[Pitch] Extending optional chains to include for loops

[tera]

@Ben_Cohen

Sure, if you're willing to teach the compiler to specially handle something, it can do all sorts. But I'm not seeing why this would be better to either of the other two options. It would just be some weird hard-coded special case that looks like a library implementable thing (deliberately echoes an existing one in fact), but isn't.

Actually I view that as an advantage. It would look exactly like a library feature (and the only thing making it different is that it can't be implemented as a pure library feature† that's why it is special-cased in the compiler), and users won't have to learn a new syntax. The "perceived" language size is not increased.

† or maybe it could?

// using MaybeSequence from https://forums.swift.org/t/pitch-extending-optional-chains-to-include-for-loops/65848/15

struct MaybeSequence<Wrapped: Sequence> {
    let wrapped: Wrapped?
}

extension MaybeSequence {
    struct Iterator {
        var wrapped: Wrapped.Iterator?
    }
}

extension MaybeSequence.Iterator: IteratorProtocol {
    mutating func next() -> Wrapped.Element? {
        wrapped?.next()
    }
}

extension MaybeSequence: Sequence {
    func makeIterator() -> Iterator {
        .init(wrapped: wrapped?.makeIterator())
    }
}

enum EmptySequence {
    case empty
}

func ?? <Wrapped> (lhs: Optional<Wrapped>, rhs: EmptySequence) -> MaybeSequence<Wrapped> {
    MaybeSequence(wrapped: lhs)
}

func test() {
    let y: [Int]? = [1, 2, 3]
    let z: [Int]? = nil
    
    for x in y ?? .empty {
        print(x)
    }
    for x in z ?? .empty {
        print(x)
    }
}

test()