列表以外的序列上的 Scala 模式匹配 [英] Scala pattern matching on sequences other than Lists

问题描述

我有以下代码，它对列表中的每个元素进行递归操作

I have the following code which recursively operates on each element within a List

def doMatch(list: List[Int]): Unit = list match {
  case last :: Nil  => println("Final element.")
  case head :: tail => println("Recursing..."); doMatch(tail)
}

现在，忽略此功能可通过 filter() 和 foreach() 使用，这很好用.但是，如果我尝试将其更改为接受任何 Seq[Int]，则会遇到问题:

Now, ignoring that this functionality is available through filter() and foreach(), this works just fine. However, if I try to change it to accept any Seq[Int], I run into problems:

• Seq 没有​​ ::，但它有 +:，据我所知，这基本上是一样的.但是，如果我尝试在 head +: tail 上进行匹配，则编译器会抱怨错误:未找到:值 +:"
• Nil 特定于 List，我不确定用什么来替换它.如果我解决了之前的问题，我将尝试 Seq()

以下是我认为代码的外观，但它不起作用:

Here is how I think the code should look, except it doesn't work:

def doMatch(seq: Seq[Int]): Unit = seq match {
  case last +: Seq() => println("Final element.")
  case head +: tail  => println("Recursing..."); doMatch(tail)
}

这么多好的答案！我接受了agilesteel 的回答，因为他是第一个指出 :: 在我的示例中不是运算符，而是案例类，因此是不同的.

So many good answers! I'm accepting agilesteel's answer as his was the first that noted that :: isn't an operator in my example, but a case class and hence the difference.

推荐答案

Scala 中有两个 ::(发音为 cons).一个是class List 中定义的操作符，一个是class(List的子类)，表示一个有头有尾的非空列表.

There are two :: (pronounced cons) in Scala. One is an operator defined in class List and one is a class (subclass of List), which represents a non empty list characterized by a head and a tail.

head :: tail 是一个构造器模式，它是从 ::(head, tail) 语法修改而来的.

head :: tail is a constructor pattern, which is syntactically modified from ::(head, tail).

:: 是一个 case 类，这意味着为它定义了一个提取器对象.

:: is a case class, which means there is an extractor object defined for it.

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