Kotlin compiler can't figure out that variable is not nullable in do-while loop

Question

I have the following method. Its logic is very simple, if right is set then call left while it has a value (not null). When I write it in the following way, it works.

fun goNext(from: Node): Node? {
    var prev : Node = from
    var next : Node? = from.right
    if (next != null) {
        prev = next
        next = next.left
        while (next != null) {
            prev = next
            next = next.left
        }
    }
    return prev
}

If, instead, I try to shorten the code using a do-while loop, it no longer smart casts next to Node. It shows this error:

Type mismatch.
Required: Node<T>
Found: Node<T>?

The code follows:

fun goNext(from: Node): Node? {
    var prev : Node = from
    var next : Node? = from.right
    if (next != null) {
        do {
            prev = next // Error is here, even though next can't be null
            next = next.left
        } while (next != null)
    }
    return prev
}

Why don't you simplify to just `while (next != null) { ... }`? — Alexey Romanov, Nov 26 '17 at 12:45

Zonico · Answer 1 · 2017-11-25T20:55:50.457

-1

The compiler probably assumes that next can be changed between the if statement and the loop from another thread. Since you can assure that next is not null, just add !! to next when using it in the loop: next!!

edited Nov 25 '17 at 20:55

answered Nov 25 '17 at 20:50

Zonico

192
1
11

Then it can change also between while condition and the assingment `prev = next` – barsdeveloper Nov 25 '17 at 20:56
@biowep prev = next!! should fit, shouldn't it? – Zonico Nov 25 '17 at 21:16
Sure, it does, but it's a work around a non coherent behaviour, also it does introduce a non necessary control. – barsdeveloper Nov 25 '17 at 21:18
Here `next` is a local variable which doesn't leak outside the function, so it cannot be changed in another thread. – Ilya Dec 30 '17 at 02:56

Kotlin compiler can't figure out that variable is not nullable in do-while loop

1 Answers1