Union types

Language Design
103

Union types would make a big difference in Kotlin. At the moment, a sealed class is the closest thing to a union type, but it has several limitations, the biggest being that you can only have subtypes defined by you as part of a sealed class.

The proposal would be for the type system to understand union types as a concept. A few examples.

Dynamic casting:

val something: Int|String|Boolean = TODO("implement")
if (something is String|Boolean) {
    // here it's a string or a boolean
} else {
  // here it's an int
}

Intersection of available functions:

class First(val value: Int) {
      
     fun speak() { println("shouting") }
}

class Second(val value: Int, val name: String) {
   
     fun speak() { println("talking") }
}

fun main() {

     val something: First|Second = TODO("implement")
     val value = something.value
     something.speak()
     // something.name doesn't compile
}

Syntax sugar for Null type:

val messageOrNull: String? = TODO("implement")
val messageOrNull2: String|Null = TODO("implement")

Covariance and contravariance:

fun main() {

     val consumerWider: (String|Int) -> Unit = TODO("implement")
     val consumerNarrower: (Int) -> Unit = consumerWider

     val producerNarrower: () -> Int = TODO("implement")
     val producerWider: () -> String|Int = producerNarrower
}

Covariant and contravariant overriding:

interface MessageProcessor {
    
     val format: String|Format

     fun process(input: String|Message): String|Message
}

class InMemoryMessageProcessor(override val format: Format) : MessageProcessor {

     override fun process(input: String|Message?): Message
}
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