Error management using the Either monad

login.kt

// This is how to manage the errors returned by a use case using the Either monad

fun doLogin() {
  val params = UseCaseCommonMainLogin.AuthenticationLoginParams(
    username = localState.data.username,
    password = localState.data.password,
  )
  useCaseCommonMainLogin.invoke(params, viewModelScope) { result ->
    result.fold(::displayError, ::navigateToHome)  // 'result' is of type Either<CommonMainFailure, Unit>
}

fun displayError(failure: CommonMainFailure) {
  val errorMessage = when (failure) {
    CommonMainFailure.ConnectionError -> "There are connection problem: please verify that the Internet connection is working"
    CommonMainFailure.BackendError -> "The backend seems to experience some problems. Please retry in few minutes"
    CommonMainFailure.LoginError -> "Login error. Please verify the username and the password"
  }
}

sealed interface CommonMainFailure {
  object ConnectionError : CommonMainFailure
  object BackendError : CommonMainFailure
  object LoginError : CommonMainFailure
}

class UseCaseCommonMainLogin {
  ...
  suspend fun invoke(
    params: AuthenticationLoginParams, 
    coroutineScope: CoroutineScope, 
    onResult: (result: Either<CommonMainFailure, Unit>) -> Unit
  ) {
    ...
  }
  ...
}