キー毎に集計する例（irofさんの http://d.hatena.ne.jp/irof/20111203/p1 をScalaにしてみた）

summary1.scala

//getOrElseUpdateを使ってみたバージョン
case class Data(code: String, name: String, var value: Int)

val input = Seq(
  Data("A01", "hoge", 100), Data("A01", "piyo", 200),
  Data("A02", "hoge", 300),
  Data("A03", "hoge", 400), Data("A03", "piyo", 500)
)

def summary1(list: Seq[Data]): Seq[Data] = {
  val map = scala.collection.mutable.LinkedHashMap[String, Data]()
  for (d <- list) {
    val data = map.getOrElseUpdate(d.code, Data(d.code, "", 0))
    data.value += d.value
  }
  map.values.toList
}

summary1(input)

summary2.scala

//Scalaのコレクションメソッドを駆使してみたバージョン
case class Data(code: String, name: String, value: Int)

val input = Seq(
  Data("A01", "hoge", 100), Data("A01", "piyo", 200),
  Data("A02", "hoge", 300),
  Data("A03", "hoge", 400), Data("A03", "piyo", 500)
)

def summary2(list: Seq[Data]): Seq[Data] = {
  list.groupBy(_.code).map{ case (code, data) => Data(code, "", data.map(_.value).sum) }.toSeq.sortBy(_.code)
}

summary2(input)