The Most Useful Kotlin Function

20 Mar 2017

Two years ago, I wrote a posted called The Most Useful Java Function. This was rather tongue-in-cheek, as the function in question lets you cast from a type to another, inferring the target type and suppressing unchecked cast exceptions.

Nevertheless, it is useful remarkably often.

With Kotlin however, you can step up your game to a whole new level. Introducing proclaim:

val list: Collection<Int> = ArrayList(list(1, 2, 3))
proclaim(list as ArrayList<Int>)
val x = list[0] // works

The trick here is actually not proclaim. It's the fact that Kotlin has "smart casts" which are essentially a form of flow typing.

The type inferencer reasons out that if the cast list as ArrayList<Int> succeeds, then it means that list is indeed an ArrayList<Int>. And since list is a val, its value can't change. So in the rest of the body where the cast appears, list is "smart-casted" to ArrayList<Int>.

Here is a possible definition for proclaim (0 run-time overhead besides the cast):

@Suppress("UNUSED_PARAMETER", "NOTHING_TO_INLINE")
inline fun proclaim (cast: Any)
    = Unit

Let's get the obvious out of the way: of course this can be misused.

On the other hand, just like the casting function, it is handy surprisingly often. Type systems just aren't powerful enough to be able to figure some things we know to be true in our code. Especially that of Java and Kotlin.

I'm not giving any examples: by their nature they would be short and easily rewritten to accomodate the type system. But if you genuinely want to know, you can search the net for "(path-)dependent types" use case or examples. Keep in mind that's just a subset of what we can't express in Java/Kotlin though.