[swift-evolution] [Proposal draft] Disallow Optionals in String Interpolation Segments
Harlan Haskins
harlan at harlanhaskins.com
Mon Oct 3 13:00:16 CDT 2016
Unfortunately, Optional-to-Any does not currently hit this case because IIRC it doesn't promote to Any in an interpolation segment. I tested this with a ToT build yesterday.
- Harlan
> On Oct 3, 2016, at 1:57 PM, Joe Groff <jgroff at apple.com> wrote:
>
> We now emit a warning whenever an optional is used as an Any. I disagree that this should be an error, but it seems reasonable to warn (if we don't already thanks to the 'Any' warning).
>
> -Joe
>
>> On Oct 3, 2016, at 10:52 AM, Harlan Haskins via swift-evolution <swift-evolution at swift.org> wrote:
>>
>> Hey all,
>>
>> Julio Carrettoni, Robert Widmann, and I have been working on a proposal to mitigate something that's burned us all since Swift 1. We'd love some feedback!
>>
>> It's available here: https://gist.github.com/harlanhaskins/63b7343e7fe4e5f4c6cfbe9413a98fdd
>>
>> I've posted the current draft below.
>>
>> Thanks,
>> Harlan Haskins
>>
>> Disallow Optionals in String Interpolation Segments
>>
>> Proposal: SE-NNNN
>> Authors: Harlan Haskins, Julio Carrettoni, Robert Widmann
>> Review Manager: TBD
>> Status: Awaiting revie
>> Introduction
>>
>> Swift developers frequently use string interpolation as a convenient, concise syntax for interweaving variable values with strings. The interpolation machinery, however, has surprising behavior in one specific case: Optional<T>. If a user puts an optional value into a string interpolation segment, it will insert either "Optional("value")" or "nil" in the resulting string. Neither of these is particularly desirable, so we propose a warning and fix-it to surface solutions to these potential mistakes.
>>
>> Swift-evolution thread: Discussion thread topic for that proposal
>>
>> Motivation
>>
>> The Swift Programming Language defines string interpolation segments as "a way to construct a new String value from a mix of constants, variables, literals, and expressions". There is one type that runs counter to this definition: Optional. The .none case in particular is used to indicate the absence of a value. Moreover, its inclusion in interpolation segments leads to the dreaded "nil" in output that is often fed to UI elements. Even barring that, interpolating a non-nil optional value yields "Optional("value")", a result that is not useful even in logged output.
>>
>> Given that the Optional type is never fit for display to the end user, and can often be a surprising find in the console, we propose that requesting an Optional's debug description be an explicit act. This proposal now requires a warning when using an expression of Optional type within a string interpolation segment.
>>
>> Proposed solution
>>
>> The user will be warned after attempting to use an expression with type Optional<T> in a string interpolation segment. They will then be offered a fixit suggesting they explicitly request the debugDescription of the Optional value instead.
>>
>> Detailed design
>>
>> Semantic analysis currently does not do much but guarantee the well-formedness of expressions in interpolation segments. These are then fed directly to String.init(stringInterpolationSegment:) and are run through the runtime reflection system to generate a description. Semantic analysis will be tweaked to inspect the result of solving an interpolation segment for an Optional and will offer a fixit in that case.
>>
>> Impact on existing code
>>
>> As this is a warning, code written before this proposal will continue to compile and run with the same semantics as before. Authors of code that makes use of this unsafe pattern will be offered a migration path to the safer, more explicit form.
>>
>> Alternatives considered
>>
>> A fixit that suggests a default value be inserted would be entirely appropriate (following the style of the fixit introduced in SE-0140).
>>
>> Forbidding this pattern by hard error would make this proposal a breaking change that is out of scope for this stage of Swift's development.
>>
>> A fixit that introduces a force-unwrapping would technically work as well, however it would be fixing a dangerous operation with yet another dangerous operation.
>>
>>
>>
>> Sent from my iPad
>> _______________________________________________
>> swift-evolution mailing list
>> swift-evolution at swift.org
>> https://lists.swift.org/mailman/listinfo/swift-evolution
>
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