You are using an immutable list. The operations on the List return a new List. The old List remains unchanged. This can be very useful if another class / method holds a reference to the original collection and is relying on it remaining unchanged. You can either use different named vals as in
val myList1 = 1.0 :: 5.5 :: Nil
val myList2 = 2.2 :: 3.7 :: mylist1
or use a var as in
var myList = 1.0 :: 5.5 :: Nil
myList :::= List(2.2, 3.7)
This is equivalent syntax for:
myList = myList.:::(List(2.2, 3.7))
Or you could use one of the mutable collections such as
val myList = scala.collection.mutable.MutableList(1.0, 5.5)
myList.++=(List(2.2, 3.7))
Not to be confused with the following that does not modify the original mutable List, but returns a new value:
myList.++:(List(2.2, 3.7))
However you should only use mutable collections in performance critical code. Immutable collections are much easier to reason about and use. One big advantage is that immutable List and scala.collection.immutable.Vector are Covariant. Don’t worry if that doesn’t mean anything to you yet. The advantage of it is you can use it without fully understanding it. Hence the collection you were using by default is actually scala.collection.immutable.List its just imported for you automatically.
I tend to use List as my default collection. From 2.12.6 Seq defaults to immutable Seq prior to this it defaulted to immutable.