Added +: and :+ extractors to mirror append/prepend.

* +: does head/tail decomposition on any Seq
* :+ does init/last decomposition on any Seq
* Both preserve specific Seq types.

Review by @odersky

1 files changed, 21 insertions, 0 deletions

diff --git a/src/library/scala/collection/SeqExtractors.scala b/src/library/scala/collection/SeqExtractors.scala
new file mode 100644
index 0000000000..cb3cb27f18
--- /dev/null
+++ b/src/library/scala/collection/SeqExtractors.scala
@@ -0,0 +1,21 @@
+package scala.collection
+
+/** An extractor used to head/tail deconstruct sequences. */
+object +: {
+  def unapply[T,Coll <: SeqLike[T, Coll]](
+      t: Coll with SeqLike[T, Coll]): Option[(T, Coll)] =
+    if(t.isEmpty) None
+    else Some(t.head -> t.tail)
+}
+
+/** An extractor used to init/last deconstruct sequences. */
+object :+ {
+  /** Splits a sequence into init :+ tail.
+   * @returns Some(init, tail) if sequence is non-empty.
+   *          None otherwise.
+   */
+  def unapply[T,Coll <: SeqLike[T, Coll]](
+      t: Coll with SeqLike[T, Coll]): Option[(Coll, T)] =
+    if(t.isEmpty) None
+    else Some(t.init -> t.last)
+}