Question

I have the following code which recursively operates on each element within a List

def doMatch(list: List[Int]): Unit = list match {
  case last :: Nil  => println("Final element.")
  case head :: tail => println("Recursing..."); doMatch(tail)
}

Now, ignoring that this functionality is available through filter() and foreach(), this works just fine. However, if I try to change it to accept any Seq[Int], I run into problems:

• Seq doesn’t have ::, but it does have +:, which as I understand is basically the same thing. If I try to match on head +: tail however, the compiler complains ‘error: not found: value +:’
• Nil is specific to List, and I’m not sure what to replace it with. I’m going to try Seq() if I ever get past the previous problem

Here is how I think the code should look, except it doesn’t work:

def doMatch(seq: Seq[Int]): Unit = seq match {
  case last +: Seq() => println("Final element.")
  case head +: tail  => println("Recursing..."); doMatch(tail)
}

Edit: So many good answers! I’m accepting agilesteel’s answer as his was the first that noted that :: isn’t an operator in my example, but a case class and hence the difference.

Answer 1

There are two :: (pronounced cons) in Scala. One is an operator defined in class List and one is a class (subclass of List), which represents a non empty list characterized by a head and a tail.

head :: tail is a constructor pattern, which is syntactically modified from ::(head, tail).

:: is a case class, which means there is an extractor object defined for it.