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Extractor for pattern matching last element of a list #2575

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scabug opened this issue Nov 4, 2009 · 3 comments

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commented Nov 4, 2009

Scala has an elegant way to match the first element of a list, but not the last one. Doing it currently seems fairly awkward:

List(1,2,3) match {
  case l: List[_] if l.last == 3 => println("last is 3")
}

Instead, Apache ESME has defined the following extractor to be able to match the last element of the list:

object ::> {
  def unapply[A] (l: List[A]) = l match {
    case Nil => None
    case _ => Some( (l.init, l.last) )
  }
}

Now one can simply do this

List(1,2,3) match {
  case _ ::> 3 => println("last is 3")
}

Does it make sense to include something like this in the standard library? Perhaps one could think of a more appropriate object name.

This feature request is definitely not of high priority.

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commented Nov 4, 2009

Imported From: https://issues.scala-lang.org/browse/SI-2575?orig=1
Reporter: Vassil Dichev (vdichev)

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commented Nov 17, 2009

@adriaanm said:
this should go to the greenhouse project that will be announced soon
you can easily do this outside of the std library

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commented Dec 4, 2012

@retronym said:

~/code/scala squala
Welcome to Scala version 2.10.1-20121203-101839-fd57069a3a (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_27).
Type in expressions to have them evaluated.
Type :help for more information.

scala> val init :+ last = List(1, 2, 3)
init: List[Int] = List(1, 2)
last: Int = 3

@scabug scabug closed this Dec 4, 2012

@scabug scabug added this to the 2.10.0 milestone Apr 7, 2017

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