[swift-evolution] Proposal: Deprecate optionals in string interpolation

[Charlie Monroe]

Hi Nicola, thanks for the feedback.

> I assume you mean "at runtime" here?

That's probably my wrong wording. :) The unexpected result is at runtime, but we'd like to catch this at compile time. I've updated it to say

"in order to prevent unexpected results already at compile time" - hopefully that's a clearer wording.

> I think the detailed design needs  some more thought. The "Uninterpolable"
> protocol, and suggesting to cast "as Any" in a Fix-it both seem hacks.

Originally, the proposal was simply to emit a warning for interpolation of Optionals, but several people made good points:

- there may be other types you may not want to use for interpolation - as mentioned in the proposal, e.g. private data structures that would expose something you want to keep private, various enum values, etc. Which is why I've started thinking about making a protocol that would indicate the type is discouraged being "interpoled". I've thought about this and decided to make a more robust and customizable solution.

An alternative to this would be to use annotations or just drop this customizability completely. But I think with Swift and its protocol-driven development, marking the type with this protocol is the most robust way to go.

- both .description and .debugDescription are mentioned in alternatives for the Fix-It. 

"as Any" seemed, however, the cleanest and most robust solution to me, since then the Uninterpolable protocol can be applied to any type without forcing the type to conform to CustomStringConvertible as well. I agree that it's kind of a hack, though.

> I'm not even sure if the general direction of a compile time warning is the
> right one, and if the problem wouldn't be better solved by simply not making
> Optional put "Optional()" around the value in its .description.

There are many people oposing this and expecting the Optional() wrap around the value, indicating the actual type. Actually, including me - I agree it can be useful for some types of debugging since in what you wrote further, there'd be no difference between description of [1, 2, 3] (i.e. [Int]) and Optional([1, 2, 3]) (i.e. [Int]?).

There are legitimate usecases where the current behavior is correct, but in most of cases, having an optional in string interpolation will lead either to unnecessary clutter in the log/console or bugs - in which case even "nil" is not correct to be used for the interpolation. Which is the basis for this proposal.

> print("\(o)")               // "Optional(1)", why??

String has several overloads for the init(stringInterpolationSegment:) initiailizer. Optional falls into the generic <T> category, which will call String(optional) - which most likely uses debugDescription.