1-Old.kt

fun main() {
    userRepository.findById(1, object: UserCallback {
        override fun onUserFound(user: User) {
            // Do something with `user`.
        }
    })
}

interface UserRepository {
    fun findById(id: Int, callback: UserCallback)
}

2-New.kt

fun main() = runBlocking {
    val user = userRepository.findById(1) // Blocking call
    // Do something with `user`.
}

interface UserRepository {
    suspend fun findById(id: Int): User
}