Scala's for syntax could have had a sane rule behind it. Why not something like this?

sane_for.scala

// WHY NOT SCALA!?!
// Why not require this for for {  } syntax?!? FOR THE LOVE OF GOD!?

trait FlatMappable[F[+_], +A] {
    def flatMap[B](fn: A => F[B]): F[B]
    def map[B](fn: A => B): F[B]
}

sealed trait List[+T] extends FlatMappable[List, T] {
  def ++[U >: T](that: List[U]): List[U]
}
case object Empty extends List[Nothing] {
  def flatMap[B](fn: Nothing => List[B]): List[B] = Empty
  def map[B](fn: Nothing => B): List[B] = Empty
  def ++[U >: Nothing](that: List[U]): List[U] = that
}
case class Cons[T](head: T, tail: List[T]) extends List[T] {
  def flatMap[B](fn: T => List[B]): List[B] =
    fn(head) ++ tail.flatMap(fn)
  def map[B](fn: T => B): List[B] = Cons(fn(head), tail.map(fn))
  def ++[U >: T](that: List[U]): List[U] =
    Cons(head, tail ++ that)
}