flat.go

package main

import (
	"fmt"
	"reflect"
)

type any = interface{}

func main() {
	arr := []any{1, 2, 3, []any{5, 6, 7, []any{9, 0}}}
	res := flat(arr)
	fmt.Printf("flat %v\n", res)
}

func flat(arr []any) []any {
	acc := []any{}
	for _, elem := range arr {
		v := reflect.ValueOf(elem)
		switch v.Kind() {
		case reflect.Slice:
			rec := flat(elem.([]any))
			acc = append(acc, rec...)
		default:
			acc = append(acc, elem)
		}
	}
	return acc
}