Comment by gf000
14 hours ago
Sealed classes/interfaces and records are proper sum and product types.
The stdlib's Option type predates this language update by a long shot, so it doesn't use sealed classes, but it is now possible to have the usual FP "Maybe" type in Java:
``` sealed class Maybe<T> permits Some, None { record Some<T>(T obj) {} record None() {} } ```
(You will probably have to write Maybe.Some and I might have messed up the generic syntax as I wrote it on my phone, but that's mostly how it looks)
Or just do as Kotlin and embrace null, but in a type aafe way.
"Funnily", having nullable types be practically `T | Null` gives you union types, not sum types (the latter is, importantly are a disjunct union!)
The main difference is that (T | Null) | Null = T | Null, while Maybe<Maybe<T>> is different from Maybe<T>
Your point being?
Except this is completely wrong.
First, a record can't extend anything, it's not even valid syntax, so a sealed class can't permit record subclasses. So no, it's not possible to create a Maybe<T> class in Java that can only represent a Some<T> or a None<T> record. You could do it with regular classes, or if it's ok for Maybe<T> to be an interface.
Secondly, regardless of the sealing, nothing in any current or near future of Java prevents you from assigning `null` to any class of any kind you might create. So you can always have `Maybe<T> x = null`, or even `Some<T> x = null`.
None of this will change with the adoption of value classes either. So no, there is absolutely no way in Java to create a real Optional/Maybe type that would guarantee that a variable is either an object of a given type or None. There is probably some way to do it for your specific project using annotation processors, of course, but that is very different from having built-in support.
It's probably a typo. He meant sealed interface. I think that should be quite obvious.
Mistyped, it's sealed interface.
> So you can always have `Maybe<T> x = null`, or even `Some<T> x = null`.
Yeah and? Practically every type system have escape hatches, like Haskell can also do side effects without the IO monad, does it make the latter useless?
The whole point of using Optional/Maybe is to prevent the possibility of accidemtally creating nulls. If you don't make mistakes, then nullability is not a problem. If you do make mistakes, then a class that only helps when you don't make mistakes is basically useless.
This also has significant impact for serialization/de serialization - a classic place where you get unexpected nulls, that Java Optional/Maybe don't help with at all.
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