update book code

eBook/exercises/chapter_11/simple_interface.go

@@ -0,0 +1,34 @@
+// simple_interface.go
+package main
+
+import (
+	"fmt"
+)
+
+type Simpler interface {
+	Get() int
+	Put(int)
+}
+
+type Simple struct {
+	i int
+}
+
+func (p *Simple) Get() int {
+	return p.i
+}
+
+func (p *Simple) Put(u int) {
+	p.i = u
+}
+
+func fI(it Simpler) int {
+	it.Put(5)
+	return it.Get()
+}
+
+func main() {
+	var s Simple
+	fmt.Println(fI(&s))  // &s is required because Get() is defined with a receiver type pointer
+}
+// Output: 5