Comment by rtpg

I'm an advocate for "both".

- `Option<T>` and `Result<T,E>` at core;

- `?T` and `T!E` as type declaration syntax that desugars to them;

- and `.?` and `.!` operators so chains like `foo()?.bar()!.baz()` can be written and all the relevant possible return branches are inserted without a fuss.

Having `Option` and `Result` be simply normal types (and not special-casing "nullable") has benefits that are... obvious, I'd say. They're just _normal_. Not being special cases is great. Then, having syntactic sugars to make the very, _very_ common cases be easy to describe is just a huge win that makes correct typing more accessible to many more people by simply minimizing keystrokes.

The type declaration sugar is perhaps merely nice to have, but I think it really does change the way the average programmer is willing to write. The chaining operators, though... I would say I borderline can't live without those, anymore.

Chaining operators can change the SLOC count of some functions by as much as... say, 75%, if we consider a language like Go with it's infamous "if err not nil" clause that is mandated to spread across three lines.

Erased generics give parametricity, which most PL people think is fairly important. See https://en.wikipedia.org/wiki/Parametricity or https://www.cl.cam.ac.uk/teaching/1617/L28/parametricity.pdf

> E.g. if you have a list finding function that returns X?, then if you give it a list of MyObject?, you don't know if you found a null element or if you found nothing.

This is a problem with the signature of the function in the first place. If it's:

  template <typename T>
  T* FindObject(ListType<T> items, std::function<bool(const T&)> predicate)

Whether T is MyObject or MyObject?, you're still using nullpointers as a sentinel value;

  MyObject* Result = FindObject(items, predicate);

The solution is for FindObject to return a result type;

  template <typename T>
  Result<T&> FindObject(ListType<T> items, std::function<bool(const T&)> predicate)

where the _result_ is responsible for the return value wrapping. Making this not copy is a more advanced exercise that is bordering on impossible (safely) in C++, but Rust and newer languages have no excuse for it

Different language, but I find this Kotlin RFC proposing union types has a nice canonical example (https://youtrack.jetbrains.com/projects/KT/issues/KT-68296/U...)

    inline fun <T> Sequence<T>.last(predicate: (T) -> Boolean): T {
        var last: T? = null
        var found = false
        for (element in this) {
            if (predicate(element)) {
                last = element
                found = true
            }
        }
        if (!found) throw NoSuchElementException("Sequence contains no element matching the predicate.")
        @Suppress("UNCHECKED_CAST")
        return last as T
    }

A proper option type like Swift's or Rust's cleans up this function nicely.

Your example produces very distinguishable results. e.g. if Array.first finds a nil value it returns Optional<Type?>.some(.none), and if it doesn't find any value it returns Optional<Type?>.none

The two are not equal, and only the second one evaluates to true when compared to a naked nil.

Well, in a language with nullable reference types, you could use something like

  fn find<T>(self: List<T>) -> (T, bool)

to express what you want.

But exactly like Go's error handling via (fake) unnamed tuple, it's very much error-prone (and return value might contain absurd values like `(someInstanceOfT, false)`). So yeah, I also prefer language w/ ADT which solves it via sum-type rather than being stuck with product-type forever.

I like go’s approach on having default value, which for struct is nil. I don’t think I’ve ever cared between null result and no result, as they’re semantically the same thing (what I’m looking for doesn’t exist)