eh.scala

case class Book(title: String, category: String, rating: Int)

val t = List(
  Book("The dark", "Mystery", 90),
  Book("What is", "Romance", 55),
  Book("This is what", "Mystery", 80),
  Book("FPIS", "Reference", 99),
  Book("Darker", "Mystery", 80),
  Book("It isn't", "Romance", 82)
)

def zer(x: List[Book]) = {
  x.groupBy(_.category).head._2
}

zer(t)

def retro(x: List[Book]) = {
//  x.groupBy(_.category).values.map(_.sortBy(_.rating).head)
  x.groupBy(_.category).values.map(_.maxBy(_.rating))
}

retro(t)
.foreach {
  case Book(t, c, _) =>
    println(s"Top book for $c is $t")
}