Simple freestyle free program example with single instruction

SimpleFreestyleFree.scala

import cats.data.State
import freestyle.free._

/* domain objects */
case class Id(id: String)
case class AcmeItem(id: Id)

/* state */
trait MapState[K, V] {
  type Type     = Map[K, V]
  type TypeA[A] = State[Type, A]

  def apply(state: Map[K, V]): Type = state
  def apply(state: (K, V)*): Type   = state.toMap
  def empty: Type                   = Map.empty[K, V]
}

object AcmeStorageState extends MapState[Id, AcmeItem]

/* freestyle free */
@free trait AcmeStorage {
  def put(item: AcmeItem): FS[Unit]
  def get(id: Id): FS[Option[AcmeItem]]
}

object FreeAcme extends App {
  implicit val acmeStorageHandler = new AcmeStorage.Handler[AcmeStorageState.TypeA] {
    def put(item: AcmeItem): AcmeStorageState.TypeA[Unit]     = State.modify(_.updated(item.id, item))
    def get(id: Id): AcmeStorageState.TypeA[Option[AcmeItem]] = State.inspect(_.get(id))
  }

  val id1   = Id("foo")
  val item1 = AcmeItem(id1)

  val ops         = AcmeStorage.to[AcmeStorage.Op]
  val inputState  = AcmeStorageState.empty
  val program     = ops.put(item1)
  val outputState = program.interpret[AcmeStorageState.TypeA].runS(inputState).value

  val assertion1 = outputState == AcmeStorageState(id1 -> item1)
  println("%s << outputState == AcmeStorageState(id1 -> item1)".format(assertion1))
  // true << outputState == AcmeStorageState(id1 -> item1)
}