Q:

Scala MergeSort

I am new to Scala and have to implement MergeSort.
The given prototype looks like this: def mergeSort(split : List[Int] => (List[Int], List[Int]), as: List[Int]): List[Int] 
I don't ask you to implement MergeSort for me, I already know how to do it. My problem is with the method definition I am not allowed to change.
I don't know how to deal with split : List[Int] => (List[Int], List[Int]), as: List[Int])
It obviously means that it splits a given List into two, but how do I access the two resulting lists?
And if I try something like this:
if (List.length < 2) List I get the error: error: value length is not a member of object List
However, I have seen examples of MergeSort doing exactly that.

A:

Hints: 

if (as.length < 2) { ... }
val (lefts, rights) = split(as)

