结合协变性质,Nil可以被视为List[Int],List[String]......(1,Cons(2,Cons(3,Nil)))
2 val ls = List("one","two","three") //> ls : ch3.list.List[String...sum(""){_ + _} //> res1: String = hello,World!...Cons(h,t) => if (f(h)) Cons(h,t.filter(f)) else t.filter(f)
12 }
1 List(1,2,3) map {_ + 10}...} //> res14: ch3.list.List[Int] = Cons(11,Cons(12,Cons(13,Nil)))
3 List(1,2,3) filter {_