Add transform and flip methods.

core/common/src/main/scala/net/liftweb/common/Box.scala

@@ -516,6 +516,30 @@ sealed abstract class Box[+A] extends Product with Serializable{
    */
   def foreach[U](f: A => U): Unit = {}
 
+  /**
+    * If this box is a `Failure`, returns a `Failure` box that results from passing this box
+    * to `fn`. Otherwise, returns this box. Used for transforming Failures without having to
+    * cover all three cases of Box, which is required by `map`.
+    *
+    * `map` is only applied if the box is full, `mapFailure` is only applied if this box is a `Failure`.
+    *
+    * @example {{{
+    *  // Returns `Failure("Changed failure")` because this box is a failure.
+    *  Failure("Original Failure") mapFailure { failed => Failure("Changed failure") }
+    *
+    *  // Returns Full("some-value") on which it was called
+    *  Full("some-value") mapFailure { failure => Failure("Changed failure") }
+    *
+    *  // Returns this Empty instance
+    *  Empty mapFailure { case value => value }
+    *  }}}
+    *
+    * @param fn the function that will be used to transform the Failure if this box is a failure.
+    * @return A `Failure` instance that results from applying `fn` if this box is a `Failure`, otherwise
+    *         the same instance on which it was called.
+    */
+  def mapFailure(fn: Failure => Failure): Box[A] = this
+
   /**
    * If this box is `Full` and contains an object of type `B`, returns a `Full`
    * of type `Box[B]`. Otherwise, returns `Empty`.
@@ -904,6 +928,8 @@ sealed case class Failure(msg: String, exception: Box[Throwable], chain: Box[Fai
 
   override def map[B](f: A => B): Box[B] = this
 
+  override def mapFailure(f: (Failure) => Failure): Box[A] = f(this)
+
   override def flatMap[B](f: A => Box[B]): Box[B] = this
 
   override def isA[B](cls: Class[B]): Box[B] = this

core/common/src/test/scala/net/liftweb/common/BoxSpec.scala

@@ -67,8 +67,8 @@ class BoxSpec extends Specification with ScalaCheck with BoxGenerator {
       Full(1) reduceLeft {(x: Int, y: Int) => x + y} must_== 1
     }
     "be used as an Option" in {
-      Full(1) orElse Some(2) must_== Some(1)
-      Empty orElse Some(2) must_== Some(2)
+      Full(1) orElse Some(2) must beSome(1)
+      Empty orElse Some(2) must beSome(2)
     }
     "be implicitly defined from an Option. The openOrThrowException method can be used on an Option for example" in {
       Some(1).openOrThrowException("This is a test") must_== 1
@@ -152,14 +152,17 @@ class BoxSpec extends Specification with ScalaCheck with BoxGenerator {
         Full(Empty).flatten must_== Empty
       }
     }
+    "define a 'mapFailure' method returning itself." in {
+      Full(1).mapFailure(identity) must_==  Full(1)
+    }
     "define an 'elements' method returning an iterator containing its value" in {
       Full(1).elements.next must_== 1
     }
     "define a 'toList' method returning a List containing its value" in {
       Full(1).toList must_== List(1)
     }
     "define a 'toOption' method returning a Some object containing its value" in {
-      Full(1).toOption must_== Some(1)
+      Full(1).toOption must beSome(1)
     }
     "return itself if asked for its status with the operator ?~" in {
       Full(1) ?~ "error" must_== Full(1)
@@ -298,14 +301,17 @@ class BoxSpec extends Specification with ScalaCheck with BoxGenerator {
     "define a 'flatten' method returning Empty" in {
       Empty.flatten must beEmpty
     }
+    "define a 'mapFailure' method returning itself." in {
+      Empty.mapFailure(identity) must beEmpty
+    }
     "define an 'elements' method returning an empty iterator" in {
       Empty.elements.hasNext must beFalse
     }
     "define a 'toList' method returning Nil" in {
       Empty.toList must_== Nil
     }
     "define a 'toOption' method returning None" in {
-      Empty.toOption must_== None
+      Empty.toOption must beNone
     }
     "return a failure with a message if asked for its status with the operator ?~" in {
       Empty ?~ "nothing" must_== Failure("nothing", Empty, Empty)
@@ -358,7 +364,11 @@ class BoxSpec extends Specification with ScalaCheck with BoxGenerator {
       Failure("error", Empty, Empty) flatMap {x: String => Full(x.toString)} must_== Failure("error", Empty, Empty)
       Failure("error", Empty, Empty).flatten must_== Failure("error", Empty, Empty)
     }
-    "return a itself when asked for its status with the operator ?~" in {
+    "define a 'mapFailure' method that transforms it into another Failure instance." in {
+      val exception = new Exception("transformed")
+      Failure("error", Empty, Empty) mapFailure { _ => Failure("new-error", Full(exception), Empty) } must_=== Failure("new-error", Full(exception), Empty)
+    }
+    "return itself when asked for its status with the operator ?~" in {
       Failure("error", Empty, Empty) ?~ "nothing" must_== Failure("error", Empty, Empty)
     }
     "create a new failure with a chained message if asked for its status with the operator ?~!" in {