diaryConverter.scala

import scala.annotation.tailrec
import scala.collection.immutable.TreeMap

val lines = List(
  "- 2018年02月08日 木曜日 20:51",
  "今日は仕事をしました",
  "お昼にパスタをつくりました",
  "- 2018年02月09日 金曜日 20:51",
  "お出かけしました"
)

val dateTimeSep = """- (\d\d\d\d)年(\d\d)月(\d\d)日 .曜日 (\d\d:\d\d)""".r

// foldLeft版
val articles = lines.foldLeft(TreeMap[String, String]()) { (_articles, line) =>
  line match {
    case dateTimeSep(year, month, day, time) =>
      _articles.insert(s"$year-$month-$day $time", "")
    case _ => {
      val (lastKey, lastValue) = _articles.last
      _articles.updated(lastKey, s"$lastValue$line\n")
    }
  }
}

println(articles)

// 再帰関数版
@tailrec
def convert(lines: List[String], articles: TreeMap[String, String]): TreeMap[String, String] = {
  lines match {
    case (head :: tail) =>
      head match {
        case dateTimeSep(year, month, day, time) =>
          convert(tail, articles.insert(s"$year-$month-$day $time", ""))
        case _ => {
          val (lastKey, lastValue) = articles.last
          convert(tail, articles.updated(lastKey, s"$lastValue$head\n"))
        }
      }
    case Nil => articles
  }
}

println(convert(lines, TreeMap[String, String]()))