[swift-evolution] [Proposal draft] Disallow Optionals in String Interpolation Segments

Kevin Ballard kevin at sb.org
Tue Oct 4 13:06:01 CDT 2016

On Tue, Oct 4, 2016, at 10:44 AM, Mark Lacey wrote:
>> On Oct 4, 2016, at 10:29 AM, Kevin Ballard via swift-evolution <swift-
>> evolution at swift.org> wrote:
>> On Tue, Oct 4, 2016, at 10:28 AM, Nate Cook wrote:
>>>> On Oct 3, 2016, at 5:49 PM, Kevin Ballard via swift-evolution <swift-
>>>> evolution at swift.org> wrote:
>>>> On Mon, Oct 3, 2016, at 03:18 PM, Jordan Rose wrote:
>>>>>> ...
>>>>> We had this at one point, but we took it out because people would
>>>>> forget to test the nil case. I think `?? ""` or `?? nil` really is
>>>>> the best answer here.
>>>> But you can't write that, unless you're dealing specifically with
>>>> an Optional<String>.  If you try you'll get an error:
>>>> unnamed.swift:2:19: error: binary operator '??' cannot be applied
>>>> to operands of type 'Int?' and 'String'
>>>>     print("x: \(x ?? "nil")")
>>>>                 ~ ^  ~~~~~
>>>> unnamed.swift:2:19: note: overloads for '??' exist with these
>>>> partially matching parameter lists: (T?, @autoclosure () throws ->
>>>> T), (T?, @autoclosure () thro
>>>> ws -> T?)
>>>>     print("x: \(x ?? "nil")")
>>>>                   ^
>>>> This leads to writing code like "…
>>>> \(x.map(String.init(describing:)) ?? "nil")" which is pretty gross.
>>> I think that if we're going to add this warning we should make it
>>> possible to provide a string as an alternative. It seems like it
>>> should be possible to build a ?? operator with a (T?, String) ->
>>> _StringInterpolationSomething signature that works only in a string
>>> interpolation context.
>>> There are some types that aren't trivially constructible, or don't
>>> have clear alternatives for the nil case. Other times it might just
>>> not make sense to build a new instance simply to turn it into a
>>> string. If we're going to make people provide an alternative for
>>> optionals in this otherwise simple-to-use construct, let's make it
>>> simple to do so.
>>> This is undoubtedly a more complex approach that could be considered
>>> separately, but I think it would be a valuable part of how
>>> developers could transition their code.
> That’s definitely more complex, and seems like a completely orthogonal
> feature request.
>> I like this idea. This combined with the warning for naively
>> interpolating an Optional would be a good solution, because now when
>> I see the warning I can trivially solve it with `?? "nil”`.
> If you can suppress the warning with `as T?` (where T? is the type of
> the thing being warned on), you wouldn’t need a form that specifically
> printed “nil”, correct?

How many times do I need to repeat myself? I'm looking for a solution to
the problem where printing Optionals sanely (e.g. no "Optional(…)"
wrapper for .some values) is a PITA right now. Getting rid of the
warning does not solve this problem. This is why I like Nate Cook's idea
to enable `?? "nil"` in string interpolations, because it *does* solve
my problem. And with this tool, now the warning on printing Optionals
becomes useful because it tells me where to add `?? "nil"`. Getting rid
of the warning without the ability to add `?? "nil"` is not helpful to
me, because I don't want to print "Optional(…)".

-Kevin Ballard
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