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)

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tsimionescu 3 hours ago | parent | next [-]

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.

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munksbeer 38 minutes ago | parent | next [-]

It's probably a typo. He meant sealed interface. I think that should be quite obvious.

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gf000 2 hours ago | parent | prev [-]

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?

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tsimionescu 2 hours ago | parent [-]

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.

	▲	joe_mwangi 18 minutes ago \| parent [-]
		Since they plan to have null-restricted types, then I don't see any issue.

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ahoka 6 hours ago | parent | prev [-]

Or just do as Kotlin and embrace null, but in a type aafe way.

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gf000 6 hours ago | parent [-]

"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>

	▲	ahoka 3 hours ago \| parent [-]
		Your point being?