scala3-Underscores-TypesFunctionsMatches.scala

// Type parameter placeholder: S[_]
def str[T, S[_]](ss: S[T]): String = ss.toString
val s1 = str(Seq(1, 2, 3))    // "List(1, 2, 3)"

// Function parameter placeholder: _.toUpperCase
val loud = Seq("hello world!").map(_.toUpperCase) // List(HELLO WORLD!)

// Match placeholder: _ in the first two case clauses
Seq(Seq('a', 'b', 'c'), Seq(1, 2, 3), Seq(1.1, 2.2)).map { 
  seq => seq match {
    case 'a' +: _ => "Found a"
    case head +: _ +: last +: _ => s"Found $head and $last"
    case head +: tail => s"Other list: $seq"
  }
}  // List("Found a", "Found 1 and 3", "Other list: List(1.1, 2.2)")