Scala 模式匹配中 case 子句中的映射表达式 [英] map expression in case clause in scala pattern matching

问题描述

我有一个配置值，它与地图中的值之一匹配，并根据它匹配的值我采取行动.这是我正在尝试做的一些示例代码

I have a configuration value that matches to one of the values in a map and depending on to which it matches i take an action. Here is some sample code of what i am trying to do

val x = 1         // or 2 or 3

val config = Map("c1"-> 1, "c2"-> 2, "c3"-> 3)

x match {
  case config("c1") =>
    println("1")
  case config("c2") =>
    println("2")
  case config("c3") =>
    println("3")
}

现在应该打印 1 因为 config("c1") 计算结果为 1 但它给出了错误

Now this should print 1 because config("c1") evaluates to 1 but it gives error

error: value config is not a case class, nor does it have an unapply/unapplySeq member
                case config("c1") =>

其他两种情况类似.为什么我要在这里取消申请?有什么指点吗?

Similarly for the other 2 cases. Why should i have an unapply here? Any pointers?

推荐答案

这样的表达式看起来像一个 extractor，因此是关于 unapply/unapplySeq 方法的消息.如果您不想使用提取器而只想匹配一个普通值，您需要将该值存储在一个稳定的标识符中——您不能使用任意表达式作为匹配案例:

An expression like that looks like an extractor, hence the message about unapply/unapplySeq methods. If you don't want to use an extractor but just want to match against a plain value, you need to store that value in a stable identifier - you can't use an arbitrary expression as a match case:

val case1 = config("c1")
x match {
  case case1 => println("1")
  ...
}

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