How exactly "case" works in partial functions in Scala?

I am just starting my seemingly steep learning curve with Scala and can't quite grasp how "case" works in partial functions exactly.

I looked at the definition of PartialFunction itself, and there I see a sample like the following:

val isEven: PartialFunction[Int, String] = { 
   case x if x % 2 == 0 => x+" is even" 
}

The part where I am getting stuck is case x if x%2 -- how does Scala know what's what here? What is a formal definition of this "case" statement/keyword?

I think one reason for my confusion is because in Lift I see things like the following(in Actor classes):

override def messageHandler = {
   case SomeKindOfUserMessageClass(id1, param1) => ....
   case AnotherKindOfUserMessageClass(id2) => ....
}

I sort of understand intuitively what's going on here, but I can't assemble some kind of unified definition of how "case" should be used. Even more puzzling to me is how Scala compiler untangles all that.

Solution

The thing you're asking about is called pattern matching. A pattern match block can be used with the match keyword, or it can be used to define either a function or a partial function, depending on the context.

Your second example uses extractors. Scala's evaluation of m match { case A(x) => } involves calling A.unapply(m). The companion object of a case class is an extractor (using case classes is more common than actually writing an unapply method).

The Scala Language Specification is a tough read, but sometimes it's worth trying to take a look at, especially if you're looking for formalism. Chapter 8 is about pattern matching. Section 8.4 introduces the idea of using if in a case clause like in your first example.