Kotlin prepend element

Question

I am searching for Kotlin alternative to:
(cons 1 '(2 3)) in lisp or
1 : [2, 3] in haskell or
1 :: List(2, 3) in scala,
(which all result in sth like [1, 2, 3])
so I can prepend an element to a List<T> (or any other list you can offer).

It will also be fine if one could provide O(1) head and tail Kotlin alternatives (I've found just first())

Answer 1

I think the easiest would be to write:

var list = listOf(2,3) println(list) // [2, 3] list = listOf(1) + list println(list) // [1, 2, 3] 

There is no specific tail implementation, but you can call .drop(1) to get the same. You can make this head\tail more generic by writing these extension properties:

val <T> List<T>.tail: List<T>   get() = drop(1)  val <T> List<T>.head: T   get() = first() 

Then:

val list = listOf(1, 2, 3) val head = list.head val tail = list.tail 

Some more info: Kotlin List tail function

Answer 2

Any class which implements Deque will suitable for you, for example LinkedList:

val linkedList = LinkedList(listOf(2, 3)) linkedList.push(1) println(linkedList) // [1, 2, 3] 

Creating lists throught constructor LinkedList(listOf(2, 3)) in many places can be annoying, so feel free to write factory method:

fun <T> linkedListOf(vararg elements: T): LinkedList<T> {     return LinkedList<T>(elements.toList()) }  // Usage: val list = linkedListOf(2, 3) list.push(1) println(list) // [1, 2, 3]