2 files changed, 35 insertions, 1 deletions
diff --git a/src/library/scala/Function1.scala b/src/library/scala/Function1.scala
index dc8e67bbb0..7517e6604b 100644
--- a/src/library/scala/Function1.scala
+++ b/src/library/scala/Function1.scala
@@ -24,10 +24,18 @@ package scala
  *    assert(succ(0) == anonfun1(0))
  *  }
  *  }}}
+ *
+ *  Note that `Function1` does not define a total function, as might
+ *  be suggested by the existence of [[scala.PartialFunction]]. The only
+ *  distinction between `Function1` and `PartialFunction` is that the
+ *  latter can specify inputs which it will not handle.
+ *
  */
 @annotation.implicitNotFound(msg = "No implicit view available from ${T1} => ${R}.")
 trait Function1[@specialized(scala.Int, scala.Long, scala.Float, scala.Double) -T1, @specialized(scala.Unit, scala.Boolean, scala.Int, scala.Float, scala.Long, scala.Double) +R] extends AnyRef { self =>
-  /** Apply the body of this function to the argument.
+  /** Apply the body of this function to the argument. It may throw an
+   *  exception.
+   *
    *  @return   the result of function application.
    */
   def apply(v1: T1): R
diff --git a/src/library/scala/PartialFunction.scala b/src/library/scala/PartialFunction.scala
index b2910c2278..ae2c343e0b 100644
--- a/src/library/scala/PartialFunction.scala
+++ b/src/library/scala/PartialFunction.scala
@@ -13,6 +13,32 @@ package scala
  *  The function `isDefinedAt` allows to test dynamically if a value is in
  *  the domain of the function.
  *
+ *  Even if `isDefinedAt` returns true for an `a: A`, calling `apply(a)` may
+ *  still throw an exception, so the following code is legal:
+ *
+ *  {{{
+ *  val f: PartialFunction[Int, Any] = { case _ => 1/0 }
+ *  }}}
+ *
+ *  The main distinction between `PartialFunction` and [[scala.Function1]] is that
+ *  an alternative behavior can be applied to those elements that are declared
+ *  to be outside its domain. For example:
+ *
+ *  {{{
+ *  val sample = 1 to 10
+ *  val isEven: PartialFunction[Int, String] = { case x if x % 2 == 0 => x+" is even" }
+ *
+ *  // the method collect can use isDefinedAt to select which members to collect
+ *  val evenNumbers = sample collect isEven
+ *
+ *  val isOdd: PartialFunction[Int, String] = { case x if x % 2 == 1 => x+" is odd" }
+ *
+ *  // the method orElse allows chaining another partial function to handle input
+ *  // outside the declared domain
+ *  val numbers = sample map (isEven orElse isOdd)
+ *  }}}
+ *
+ *
  *  @author  Martin Odersky
  *  @version 1.0, 16/07/2003
  */