EitherActivity.kt

class EitherActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_input)

        val viewModel = ViewModelProviders.of(this)[EitherViewModel::class.java]

        signup_button.setOnClickListener {
            viewModel.validate(email.text.toString(), number.text.toString())
        }

        viewModel.validation.observe(this, Observer {
            it?.fold(this::handleLeft, this::handleRight)
        })
    }
}