语法：for (enumerators) yield e
enumerators为用分号间隔开的变量生成器 或者过滤器（if），yield返回值，两个例子：
def foo(n: Int, v: Int) =
for (i <- 0 until n;
j <- 0 until n if i + j == v)
yield (i, j)
case class User(name: String, age: Int)
val userBase = List(User("Travis", 28),
User("Kelly", 33),
User("Jennifer", 44),
User("Dennis", 23))
val twentySomethings = for (user <- userBase if (user.age >=20 && user.age < 30))
yield user.name // i.e. add this to a list
twentySomethings.foreach(name => println(name)) // prints Travis Dennis
