What's the difference between :: and ::: in Scala

val list1 = List(1,2) val list2 = List(3,4) 

then

list1::list2 returns: List[Any] = List(List(1, 2), 3, 4) list1:::list2 returns: List[Int] = List(1, 2, 3, 4) 

I saw the book writes that when use :: it also results List[Int] = List(1, 2, 3, 4). My Scala version is 2.9.

2 Answers

:: prepends a single item whereas ::: prepends a complete list. So, if you put a List in front of :: it is taken as one item, which results in a nested structure.

In general:

• :: - adds an element at the beginning of a list and returns a list with the added element
• ::: - concatenates two lists and returns the concatenated list

For example:

1 :: List(2, 3) will return List(1, 2, 3) List(1, 2) ::: List(3, 4) will return List(1, 2, 3, 4) 

In your specific question, using :: will result in list in a list (nested list) so I believe you prefer to use :::.

Reference: class List int the official site