wallet: improve assertError again by creating a custom error type

wallet/wallet.go

@@ -7,6 +7,8 @@ import (
 
 type Bitcoin int
 
+var ErrInsufficientFunds = errors.New("cannot withdraw, insufficient funds")
+
 func (b Bitcoin) String() string {
 	return fmt.Sprintf("%d BTC", b)
 }
@@ -21,7 +23,7 @@ func (w *Wallet) Deposit(amount Bitcoin) {
 
 func (w *Wallet) Withdraw(amount Bitcoin) error {
 	if w.balance < amount {
-		return errors.New("not enough to withdraw")
+		return ErrInsufficientFunds
 	}
 	w.balance -= amount
 	return nil

wallet/wallet_test.go

@@ -3,26 +3,6 @@ package wallet
 import "testing"
 
 func TestWallet(t *testing.T) {
-	assertBalance := func(t testing.TB, wallet Wallet, want Bitcoin) {
-		t.Helper()
-
-		got := wallet.Balance()
-		if got != want {
-			t.Errorf("got %q, want %q", got, want)
-		}
-	}
-
-	assertError := func(t testing.TB, got error, want string) {
-		t.Helper()
-
-		if got == nil {
-			t.Fatal("didn't get an error but wanted one")
-		}
-		if got.Error() != want {
-			t.Errorf("got %q, want %q", got, want)
-		}
-	}
-
 	t.Run("deposit", func(t *testing.T) {
 		wallet := Wallet{}
 		wallet.Deposit(10)
@@ -40,7 +20,27 @@ func TestWallet(t *testing.T) {
 		wallet := Wallet{balance: startingBalance}
 		err := wallet.Withdraw(100)
 
-		assertError(t, err, "not enough to withdraw")
+		assertError(t, err, ErrInsufficientFunds)
 		assertBalance(t, wallet, startingBalance)
 	})
 }
+
+func assertBalance(t testing.TB, wallet Wallet, want Bitcoin) {
+	t.Helper()
+
+	got := wallet.Balance()
+	if got != want {
+		t.Errorf("got %q, want %q", got, want)
+	}
+}
+
+func assertError(t testing.TB, got error, want error) {
+	t.Helper()
+
+	if got == nil {
+		t.Fatal("didn't get an error but wanted one")
+	}
+	if got != want {
+		t.Errorf("got %q, want %q", got, want)
+	}
+}