[swift-evolution] 100% bikeshed topic: DictionaryLiteral

[Nate Cook]

Nate

> On Jan 9, 2018, at 12:12 AM, Chris Lattner via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
>> On Jan 8, 2018, at 4:29 PM, Ben Cohen via swift-evolution <swift-evolution at swift.org> wrote:
>> There exists in the standard library a type `DictionaryLiteral` that deserves naming re-consideration before we declare ABI Stability, because it’s confusingly misnamed, being neither a Dictionary (it doesn’t provide key-based lookup of values) nor a Literal. 
>> 
>> Instead, it’s just an immutable collection of key-value pairs you can create _from_ a literal.
> 
> Wow.  This is really gross, I didn’t know it existed :-)
> 
> Random question for you.  DictionaryLiteral has this doc comment:
> 
> /// You initialize a `DictionaryLiteral` instance using a Swift dictionary
> /// literal. Besides maintaining the order of the original dictionary literal,
> /// `DictionaryLiteral` also allows duplicates keys. For example:
> 
> why is maintaining duplicate keys a feature?
> 
> 
> It also has this one:
> 
> /// Some operations that are efficient on a dictionary are slower when using
> /// `DictionaryLiteral`. In particular, to find the value matching a key, you
> /// must search through every element of the collection. The call to
> /// `index(where:)` in the following example must traverse the whole
> /// collection to find the element that matches the predicate:
> 
> Since it is immutable, why not sort the keys in the initializer, allowing an efficient binary search to look up values?
> 
> 
>> I’m canvassing for opinions on what it ought to be called.  Some suggestions so far:
>> 
>> - `AssociationCollection`: Following the term of art from some other languages. Slightly obscure-sounding to developers not already familiar. Also “association” and “associative” are confusingly similar, which brings back the is-this-a-dictionary problem.
>> - `KeyValueCollection`: Problematic because key-value comes up in a totally different context in Cocoa.
>> - `PairCollection`: “Pair” is kinda nondescript.
>> - Do nothing. It’s not so bad.
>> 
>> The old name can live on indefinitely via a typealias (which has no ABI consequences, so could be retired at a later date once everyone has had plenty of time to address the deprecation warnings). Removing it as not carrying its weight (and instead using `[(Key,Value)]`, which is basically what it’s a wrapper for) is probably off the table for source stability reasons.
> 
> I’m not familiar with this type at all, so I apologize for the dumb question but… why was this added in the first place?  If it is the wrong thing, why not just deprecate it in Swift 5 and remove it in a future release?  That avoids it being an ABI concern, because we could make it be force inlined into any client code.

According to now-gone doc comments, it's intended to be used when providing the children for a custom mirror (that's the only API I know of that uses the type). The order of elements there is important, but given how scarcely used the type is, an array of tuples might be a reasonable (though source-breaking) substitute. 

> Finally, is anyone actually using this type?

In the standard library, we have this Mirror initializer: https://developer.apple.com/documentation/swift/mirror/1540408-init. If anyone is customizing their type's mirror, it's a good bet that they're calling that init and passing a literal that would break if the argument changed to an array. The translation should be mechanical, so maybe it could be a migrator fix? (I don't know enough about how that works to know.)

> 
> -Chris
> 
> 
> 
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20180109/0cc87e89/attachment-0001.html>