Question

I have often the need to check if many values are equal and in case extract the common value. That is, I need a function that will work like follows:

extract(List()) // None
extract(List(1,2,3)) // None
extract(List(2,2,2)) // Some(2)

Assuming one has a pimp that will add tailOption to seqs (it is trivial to write one or there is one in scalaz), one implementation looks like

def extract[A](l: Seq[A]): Option[A] = {

  def combine(s: A)(r: Seq[A]): Option[A] =
    r.foldLeft(Some(s): Option[A]) { (acc, n) => acc flatMap { v =>
      if (v == n) Some(v) else None
    } }

  for {
    h <- l.headOption
    t <- l.tailOption
    res <- combine(h)(t)
  } yield res
}

Is there something like that – possibly more general – already in Scalaz, or some simpler way to write it?

Answer 1

This seems like a really complicated way to write

def extract[A](l:Seq[A]):Option[A] = l.headOption.flatMap(h =>
  if (l.tail.forall(h==)) Some(h) else None)

You don’t need tailOption, since the anonymous function that gets passed as an argument to flatMap is only executed if l is not empty.