If you take the first example program from https://live.gnome.org/Vala/AsyncSamples and replace

    list_dir.begin((obj, res) => {
            list_dir.end(res);
            loop.quit();
        });

with
    list_dir.begin(() => {
            list_dir.end();
            loop.quit();
        });
:


// Example with GIO asynchronous methods:
// Build with: valac --pkg=gio-2.0 example.vala

async void list_dir() {
    var dir = File.new_for_path (Environment.get_home_dir());
    try {
        var e = yield dir.enumerate_children_async(
            FileAttribute.STANDARD_NAME, 0, Priority.DEFAULT, null);
        while (true) {
            var files = yield e.next_files_async(
                 10, Priority.DEFAULT, null);
            if (files == null) {
                break;
            }
            foreach (var info in files) {
                print("%s\n", info.get_name());
            }
        }
    } catch (Error err) {
        warning("Error: %s\n", err.message);
    }
}

void main() {
    var loop = new MainLoop();
    list_dir.begin(() => {
            list_dir.end();
            loop.quit();
        });
    loop.run();
}

The error message you get is "0 missing arguments for `void list_dir ()'" which is not helpful.