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

:: 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