Vavr: Why using abstract classes?

[ivan-moto]

Quick design question: why go from interfaces to abstract classes (for Try and Option, for example)?

[danieldietrich]

Interfaces can be implemented by everyone, therefore it is a security hole. Option, Try etc only have two implementations (Some/None, Success/Failure etc). Because Java does not have sealed interfaces, we can only use an abstract class to hide the constructor. Alternatively we could have used a final class without inheritance (like Java‘s Optional).

[danieldietrich]

Allowing only specific implementations is essential when it comes to pattern matching. Otherwise the compiler could not guarantee exhaustive checks. (Look at Scala)

// a future Java switch expression
var res = switch myOption {
  case Some(t) -> doSomeThingWith(t)
  case None -> doSthElse()
  // no other case possible if Option is sealed!
}

// can't guarantee that there are 
interface Option<T> {
    static class Some<T> implements Option<T> {}
    static class None<T> implements Option<T> {}
}

// only two impls possible!
abstract class Option<T> {
    private Option() {} // sealed!
    static class Some<T> extends Option<T> {}
    static class None<T> extends Option<T> {}