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Last active September 13, 2021 22:48
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Kotlin DI with receivers & interface delegation
import arrow.core.*
import memeid.UUID
data class User(val email: String, val name: String) {
companion object
data class ProcessedUser(val id: UUID, val email: String, val name: String) {
companion object
object ProcessingError
interface Repo {
suspend fun fetchUsers(): List<User>
fun MockRepo(): Repo = object : Repo {
override suspend fun fetchUsers(): List<User> =
User("", "Simon"),
User("", "Raul"),
User("", "Jorge")
interface Persistence {
suspend fun User.process(): Either<ProcessingError, ProcessedUser>
suspend fun List<User>.process(): Either<ProcessingError, List<ProcessedUser>> =
traverseEither {
fun MockPersistence(): Persistence = object : Persistence {
override suspend fun User.process(): Either<ProcessingError, ProcessedUser> =
if (email.contains(Regex("^(.+)@(.+)$"))) Either.Right(ProcessedUser(UUID.V4.squuid(), email, name))
else Either.Left(ProcessingError)
* When using the generic method as shown below you define top-level functions
* which could be considered a "Service" which is typically composed by repos
* from the network or persistence layer.
* These functions are your business logic, which are in turn typically called by
* your routes or controllers.
* The biggest advantage of this approach is that this function explicitly defines
* everything that it uses in its signature.
* Because of that you can call if from anywhere that satisfies those constraints.
* The generic top-level function
* Enables the [getProcessUsers] function (or syntax) when the [Persistence] & [Repo] constraints are met.
suspend fun <R> R.getProcessUsers(/* add any arguments as needed */): Either<ProcessingError, List<ProcessedUser>>
where R : Repo,
R : Persistence =
* We define a class that satisfies both [Persistence] & [Repo] such that the
* [getProcessUsers] function automatically gets added through the top-level definition.
* We use Kotlin's interface delegation system to automatically implement the interfaces
* by passing runtime representations of them.
class DataModule(
persistence: Persistence,
repo: Repo
) : Persistence by persistence, Repo by repo
suspend fun main(): Unit {
// This is your router { get { } } router definition or
// your Android launch { } or compose function.
// Generic top-level function automatically got enabled
val processedUsers = DataModule(MockPersistence(), MockRepo()).getProcessUsers()
// Call the alternative approach
val processedUsers2 = DataModule2(MockPersistence(), MockRepo()).getProcessUsers2()
* Another approach to defining [DataModule] could be using an interface and a function.
interface DataModule2 : Persistence, Repo
fun DataModule2(persistence: Persistence, repo: Repo): DataModule2 =
object : DataModule2, Repo by repo, Persistence by persistence {}
* When using this style you could also consider an alternative way of defining [getProcessUsers]
suspend fun DataModule2.getProcessUsers2(/* add any arguments as needed */): Either<ProcessingError, List<ProcessedUser>> =
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nomisRev commented Jul 6, 2021

Discussion on Kotlin Slack concerning some of this:

suspend fun <R> R.processUser(userId: Long): Unit
   where R : Persistence<User>,
         R : Repo<User>,
         R : Logger,
         R : Config,
         R : Processor<User> /*, R : ... */ {
  val user = userForId(userId)
  log("user for id: $user")
  val info = fetchInfoForUser(user)
  process(config, user, info)

So to satisfies this you need a larger composition of interfaces, which is often referred to as "god-class" or "god-dependency".
This is not actually an issue for this style of programming because the function and the "god-class" are completely decoupled and completely unaware of each other. Meaning that it automatically is also 100% testable with whatever techniques you prefer.

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