[swift-evolution] Optional Argument Chaining

Kelvin Ma kelvin13ma at gmail.com
Mon Dec 11 17:42:36 CST 2017

i’ve run into this problem enough to wish this was a thing in Swift a lot,
but when it comes to solutions, i think the proposed syntax just isn’t
clear or easy to read, especially if there’s multiple layers involved.

On Mon, Dec 11, 2017 at 1:28 PM, Xiaodi Wu via swift-evolution <
swift-evolution at swift.org> wrote:

> This topic has been discussed at least two and maybe more times in the
> past. It’s hard for me to post links at the moment, but it should be
> possible to find on Google.
> One major challenge to this idea, for which no satisfactory answer has
> been achieved after all these years, is the following issue:
> f(g()?, h()?, i(), j()?)?
> If g() evaluates to nil, is h() called or not? How about i(), which is not
> failable? Since any function or property access can have side effects, in
> what order are the arguments evaluated, and how does one reason about this
> code flow?
> To my mind, in the absence of an intuitive answer to the above—which does
> not appear to be possible—this idea is not feasible.
> On Mon, Dec 11, 2017 at 12:34 Magnus Ahltorp via swift-evolution <
> swift-evolution at swift.org> wrote:
>> > 12 Dec. 2017 02:58 Jared Khan <jaredkhan at me.com> wrote:
>> >
>> > 2. It felt natural to me. It’s analogous to the existing optional
>> chaining scenarios and composes nicely. I think it’s about as
>> understandable as existing chaining, a newbie would have to look it up to
>> discover its meaning. What are your thoughts on this particular syntax
>> (ignoring 3. momentarily)? Hopefully others in this thread can share their
>> views too.
>> Chaining methods is linear, while nesting fills a similar purpose when we
>> use function calls. This of course affects the way existing Swift code is
>> written, but that is something we have to live with if we want to use
>> familiar syntax patterns. However, I think we have to consider this
>> difference in this case, since the syntax becomes more convoluted. Your
>> suggestion is definitely not as easy to read as the optional chaining
>> syntax, and maybe it can't be.
>> > As for how common I’d expect it to be, it’s something I’ve run into
>> myself a few times. Again, I hope members of this list can give their view
>> on if this would be useful to them.
>> I don't have any real examples, but I certainly think that I have run
>> into it, so I'm quite open to solving the problem. For me, it is probably
>> only a matter of finding a syntax that is acceptable.
>> > 3. I’m not entirely sure what the grammar situation is yet but afaik
>> ‘?’ has never been available as a postfix operator. Perhaps I’m missing
>> your point, could you demonstrate where it is allowed?
>> I did not expect that you would be able to answer that, it was more a
>> question directed to people who are more connected to the inner workings of
>> the parsing of Swift than I am. It is not allowed, but the error message is
>> not the one I expect, something that gives me a hint that it does have some
>> meaning early in the parsing.
>> /Magnus
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