> On Jan 8, 2018, at 3:23 PM, Gregg Wonderly via swift-evolution
> <swift-evolution@swift.org> wrote:
>
> But why wouldn’t the key also be available so that you might be able to have
> selective handling of first vs last for each key?
Having only the values allows you to do some nice higher-order things, like:
let histogram = Dictionary(values.map { ($0, 1) }, uniquingValuesWith:
+)
In other words, the same functions you could use with `reduce` can be used with
`init(_:uniquingValuesWith:)`. If the function took a third parameter, we would
lose this property.
There's a bunch of very reasonable things you can't do with
`Dictionary(_:uniquingValuesWith:)`; for instance, you can't cause the
initializer to fail by returning `nil`, only by throwing an error. Supporting
these features would either complicate cases where you don't need them, or
require us to provide another overload. Given that
`Dictionary(_:uniquingValuesWith:)` and friends are just conveniences, and
reimplementing them requires only a few lines of straightforward code, it's
just not worth supporting every plausible variant.
(For what it's worth, the Swift Evolution proposal which added this
initializer, [SE-0165 Dictionary & Set
Enhancements](https://github.com/apple/swift-evolution/blob/master/proposals/0165-dict.md),
touches on some of these issues. A big part of a proposal's role is to
document the reasons for Swift's designs; if you ever wonder why something in
Swift is designed the way it is, the feature's evolution proposal is a good
place to start.)
--
Brent Royal-Gordon
Architechies
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