Hi, I'm trying to create a function which produces...

# codereview

[stoyicker]

Hi, I'm trying to create a function which produces a value of a type parameter, which may or may not be nullable. This is what I have:

private fun <T> runInTheFutureButAlsoWaitForResult(block: Foo.() -> T): T {
    var ret: T? = null
    var done = false
    val condition = reentrantLock.newCondition()
    someOtherThread.runAtSomePointInTheFuture {
        ret = delegate.block()
        done = true
        reentrantLock.withLock { condition.signal() }
    }
    reentrantLock.withLock {
        while (!done) {
            condition.await()
        }
    }
    return ret as T
}

While this does work at runtime for both cases in which T is nullable and non-nullable, casting ret to T is marked as unsafe, which does make sense, but if I define ret as type T, I can't assign null to it, which makes sense too, but I also can't assign anything else because I don't know what T is so I can't instantiate it. Therefore, I could try to make it lateinit, but mind you, lateinit can't be used with types whose upper bound is nullable, as is the case implicitly for T. Is there a way to write this code "correctly"?

[hho]

I don't think I understand … If you have the nullable case, you have to make everything nullable, right?

fun <T> runInTheFutureButAlsoWaitForResult(block: Foo.() -> T?): T? {
    …
    return ret
}

[stoyicker]

no you don't have to. T implicitly inherits from Any?, so by specifying T you're saying "whatever the caller uses as type parameter, which may or may not be nullable". The problem is not with the method signature, but with the type of ret

[ascii]

I'd just suppress the unsafe warning

[ascii]

You own whatever block's code is, right? You could write tests to ensure it's not nullable.

[stoyicker]

the thing is it might be nullable, and that does work at runtime, because T in "ret as T" is nullable. suppressing is of course an option, I was just thinking maybe there's a way that doesn't involve doing that

[Daniel Pitts]

Heh, I ran into basically the same thing recently, and just ended up suppressing the unsafe cast complaint.

[Daniel Pitts]

Note, your code here will deadlock if delegate.block() throws an exception.

[stoyicker]

true that, thank you