Question

I see that Scala standard library misses the method to get ranges of objects in the collection, that satisfy the predicate:

def <???>(p: A => Boolean): List[List[A]] = {
  val buf = collection.mutable.ListBuffer[List[A]]()
  var elems = this.dropWhile(e => !p(e))
  while (elems.nonEmpty) {
    buf += elems.takeWhile(p)
    elems = elems.dropWhile(e => !p(e))
  }
  buf.toList
}

What would be the good name for such method? And is my implementation good enough?

Answer 1

I’d go for chunkWith or chunkBy

As for your implementation, I think this cries out for recursion! See if you can fill out this

@tailrec def chunkBy[A](l: List[A], acc: List[List[A]] = Nil)(p: A => Boolean): List[List[A]] = l match {
  case Nil => acc
  case l    =>
    val next = l dropWhile !p
    val (chunk, rest) = next span p
    chunkBy(rest, chunk :: acc)(p)
}

Why recursion? It’s much easier to understand the algorithm and more likely to be bug-free (given the absence of vars).

The syntax !p for the negation of a predicate is achieved via an implicit conversion

implicit def PredicateW[A](p: A => Boolean) = new {
  def unary_! : A => Boolean = a => !p(a)
}

I generally keep this around as it’s astoundingly useful