scala – 如何实现reader monad访问数据库

我想使用monadic样式来查询数据库.

用例1：用户与同事有一对一的关系.伪代码是getUserById(getUserById(id).getColleague())

用例2：按ID检索用户列表.伪代码是List(getUserById(id1),getUserById(id2))

这似乎是monad的好用例.我的目标是看看我是否可以利用monads来改进我的代码

PS：请提供至少一个没有scalaz的答案.

这是代码：

package monad
import com.mongodb.casbah.Imports._

object Monad {
  type UserId = Int
  case class User(id: UserId,name: String,colleagueId: UserId)

  trait Reader[I,A] { self =>
    def run(id: I) : A
    def map[B](f: A => B) : Reader[I,B] = 
      new Reader[I,B] { def run(id: I) = f(self.run(id)) }
    def flatMap[B](f: A => Reader[I,B]) : Reader[I,B] =
      new Reader[I,B] { def run(id: I) = f(self.run(id)).run(id) }
  }

  def coll = MongoClient()("test")("user")

  def DBObject2User(o: DBObject) : User = User(o.as[Double]("id").toInt,o.as[String]("name"),o.as[Double]("colleague").toInt)

  // Strange design,id is not used…
  def User2Colleague(u: User) : Reader[UserId,DBObject] = 
    unit(coll.findOne(MongoDBObject("id" -> u.colleagueId)).get)

  def GetUserById : Reader[UserId,DBObject] = 
    Reader { id: UserId => coll.findOne(MongoDBObject("id" -> id)).get }

  def GetUserById2 : Reader[UserId,User] = GetUserById.map(DBObject2User)

  def unit[A](a: => A) = Reader { id: UserId => a }

  object Reader {
    def apply[I,A](f: I => A) = new Reader[I,A] { def run(i: I) = f(i) }
  }

  def main(args: Array[String]) {
    // I can do
    println(GetUserById2.run(1))

    // Same with for comprehension
    val userReader = for (io <- GetUserById2) yield io
    println(userReader.run(1))

    //Combination to explore one-to-one relation
    val user = GetUserById2.run(1)
    val colleague = GetUserById2.run(user.colleagueId)

    // Same with flatMap
    println(GetUserById2.flatMap(User2Colleague).run(1))

    // Same with for-comprehension but doesn't work
    val io = for {io  <- GetUserById2
                  io2 <- User2Colleague(io).map(DBObject2User)} yield io2
    println(io.run(1))

    //TODO: List[Reader] to Reader[List]
  }
}

这是好方法吗？我有一些疑问,参见我的评论奇怪的设计

我怎么能改进我的代码？