for-comprehension

As I understand it, Scala "for" syntax is extremely similar to Haskell's monadic "do" syntax. In Scala, "for" syntax is often used for List s and Option s. I'd like to use it with Either s, but the necessary methods are not present in the default imports. for { foo <- Right(1) bar <- Left("nope") } yield (foo + bar) // expected result: Left("nope") // instead I get "error: value flatMap is not a member..." Is this functionality available through some import? There is a slight hitch: for { foo <- Right(1) if foo > 3 } yield foo // expected result: Left(???) For a List, it would be List() . For

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. This approach works: val schoolFuture = for { ud <- userStore.getUserDetails(user.userId) sid = ud.right.toOption.flatMap(_.schoolId) s <- schoolStore.getSchool(sid.get) if sid.isDefined } yield s However I'm not happy with having the "if" in there, it seems that I should be able to use a map instead. But when I try with a map: val schoolFuture: Future[Option[School]] = for { ud <- userStore.getUserDetails(user.userId) sid = ud.right.toOption

Given a map where a digit is associated to several characters scala> val conversion = Map("0" -> List("A", "B"), "1" -> List("C", "D")) conversion: scala.collection.immutable.Map[java.lang.String,List[java.lang.String]] = Map(0 -> List(A, B), 1 -> List(C, D)) I want to generate all possible character sequences based on a sequence of digits. Examples: "00" -> List("AA", "AB", "BA", "BB") "01" -> List("AC", "AD", "BC", "BD") I can do this with for comprehensions scala> val number = "011" number: java.lang.String = 011 Create a sequence of possible characters per index scala> val values = number

Does anyone know how to get the (Scala part only) desugared translation of a for/comprehension expression before it actually tries to compile in the REPL (or compiler)? The only thing I've found so far is the compiler "-print" flag but that gives you the full Scala translation… As I already said in the other topic, scalac -print prints out scala code, not java. It translates all scala keywords that are not directly compatible with java to normal scala code. It is not possible to let the compiler translate only parts afaik. But basically a for-comprehension is always translated the same way. A

Why does this construction cause a Type Mismatch error in Scala? for (first <- Some(1); second <- List(1,2,3)) yield (first,second) <console>:6: error: type mismatch; found : List[(Int, Int)] required: Option[?] for (first <- Some(1); second <- List(1,2,3)) yield (first,second) If I switch the Some with the List it compiles fine: for (first <- List(1,2,3); second <- Some(1)) yield (first,second) res41: List[(Int, Int)] = List((1,1), (2,1), (3,1)) This also works fine: for (first <- Some(1); second <- Some(2)) yield (first,second) For comprehensions are converted into calls to the map or