Is Swift optional chaining lazily evaluated left-to-right?

Question

The documentation doesn't seem to mention anything explicit about the order of optional chaining evaluation, just that:

Multiple queries can be chained together, and the entire chain fails gracefully if any link in the chain is nil.

Might seem obvious but I needed to confirm exactly what happens and thought it might be helpful for others. e.g. are we able to safely do this kind of thing?

    opVar?.someObject.someMethod(opVar!)

Answer 1

Optional chains are lazily evaluated left-to-right as you'd expect. The first optional in the chain to fail stops any further evaluation of the chain.

// Playground test for left-to-right lazy evaluation.
struct C {
    static var i = 0
    func chain(count: Int) -> C? {
        print("Expected \(count) -> \(C.i)")
        assert(count == C.i)
        C.i += 1
        return C.i > 2 ? nil : self
    }
}

let c = C()
c.chain(0)?.chain(1)?.chain(2)?.chain(3)!.chain(4)!

Output

Expected 0 -> 0
Expected 1 -> 1
Expected 2 -> 2