gistfile1.txt

scala> val o: Option[Int] = Some(5)
o: Option[Int] = Some(5)

scala> val l: List[Int] = List(10)
l: List[Int] = List(10)

scala> l flatMap (_ => o)
res1: List[Int] = List(5)

scala> o flatMap (_ => l)
<console>:10: error: type mismatch;
 found   : List[Int]
 required: Option[?]
              o flatMap (_ => l)
                              ^

scala> val m: Map[Int, Int] = Map(1 -> 2)
m: Map[Int,Int] = Map(1 -> 2)

scala> l flatMap (_ => m)
res3: List[(Int, Int)] = List((1,2))

scala> m flatMap (_ => l)
res4: scala.collection.immutable.Iterable[Int] = List(10)