[swift-evolution] Custom summary for Mirrors?

[Dave Abrahams]

on Tue Jan 05 2016, Austin Zheng via swift-evolution <swift-evolution-m3FHrko0VLzYtjvyW6yDsg-AT-public.gmane.org> wrote:

> Here are a couple of examples I had in mind.
>
> * Arrays (from test/1_stdlib/Runtime.swift:1348), dumping an array with 5
> elements:
>
> BEFORE:
> ▿ 5 elements
> - [0]: a.MultiPayloadTagBitsSmallNonGenericEnumWithDefaultMirror.MacWrite
> - [1]: a.MultiPayloadTagBitsSmallNonGenericEnumWithDefaultMirror.MacPaint
> - [2]: a.MultiPayloadTagBitsSmallNonGenericEnumWithDefaultMirror.FileMaker
> ▿ [3]: a.MultiPayloadTagBitsSmallNonGenericEnumWithDefaultMirror.ClarisWorks
>     - ClarisWorks: true
>        ▿ [4]:
> a.MultiPayloadTagBitsSmallNonGenericEnumWithDefaultMirror.HyperCard
>     - HyperCard: false
>
> AFTER:
> ▿ [a.MultiPayloadTagBitsSmallNonGenericEnumWithDefaultMirror.MacWrite,
> a.MultiPayloadTagBitsSmallNonGenericEnumWithDefaultMirror.MacPaint,
> a.MultiPayloadTagBitsSmallNonGenericEnumWithDefaultMirror.FileMaker,
> a.MultiPayloadTagBitsSmallNonGenericEnumWithDefaultMirror.ClarisWorks(true),
> a.MultiPayloadTagBitsSmallNonGenericEnumWithDefaultMirror.HyperCard(false)]
> - [0]: a.MultiPayloadTagBitsSmallNonGenericEnumWithDefaultMirror.MacWrite
> - [1]: a.MultiPayloadTagBitsSmallNonGenericEnumWithDefaultMirror.MacPaint
> - [2]: a.MultiPayloadTagBitsSmallNonGenericEnumWithDefaultMirror.FileMaker
> ▿ [3]:
> a.MultiPayloadTagBitsSmallNonGenericEnumWithDefaultMirror.ClarisWorks(true)
>     - ClarisWorks: true
>     ▿ [4]:
> a.MultiPayloadTagBitsSmallNonGenericEnumWithDefaultMirror.HyperCard(false)
>     - HyperCard: false
>
> * Dictionaries (from test/1_stdlib/ReflectionHashing.swift:43):
>
> BEFORE:
> ▿ 5 key/value pairs
>   ▿ [0]: (2 elements)
>     - .0: Four
>     - .1: 4
>   ▿ [1]: (2 elements)
>     ...
>
> AFTER:
> ▿ ["Four": 4, "One": 1, "Two": 2, "Five": 5, "Three": 3]
>   ▿ [0]: ("Four", 4)
>     - .0: "Four"
>     - .1: 4
>   ▿ [1]: ("One", 1)
>     ...
>
> * Dumping a CGRect (from test/1_stdlib/Reflection_objc.swift):
>
> BEFORE:
> (50.0, 60.0, 100.0, 150.0)
>
> AFTER:
> __C.CGRect(origin: __C.CGPoint(x: 50.0, y: 60.0), size: __C.CGSize(width:
> 100.0, height: 150.0))
>
> Let me know if you'd like more, although most are variants on the above.
>
> Best,
> Austin
>
> On Tue, Jan 5, 2016 at 5:37 PM, Dave Abrahams <dabrahams at apple.com> wrote:
>
>>
>> On Jan 5, 2016, at 5:28 PM, Austin Zheng via swift-evolution <
>> swift-evolution at swift.org> wrote:
>>
>> Hi Joe,
>>
>> I respect the choice of the team to use Custom[Debug]StringConvertible in
>> lieu of summary. At the same time, in my opinion the output of dump() has
>> become significantly more difficult to read (c.f. unit tests in
>> https://github.com/apple/swift/pull/838/files).
>>
>>
>> Specific examples of readability regressions, please?
>>
>> Would you and the team be open to exploring alternative solutions that
>> improve the readability of dump() without increasing API surface area?
>>
>>
>> Sure.
>>
>> For example, perhaps the reflection machinery itself should have special
>> handling for some of the built-in types. If not, I'll consider this
>> discussion thread complete.
>>
>> Thanks,
>> Austin
>>
>>
>>
>> On Tue, Jan 5, 2016 at 3:22 PM, Jordan Rose <jordan_rose at apple.com> wrote:
>>
