Isolate domain models from transaction management

nyankoro.scala

// dummy session classes for sample codes
final class AnormSession {
  def anormMethod = {
    println("anorm")
  }

  def start = {}
  def commit = {}
}

final class SlickSession {
  def slickMethod = {
    println("slick")
  }

  def start = {}
  def commit = {}
}

// domain layer
trait Entity
trait Repo

case class IoContext[T](session: T)

trait IoContextSupport[T] {
  implicit def contextTo(context: IoContext[_]): T = {
    context.session.asInstanceOf[T] // actually don't want to do this... this causes runtime casting error
  }
}

case class User(id: Int, name: String) extends Entity

trait UserRepo extends Repo {
  def find(id: Int)(implicit ctx: IoContext[_]): User
  def store(name: String)(implicit ctx: IoContext[_]): User
}

// infra layer for persistence
class SlickUserRepo extends UserRepo with IoContextSupport[SlickSession] {
  def find(id: Int)(implicit ctx: IoContext[_]): User = {
    ctx.slickMethod
    User(2, "slick")
  }

  def store(name: String)(implicit ctx: IoContext[_]): User = {
    ctx.slickMethod
    User(2, name)
  }
}

class AnormUserRepo extends UserRepo with IoContextSupport[AnormSession]{
  def find(id: Int)(implicit ctx: IoContext[_]): User = {
    ctx.anormMethod
    User(2, "anorm")
  }

  def store(name: String)(implicit ctx: IoContext[_]): User = {
    ctx.anormMethod
    User(2, name)
  }
}

// app layer
object App {
  def doSomethingWithSlick() = {
    val session = new SlickSession
    implicit val ctx = IoContext(session)

    session.start

    val repo = new SlickUserRepo

    repo.store("slick")
    repo.find(2)

    session.commit
  }

  def doSomethingWithAnorm() = {
    val session = new AnormSession
    implicit val ctx = IoContext(session)

    session.start

    val repo = new AnormUserRepo

    repo.store("anorm")
    repo.find(2)

    session.commit
  }

  def doSomethingButError() = {
    case class InvalidSessionType()
    val session = new InvalidSessionType
    implicit val ctx = IoContext(session)

    val repo = new SlickUserRepo

    repo.store("slick")
    repo.find(2)
  }
}

• ランタイムにキャストエラーが発生するのが微妙
• 実質 Repository は IoContextSupport なしには利用できないインターフェースになっているので、Repository と IoContextSupport が分かれている(お作法になってしまっている)のが微妙

App layer で implicit とか駆使すれば cast error は防げるかも