Flatten a list of lists and integers

flatten.go

// Given a list that contains integers and lists of integers,
// build a list containing all the integers.
//
// Example:
//      given  L = [1 2 [7 [8 9]] 3 [4 5] 6]
//      obtain M = [1 2 7 8 9 3 4 5 6]

package main

import (
	"fmt"
	"reflect"
)

func flatten(list interface{}) []int64 {
	if reflect.TypeOf(list).Kind() != reflect.Slice {
		fmt.Println("err: expected slice")
		return nil
	}

	var m []int64

	l := reflect.ValueOf(list)
	for i := 0; i < l.Len(); i++ {
		val := l.Index(i).Interface()

		t := reflect.TypeOf(val).Kind()
		switch t {
		case reflect.Slice:
			temp := flatten(val)
			m = append(m, temp...)
		case reflect.Int:
			fallthrough
		case reflect.Int64:
			m = append(m, reflect.ValueOf(val).Int())
		default:
			fmt.Println("unknown:", t)
		}
	}

	return m
}

func main() {
	list := []interface{}{1, 2, []interface{}{7, []int64{8, 9}}, 3, []int64{4, 5}, 6}
	fmt.Println(list)
	fmt.Println(flatten(list))
}

https://play.golang.com/p/NJDleDwg82