a.go

package main

import "fmt"

func cached(
	fn func(int) int,
) (
	decorated func(int) int,
) {
	cache := make(map[int]int)
	return func(n int) int {
		if res, ok := cache[n]; ok {
			return res
		}
		res := fn(n)
		cache[n] = res
		return res
	}
}

func main() {
	var fib func(int)int
	fib = cached(func(n int) int {
		if n <= 1 {
			return n
		}
		return fib(n-1) + fib(n-2)
	})
	fmt.Printf("%d\n", fib(10))
}