https://tour.golang.org/moretypes/26

exercise-fibonacci-closure.go

package main

import "fmt"

// fibonacci is a function that returns
// a function that returns an int.
func fibonacci() func() int {
	a := 0
	b := 1
	c := 0
	return func() int {
		c += a
		a = b
		b = c
		return c
	}
}

func main() {
	f := fibonacci()
	//for i := 0; i < 10; i++ {
	for i := 0; i < 20; i++ {
		fmt.Println(f())
	}
}