偏函数 
PartialFunction[A, B]
A 输入参数类型
B 返回结果类型

val pf01: PartialFunction[Int, String] = {
  case 1 => "a"
  case 2 => "b"
  case _ => "c"
}
println(pf01(0))
println(pf01(1))
println(pf01(2))
println(pf01(3))

val list = (0 to 7).toList
list.map(pf01).foreach(print)