♻️ (bitcoin-wallet): refactor with error type

Author: yuanyu90221

pointers-and-errors/bitcoin-and-wallet.go

@@ -1,6 +1,11 @@
 package pointers_and_errors
 
-import "fmt"
+import (
+	"errors"
+	"fmt"
+)
+
+var ErrInsufficientFunds = errors.New("cannot withdraw, insufficient funds")
 
 type Bitcoin int
 
@@ -26,7 +31,7 @@ func (w *Wallet) Balance() Bitcoin {
 // Withdraw - deposit amount from wallet balance
 func (w *Wallet) Withdraw(amount Bitcoin) error {
 	if w.balance < amount {
-		return fmt.Errorf("oh no")
+		return ErrInsufficientFunds
 	}
 	w.balance -= amount
 	return nil

pointers-and-errors/bitcoin-and-wallet_test.go

@@ -14,11 +14,15 @@ func TestWallet(t *testing.T) {
 		}
 	}
 
-	assertError := func(t testing.TB, err error) {
+	assertError := func(t testing.TB, err, want error) {
 		t.Helper()
 		if err == nil {
 			t.Error("wanted an error but didn't get one")
 		}
+
+		if err != want {
+			t.Errorf("got %q want %q", err.Error(), want)
+		}
 	}
 
 	t.Run("deposit", func(t *testing.T) {
@@ -42,7 +46,7 @@ func TestWallet(t *testing.T) {
 		wallet := Wallet{startingBalance}
 		err := wallet.Withdraw(Bitcoin(100))
 
-		assertError(t, err)
+		assertError(t, err, ErrInsufficientFunds)
 		assertBalance(t, wallet, startingBalance)
 
 	})