specific error message

Author: SUSTAPLE117

pointers/wallet.go

@@ -26,7 +26,7 @@ func (w *Wallet) Balance() Bitcoin {
 
 func (w *Wallet) Withdraw(amount Bitcoin) error {
 	if amount > w.balance {
-		return errors.New("oh no")
+		return errors.New("cannot withdraw, insufficient funds")
 	}
 	w.balance -= amount
 	return nil

pointers/wallet_test.go

@@ -15,10 +15,14 @@ func TestWallet(t *testing.T) {
 		}
 	}
 
-	assertError := func(t *testing.T, err error) {
+	assertError := func(t *testing.T, got error, want string) {
 		t.Helper()
-		if err == nil {
-			t.Error("wanted an error but didn't get one")
+		if got == nil {
+			t.Fatal("wanted an error but didn't get one")
+		}
+
+		if got.Error() != want {
+			t.Errorf("got %q, want %q", got, want)
 		}
 	}
 
@@ -39,6 +43,6 @@ func TestWallet(t *testing.T) {
 		err := wallet.Withdraw(Bitcoin(100))
 
 		assertBalance(t, wallet, Bitcoin(20))
-		assertError(t, err)
+		assertError(t, err, "cannot withdraw, insufficient funds")
 	})
 }