Nullable of nullable in Kotlin?

user_1993650 0
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I have a use case where I need double level of nullable, something like x: SomeClass??. See prefetchedRecord argument in java code below

public Optional<SomeClass> fetchRecord(Long id) {
   ...
}

public void process(
    Long id,
    Optional<Optional<SomeClass>> prefetchedRecord
) {
  Optional<SomeClass> fetchedRecord = prefetchedResult.orElseGet( () -> fetchRecord(id) )
  if (fetchedRecord.isPresent()) { ... process ... }
  else { ... do something else ... }
}

So for the prefetchedRecord the inner optional signifies the result of the fetching and the outer optional shows whether the fetching was ever done before calling the process function. I use two optional to avoid refetching the record even if the fetching returned nothing.

How would you write this in Kotlin? I realize using double optional is not that clear and I could use a custom class instead, smth like 

data class FetchResult<T>(val result: T, val fetched: boolean)

but I wonder if there is something built in into Kotlin to support this use case.

kotlin
2 Answers

There is no way to do nullable of nullable in Kotlin. I think Kotlin way is avoid null in most cases. You can use sealed-classes for more expressive code.

For example:

sealed class Result<out T> {
    data class Fetched<T>(val result: T): Result<T>()
    data class Prefetched<T>(val result: T): Result<T>()
    object None: Result<Nothing>()
}

Sealed classes provides algebraic data types support and you can make very expressive types with this powerful language feature. You also can use Kotlin standard Result in some cases

user_77204970

No, there is nothing built-in in Kotlin to handle your requirements. If you are targeting Java platform you can use java.util.Optional<T> class:

fun process(id: Long, prefetchedRecord: Optional<Optional<SomeClass>>) {
    ...
}

Or you can try to use java.util.Optional<T> class like this:

fun process(id: Long, prefetchedRecord: Optional<SomeClass>?) {
    ...
}
user_17316260
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