Golang slice flatten

main.go

package main

import (
	"errors"
	"fmt"
)

func Flatten(arr interface{}) ([]int, error) {
	return doFlatten([]int{}, arr)
}

func doFlatten(acc []int, arr interface{}) ([]int, error) {
	var err error

	switch v := arr.(type) {
	case []int:
		acc = append(acc, v...)
	case int:
		acc = append(acc, v)
	case []interface{}:
		for i := range v {
			acc, err = doFlatten(acc, v[i])
			if err != nil {
				return nil, errors.New("not int or []int given")
			}
		}
	default:
		return nil, errors.New("not int given")
	}

	return acc, nil
}

func main() {
	res, err := Flatten([]interface{}{[]interface{}{1, 2, []int{3}}, 4})

	fmt.Println(res, err)
}

So Go 1.18 already have generics, what you gonna do now?