signature};
-use crate::ty::{Predicate, Type};
+use crate::ty::{HasType, Predicate, Type};
 
 pub fn ordinal_num(n: usize) -> String {
     match n.to_string().chars().last().unwrap() {
@@ -109,7 +109,7 @@ pub fn readable_name(name: &str) -> &str {
     }
 }
 
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 pub struct CompileError {
     pub core: Box<ErrorCore>, // ErrorCore is large, so box it
     pub input: Input,
@@ -1458,6 +1458,33 @@ impl LowerError {
         )
     }
 
+    pub fn unused_expr_warning(input: Input, errno: usize, expr: &Expr, caused_by: String) -> Self {
+        let desc = switch_lang!(
+            "japanese" => format!("式の評価結果(: {})が使われていません", expr.ref_t()),
+            "simplified_chinese" => format!("表达式评估结果(: {})未使用", expr.ref_t()),
+            "traditional_chinese" => format!("表達式評估結果(: {})未使用", expr.ref_t()),
+            "english" => format!("the evaluation result of the expression (: {}) is not used", expr.ref_t()),
+        );
+        let hint = switch_lang!(
+            "japanese" => "値を使わない場合は、discard関数を使用してください",
+            "simplified_chinese" => "如果您不想使用该值，请使用discard函数",
+            "traditional_chinese" => "如果您不想使用該值，請使用discard函數",
+            "english" => "if you don't use the value, use discard function",
+        )
+        .to_owned();
+        Self::new(
+            ErrorCore::new(
+                vec![SubMessage::ambiguous_new(expr.loc(), vec![], Some(hint))],
+                desc,
+                errno,
+                UnusedWarning,
+                expr.loc(),
+            ),
+            input,
+            caused_by,
+        )
+    }
+
     pub fn duplicate_decl_error(
         input: Input,
         errno: usize,
@@ -2234,7 +2261,7 @@ impl LowerError {
     }
 }
 
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 pub struct CompileErrors(Vec<CompileError>);
 
 impl std::error::Error for CompileErrors {}
diff --git a/compiler/erg_compiler/lower.rs b/compiler/erg_compiler/lower.rs
index c334a595..bd13f8a9 100644
--- a/compiler/erg_compiler/lower.rs
+++ b/compiler/erg_compiler/lower.rs
@@ -31,7 +31,7 @@ use crate::context::{
     ClassDefType, Context, ContextKind, ContextProvider, RegistrationMode, TypeRelationInstance,
 };
 use crate::error::{
-    CompileError, CompileErrors, LowerError, LowerErrors, LowerResult, LowerWarnings,
+    CompileError, CompileErrors, LowerError, LowerErrors, LowerResult, LowerWarning, LowerWarnings,
     SingleLowerResult,
 };
 use crate::hir;
@@ -179,26 +179,11 @@ impl ASTLowerer {
     /// OK: exec `None`
     fn use_check(&self, expr: &hir::Expr, mode: &str) -> SingleLowerResult<()> {
         if mode != "eval" && !expr.ref_t().is_nonelike() && !expr.is_type_asc() {
-            Err(LowerError::syntax_error(
+            Err(LowerWarning::unused_expr_warning(
                 self.cfg.input.clone(),
                 line!() as usize,
-                expr.loc(),
+                expr,
                 String::from(&self.ctx.name[..]),
-                switch_lang!(
-                    "japanese" => format!("式の評価結果(: {})が使われていません", expr.ref_t()),
-                    "simplified_chinese" => format!("表达式评估结果(: {})未使用", expr.ref_t()),
-                    "traditional_chinese" => format!("表達式評估結果(: {})未使用", expr.ref_t()),
-                    "english" => format!("the evaluation result of the expression (: {}) is not used", expr.ref_t()),
-                ),
-                Some(
-                    switch_lang!(
-                        "japanese" => "値を使わない場合は、discard関数を使用してください",
-                        "simplified_chinese" => "如果您不想使用该值，请使用discard函数",
-                        "traditional_chinese" => "如果您不想使用該值，請使用discard函數",
-                        "english" => "if you don't use the value, use discard function",
-                    )
-                    .to_owned(),
-                ),
             ))
         } else {
             Ok(())
@@ -1939,8 +1924,8 @@ impl ASTLowerer {
         };
         // TODO: recursive check
         for chunk in hir.module.iter() {
-            if let Err(err) = self.use_check(chunk, mode) {
-                self.errs.push(err);
+            if let Err(warn) = self.use_check(chunk, mode) {
+                self.warns.push(warn);
             }
         }
         if self.errs.is_empty() {