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[SR-12255] Mirror behaviour is unspecified and inconsistent for enums with zero-size or no associated values #54682

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swift-ci opened this issue Feb 22, 2020 · 3 comments
Closed

[SR-12255] Mirror behaviour is unspecified and inconsistent for enums with zero-size or no associated values #54682

swift-ci opened this issue Feb 22, 2020 · 3 comments
Labels
bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. standard library Area: Standard library umbrella

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@swift-ci
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Previous ID SR-12255
Radar None
Original Reporter SAJim (JIRA User)
Type Bug
Status Resolved
Resolution Done
Environment

Xcode 11.3.1, running in a Swift Playground. (Not sure if this runs Swift 5.1 / 5.2)

Additional Detail from JIRA
Votes 0
Component/s Standard Library
Labels Bug
Assignee None
Priority Medium

md5: 31b5e4ee726e4a8090abd3eff30a75e5

Issue Description:

My expected behaviour, based on the existing results for non-zero-size associated values, would be that Mirror maps cases with associated values to a tuple of the case name and any associated values. If the enum has no associated values, it's not obvious what the result should be, but I'd most expect a plain String case name.

The ideal behaviour, on the other hand, is subject to interpretation of how we should compromise with SE-0155 per the decision not to fully implement it. Under this proposal as written, I would have expected labels to be available as Strings in the reflected representation.

The current state of affairs is illustrated by this code sample:

struct NonEmpty {
    let value = ""
}
struct Empty {}

enum MyEnum {
    case nonEmpty(NonEmpty)
    case duplicateNonEmpty(NonEmpty, s: NonEmpty)
    case empty(Empty)
    case duplicateEmpty(Empty, s: Empty)
    case paddedEmpty(Empty, NonEmpty)
    case noAssociatedValue
}

Mirror(reflecting: MyEnum.nonEmpty(NonEmpty())).children.first
// (label: "nonEmpty", value: NonEmpty(""))

Mirror(reflecting: MyEnum.duplicateNonEmpty(NonEmpty(), s: NonEmpty())).children.first
// (label: "duplicateNonEmpty", (NonEmpty(""), s: NonEmpty("")))

Mirror(reflecting: MyEnum.empty(Empty())).children.first
// nil

Mirror(reflecting: MyEnum.duplicateEmpty(Empty(), s: Empty())).children.first
// nil

Mirror(reflecting: MyEnum.paddedEmpty(Empty(), NonEmpty())).children.first
// (label: "paddedEmpty", (Empty(), NonEmpty("")))

Mirror(reflecting: MyEnum.noAssociatedValue).children.first
// nil

SR-6921 is partly related, referring to the lack of reflection information for enum cases without associated values.

SR-10272 appears to be an alternative example of an associated value of zero size.
@swift-ci
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Comment by James Froggatt (JIRA)

This issue came up while using EnumKit to access enum associated values via subscripts: gringoireDM/EnumKit#4

@theblixguy
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Related: SR-12044 which was recently fixed. Can you try with the latest development snapshot?

@swift-ci
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Comment by James Froggatt (JIRA)

The fix works for me as described - label-value pairs are printed for enum cases with zero-size associated values, which fixes SR-10272 and solves my use-case. That's a big win!

As I understand it, enum cases still can't be properly enumerated/compared without making Strings available for both argument labels, and names of cases with no associated values. I imagine a solution would benefit use-cases such as EnumKit and Case Paths. However, preserving argument labels as strings is probably more in the realm of a Swift Evolution proposal. If others agree I'm happy for this ticket to be closed, and SR-6921 can continue to track the no-associated-values behaviour.

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
This issue was closed.
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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. standard library Area: Standard library umbrella
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