Finished with pointers

learn_go_with_tests/pointers_errors/README.md

@@ -2,3 +2,22 @@
 
 https://quii.gitbook.io/learn-go-with-tests/go-fundamentals/pointers-and-errors
 
+## Handle Errors Gracefully
+
+A must read:
+
+https://dave.cheney.net/2016/04/27/dont-just-check-errors-handle-them-gracefully
+
+
+The author has a package https://pkg.go.dev/github.com/pkg/errors that you can use to implement the error handling pattern he outlines in the article.
+
+## Use errcheck
+
+You can use this static analysis tool to check if you have
+potentially not handled an error case in your code
+
+
+```
+go install github.com/kisielk/errcheck@latest
+errcheck <src dir>
+```

learn_go_with_tests/pointers_errors/wallet.go

@@ -5,10 +5,16 @@ import (
 	"fmt"
 )
 
+
+// The var keyword allows us to define values global to the package.
+var ErrInsufficientFunds = errors.New("cannot withdraw, insufficient funds")
+
 type Stringer interface {
 	String() string
 }
 
+errors.Wrap()
+
 // creating a new type from an existing on
 // The idea being it could be more descriptive
 // but also you can declare methods on them
@@ -46,7 +52,7 @@ func (w *Wallet) Balance() Bitcoin {
 
 func (w *Wallet) Withdraw(amount Bitcoin) error {
 	if amount > w.balance {
-		return errors.New("Withdraw operation failed. Insufficient funds")
+		return ErrInsufficientFunds
 	}
 	w.balance -= amount
 	return nil

learn_go_with_tests/pointers_errors/wallet_test.go

@@ -4,6 +4,13 @@ import (
 	"testing"
 )
 
+func assertNoError(t testing.TB, got error) {
+	t.Helper()
+	if got != nil {
+		t.Fatal("got an error but didn't want one")
+	}
+}
+
 func TestWallet(t *testing.T) {
 
 	assertBalance := func(t *testing.T, wallet Wallet, want Bitcoin) {
@@ -15,6 +22,23 @@ func TestWallet(t *testing.T) {
 		}
 	}
 
+	assertError := func(t testing.TB, got, want error) {
+		t.Helper()
+		if got == nil {
+			// Fata; will stop the test if called. We do this because we
+			// don't want to make any more assertions, otherwise we would try
+			// to access a nil pointer and hit a panic
+			t.Fatal("wanted error but didn't get one")
+		}
+		// cast error to string and compare
+		// if got.Error() != want {
+		// 	t.Errorf("got %q, want %q", got, want)
+		// }
+		if got != want {
+			t.Errorf("got %q, want %q", got, want)
+		}
+	}
+
 	t.Run("deposit", func(t *testing.T) {
 		wallet := Wallet{}
 
@@ -26,8 +50,13 @@ func TestWallet(t *testing.T) {
 	t.Run("withdraw", func(t *testing.T) {
 		wallet := Wallet{balance: Bitcoin(20)}
 
-		wallet.Withdraw(Bitcoin(10))
+		// This was found by using
+		// go install github.com/kisielk/errcheck@latest
+		// and using `errcheck .` on our code. Could make
+		// a great pre-commit hook.
+		err := wallet.Withdraw(Bitcoin(10))
 
+		assertNoError(t, err)
 		assertBalance(t, wallet, Bitcoin(10))
 
 	})
@@ -38,10 +67,7 @@ func TestWallet(t *testing.T) {
 		err := wallet.Withdraw(Bitcoin(100))
 
 		assertBalance(t, wallet, startingBalance)
-
-		if err == nil {
-			t.Error("Wanted an error but didn't get one")
-		}
+		assertError(t, err, ErrInsufficientFunds)
 
 	})
 }