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

Harlan Haskins harlan at harlanhaskins.com
Mon Oct 3 12:52:39 CDT 2016

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.

Harlan Haskins

Disallow Optionals in String Interpolation Segments
Proposal: SE-NNNN
Authors: Harlan Haskins, Julio Carrettoni, Robert Widmann
Review Manager: TBD
Status: Awaiting revie

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


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
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