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

GoLang Tour - Fibonacci Closure v2

package main

import "fmt"

// fibonacci is a function that returns
// a function that returns an int.
//
// (aru): I don't believe this is a true closure either?
// or if it is...that it is more complex than it needs to be.
func fibonacci_naive() func(int) int {
  // Seed values for current and previous fibonacci numbers.
  curr, prev := 0, 1
  return func(x int) int {
     // O is always the starting number, so if x == 0 just return 0
     if x == 0 {
         return x
     // When x isn't 0 let's perform the Fibonacci sequence magic
     } else {
         // New value of x becomes sum of curr + prev values
         x = curr + prev
         // Set prev to current curr for next call of function
         prev = curr
         // Set curr to x(sum of curr + prev) so we can return it
         curr = x
         return curr
     }
  }
}

// This is closing over the curr and next values (keeping state across function calls)
func fibonacci_proper_closure() func() int {
  // Seed values
  curr, next := 0, 1
  return func() int {
    // Set fibsum to curr to set return value
    fibsum := curr
    // Set curr and next values to appropriate/correct fib seq values.
    curr, next = next, next + curr
    return fibsum
  }
}

func main() {
        // Call naive closure.
	fn := fibonacci_naive()
	for i := 0; i < 20; i++ {
		fmt.Printf("%d, ", fn(i))
	}
	
	// Make the output pretty.
	fmt.Println()
	
	// Call decent(proper) closure.
        fpc := fibonacci_proper_closure()
	for i := 0; i < 20; i++ {
		fmt.Printf("%d, ", fpc())
	}
}