Types and pointers

learn_go_with_tests/pointers_errors/wallet.go

@@ -1,12 +1,46 @@
 package pointers_errors
 
+import "fmt"
+
+type Stringer interface {
+	String() string
+}
+
+// creating a new type from an existing on
+// The idea being it could be more descriptive
+// but also you can declare methods on them
+// which can be used to add domain specific functionality
+type Bitcoin int
+
+// this let's us use fmt strings with %s on the Bitcoin type
+func (b Bitcoin) String() string {
+	return fmt.Sprintf("%d BTC", b)
+}
+
 type Wallet struct {
-	balance int
+	balance Bitcoin
 }
 
-func (w Wallet) Deposit(amount int) {
+func (w *Wallet) Deposit(amount Bitcoin) {
+	// Need a pointer because the func args are copied
+	// so w Wallet is a copy of wallet. Not the actually refernce to the
+	// wallet that Deposit was called on.
+
+	// Notice we don't have to derefence the pointer like in Balance because
+	// in go https://go.dev/ref/spec#Method_values struct pointers are automatically
+	// derefenced
+	w.balance += amount
+}
+
+// Technically you do not need to change Balance to use a pointer receiver as taking a copy of the balance is fine. However, by convention you should keep your method receiver types the same for consistency.
+
+func (w *Wallet) Balance() Bitcoin {
+	// dereferences the pointer
+	// which as stated above didn't need to happen because go automatically
+	// dereferences struct pointers.
+	return (*w).balance
 }
 
-func (w Wallet) Balance() int {
+func (w *Wallet) Withdraw(amount Bitcoin) {
-	return 0
+	w.balance -= amount
 }

learn_go_with_tests/pointers_errors/wallet_test.go

@@ -1,17 +1,40 @@
 package pointers_errors
 
-import "testing"
+import (
+	"fmt"
+	"testing"
+)
 
 func TestWallet(t *testing.T) {
 
+	t.Run("deposit", func(t *testing.T) {
 		wallet := Wallet{}
 
 		wallet.Deposit(10)
 
 		got := wallet.Balance()
-	want := 10
+
+		// %p is placeholder for address in memory
+		fmt.Printf("address of balance in test is %p \n", &wallet.balance)
+		want := Bitcoin(10)
 
 		if got != want {
-		t.Errorf("got %d want %d", got, want)
+			// t.Errorf("got %d want %d", got, want)
+			t.Errorf("got %s want %s", got, want)
 		}
+	})
+
+	t.Run("withdraw", func(t *testing.T) {
+		wallet := Wallet{balance: Bitcoin(20)}
+
+		wallet.Withdraw(Bitcoin(10))
+
+		got := wallet.Balance()
+		want := Bitcoin(10)
+
+		if got != want {
+			t.Errorf("got %s want %s", got, want)
+		}
+
+	})
 }