update book code

eBook/exercises/chapter_11/interface_nil.go

@@ -0,0 +1,46 @@
+// interface_nil.go
+package main
+
+import "fmt"
+
+type Any interface {}
+type Anything struct {}
+
+func main() {
+	any := getAny()
+	if any == nil {
+		fmt.Println("any is nil")
+	} else {
+		fmt.Println("any is not nil")
+	}
+/*
+	// to get the inner value:
+	anything := any.(*Anything) 
+	if anything == nil {
+		fmt.Println("anything is nil")
+	} else {
+		fmt.Println("anything is not nil")
+	}
+*/
+}
+
+func getAny() Any {
+	return getAnything()
+}
+
+func getAnything() *Anything {
+	return nil
+}
+
+/* Output:
+any is not nil
+WHY?
+you would perhaps expect: any is nil,because getAnything() returns that
+BUT:
+the interface value any is storing a value, so it is not nil. 
+It just so happens that the particular value it is storing is a nil pointer.
+The any variable has a type, so it's not a nil interface, 
+rather an interface variable with type Any and concrete value (*Anything)(nil).
+To get the inner value of any, use: anything := any.(*Anything)
+now anything contains nil !
+*/