java - Why doesn't the Scala List have a size field?

Question

Coming from a Java background, I'm wondering why List in Scala doesn't have a size field like its Java equivalent LinkedList. After all, with a size field you'll be able to determine the size of the list in constant-time, so why was the size field dropped?

(This question refers to the new collection classes in Scala 2.8 and later. Also, I'm referring to the immutable List, not the mutable one.)

Answer 1

One cannot say the size field was dropped, as such list without the size have existed for 50 years since LISP where they are ubiquitous and they are very common in ML and Haskell too, both influential in scala.

The basic reason is that list is a recursive structure. A non empty List is Cons(head: A, tail: List[A]) — except that Cons is in fact called :: to allow a convenient infix notation. You can access the tail (the list without its head element) and that is a list too. And this is done just about all the time. So having the count in the list would not mean adding just one integer, but as many integers as there are elements. This is feasible, but certainly not free.

If you compare with java's LinkedList, LinkedList has a recursive implementation (based on Node, which is more or less like Cons, but with links in both direction). But a LinkedList is not a Node, it owns them (and keep their count). So while it has a recursive implementation, you cannot treat it recursively. It you wanted the tail of a LinkedList as a LinkedList, you would have to either remove the head and have your list changed or else copy all the tail elements to a new LinkedList. So scala's List and java's LinkedList are very different structures.