Alasca: Traits, Sealed Traits and Enum Traits

Published on 2018-08-31.

Traits

traits are interfaces with implementations (and type classes (todo))

trait Name
	fun name: String

traits can have constructor fields:

trait Age(age: Int32)

Sealed Traits

@sealed requires that all classes/values/modules of a trait are defined in the trait’s companion module:

@sealed
trait Car(expensive: Boolean)
	fun model: String

module Car
	module NoCar                extends Car
		fun model: String = "none"
	class Jaguar(model: String) extends Car(true)
	value Nissan(model: String) extends Car(true)

Enum Traits

an @enum trait is more restrictive than a sealed trait: it can only contain modules

module Friend
	module Joe    extends Age(23), Friend
	module Joanna extends Age(42), Friend
	module John   extends Age(17), Friend

@enum
trait Friend

in return, an enum value receives some helper methods by default:

Friends.values == ImmutableArray(Joe, Joanna, John)
Friends.withName("Joe") == Friends.Joe
Set[Friend](...) // implementation uses a bit field