From 3ff0eb8f656b64ef27e8e659fbe5a7c348b95b83 Mon Sep 17 00:00:00 2001
From: Shunsuke Shibayama <sbym1346@gmail.com>
Date: Fri, 17 Mar 2023 00:13:43 +0900
Subject: [PATCH] fix: improve `match` error messages

---
 crates/erg_compiler/context/inquire.rs |  4 ++++
 crates/erg_compiler/error/tycheck.rs   | 16 ++++++++++++----
 2 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/crates/erg_compiler/context/inquire.rs b/crates/erg_compiler/context/inquire.rs
index cc858818..12a558cf 100644
--- a/crates/erg_compiler/context/inquire.rs
+++ b/crates/erg_compiler/context/inquire.rs
@@ -304,6 +304,7 @@ impl Context {
         let match_target_expr_t = pos_args[0].expr.ref_t();
         // Never or T => T
         let mut union_pat_t = Type::Never;
+        let mut arm_ts = vec![];
         for (i, pos_arg) in pos_args.iter().skip(1).enumerate() {
             let lambda = erg_common::enum_unwrap!(&pos_arg.expr, hir::Expr::Lambda); // already checked
             if !lambda.params.defaults.is_empty() {
@@ -334,6 +335,7 @@ impl Context {
                 ParamKind::NonDefault,
             )?;
             union_pat_t = self.union(&union_pat_t, &rhs);
+            arm_ts.push(rhs);
         }
         // NG: expr_t: Nat, union_pat_t: {1, 2}
         // OK: expr_t: Int, union_pat_t: {1} or 'T
@@ -347,6 +349,8 @@ impl Context {
                 pos_args[0].loc(),
                 self.caused_by(),
                 match_target_expr_t,
+                &union_pat_t,
+                arm_ts,
             )));
         }
         let branch_ts = pos_args
diff --git a/crates/erg_compiler/error/tycheck.rs b/crates/erg_compiler/error/tycheck.rs
index 67c66c67..e2f2eb40 100644
--- a/crates/erg_compiler/error/tycheck.rs
+++ b/crates/erg_compiler/error/tycheck.rs
@@ -409,15 +409,23 @@ impl TyCheckError {
         loc: Location,
         caused_by: String,
         expr_t: &Type,
+        union_pat_t: &Type,
+        arm_ts: Vec<Type>,
     ) -> Self {
+        let arms = arm_ts
+            .into_iter()
+            .enumerate()
+            .fold("".to_string(), |acc, (i, t)| {
+                acc + &format!("{} arm type: {t}\n", ordinal_num(i + 1))
+            });
         Self::new(
             ErrorCore::new(
                 vec![SubMessage::only_loc(loc)],
                 switch_lang!(
-                    "japanese" => format!("{expr_t}型の全パターンを網羅していません"),
-                    "simplified_chinese" => format!("并非所有{expr_t}类型的模式都被涵盖"),
-                    "traditional_chinese" => format!("並非所有{expr_t}類型的模式都被涵蓋"),
-                    "english" => format!("not all patterns of type {expr_t} are covered"),
+                    "japanese" => format!("{expr_t}型の全パターンを網羅していません\nunion type: {union_pat_t}\n{arms}"),
+                    "simplified_chinese" => format!("并非所有{expr_t}类型的模式都被涵盖\nunion type: {union_pat_t}\n{arms}"),
+                    "traditional_chinese" => format!("並非所有{expr_t}類型的模式都被涵蓋\nunion type: {union_pat_t}\n{arms}"),
+                    "english" => format!("not all patterns of type {expr_t} are covered\nunion type: {union_pat_t}\n{arms}"),
                 ),
                 errno,
                 TypeError,