Match Error on List.collect - scala

Question

What's wrong with the second implementation?
Works:
scala> List(1,2,3).collect{ case i: Int if i % 2 == 0 => i }
res1: List[Int] = List(2)
Doesn't:
scala> val evens = PartialFunction[Any, Int]{
| case i: Int if i % 2 == 0 => i
| }
evens: PartialFunction[Any,Int] = <function1>
scala> List(1,2,3).collect{evens}
scala.MatchError: 1 (of class java.lang.Integer)
at $anonfun$1.apply(<console>:7)
at $anonfun$1.apply(<console>:7)
at scala.PartialFunction$$anonfun$apply$1.applyOrElse(PartialFunction.scala:242)
at scala.runtime.AbstractPartialFunction.apply(AbstractPartialFunction.scala:33)
at scala.collection.TraversableLike$$anonfun$collect$1.apply(TraversableLike.scala:278)
at scala.collection.immutable.List.foreach(List.scala:318)
...

Answer 1

You should create evens PartialFunction like this:
val evens: PartialFunction[Any, Int] = {
case i: Int if i % 2 == 0 => i
}
With PartialFunction[Any, Int]{ ... } you are calling an apply method of object PartialFunction. It defined like this:
def apply[A, B](f: (A) ⇒ B): PartialFunction[A, B]
So { case i: Int if i % 2 == 0 => i } is used as a function, not a PartialFunction and evens is defined at any argument:
scala> val evens = PartialFunction[Any, Int]{
| case i: Int if i % 2 == 0 => i
| }
scala> evens.isDefinedAt(1)
res1: Boolean = true
scala> val evens: PartialFunction[Any, Int] = {
| case i: Int if i % 2 == 0 => i
| }
scala> evens.isDefinedAt(1)
res5: Boolean = false

Answer 2

Your code actually does work on Scala 2.9.2 (modulo incorrect partial function declaration). However, in any case the reason is probably the type signature:
PartialFunction[Any, Int]
Since the first type parameter is Any, the collection elements get autoboxed to java.lang.Integer instances - it even says so in the stack trace (by necessity, since Any corresponds to Object). However, you're matching for Int, and this causes the error.
Using PartialFunction[Int, Int] should solve the problem, i.e.:
val evens:PartialFunction[Int, Int] = {case i: Int if i % 2 == 0 => i}