>>> Getting custom summaries for the common CG types certainly seems
>>> reasonable. We'd have to get approval from the appropriate teams at Apple,
>>> but I can't see any objections.
>>>
>>> Jordan
>>>
>>>
>>> On Dec 30, 2015, at 9:55, Joe Groff via swift-evolution <
>>> swift-evolution at swift.org> wrote:
>>>
>>> I believe 'summary' is obsolete, and you're supposed to use
>>> Custom[Debug]StringConvertible to customize your type's reporting now.
>>>
>>> -Joe
>>>
>>> On Dec 29, 2015, at 10:38 PM, Austin Zheng via swift-evolution <
>>> swift-evolution at swift.org> wrote:
>>>
>>> Hi all,
>>>
>>> I'd like to gauge reaction for a proposal I was considering: adding to
>>> the standard library's Mirror type a 'summary' property, and the option to
>>> initialize a Mirror with a custom summary. If no custom summary is
>>> provided, the summary would default to the string produced by calling
>>> String(reflecting: subject) on the subject at the time of mirror creation.
>>>
>>> Some context: right now, there are two APIs for mirrors in the standard
>>> library: CustomReflectable, which is publicly exposed and relies on the
>>> conforming type creating a Mirror object, and _Reflectable, which relies on
>>> the conforming type having a companion type conforming to _MirrorType. A
>>> short-term goal is to migrate the standard library's types off the
>>> _Reflectable API and have them use the CustomReflectable API, and changing
>>> dump() accordingly.
>>>
>>> The extant implementation of dump() uses a property on _MirrorType called
>>> "summary". (This is where e.g. "4 elements" comes from when you dump() an
>>> array.) "summary" is absent from Mirror or any types related to
>>> CustomReflectable. I asked Joe Groff about this and the rationale was that
>>> it was deemed too similar to debugDescription (or String(reflecting: foo))
>>> to be worth carrying over.
>>>
>>> I would like to suggest that there might be a purpose for "summary":
>>>
>>> - Types with children, especially container types like arrays, often
>>> print out a description of their children as part of their debugDescription
>>> or description, redundant when using an API like dump() which provides a
>>> structural representation of the children of the subject. In such cases a
>>> lighter-weight description (like "3 elements") might be more appropriate to
>>> represent to the user.
>>>
>>> - Certain types like CGRect don't conform to CustomStringConvertible,
>>> CustomDebugStringConvertible, Streamable, etc. Having a custom summary for
>>> these types customized by the corresponding Mirror would allow for a
>>> 'pretty' representation during reflection in lieu of the ugly one generated
>>> by the runtime without making more substantial changes to the API which
>>> might break third-party code (such as conforming CGRect to any of the
>>> aforementioned protocols).
>>>
>>> I know that Mirror (and reflection as a whole) are being considered for
>>> major design changes, so this would be a minor transient change to make the
>>> API easier to work with in the meantime.
>>>
>>> Please let me know whether or not you think this proposed change is
>>> meaningful and worthwhile, or if you have any questions.
>>>
>>> Best,
>>> Austin

Hey, Austin,

Is this still something we need to discuss, or did it get resolved
somehow?

Thanks,
Dave