flatten.go

package main

import (
	"fmt"
)

func fList(v ...interface{}) []interface{} {
	return v
}

func main() {
	a := fList(1, fList(2, 3, 4), 5, 6, 8, fList(7, 8, 9))
	fmt.Println(a)
	fmt.Println(flatten(a))
}

func flatten(s []interface{}) []int {
	result := []int{}

	for _, v := range s {
		switch i := v.(type) {
		case int:
			result = append(result, i)
		case []interface{}:
			result = append(result, flatten(i)...)
		}
	}

	return result
}