Onion layers

provider.kt

interface Provider<T, U> {
  fun provide(params: U?): T
}

class DatabaseProvider: Provider<User, string> {
  override fun provide(id: string?): User {
    // Query DB here
  }
}

class CachingProvider<T, U>: Provider<T, U>(
  private val source: Provider<T, U>
) {
  
  private cache: Map<U, T>

  override fun provide(params: U?): T {
    // Check and update cache
  }
}

class FetchUser(private val provider: Provider<User, string>) {
}

val provider = CachingProvider(DatabaseProvider())
val fetchUser = FetchUser(provider)