Hi! I was missing a collection function `sumOfOrNull` so I wrote my own very simple one: ```inline ...

# kontributors

[Brutus5000]

Hi! I was missing a collection function sumOfOrNull so I wrote my own very simple one:

inline fun <T> Iterable<T>.sumOfOrNull(selector: (T) -> Double?): Double? =
    this.mapNotNull(selector)
        .reduceOrNull { acc, i -> acc + i }

Does it make sense to add it to the standard library?
Is this even suitable? I see the standard library uses mostly iterators, for or while, and not that much high level stuff (I guess due to performance?)

[AJ Alt]

Do you really need an orNull variant? sumOf doesn't throw an exception if the iterable is empty.

[Brutus5000]

Yes, I need an orNull variant. I am summing over a List<Double?> and if all elements are null, the result should be null not 0. Regular sumOf would return 0.

[Brutus5000]

Well technically it's not a List<Double?>, but my selector returns a Double?

[rnett]

Could you not simply use something like sumByDouble{ it ?: 0.0 }

[Brutus5000]

No. In my use case null and 0 is semantically different

[rnett]

Oh I see, you want your sum function to return null if all elements are null?

[Brutus5000]

Exactly. Just like reduceOrNull or all of the other functions that work like this 🙂

[AJ Alt]

Combining a specific type of filtering and summing seems like a very niche use case. It's probably not common enough to add to the stdlib.