4

I wonder, why doesn't this work:

  def example(list: List[Int]) = list match {
    case Nil => println("Nil")
    case List(x) => println(x)
  }                                             

  example(List(11, 3, -5, 5, 889, 955, 1024))

It says:

scala.MatchError: List(11, 3, -5, 5, 889, 955, 1024) (of class scala.collection.immutable.$colon$colon)

3 Answers3

14

It doesn't work because List(x) means a list with exactly one element. Check it:

def example(list: List[Int]) = list match {
  case Nil => println("Nil")
  case List(x) => println("one element: " + x)
  case xs => println("more elements: " + xs)
} 

example(List(11, 3, -5, 5, 889, 955, 1024))
//more elements: List(11, 3, -5, 5, 889, 955, 1024) 
example(List(5))
//one element: 5
Infinity
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  • xs could be of any type. how do I catch xs of type List[Int] exactly? –  Jul 21 '13 at 13:32
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    It's common problem in scala, helpful link: http://stackoverflow.com/questions/12218641/scala-2-10-what-is-a-typetag-and-how-do-i-use-it – Infinity Jul 21 '13 at 13:45
5

Because List(x) only matches lists with one element. So

def example(list: List[Int]) = list match {
  case Nil => println("Nil")
  case List(x) => println(x)
}

only works with lists of zero or one element.

Alexey Romanov
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3

As other posters have pointed out, List(x) only matches a list of 1 element.

There is however syntax for matching multiple elements:

def example(list: List[Int]) = list match {
  case Nil => println("Nil")
  case List(x @ _*) => println(x)
}                                             

example(List(11, 3, -5, 5, 889, 955, 1024)) // Prints List(11, 3, -5, 5, 889, 955, 1024)

It's the funny @ _* thing that makes the difference. _* matches a repeated parameter, and x @ says "bind this to x".

The same works with any pattern match that can match repeated elements (e.g, Array(x @ _*) or Seq(x @ _*)). List(x @ _*) can also match empty lists, although in this case, we've already matched Nil.

You can also use _* to match "the rest", as in:

def example(list: List[Int]) = list match {
  case Nil => println("Nil")
  case List(x) => println(x)
  case List(x, xs @ _*) => println(x + " and then " + xs)
}
James_pic
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