I think the main issue with the Rust option is they ended up with language syntactic sugar anyways. So in some ways, it's worse than the kotlin-style because reading `let baz = foo?.bar` could have all sorts of weird type implications.

What is baz? Is it Option<Bar>? Is it Result<Bar>? Is it some user defined enum<Bar>? Who knows! You have to find that by looking up the foo definition.

For kotlin, the answer is simple. "baz is a nullable Bar"

Rust did this because interacting with the enum directly was cumbersome.

The concept of "no value" is so integral to day to day programming that elevating it into the type system with "nullable types" makes more sense to me vs using generics trickery. Even if it's slightly less "pure".

> So in some ways, it's worse than the kotlin-style because reading `let baz = foo?.bar` could have all sorts of weird type implications.

Syntax sugar that works with a normal library type is much nicer than dedicated syntax for a special-case builtin, IME.

> What is baz? Is it Option<Bar>? Is it Result<Bar>? Is it some user defined enum<Bar>? Who knows! You have to find that by looking up the foo definition.

Sure, or if you have a decent IDE you just mouseover it. But that's no different than any other method. `let baz = foo.add(bar)` doesn't tell you what type foo or bar is, and I don't think I've ever seen anyone argue that it should (e.g. by requiring method names to be globally unique).

> The concept of "no value" is so integral to day to day programming that elevating it into the type system with "nullable types" makes more sense to me vs using generics trickery. Even if it's slightly less "pure".

I've found this isn't really true. Once you don't have language-level support nudging you to use it all the time, wanting to have a possibly-absent value is actually pretty rare. (E.g. a lot of the time you want to include a "reason" for why it's absent, so you want an Either/Result-like type - but if you're using Kotlin you end up using a nullable type because you're lazy and the language makes that easier. That's bad for long-term maintainability IME, especially because if you want to switch the nullable type for a result you have to change all your code - unlike Rust where you can switch fairly easily because ?. works the same way for both types).

I rarely if ever use the question mark with 'Option' when working with rust, and it's been just fine.

I do use it a lot with Result though. But if you have any question about the return type you could[1] just look at the return type for the current function you're in.

[1] I think there's a way to define custom types on nightly, and I wouldn't be surprised if it let you map custom types on Result/Option, but I don't actually know.

I am a fan of Rust, but I feel obligated to point out the return type of the current function does not provide any guarantees about the type which '?' was used on, even on stable. Since the generic requirement to use '?' with a return type of `Result<T,E>` is just `Into<E>`, you would still need to look at the called function since `From<E> for T` could be satisfied for (almost) any T.

Ah, I did not realize that. Thanks for pointing that out!

Is there a difference between excluding null from a type and having basically syntactic sugar for ‘T | Null’ in the form of ‘T?’ ?

Yes. Having an explicit `Option<T>` type allows you to write things like `Option<Option<int>>`. This type is not possible in, say, Kotlin or Typescript. (Well, technically it is, but not using the native null types provided by the language.)

Most of the time this isn't a very useful type to have, but it has the big advantage of being mechanically obvious. For example, consider a hashmap of optional types, something like `Map<String, Option<Player>>`. If we write a get method for the map, it should return an Option to indicate whether the value was present or not. But what happens if the value is present, but it is explicitly Null?

The advantage of having an explicit option type is basically that it's easier to compose generics without having to understand what the values might be, which is usually what you want when using generics. That said, most of the time, if you've got an option of options of something, you're just going to flatten that type down anyway.

E: Thinking about it, the other advantage is that it's easier to create a separate namespace for "methods that should exist when T might be null but are meaningless the rest of the time". For example, Rust's `Option` type has methods to map the internal value into another value, unwrap the value and panic if it was null, swap the value with a different one, etc. In languages which use null | T, the equivalent is usually to use Elvis operators and similar (obj?.field), but that requires more special casing.

I guess one could have ‘(T | None) | None’, which can automatically be flattened to ‘T | None’, couldn’t it?

Also, the Map::get method that can return null is the problem here (even though your example is great and thanks for that, I didn’t think of it), something like ‘Option<Player?>’ should allow to differentiate between those cases.

But I do remember reading that a bottom type does make typing rules harder (e.g. scala 3’s explicit nulls feature which is basically “T is non-nullable, write it as T | Null” is not sound)

The other commenter related the types to normal binary operations, which I think explains why (T | Null) | Null can't be distinguished from T | (Null | Null). Another way of thinking about it is by asking what "untagged unions" actually means, and in the context of languages like Typescript, it generally means "unions where the tag is the type of the object". ("Untagged" here is a bit of a misnomer.) But if the tag is the type of the object, how do I distinguish between two different nulls? They both have the same type (the Null-Type), which means they both have the same tag in the union, which means they're the same.

What you say about `Option<Player?>` is a good point though. If we want to distinguish between the different results, we need an explicit Option type. But now we've got Option _and_ we've got nullable types. Which should we use? They're both doing the same thing (i.e. marking where a type may be present but might not be), so why do we need both?

In practice, my impression that nullable types are really good for integrating with languages that already have unchecked nulls in then, either for historical reasons (like Java) or because they're dynamic languages (like Python or Javascript). It's a way of acknowledging the null value in the type system without demanding that all the code that been interacting with nulls be rewritten.

However, if you were going to write your own language from scratch, it's difficult to see why you would allow nullables to exist when the Option type does pretty much everything that nullables can, but with more clarity for cases of "nested nullability". You can also still add syntax sugar for it (Rust is going down this route, for example), but you don't have to special case nullability to the same extent.

Thanks for the answer, I was genuinely curious about the advantages/disadvantages of these approaches. But indeed, types that completely exclude ‘nulls’ seem to be the cleaner solution — though I wonder whether that is even possible to retrofit to Java.

No, you cannot.

var foo: (T | None) | None = None

Which None do you mean here?

This is just basic logic: (X or Y) or Y <=> X or Y

Untagged unions cannot model that. Contrast it with tagged unions:

var foo: Option<Option<T>> = Some(None)

var bar: Option<Option<T>> = None

Now you can reason about where the None means.

I may have not been clear, but that “flattening” is what I meant, and mentioned it as a “feature” (when Some(None) and None is equally useful/useless).

Surely, if you want to distinguish between 3 states you need more data, hence my recommendation of Option<T?>

I really don't think that you want null in a modern language, at the runtime level there will eventually be a zeroed out reference field - but a programmer shouldn't have to deal with that.

So I think we largely agree.

(I left example syntax in another comment.)

Having nil (NULL, undefined whatever) that is falsey as is customary in Lisp/ Clojure is quite useful - the language family depends on it. Most things deal with it very well and it is a useful distinction that something wasn't setup or didn't return any meaningful value for whatever reason.

Clojure has a powerful meta-data system which could be used for checking for errors if Clojure didn't depend on the host platform for that. Also, it seems, Common Lisp also has some ideas about how to handle things more or less in-line with other code without having a special system that you can't see most of the time.

Btw. some of this can go very deep in the technology stack, there were computers (https://en.wikipedia.org/wiki/Ternary_computer) with ternary logic (https://en.wikipedia.org/wiki/Three-valued_logic). Such a computer could improve many aspects of computing, e.g. density and therefore efficiency. It could improve reliability (things could be more explicit). You could divide by 3 precisely and efficiently. It could improve our understanding of logic by making this for most of us alien concept to a more practical and widespread tool.