update book code

eBook/exercises/chapter_6/fibonacci_closure.go

@@ -0,0 +1,20 @@
+package main
+
+// fib returns a function that returns
+// successive Fibonacci numbers.
+func fib() func() int {
+	a, b := 1, 1
+	return func() int {
+		a, b = b, a+b
+		return b
+	}
+}
+
+func main() {
+	f := fib()
+	// Function calls are evaluated left-to-right.
+	// println(f(), f(), f(), f(), f())
+	for i:=0; i<=9; i++ {
+		println(i+2, f() )
+	}
+}