scala - 模式匹配scala中的常量表达式？

Question

是否可以重写下面的代码

for (i <- x) {
  if (i==x.first) {
    // do sth
  } else if (i==x.last) {
    // do sth
  } else {
    // do sth
  }
}

像这样使用模式匹配

for (i <- x) i match { 
  case `x.first` => // do sth
  case `x.last` => // do sth
  case _ => // do sth
}

我知道我们可以使用guard，或者预先对x.first和x.last进行求值，然后将它们存入其他的val中去在这里引用，但这很丑陋。有任何想法吗？谢谢!

Answer 1

一种简洁的方法是为自己定义提取器 +: 和 :+:

object +: {
  def unapply[CC, A, That](seq: CC)(implicit asSeq: CC => Seq[A], cbf: CanBuildFrom[CC, A, That]): Option[(A, That)] = {
    if (seq.nonEmpty)
      Some(seq.head, cbf(seq) ++= seq.tail result)
    else
      None
  }
}

object :+ {
  def unapply[CC, A, That](seq: CC)(implicit asSeq: CC => Seq[A], cbf: CanBuildFrom[CC, A, That]): Option[(That, A)] = {
    if (seq.nonEmpty)
      Some(cbf(seq) ++= seq.dropRight(1) result, seq.last)
    else
      None
  }
}

然后你可以简单地做:

val x = Seq(1, 2, 3, 4)
val first +: middle :+ last = x

println("first is %s".format(first))
for (y <- middle)
  println("middle contains %s".format(y))
println("last is %s".format(last))

打印:

first is 1
middle contains 2
middle contains 3
last is 4