FIzzBuzz.scala

object Main extends App {
  def myif(p: => Boolean)(thenS: => Any)(elseS: => Any): Any = {
    var res: Any = null
    val result = p && { res = thenS; true} || { res = elseS; false}
    res
  }

  def myelse(elseS: => Any): Any = {
    elseS
  }

  println(myif(1 != 1){
    "aaaaa"
  }( myelse {
    "bbbbb"
  }))
}

else つけてもつけなくてもOKだしAnyにならない版です

scala> :paste
// Entering paste mode (ctrl-D to finish)

object MyIfElse {

  sealed trait IfResult[+A] {
    def `else`[B >: A](exp: => B): B = this match {
      case Then(result) => result
      case Else         => exp
    }
  }
  private case class Then[+A](result: A) extends IfResult[A]
  private case object Else extends IfResult[Nothing]

  def `if`[A](p: Boolean)(exp: => A): IfResult[A] = p match {
    case true => Then(exp)
    case _    => Else
  }

}

// Exiting paste mode, now interpreting.

defined module MyIfElse

scala> import MyIfElse._
import MyIfElse._

scala> val a = `if` (10 > 11) { println("then"); "AAA" } `else` { println("else"); "BBB" }
else
a: String = BBB