update book code

eBook/exercises/chapter_13/panic_defer.go

@@ -0,0 +1,44 @@
+// panic_defer.go
+package main
+
+import "fmt"
+
+func main() {
+	f()
+	fmt.Println("Returned normally from f.")
+}
+
+func f() {
+	defer func() {
+		if r := recover(); r != nil {
+			fmt.Println("Recovered in f", r)
+		}
+	}() 
+	fmt.Println("Calling g.")
+	g(0)
+	fmt.Println("Returned normally from g.")
+}
+
+func g(i int) {
+	if i > 3 {
+		fmt.Println("Panicking!")
+		panic(fmt.Sprintf("%v", i))
+	}
+	defer fmt.Println("Defer in g", i)
+	fmt.Println("Printing in g", i)
+	g(i + 1)
+}
+/* Output:
+Calling g.
+Printing in g 0
+Printing in g 1
+Printing in g 2
+Printing in g 3
+Panicking!
+Defer in g 3
+Defer in g 2
+Defer in g 1
+Defer in g 0
+Recovered in f 4
+Returned normally from f.
+*/