// 1. singleton based
import Mailer;
class RegistrationForm {
	public void register() {
		Mailer.getInstance().sendEmail(...);
		assert(typeof(Mailer.getInstance()) === 'ImapMailer');
	}
}

// 2. service locator based
import Mailer;
class RegistrationForm {
	public void register() {
		// application is global var
		application.get('mailer').sendEmail(...);
		assert(typeof(application.get('mailer')) === 'ImapMailer');
	}
}

// 3. service container + dependency injection based
import Mailer;
class RegistrationForm {
	private Mailer mailer;
	public RegistrationForm(Mailer m) {
		this.mailer = m;
	}
	public void register() {
		this.mailer.sendEmail(...);
		assert(typeof(this.mailer) === 'ImapMailer');
	}
}