union type for exception

main.cs

// File API
get_file(string filename) -> (File | NotFoundException) { ... }
close_file(File file) -> void  { ... }
read_file(File file) -> (string | IOFoundException)  { ... }

// File helper
<T>with_file(filename, Func<file, T> func) -> (T | NotFoundException) {
    var file = get_file(filename);
    if (file is NotFoundException) { return result; }

    var result = func(file);
    close_file(file);
    return result;
}

// usage
get_content_from_file(filename) -> (string | NotFoundException | IOFoundException )  {
    return with_file(filename, (file)-> {
        return read_file(file);
    });
}

skip_check_exception(filename) -> (void | Exception) {
    var result = get_content_from_file(filename);
    if (result is Exception) { return (Exception)result; }
    // keep process result
    return;
}