误解scala'match'编译规则 [英] Misunderstanding of scala 'match' compile rules

问题描述

我必须对Scala的match语义或编译器逻辑有一些基本的误解。此代码：

I must have some basic misunderstanding of the Scala 'match' semantics or the compiler logic. This code:

val stageStart:Int = 0
val stageShutDown:Int = Int.MaxValue
val stageErrorReport:Int = Int.MinValue

def stageString(stage:Int):String = stage match {
  case stageStart       => "Start"
  case stageShutDown    => "End"
  case stageErrorReport => "Error"
  case _                => "Step " + String.valueOf(stage)
}

会导致Unreachable Code错误在最后3'case'语句？如果不是用名称替换实际值（0，Int.MaxValue，Int.MinValue），它会编译 - 但现在我已经硬编码的值应该由他们的名字引用（因为所有通常的原因）。因为'val'永远不会改变，第一个版本是否也能工作？

results in "Unreachable Code" errors on the last 3 'case' statements? If instead of the names you substitute the actual values (0, Int.MaxValue, Int.MinValue) it compiles -- but now I've hard-coded values that should be referenced by their names (for all the usual reasons). Since a 'val' can never change, shouldn't the first version also work?

推荐答案

：如果 case 规则中的标识符以小写字母开头，则它们始终被视为变量。因此，第一个 case 总是匹配（存储 stage 到变量 stageStart ），其余3个不可达。您需要使用大写字母定义常量

There is a subtle yet important feature: If the identifier in case rules start with a lower-case letter, they're always treated as variables. So the first case matches always (storing stage into variable stageStart) and the rest 3 are unreachable. You need to define the constants with upper case letters as

val StageStart:Int = 0
val StageShutDown:Int = Int.MaxValue
val StageErrorReport:Int = Int.MinValue

def stageString(stage:Int):String = stage match {
  case StageStart       => "Start"
  case StageShutDown    => "End"
  case StageErrorReport => "Error"
  case _                => "Step " + String.valueOf(stage)
}

另请参见此模板匹配的常量。 answer Scala常量的命名约定？

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