Add unsafeHead and unsafeTail for optimizations

src/main/scala/strawman/collection/Seq.scala

@@ -23,11 +23,7 @@ trait LinearSeq[+A] extends Seq[A] with LinearSeqLike[A, LinearSeq] { self =>
   def iterator() = new Iterator[A] {
     private[this] var current: LinearSeq[A] = self
     def hasNext = !current.isEmpty
-    def next() = {
-      val Some((head, tail)) = current.uncons
-      current = tail
-      head
-    }
+    def next() = { val r = current.unsafeHead; current = current.unsafeTail; r }
   }
 
   /** `length` is defined in terms of `iterator` */
@@ -39,7 +35,8 @@ trait LinearSeq[+A] extends Seq[A] with LinearSeqLike[A, LinearSeq] { self =>
   override def apply(n: Int): A = {
     if (n < 0) throw new IndexOutOfBoundsException(n.toString)
     val skipped = drop(n)
-    skipped.uncons.fold(throw new IndexOutOfBoundsException(n.toString))(_._1)
+    if (skipped.isEmpty) throw new IndexOutOfBoundsException(n.toString)
+    skipped.unsafeHead
   }
 }
 
@@ -81,6 +78,10 @@ trait LinearSeqLike[+A, +C[+X] <: LinearSeq[X]] extends SeqLike[A, C] {
   /** Extract the head and tail, if the collection is not empty */
   def uncons: Option[(A, C[A])]
 
+  /** To be overriden for performance in subclasses */
+  protected def unsafeHead: A = uncons.get._1
+  protected def unsafeTail: C[A] = uncons.get._2
+
   /** Optimized version of `drop` that avoids copying
     *  Note: `drop` is defined here, rather than in a trait like `LinearSeqMonoTransforms`,
     *  because the `...MonoTransforms` traits make no assumption about the type of `Repr`
@@ -89,17 +90,13 @@ trait LinearSeqLike[+A, +C[+X] <: LinearSeq[X]] extends SeqLike[A, C] {
     */
   override def drop(n: Int): C[A] = {
     def loop(n: Int, s: C[A]): C[A] = {
+      if (n <= 0) s
       // implicit contract to guarantee success of asInstanceOf:
       //   (1) coll is of type C[A]
       //   (2) The tail of a LinearSeq is of the same type as the type of the sequence itself
       // it's surprisingly tricky/ugly to turn this into actual types, so we
       // leave this contract implicit.
-      if (n <= 0) s else {
-        s.uncons match {
-          case None => s
-          case Some((_, t)) => loop(n - 1, t.asInstanceOf[C[A]])
-        }
-      }
+      else loop(n - 1, s.unsafeTail.asInstanceOf[C[A]])
     }
     loop(n, coll)
   }

src/main/scala/strawman/collection/immutable/List.scala

@@ -1,16 +1,16 @@
 package strawman.collection.immutable
 
 import scala.annotation.unchecked.uncheckedVariance
-import scala.{Option, None, Nothing, Some}
+import scala.{Option, None, NoSuchElementException, Nothing, Some, UnsupportedOperationException}
 import scala.Predef.???
-import strawman.collection.{InhabitedLinearSeqFactory, InhabitedLinearSeqOps, InhabitedSeq, Iterable, IterableFactory, IterableOnce, LinearSeq, SeqLike, View}
+import strawman.collection.{InhabitedLinearSeqFactory, InhabitedLinearSeqOps, InhabitedSeq, Iterable, IterableFactory, IterableOnce, LinearSeq, LinearSeqLike, View}
 import strawman.collection.mutable.{Buildable, ListBuffer}
 
 
 /** Concrete collection type: List */
 sealed trait List[+A]
   extends LinearSeq[A]
-    with SeqLike[A, List]
+    with LinearSeqLike[A, List]
     with Buildable[A, List[A]] {
 
   def fromIterable[B](c: Iterable[B]): List[B] = List.fromIterable(c)
@@ -22,10 +22,8 @@ sealed trait List[+A]
 
   /** Prepend operation that avoids copying this list */
   def ++:[B >: A](prefix: List[B]): List[B] =
-    prefix match {
-      case Nil => this
-      case h :: t => h :: (t ++: this)
-    }
+    if (prefix.isEmpty) this
+    else (prefix.unsafeHead :: prefix.unsafeTail) ++: this
 
   /** When concatenating with another list `xs`, avoid copying `xs` */
   override def ++[B >: A](xs: IterableOnce[B]): List[B] = xs match {
@@ -40,15 +38,22 @@ case class :: [+A](x: A, private[collection] var next: List[A @uncheckedVariance
   extends List[A]
     with InhabitedSeq[A]
     with InhabitedLinearSeqOps[A, List[A]] {
+
   override def isEmpty = false
   def head = x
   def tail = next
+
   def uncons: Option[(A, List[A])] = Some((x, next))
+  override protected def unsafeHead: A = x
+  override protected def unsafeTail: List[A] = next
 }
 
 case object Nil extends List[Nothing] {
   override def isEmpty = true
+
   def uncons = None
+  override protected def unsafeHead: Nothing = throw new NoSuchElementException
+  override protected def unsafeTail: Nothing = throw new UnsupportedOperationException
 }
 
 object List extends IterableFactory[List] with InhabitedLinearSeqFactory[::] {