I am playing with `DelimitedScope` to implement `option {}` continuation and I realised that `DelimC...

# arrow-contributors

[kioba]

I am playing with DelimitedScope to implement option {} continuation and I realised that DelimContScope can not manage null as a shift return value.
the implementation already using nullable to show that the block is done. I replaced the nullable type with a quick GADT to represent empty state and allow nullable value:

private val nextShift = atomic<Shift<(suspend () -> R)>>(Empty)

is there any better way to avoid constant boxing for the return values of shifts?

[Jannis]

Yes, you could use a custom object and use any. Same principle as null just fixed to an internal. Still doesn't allow nesting but allows null

[Jannis]

I used null simply as convenience cause at the time of writing most of that package was experimental^^ also feel free to ask me if anything in there is unclear 😅

[raulraja]

@kioba there will be an option continuation as well so this one should be named nullable for consistency

[raulraja]

All computation expressions are now named after the data type decapitalized

[kioba]

sorry @Jannis didn’t mean to criticise, the implementation is super great 🎉 I have only a limited understanding of the DelimitedScope and don’t want to make any changes which could impact the performance on a bad way.
@raulraja thanks, I will rename it 👍 naming makes sense especially to have both of them 🙂

[Jannis]

@kioba no worries and please do criticise^^
What I meant was more along the lines of "everything is pretty hacky and not all that well tested so any improvement or found bug is great!" anyway 🙈
Either way, the atomic containing the result needs to allow nullables to work properly in a generic setting. Best is probably a singleton object for which we check with reference equality since we don't need to be concerned with nesting here.

[kioba]

you mean a separate atomic variable l to keep track of the end of the block?

[kioba]

create a draft PR to be able to show the current implementation.
https://github.com/arrow-kt/arrow-core/pull/251
Added a few questions regarding the code arrangement but happy to take any feedback regarding these changes if you have time 🙂 👌

[simon.vergauwen]

@kioba this is the trick @Jannis is referring to. It's comming in a lot of Kotlin generic code when not using Option to optimise over allocations in hot-spot areas like this. It sadly requires Any? and thus also casting in some places.

private object MY_NULL

class Generic<T>(val value: T) {

    val atomicValue: Any? = value

    fun isMyNull(): Boolean =
        atomicValue === MY_NULL
}

[kioba]

this is definitely a neat trick! Reminds me the old days for C++ boolean bitwise 😅.
thanks guys for teaching this 👨‍🎓 I will go and apply the changes today 👍

[simon.vergauwen]

No problem, it's a bit dirty but a good way to optimise low-level code to support generic ? 🙂

[Jannis]

It has the same nesting problems as nullables but since it uses a different reference it doesn't lead to nested null cases^^
Sadly we can't do this in general for option because of those drawbacks