Functional Kotlin

Synchronous implementation

Synchronous code is easy to write, predictable, and easy to test, but in some cases, it doesn't use system resources in an optimal manner: 

class SynchronousUserService(private val userClient: UserClient,                      private val factClient: FactClient,                      private val userRepository: UserRepository,                      private val factRepository: FactRepository) : UserService {   override fun getFact(id: UserId): Fact {      val user = userRepository.getUserById(id)      return if (user == null) {         val userFromService = userClient.getUser(id)         userRepository.insertUser(userFromService)         getFact(userFromService)      } else {         factRepository.getFactByUserId(id) ?: getFact(user)      }   }   private fun getFact(user: User): Fact {      val fact = factClient.getFact(user)      factRepository ...