PlayingWithFreestyle.scala

import cats.Id
import cats.free.Free
import freestyle._
import freestyle.implicits._
import cats.implicits._

object Main extends App {

  case class Foo(id: Int, content: String)

  @free
  trait CrudActions[F[_]] {
    def create(a: Foo): FreeS[F, Int]

    def read(id: Int): FreeS[F, Foo]

    def update(a: Foo): FreeS[F, Unit]

    def delete(id: Int): FreeS[F, Unit]
  }

  @free
  trait SumActions[F[_]] {
    def concat(a: Foo, b: Foo): FreeS[F, Foo]

    def sum(a: Foo, b: Foo): FreeS[F, Foo]
  }

  @free
  trait FooReader[F[_]] {
    def getFoo: FreeS[F, Foo]
  }

  @module
  trait Application[F[_]] {
    val crud: CrudActions[F]
    val sum: SumActions[F]
    val reader: FooReader[F]
  }

  def program[F[_]](implicit A: Application[F]): FreeS[F, List[Foo]] = {
    import A._

    for {
      f <- reader.getFoo
      id <- crud.create(f)
      created <- crud.read(id)
      _ <- crud.update(created.copy(content = "bar"))
      updated <- crud.read(id)
      concated <- sum.concat(created, updated)
      summed <- sum.sum(created, updated)
    } yield List(concated, summed)
  }

  implicit val idCrud = new CrudActions.Interpreter[Id] {
    var store = Map.empty[Int, Foo]
    var ai = 1

    def createImpl(a: Foo): Id[Int] = {
      val aId = a.copy(id = ai)
      store = store + (ai -> aId)
      ai = ai + 1
      aId.id
    }

    def readImpl(id: Int): Id[Foo] = store(id)

    def updateImpl(a: Foo): Id[Unit] = {
      store = store.updated(a.id, a)
      ()
    }

    def deleteImpl(id: Int): Id[Unit] = {
      store = store - id
      ()
    }
  }

  implicit val idSum = new SumActions.Interpreter[Id] {
    def concatImpl(a: Foo, b: Foo): Id[Foo] = Foo(a.id, a.content + b.content)

    def sumImpl(a: Foo, b: Foo): Id[Foo] = Foo(a.id + b.id, a.content)
  }

  implicit val idReader = new FooReader.Interpreter[Id] {
    def getFooImpl = Foo(-100, "content")
  }

  val freeProgResult = program[Application.T].exec[Id]

  assert {
    freeProgResult == List(Foo(1, "contentbar"), Foo(2, "content"))
  }

}