is there a nicer way of writing this in kotlin: ``` val NullableData? = firstNullable?.let { ...

# announcements

[David Smith]

is there a nicer way of writing this in kotlin:

val NullableData? = firstNullable?.let { first ->
            secondNullable?.let { second ->
                NullableData(first, second)
            }
        }

I have something like that but with many more fields so it is massively nested. I suppose I could turn nulls into exceptions and surround the whole thing in a try catch? In Haskell/Purescript/Scala we would have do notation for this and I think arrow had done something like this but I couldn’t find it?

[Javier]

a normal if or creating a general extension function which hides this under it but it has to have the exact number of nullables

[David Smith]

yeah, I need it to be a bit more flexible than that, I’m going through some weird json constructing various data classes so this is going to come up with different data classes many times

[Javier]

you can just just fill all with !!

[Javier]

and later catch the exception

[David Smith]

oh!

[David Smith]

nicel

[David Smith]

and !! throws a null pointer?

[David Smith]

so I have

fun <A> comprehend(f: () -> A): A? = try {
    f()
} catch (e: NullPointerException) {
    null
}

fun myFunction(json: ObjectNode): Pair<String, Percent>? = comprehend {
    val a = json["name"].nullString!!
    val b = json["allocated_percentage"].nullDouble?.toPercent()!!
    a to b
}

[Javier]

Not sure the exact exception, i think it can be KotlinNullPointerException

[Javier]

but I recommend you to check it

[David Smith]

cool, I’ll check the docs

[David Smith]

thanks for this, I’m happy with that

[Javier]

I think runCatching allows you can just not creating the comprehend function

[David Smith]

just had a look, it doesn’t really help. I think I will change the name of comprehend to runNull though

[Nir]

why doesn't runCatching help?

[Nir]

runCatching { ... }.getOrNull()

[Nir]

well, I guess if you want to be sure you don't accidentally catch another kind of exception

[Nir]

which is fair enough

[David Smith]

oh, nice

[David Smith]

didn’t know getOrNull

[David Smith]

runCatching { something!! }.getOrNull()

[David Smith]

excellent thank you both for your help

[ephemient]

val nullableData: NullableData? =
    if (first != null && second != null) {
        NullableData(first, second)
    } else null

smart-casting

[ephemient]

or yeah, Arrow does have a helper

val nullableData: NullableData? = nullable {
    NullableData(first.bind(), second.bind())
}

[CLOVIS]

If you're searching for a do-notation equivalent, check out the #C5UPMM0A0 library, they add Either and a direct equivalent to do (the either block)

In Kotlin, you should not use exceptions for business logic, for many reasons, one of them is how slow they are. https://elizarov.medium.com/kotlin-and-exceptions-8062f589d07 explains when you should and shouldn't use exceptions (written by Kotlin's lead designer)

[David Smith]

it’s not as simple as that, as you can see in this thread, I started without exceptions however kotlin’s nullable types do not have the tools to make my situation very readable. After some suggestions in this thread I ~ab~used exceptions in a safe and isolated way to do something very similar to do. The problem with arrow is that kotlin doesn’t have ad-hoc polymorphism so I can’t add the tools I need to the APIs I’m using, nullable types are “the kotlin way” and in the end @Javier and @Nir helped me achieve something similar to a Maybe monad. Just a shame you can’t generalize it over all monads in kotlin!

[CLOVIS]

Do keep in mind the performance impact of exceptions.

[David Smith]

@CLOVIS I saw nullable in arrow just now however that requires suspended functions. I wondered what the performance cost is of something like runBlocking { nullable { … } }?

[David Smith]

performance isn’t an issue for me right now but at the same time I don’t want to throw exceptions

[CLOVIS]

‘suspend functions' is just a fancy name for ‘nested lambdas', it's pretty much free.

You can use nullable.eager {} (same syntax) that doesn't require the external function to suspend: https://arrow-kt.io/docs/apidocs/arrow-core/arrow.core.computations/nullable/index.html#nullable

[David Smith]

ah, nice. I think I’m happy to leave it as a single runBlocking over all my code though, I have a lot of code scraping a load of json so there’s loads of these functions

[David Smith]

thanks for your help!

[elizarov]

If it's massively nested (more than two), then the following wins:

val nullableData: NullableData? = run {
    val first = firstNullable ?: return@run null
    val second = secondNullable ?: return@run null
    // ... can do more, no further nesting
    NullableData(first, second)
}

[David Smith]

I’m basically doing that with nullable and bind()

[elizarov]

Yes, but for the case of plain nullable you can do it with standard Kotlin. No need to be fancy.

[David Smith]

oh I see, can you explain what reaturn@run null is doing?

[elizarov]

https://kotlinlang.org/docs/returns.html

[David Smith]

ah, I see
Ioh,

[David Smith]

hmm, I think I prefer nullable although it’s nice that there is a kotlin way