[swift-evolution] Custom summary for Mirrors?

Joe Groff jgroff at apple.com
Wed Dec 30 11:55:22 CST 2015

I believe 'summary' is obsolete, and you're supposed to use Custom[Debug]StringConvertible to customize your type's reporting now.


> 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
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