Question

<https twitter com pablisc0 status 1496534350661357572 s=20 amp t=sqSAOAfiI63EY1gPHNKsUw|In case> people didn t know slightly smiling face thread

Grégory Lureau · Answer

I feel like it s a bit against the value class mechanism that is just about wrapping a known type to adds build time constraints gt so being more specific Can I ask your use case for using generics on value class

pablisco · Answer

Same pattern is used in the stdlib <https github com JetBrains kotlin blob master libraries stdlib src kotlin util Result kt L22>

pablisco · Answer

Also the same with custom errors <https twitter com pablisc0 status 1496534359649755140>

Grégory Lureau · Answer

Thanks I always felt it was a bad example of value class compared to a more structured sealed class like we know there is exactly always 2 possible cases and methods are related to those cases so shouldn t be always available with default returns maybe there is a performance reason behind that I don t understand thinking face

ephemient · Answer

yes it s to avoid an extra allocation in the regular non exceptional path

pablisco · Answer

TBF a combination of sealed interfaces and inline classes can provide something similar that also works with when clauses ```sealed interface Result lt out T gt value class Success lt T gt val value T Result lt T gt value class Failure val error Throwable or custom Result lt Nothing gt ``` Kotlin 1 7 <https twitter com relizarov status 1496742249379553287 s=20 amp t=nArJSZuE98nU3u6ybT0tMw|will allow for generics> on value classes so this kind of pattern will be even more powerful slightly smiling face

pablisco · Answer

My personal favourite will be ```sealed interface Lce lt out L out C out E gt value class Loading lt L gt val query L Lce lt L Nothing Nothing gt value class Content lt C gt val content C Lce lt Nothing C Nothing gt value class Error lt E gt val error E Lce lt Nothing Nothing E gt ``` Although if we don t need a query for loading and we are happy with exceptions this is the same as a typealias of `Result lt T gt ` slightly smiling face

ephemient · Answer

upcasting a value class to an interface incurs allocation

Grégory Lureau · Answer

So in this last example Loading becomes a class instead of a value class If so is there a lint warning or something

ephemient · Answer

if you pass around a `Loading` value with type `Lce` then it gets auto boxed just like how ```val x Int = 1 val y Number = x``` will auto box `Int`

ephemient · Answer

this may not be how it works in the future <https youtrack jetbrains com issue KT 27576> but it is true for now

pablisco · Answer

Oh yeah that s a good point Forgot about the Auto boxing disappointed Although that s only for primitives right inlined sealed classes will be tophat ok hand Finally can have headless union types crossed fingers If we add multiple params to value classes i e tuples shame they were removed I will be a very happy person slightly smiling face

ephemient · Answer

value classes act very much like primitives there is a non value class generated for each one which is used to hold it whenever it is used in a position where a value class is not a valid option e g nullable upcasted generic types

ephemient · Answer

e g ``` JvmInline value class Foo val foo Any val x = Foo hello x as Any == x as Any true x as Any === x as Any false because of boxing```

ephemient · Answer

just like how ```val y = 1000 y as Any == y as Any true y as Any === y as Any false because of boxing```