CODEWITHSUNDEEP
kotlingenerics
user22733269
user22733269

Reputation: 11

How to typesafely transform sumtypes in kotlin

Given the following sumtype:

sealed interface Node
data class Values(...) : Node
data class Container<V : Node>(val nodes: Map<String, V>) : Node

it's possible to construct types like Container<Container<Values>>.

Given another sumtype with a similar structure

sealed interface Diff
data class ValuesDiff(...) : Diff
data class ContainerDiff<V : Diff>(val nodes: Map<String, V>) : Diff

types with a similar structure can be created e.g. ContainerDiff<ContainerDiff<ValuesDiff>>.

I am looking for a method to transform the 1st sumtype into the 2nd, while keeping the structure. Something like this:

val node: Container<Container<Values>>
val diff: ContainerDiff<ContainerDiff<ValuesDiff>> = myFunction(node)

In Scala 3 there are Match Types.

Is there a way to do that in kotlin?

The best approach I found so far is to use a generic parameter.

sealed interface Diff<T : Node>
data class ValuesDiff(...) : Diff<Values>
data class ContainerDiff<V : Node>(val nodes: Map<String, Diff<V>>) : Diff<Container<V>>

Based on the interface and the generic parameter i provide extension methods to do the casting.

val myDiff: ContainerDiff<Container<Values>>
val theFirstContainer: Diff<Container<Values>> = myDiff.nodes.values.first() // here I would like to have the type ContainerDiff<ValuesDiff> 
// so to cast this I use 2 extension methods
fun Diff<Values>.cast() = this as ValuesDiff
fun <V : Node> Diff<Container<V>>.cast() = this as ContainerDiff<V>
// using the cast() extension method
val castedContainer: ContainerDiff<Values> = theFirstContainer.cast()
// and again for the nested Node
val castedValue: ValuesDiff = castedContainer.nodes.values.first().cast()

Is there a way to get the desired type directly without using the cast() extension methods? Have you had this problem and how did you solve it?

Upvotes: 1

Views: 50

Answers (1)

user3252407
user3252407

Reputation: 45

Boopidy Beep Beep. All facists are psychopaths, including those that think they are special because of their fake authority.

Upvotes: 0

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