Pattern matching list prefixes in Scala - list

Question

I have questions regarding pattern matching of list prefixes (i.e. the first couple of elements of a list).
This compiles, but it does not work as expected:
val l = List(1,2,3)
val test = { m: List[Int] =>
m match {
case l :: tail => println("tail: "+tail.mkString(","))
case _ => println("no match")
}
}
test(List(1,2,3,4,5))
Output is tail: 2,3,4,5. I'd expect it to say either tail: 4,5, or to fail to match, or to fail at compile time. What makes this work as it does?
My second question is: How can I match a list prefix using a list? I know that this works as I expect:
case 1 :: 2 :: 3 :: tail => println("tail: "+tail.mkString(","))
I have, however, my prefixes as lists, and cannot hard-code them. Is pattern matching even the right thing here?
I know I could do something like
if (m startsWith l) {
val tail = m drop l.size
}
But a long list of these seems rather inelegant, especially in Scala.

Answer 1

Regarding the output of your first code snippet, the l inside of the match is actually a new value that shadows the outer scoped l and captures 1 during execution.
The problem you are encountering is that :: is the unapply for List to break it into exactly a single head value and the tail, deconstructing the linked list.
While there is a ::: operation to go along with ::: to concatenate two lists, it does not have a corresponding unapply which would let you use it in a pattern match in the way you desire.

Answer 2

I don't think this is possible. Closest syntax i could propose based on this workaround:
import collection.SeqLike
implicit class PrefixMatcher[T](prefix: Seq[T]) {
object then {
def unapply[S <: SeqLike[T,S]](seq: S): Option[S] =
if (seq startsWith prefix) Some(seq drop prefix.length) else None
}
}
Then you could use it as
val test: List[Int] => Unit = {
case l.then(tail) => println("tail: " + tail.mkString(","))
case _ => println("no match")
}

Answer 3

Addresing the first part of your question: As #Arne mentions, case l is not being matched against your list, but captures a new list. For the former you'd need to enclose it on backticks, but even then I don't see how you can achieve what you want, the closes I can think of is:
case `l` :+ x => println(s"tail is $s") //but this just works when the tail is just one element long.
For the final part of your question, maybe pattern matching is not the right thing to do here, how about:
val prefixes = List (List (1,2,3), List(4,5,6)...)
def findTail(li:List[Int]) = prefixes.collectFirst{ case p if li.startsWith(p) => li.drop(p.size) } //Option[List[Int]]
This will find the tail for the first prefix that matches the testList or None if there was no match.You can easily generalize it to work with more than just Ints.