Showoff code that I probably wouldn't use on production, but looks smarter than whatever stuff is actually required in our everyday coding :P

circe-inmemory-ttfi-di.sc

//
// @author: Mateusz Kubuszok
//
// requirements: ammonite 1.1.0
// usage:        run `amm` and copy paste into REPL

import $ivy.`org.typelevel::cats-core:1.3.1`, cats._, cats.syntax.all._
import $ivy.`org.typelevel::cats-effect:1.0.0`, cats.effect._, cats.effect.syntax._
import $ivy.`io.circe::circe-core:0.9.3`, io.circe._, io.circe.syntax._
import $ivy.`io.circe::circe-generic:0.9.3`, io.circe.generic.auto._
import $ivy.`io.circe::circe-parser:0.9.3`, io.circe.parser._
import $ivy.`io.scalaland::pulp:0.0.5`, io.scalaland.pulp._
import $ivy.`io.monix::monix:3.0.0-M3`, monix.eval._, monix.execution.Scheduler.Implicits.global
interp.load.plugin.ivy("org.scalamacros" % "paradise_2.12.4" % "2.1.1")

final case class User(name: String, surname: String, email: String)

{
@ImplementedAs[UserRepoInMemory]
trait UserRepo {
  def fetchByEmail(email: String): Option[User]
  def save(user: User): Unit
}

@Singleton
class UserRepoInMemory extends UserRepo {
  private val users = scala.collection.mutable.Map.empty[String, User]
  def fetchByEmail(email: String): Option[User] = users.get(email)
  def save(user: User): Unit = users += (user.email -> user)
}
}

@Wired
class UserServices[F[_]: Sync](repo: UserRepo) {
  def parseJson(user: String): F[Option[User]] = Sync[F].delay(decode[User](user).toOption)
  def asJson(user: User): F[String] = Sync[F].delay(user.asJson.noSpaces)
  def fetchByEmail(email: String): F[Option[User]] = Sync[F].delay(repo.fetchByEmail(email))
  def save(user: User): F[Unit] = Sync[F].delay(repo.save(user))
}

@Wired
class Program[F[_]: Monad](userServices: UserServices[F]) {

  def store(json: String): F[Unit] = for {
    parsed <- userServices.parseJson(json)
    _ <- parsed.map(userServices.save).getOrElse(().pure[F])
  } yield ()

  def retrieve(email: String): F[String] = for {
    userOpt <- userServices.fetchByEmail(email)
    json <- userOpt.map(userServices.asJson _).getOrElse(Json.obj().noSpaces.pure[F])
  } yield json
}

// synchronously parse json, and store User to in-memory repo
Provider.get[Program[Coeval]].store("""{"name":"John","surname":"Smith","email":"john.smith@mail.com"}""").value

// asynchronously retrieve user from in-memory repo and serialize it
Provider.get[Program[Task]].retrieve("john.smith@mail.com").runAsync.onComplete(println)