So T? doesn't compose, while Option<T> does, which I'd consider a big difference.
However, without a builtin option type, duck-typing, or a pleasant way of converting the types, Option<T> may become a hassle (especially when different dependencies ship their own). And as T? is shipped with the language this is probably why it is used when the composability is not required.
P.S.: In C# T? even neatly composes.
return obj?.a?.b
if (obj != null && obj.a != null) {
return obj.a.b
} else return null;Which leads to a bit of an argument - nesting gives poor ergonomics, while collapsing impacts the ability to write generic code.
Does nesting Option's really has practical use or does it quickly become confusing?
In your example, Option<Option<T>> return type doesn't tell me by itself that this differentiates between a missing file or a missing setting. I would need to get this information from somewhere else.
In other words, nested optionals might not be very readable, but they're a necessary product of having a highly modular, reusable toolkit, and you can always massage them into more informative, domain-specific types at whatever point that becomes appropriate.
The thing is that nullable covers majority of cases and is quicker to read and write (Foo? vs Option<Foo>). Doing a?.b?.c is also more elegant than anything equivalent using an Option type.
Is there any languages that successfully combines both, nullable '?' syntax and an Option type?
Rust has dozens of helper methods for option - like map, map_or, map_or_else, and so on. When you’re starting out, it’s quite hard to find what you want in the morass of options. And many of the helper functions take a closure, which messes up your ability to do control flow (return, break, continue) from the containing function. In Swift, like typescript, I found the syntax sugar around options to be much easier to learn. Eg obj?.a?.b rather than checks docs obj.and_then(|o| o.a).and_then(|a| a.b).
“If let” in rust helps. (Similar to guard let in Swift) Ie, you can write if let Some(x) = x { … }. But you still can’t combine that with other conditions in the if statement - which drives me nuts.