Kotlin Shape Class

1

Could you please help, are any Youtrack issues for shape classes support? I’d like to extend existing or create new.

There are plans for it at the .Net Core - Exploration: Shapes and Extensions · Discussion #164 · dotnet/csharplang · GitHub.

What features I expect:

  1. Ability to extend the existing companion objects or Java Static layer with statefull logic. For example, I’d like to have something like this:
shape object Duration { // <-- these methods will be attached to this class
      val MAX: Duration = Duration.ofHours(999); // e.g. we extend static members of class
} 

fun getTimeout(input: String): Duration {
    return when(input) {
          ...
         else -> Duration.MAX
    }
}
  1. Ability to omit companion objects in generic declaration
shape object Duration : Converable<Long, Duration> {
      override fun convert(from: Long): Duration = Duration.ofNanos(from)
} 

fun <TResult> fromLong(input: Long): TResult
  where TResult.Companion: Convertible<Long, TResult>  { // so Kotlin Compiler can attach necessary factory-like objects
      return TResult.convert(input) // we called method from shape class or from companion object
}
2

Your first example could be easily implemented by companion extensions:

val Duration.Companion.MAX get() = Duration.ofHours(999)

The second one is about contracts on Companions. It is not possible right now, but it was discussed multiple times. I call them companion interfaces. I can’t find one clear proposal right now, but there were several discussions here.

3

Duration class is from Java, it doesn’t have companion object.

And I pre-created value before, so right code сould be like this:

private val maxDuration = Duration.ofHours(999)

val Duration.Companion.MAX get() = maxDuration
4

For that you need KT-11968.Pre-created values should be stored externally, because otherwise it will produce a lot of problems with initialization order (there are already a lot of them).

1 Like
5

Interesting. However as I can see I couldn’t attach the interface implementation to existing Companion object.

Moreover, I have explicitly put the companion object into the methods like fun <TResult> fromLong(input: Long) above, however compiler can resolve them automatically.

6

Companion interfaces are very important for future. They could allow to solve most of HKT use-cases and are really useful for a lot of things including serialization. But they require careful design, study and discussion.

1 Like