Unapply in docs, List. List.headOption

doc/purescala.rst

@@ -182,16 +182,32 @@ Pattern matching
 .. code-block:: scala
 
  expr match {
-    // Simple (nested) patterns:
-    case CaseClass( .. , .. , ..) => ...
-    case v @ CaseClass( .. , .. , ..) => ...
-    case v : CaseClass => ...
-    case (t1, t2) => ...
-    case 42 => ...
-    case _ => ...
-
-    // can also be guarded, e.g.
-    case CaseClass(a, b, c) if a > b => ...
+   // Simple (nested) patterns:
+   case CaseClass( .. , .. , ..) => ...
+   case v @ CaseClass( .. , .. , ..) => ...
+   case v : CaseClass => ...
+   case (t1, t2) => ...
+   case 42 => ...
+   case _ => ...
+
+   // can also be guarded, e.g.
+   case CaseClass(a, b, c) if a > b => ...
+ }
+
+Custom pattern matching with ``unapply`` methods are also supported:
+
+.. code-block:: scala
+
+ object :: {
+   def unapply[A](l: List[A]): Option[(A, List[A])] = l match {
+     case Nil() => None()
+     case Cons(x, xs) => Some((x, xs))
+   }
+ }
+  
+ def empty[A](l: List[A]) = l match {
+   case x :: xs => false
+   case Nil() => true
  }
 
 Values

library/collection/List.scala

@@ -35,6 +35,11 @@ sealed abstract class List[T] {
     (that != Nil[T]() || res == this)
   }
 
+  def headOption: Option[T] = this match {
+    case Nil() => None[T]()
+    case Cons(h, _) => Some(h)
+  }
+
   def head: T = {
     require(this != Nil[T]())
     val Cons(h, _) = this
@@ -537,6 +542,15 @@ object ListOps {
 case class Cons[T](h: T, t: List[T]) extends List[T]
 case class Nil[T]() extends List[T]
 
+// 'Cons' Extractor
+object :: {
+  def unapply[A](l: List[A]): Option[(A, List[A])] = l match {
+    case Nil() => None()
+    case Cons(x, xs) => Some((x, xs))
+  }
+}
+
+
 @library
 object ListSpecs {
   def snocIndex[T](l : List[T], t : T, i : BigInt) : Boolean = {

library/lang/string/package.scala

@@ -16,7 +16,7 @@ package object string {
 
   @ignore
   def listToList[A](s: ScalaList[A]): List[A] = s match {
-    case h :: t =>
+    case scala.::(h, t) =>
       Cons(h, listToList(t))
     case _ =>
       Nil[A]()