Sink.scala

package io.asuna.cantor.model

import cats._, implicits._
import monix.eval.Task
import monix.cats._

/**
  * A model that supports writes.
  */
trait Sink[K, V] {
  /**
    * Writes the given map of keys to values.
    */
  def writeMany(els: Map[K, V]): Task[Unit]

  /**
    * Clears the keys in the sink. Usually deletion.
    */
  def clear(keys: List[K]): Task[Unit]
}

object Sink extends SinkInstances

private[model] sealed trait SinkInstances {

  /**
    * `Apply` instance for `Source`. Allows us to take products of sources.
    */
  implicit def cvcartesianInstance[K, V]: ContravariantCartesian[Sink[K, ?]] = new ContravariantCartesian[Sink[K, ?]] {

    def contramap[A, B](fa: Sink[K, A])(fn: B => A): Sink[K, B] = new Sink[K, B] {
      override def writeMany(els: Map[K, B]): Task[Unit] = fa.writeMany(els.mapValues(fn))
      override def clear(keys: List[K]): Task[Unit] = fa.clear(keys)
    }

    def product[A, B](fa: Sink[K, A], fb: Sink[K, B]): Sink[K, (A, B)] = new Sink[K, (A, B)] {
      override def writeMany(els: Map[K, (A, B)]): Task[Unit] =
        fa.writeMany(els.mapValues(_._1))
          .product(fb.writeMany(els.mapValues(_._2))).map(_ => ())

      override def clear(keys: List[K]): Task[Unit] =
        fa.clear(keys).product(fa.clear(keys)).map(_ => ())
    }

  }

}