Kotlin DI with receivers & interface delegation

Example.kt

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> =
    listOf(
      User("[email protected]", "Simon"),
      User("[email protected]", "Raul"),
      User("[email protected]", "Jorge")
    )
}

interface Persistence {
  suspend fun User.process(): Either<ProcessingError, ProcessedUser>

  suspend fun List<User>.process(): Either<ProcessingError, List<ProcessedUser>> =
    traverseEither {
      it.process()
    }
}

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 =
  fetchUsers().process()

/**
 * 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()
  println(processedUsers)

  // Call the alternative approach
  val processedUsers2 = DataModule2(MockPersistence(), MockRepo()).getProcessUsers2()
  println(processedUsers2)
}

/**
 * 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>> =
  fetchUsers().process()

Discussion on Kotlin Slack concerning some of this:

https://kotlinlang.slack.com/archives/C5UPMM0A0/p1624026498160200

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.