Problems with Pattern matching, implementing SplitAt in scala

问题

I am trying to implement the scala splitAt using pattern matching and this is what I am trying to do:

def split[T](someIndex:Int,someList:List[T]):(List[T],List[T]) = {
     def splitHelper[T](currentIndex:Int,someList:List[T],headList:List[T]):(List[T],List[T])= {
     (currentIndex,someList) match {
        case (someIndex,x::tail) => (x::headList,tail)
        case (currentIndex,x::y) => splitHelper(currentIndex+1,y,x::headList)
        case _ => (headList,headList)
        }
     }
     splitHelper(0,someList,List[T]())
}

The compiler is complaining by saying:

<console>:15: error: unreachable code
 case (currentIndex,x::y) => splitHelper(currentIndex+1,y,x::headList)

Can someone point out what I am doing wrong here and why I am getting the unreachable code error.

Thanks

回答1:

You should use `someIndex` and `currentIndex` (constants) in pattern matching.

scala> val a = 1
a: Int = 1

scala> 2 match {
     |   case a => println(a)
     | }
2

scala> 2 match {
     |   case `a` => println("a")
     |   case _ => println("Oops")
     | }
Oops

Chapter 15 of Programming in Scala, First Edition. "Case Classes and Pattern Matching" by Martin Odersky, Lex Spoon, and Bill Venners:

If you need to, you can still use a lowercase name for a pattern constant, using one of two tricks. First, if the constant is a field of some object, you can prefix it with a qualifier. For instance, pi is a variable pattern, but this.pi or obj.pi are constants even though they start with lowercase letters. If that does not work (because pi is a local variable, say), you can alternatively enclose the variable name in back ticks.

来源：https://stackoverflow.com/questions/10863067/problems-with-pattern-matching-implementing-splitat-in-scala

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scala

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