Is it possible to compose two sealed classes?

Question

Image that I have two distinct Kotlin sealed classes.

sealed class Fruit {
  object Apple : Fruit()
  object Orange : Fruit()
  object Banana : Fruit()
}

sealed class Vegetables {
  object Broccoli : Vegetable()
  object Carrot : Vegetable()
  object Spinach : Vegetable()
}

Is it possible to define a type that is encompasses Fruits and Vegetables? Something like Produce = Fruit | Vegetable

So that you could write something like

fun lookAtProduce(produce: Produce) {
  when (produce) {
    is Carrot -> {
      return "Orange"
    }
    is Apple -> {
      return "Red"
    }
    ....
  }
}

fun putItInSalad(vegetable: Vegetable) {
  when (vegetable) {
    is Spinach -> {
      return true
    }
    is Carrot -> {
      return false
    }
    .....
  }
}

score 4 · Accepted Answer · answered Jun 10 '21 at 21:29

I think you're asking about a union type. There have been long discussions about adding one to Kotlin, but it doesn't look likely to happen soon.

Meanwhile, you can of course have an explicit parent class:

sealed class Produce

The rest then works pretty much as you'd expect:

sealed class Fruit : Produce() {
    object Apple : Fruit()
    object Orange : Fruit()
    object Banana : Fruit()
}

sealed class Vegetable : Produce() {
    object Broccoli : Vegetable()
    object Carrot : Vegetable()
    object Spinach : Vegetable()
}

fun lookAtProduce(produce: Produce) =
    when (produce) {
        is Vegetable.Carrot ->  "Orange"
        is Fruit.Apple ->  "Red"
        else -> TODO()
    }
}

fun putItInSalad(vegetable: Vegetable) =
    when (vegetable) {
        is Vegetable.Spinach -> true
        is Vegetable.Carrot -> false
        else -> TODO()
    }
}

(You have to qualify Carrot etc. unless you add static imports for them.)

(The nice thing about TODO() is that it works both as documentation and as a way to get incomplete code to compile!)

Of course, this requires you to have access to both the types you want to combine, so you can add the common parent class. If you can't do that, a more long-winded option would be to use an Either class (see e.g. here). That would need the produce to be explicitly wrapped and unwrapped, but is probably the nearest you can get in that case.

You're right that this solves the problem I posed in the question, sadly it doesn't solve my real life problem (I oversimplified for the purposes of posing the question quickly) What I'm really interested in in having multiple variants of a base sealed class. Union types is absolutely what I want. I think that I could accomplish this by extending the base sealed class, but it isn't allowed in the version of Kotlin I'm using because they are not all defined in the same file. My understanding is that in newer versions it could be done within a package. — Jaxsun, Jun 10 '21 at 22:01

score 2 · Answer 2 · answered Jun 10 '21 at 22:11

You can't create a union type, but you can always emulate it with another sealed type.

sealed class Produce {
  class FruitProduce(val fruit: Fruit): Produce()
  class VegetableProduce(val vegetable: Vegetable): Produce()
}

...and write

fun color(produce: Produce): String {
  return when (produce) {
    is Fruit -> when (produce.fruit) {
      is Orange -> "Orange"
      ...
    }
    is Vegetable -> when (produce.vegetable) {
      is Broccoli -> "Green"
      ...
    }
  }
}

Nothing better than this is possible today: you will need to wrap your fruit and vegetable objects in FruitProduce and VegetableProduce.

score 0 · Answer 3 · answered Jun 10 '21 at 21:28

0

Just create a super class:

sealed class Produce
sealed class Fruit : Produce()
sealed class Vegetable : Produce()

answered Jun 10 '21 at 21:28

broot

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Is it possible to compose two sealed classes?

3 Answers3