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POFuture[Option] in Scala for-comprehensions
 

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    <p>I have two functions which return Futures. I'm trying to feed a modified result from first function into the other using a for-yield comprehension. </p> <p>This approach works:</p> <pre><code> val schoolFuture = for { ud &lt;- userStore.getUserDetails(user.userId) sid = ud.right.toOption.flatMap(_.schoolId) s &lt;- schoolStore.getSchool(sid.get) if sid.isDefined } yield s </code></pre> <p>However I'm not happy with having the "if" in there, it seems that I should be able to use a map instead. </p> <p>But when I try with a map:</p> <pre><code> val schoolFuture: Future[Option[School]] = for { ud &lt;- userStore.getUserDetails(user.userId) sid = ud.right.toOption.flatMap(_.schoolId) s &lt;- sid.map(schoolStore.getSchool(_)) } yield s </code></pre> <p>I get a compile error:</p> <pre><code>[error] found : Option[scala.concurrent.Future[Option[School]]] [error] required: scala.concurrent.Future[Option[School]] [error] s &lt;- sid.map(schoolStore.getSchool(_)) </code></pre> <p>I've played around with a few variations, but haven't found anything attractive that works. Can anyone suggest a nicer comprehension and/or explain what's wrong with my 2nd example?</p> <p>Here is a minimal but complete runnable example with Scala 2.10:</p> <pre><code>import concurrent.{Future, Promise} case class User(userId: Int) case class UserDetails(userId: Int, schoolId: Option[Int]) case class School(schoolId: Int, name: String) trait Error class UserStore { def getUserDetails(userId: Int): Future[Either[Error, UserDetails]] = Promise.successful(Right(UserDetails(1, Some(1)))).future } class SchoolStore { def getSchool(schoolId: Int): Future[Option[School]] = Promise.successful(Option(School(1, "Big School"))).future } object Demo { import concurrent.ExecutionContext.Implicits.global val userStore = new UserStore val schoolStore = new SchoolStore val user = User(1) val schoolFuture: Future[Option[School]] = for { ud &lt;- userStore.getUserDetails(user.userId) sid = ud.right.toOption.flatMap(_.schoolId) s &lt;- sid.map(schoolStore.getSchool(_)) } yield s } </code></pre>
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