[qammm]

Only replying partially as I don't want to repeat myself.

Ok, then let's just ignore continuations as they are somehow available in both languages although the Scala one feels a bit more officially endorsed to me.

> def upcase(s: String): UppercasedString

Even with that type signature of course the compiler can not check that for input "a" the output will be "A". Your code could have a bug and upcase "a" to "B" (Off-by-one errors are quite common) and your compiler would be perfectly happy. That is the whole point why unit test are more valuable than compiler type checks: checking specific code behavior and not only static types.