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copied!<p>I have tried to rework your proposal making the collection the Input for the reader, reworking the naming a bit as I went.</p> <pre><code>package monad package object reader{ type UserId = Int } package reader { case class User(id: UserId, name: String, colleagueId: UserId) import com.mongodb.casbah.Imports._ import com.mongodb.casbah trait Reader[I, A] { self => val run = apply _ def apply(id:I):A def map[B](f: A => B): Reader[I, B] = new Reader[I, B] { def apply(id: I) = f(self.run(id)) } def flatMap[B](f: A => Reader[I, B]): Reader[I, B] = new Reader[I, B] { def apply(id: I) = f(self(id)).run(id) } } object Reader { def unit[A](a: => A) = apply { id: UserId => a } def apply[I, A](f: I => A) = new Reader[I, A] { def apply(i: I) = f(i) } } object Users { def asUser(o: DBObject): User = User(o.as[Double]("id").toInt, o.as[String]("name"), o.as[Double]("colleague").toInt) def colleague(u: User): Reader[MongoCollection, User] = Reader{ coll => asUser(coll.findOne(MongoDBObject("id" -> u.colleagueId)).get) } def getUserById(id:UserId): Reader[MongoCollection, User] = Reader { coll => asUser(coll.findOne(MongoDBObject("id" -> id)).get) } } object Client extends App { import Users._ def coll: casbah.MongoCollection = MongoClient()("test")("user") // I can do println(getUserById(1)(coll)) // Same with for comprehension val userReader = for (user <- getUserById(1)) yield user println(userReader(coll)) //Combination to explore one-to-one relation val user = getUserById(1)(coll) val otherUser = getUserById(user.colleagueId)(coll) // Same with flatMap println(getUserById(1).flatMap(colleague)(coll)) // Same with for-comprehension but doesn't work val coworkerReader = for {user <- getUserById(1) coworker <- colleague(user)} yield coworker println(coworkerReader(coll)) } } </code></pre> <p>Using this approach I think the code is easier to test as you can pass around the dependency(the MongoCollection) while manipulating only values and functions in your signatures. Read more at <a href="http://blog.originate.com/blog/2013/10/21/reader-monad-for-dependency-injection/" rel="nofollow">http://blog.originate.com/blog/2013/10/21/reader-monad-for-dependency-injection/</a> (I am not the author but it is a clear explanation)</p>

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