What are the differences between asInstanceOf[T] and (o: T) in Scala?

Question

I saw that there are two methods to cast an object in Scala:

foo.asInstanceOf[Bar]
(foo: Bar)

When I tried, I found that asInstanceOf

Answer 1

Programming in Scala covers this in a bit of detail in Chapter 15 - Case Classes and Pattern Matching.

Basically the second form can be used as Typed Pattern in a pattern match, giving the isInstanceOf and asInstanceOf functionality. Compare

if (x.isInstanceOf[String]) {
  val s = x.asInstanceOf[String]
  s.length
} else ...

vs.

def checkFoo(x: Any) = x match {
  case s: String => s.length
  case m: Int => m
  case _ => 0
}

The authors hint that the verbosity of the isInstance* way of doing things is intentional to nudge you into the pattern matching style.

I'm not sure which pattern is more effective for a simple type cast without a test though.