Question

A message class:

case class Message(username:String, content:String)

A message list:

val list = List(
    Message("aaa", "111"), 
    Message("aaa","222"), 
    Message("bbb","333"),
    Message("aaa", "444"),
    Message("aaa", "555"))

How to group the messages by name and get the following result:

List( "aaa"-> List(Message("aaa","111"), Message("aaa","222")), 
      "bbb" -> List(Message("bbb","333")),
      "aaa" -> List(Message("aaa","444"), Message("aaa", "555")) )

That means, if a user post several messages, then group them together, until another user posted. The order should be kept.

Answer 1

I can’t think of an easy way to do this with the provided Seq methods, but you can write your own pretty concisely with a fold:

def contGroupBy[A, B](s: List[A])(p: A => B) = (List.empty[(B, List[A])] /: s) {
  case (((k, xs) :: rest), y) if k == p(y) => (k, y :: xs) :: rest
  case (acc, y) => (p(y), y :: Nil) :: acc
}.reverse.map { case (k, xs) => (k, xs.reverse) }

Now contGroupBy(list)(_.username) gives you what you want.