Pattern Matching on lists behaving weird? - scala

Question

While practicing scala, I am trying to insertion-sort an integer list using pattern matching. Previously, the following code for printing the list worked absolutely fine:
object PrintList {
def iPrint(xs: List[Int]):Unit = xs match {
case x :: ys => {
print(x+" -> ")
iPrint(ys)
}
case _ => println("Nil")
}
def main(args: Array[String]) {
//val l = Nil.::(1).::(2).::(3).::(4)
val l = 4 :: 3 :: 2 :: 1 :: Nil
iPrint(l)
}
}
However, the following code to sort a list doesn't compile:
def insert(x : Int, l1 : List[Int]):List = {
//stubbed
List()
}
def iSort(l : List[Int]):List = l match {
case x :: ys => insert(x , iSort(ys))
case Nil => Nil
}
Am I missing something really trivial here??
EDIT:
Modified the code as below:
def insert(x : Int , l1 : List[Int]):List[Int] = {
//stubbed
List(0)
}
def iSort(l : List[Int]):List[Int] = l match {
case (x:Int) :: (ys:List[Int]) => insert(x , iSort(ys))
case _ => List(0)
}
Still getting error in the first case statement - Pattern type is incompatible with expected type. Found: ::[B], expected: List[Int]
Using Intellij Idea with scala plugin - 2.11.7.

Answer 1

Looking at your screenshot, you are defining your own List class in that same package Week04. It's visible in your project browser on the left-hand side. So in your code
def iSort(l: List[Int]) = ???
you have an argument of type Week04.List[Int]. You try to destructure that using the :: list-cons class. I presume you have not defined your version of ::, least I don't remember that this was defined the Coursera class. So you have a scala.:: here which is a sub-type of scala.List. So you are trying to pattern match against some completely different type. If you replaced every occurance of List with scala.List you would use Scala's standard list class instead and it should work. If you want that to work with your own list implementation, you need to define your own extractor.