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

[Kevin Ballard]

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.

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"`.

-Kevin Ballard
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