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POHow to implement a reader monad to access a database
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copied!<p>I try to implement a reader monad for the first time.</p> <p>I want to use monadic style to query the database.</p> <p>use case 1 : a user has a one-to-one relation with a colleague. Pseudo code is <code>getUserById(getUserById(id).getColleague())</code></p> <p>use case 2 : retrieve a list of users by id. Pseudo code is <code>List(getUserById(id1), getUserById(id2))</code></p> <p>It seems that this is good use cases for monads. My goal is to see if I can take advantage of monads to improve my code</p> <p>PS : Please provide at least one answer without scalaz.</p> <p>Here is the code :</p> <pre><code>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] } } </code></pre> <p>Is it the good way ? I have some doubt, cf my comment <code>Strange design</code></p> <p>How could I improve my code ?</p>

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