Is it possible to seamlessly wrap something like an `Optiona

Question

Is it possible to seamlessly wrap something like an `Optional` Where there are three valid values regular value null nothing I m using a data class so a `lateinit var` isn t exactly the best solution

jw · Answer

Seamlessly No Always Otherwise yes Most optionals do not distinguish between null and absence so you ll probably need to write one that supports nullable values and absence as orthogonal concepts

nwh · Answer

I d just like to avoid the boilerplate of `object value = uselessWrapper value ` and `object value get ` it gets so crowded

streetsofboston · Answer

What about `val value Optional lt T gt ` where a `null` value is nothing unknown a `Optional None` value is not present and `Optional Some lt T gt ` is an actual regular value

jw · Answer

yeah i mean it sounds like what you want is probably a nullable sealed type where nothing is more semantically represented in the type system instead of with a generic box

nwh · Answer

I suppose that could work but I m not sure it s superior And yeah that s about right jw Or first class support for Optionals in a similar way to nullables with an isPresent variable

groostav · Answer

The fact that your domain has a different meaning for `null` than it does for `empty nothing` to me means this is a very subtle problem What does `null` encode What does `empty` encode At this point I d probably make a sealed class that names those two concepts and go from there Downstack where you can be certain you don t care about one of those cases eg in a parameter use a simple nullable type instead of this sealed class But if its a field for a state machine you would need to completely eliminate one of those states to avoid the useless boilerplate

nwh · Answer

I mostly agree < groostav> it s mostly a semantic choice For pushing values to an API it merges incoming data with what s present So if a field is completely omitted in the request then no changes are made while null empties out the value and all other values are set directly Basically passing would be the same as an empty optional empty stored value while passing null would be equivalent to passing a null optional noop Which is a totally fine solution I was just curious about other possibilities But an empty optional is a more clear statement of deliberately passing nothing if that makes sense

streetsofboston · Answer

< nwh> Something like this ``` sealed class Optional lt out T Any gt object None Optional lt Nothing gt object Null Optional lt Nothing gt data class Some lt out T Any gt val value T Optional lt T gt ```

nwh · Answer

That s a good solution but it s very similar to using Optional in terms of the boilerplate

streetsofboston · Answer

There are some tricks around the boiler plate using operator functions slightly smiling face

streetsofboston · Answer

Some tricks using the `not` or `component1` operator use one of them where you tease out values if it is a Some and short cut a None value all together `bind` ``` private object IsNoneException Exception sealed class Optional lt out T gt object None Optional lt Nothing gt data class Some lt out T gt val value T Optional lt T gt companion object fun lt T gt bind block gt T Optional lt T gt = try Some block catch IsNoneException None operator fun lt T gt Optional lt T gt not T return when this is Optional Some gt value is Optional None gt throw IsNoneException operator fun lt T gt Optional lt T gt component1 T return when this is Optional Some gt value is Optional None gt throw IsNoneException fun main val optional Optional lt Int gt = Optional Some 2 val result1 = Optional bind val value = optional if value = null value 2 else 0 val result2 = Optional bind val value = optional if value = null value 2 else 0 println $result1 and $result2 ```

nwh · Answer

Interesting thanks