Question

Why is the + operator for List deprecated in Scala?

http://www.scala-lang.org/docu/files/api/scala/List.html#%2B%28B%29

Answer 1

Good question, so I looked it up in the book by Odersky et al. It says the following (hopefully it’s not a copyright infringement to quote this here ;-)):

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Why not append to lists?

Class Listdoes not offer an append operation because the time it takes to append to a list grows linearly with the size of the list, whereas prepending with :: takes constant time. Your options if you want to build a list by appending elements is to prepend them, then when you’re done call reverse; or use a ListBuffer, a mutable list that does offer an append operation, and when you’re done call toList.

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As far as I understand FP, prepending to a list is much more common than appending, at least in pure functional languages. I can only assume that the designers of Scala added the + operator as a convenience for Java developers, who are used to appending using add(), and then had second thoughts about it.