flatten arguments list, where arguments might be s

If I have a func like this:

func AcceptsAnything(v ...interface{}){
  args =: FlattenDeep(v);  // flatten any arrays or slices
}

I am trying to implement FlattenDeep:

func getKind(v interface{}) string {

    rt := reflect.TypeOf(v)
    switch rt.Kind() {
    case reflect.Slice:
        return "slice"
    case reflect.Array:
        return "array"
    default:
        return "unknown"
    }

}

func FlattenDeep(args ...interface{}) []interface{} {
    list := []interface{}{}

   for _, v := range args {

     kind := getKind(v);

     if kind != "unknown" {
        list = append(list, FlattenDeep(v)...)  // does not compile
     } else{
        list = append(list, v);
       }
    }
   return list;
}

but I don't know how to append multiple items to the list at once. Should I just loop over the results of FlattenDeep or is there a way to spread the results and append them to the list?

This might work:

func FlattenDeep(args ...interface{}) []interface{} {
    list := []interface{}{}

    for _, v := range args {

        kind := getKind(v);
        if kind != "unknown" {
            for _, z := range FlattenDeep((v.([]interface{})...) {
                list = append(list, z)
            }

        } else {
            list = append(list, v);
        }
    }
    return list;
}

but I am looking for something a little less verbose if possible

Here's how to flatten arbitrary slices and arrays to a []interface{}:

func flattenDeep(args []interface{}, v reflect.Value) []interface{} {

    if v.Kind() == reflect.Interface {
        v = v.Elem()
    }

    if v.Kind() == reflect.Array || v.Kind() == reflect.Slice {
        for i := 0; i < v.Len(); i++ {
            args = flattenDeep(args, v.Index(i))
        }
    } else {
        args = append(args, v.Interface())
    }

    return args
}

func AcceptsAnything(v ...interface{}) {
    args := flattenDeep(nil, reflect.ValueOf(v))
    fmt.Println(args)
}

Run it on the Playground

If the function must handle slice and array types with an arbitrary element type, then the application must iterate through the slice or array using the reflect API to get the values into an []interface{}.

If you only need to flatten []interface{}, then the reflect API is not needed:

func flattenDeep(args []interface{}, v interface{}) []interface{} {
    if s, ok := v.([]interface{}); ok {
        for _, v := range s {
            args = flattenDeep(args, v)
        }
    } else {
        args = append(args, v)
    }
    return args
}

func AcceptsAnything(v ...interface{}) {
    args := flattenDeep(nil, v)
    fmt.Println(args)
}

Run it on the Playground.