Plus-colon extractor for collections.

I often find myself writing the following for helpful head-tail decomposition:

package scala.collection

object +: {
  def unapply[T,Coll <: SeqLike[T, Coll]](
      t: Coll with SeqLike[T, Coll]): Option[(T, Coll)] =
    t.headOption map ( h => (h, t.tail))
}

And then using it for head-tail decomposition like this:

scala> val x = Vector(1,2,3)
x: scala.collection.immutable.Vector[Int] = Vector(1, 2, 3)

scala> x match {
     |   case x +: xs => println(x); xs
     | }
1
res1: scala.collection.immutable.Vector[Int] = Vector(2, 3)

rather than the 'classical' variant:

scala> x match {
     |   case Seq(x, xs @ _*) => println(x); xs
     | }
1
res2: Seq[Int] = Vector(2, 3)

I was curious if anyone else desired a parallelism to +: in pattern matching that mimics what's available for Lists (::). If you're like me, feel free to borrow the code, but also speak up! I'll find out about adding it into the standard library.

Note: We could also have a matching :+ operator for (x.init, x.last) decomposition as well. (Only really efficient for IndexedSeq, I would guess).

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