abstracting-over-constraints.scala

trait Cache[R[_], W[_], Eff[_]] {
  def save[V](key: String, value: V)(implicit writes: W[V]): Eff[Unit]
  def retrieve[V](key: String)(implicit reads: R[V]): Eff[Option[V]]
}

// In-memory cache. Potentially backed by a Map[String, Any].
// Needs no reads or writes.

class InMemoryCache extends Cache[Tautology1, Tautology1, Id] {
  def save[V](key: String, value: V)(implicit writes: Tautology1[V]): Unit = ???
  def retrieve[V](key: String)(implicit reads: Tautology1[V]): Option[V] = ???
}

class Tautology1[+A] private()
object Tautology {
  implicit val tautology1 = new Tautology[Nothing]
}

// Zookeeper-based cache

class ZkCache extends Cache[ZkReads, ZkWrites, Future] {
  def save[V](key: String, value: V)(implicit writes: ZkWrites[V]): Future[Unit] = ???
  def retrieve[V](key: String)(implicit reads: ZkReads[V]): Future[Option[V]] = ???
}