Scalaz traverse example

gistfile1.scala

/* traverse takes an F[A] and an A => G[B] and returns a G[F[B]].
 * In this case List[Long] and Long => Option[String] result in Option[List[String]]
 */
scala> val usernames = Map(1L -> "susie", 2L -> "bob")
usernames: scala.collection.immutable.Map[Long,String] = Map(1 -> susie, 2 -> bob)

scala> List(1L, 2L).traverse(usernames.get)
res2: Option[List[String]] = Some(List(susie, bob))

scala> List(1L, 2L, 3L).traverse(usernames.get)
res3: Option[List[String]] = None