[swift-evolution] Pitch: Support for map and flatMap with smart key paths

[Tony Allevato]

+1, I really like this. It would also align nicely with the method type
flattening in SE-0042
<https://github.com/apple/swift-evolution/blob/master/proposals/0042-flatten-method-types.md>
(once it gets implemented), because passing keypaths (i.e., unbound
property references) and unbound parameterless method references to
map/flatMap would look nearly the same:

```
struct Person {
  let firstName: String
  let lastName: String
  func fullName() -> String { return "\(firstName) \(lastName)" }
}

let people: [Person]
let firstNames = people.map(\.firstName)
let fullNames = people.map(Person.fullName)  // because after SE-0042, this
will be (Person) -> String, not (Person) -> () -> String
```

Especially if there's a move in the future to also use \. to denote unbound
methods references, which was discussed during the keypath reviews. (Even
with that, I believe it would be more work though to get rid of the
explicit type name in the function case.)


On Wed, Jun 7, 2017 at 6:11 PM Xiaodi Wu via swift-evolution <
swift-evolution at swift.org> wrote:

> +1. Would think that all variants should exist on Optional too unless it
> would be harmful.
> On Wed, Jun 7, 2017 at 20:13 Michael J LeHew Jr via swift-evolution <
> swift-evolution at swift.org> wrote:
>
>> This is a great idea, and ought to be easy enough to bring forward!  +1
>> from me!
>>
>> -Michael
>>
>> > On Jun 7, 2017, at 11:18 AM, Matt Diephouse via swift-evolution <
>> swift-evolution at swift.org> wrote:
>> >
>> > 💯
>> >
>> >> On Jun 7, 2017, at 10:35 AM, Adam Sharp via swift-evolution <
>> swift-evolution at swift.org> wrote:
>> >>
>> >> The new smart key path feature is really lovely, and feels like a
>> great addition to Swift.
>> >>
>> >> It seems like it might be straightforward to add overloads of `map`
>> and `flatMap` to the standard library to make use of the new functionality:
>> >>
>> >>      let managers = flatOrganisation.managers
>> >>      let allEmployees = Set(managers.flatMap(\.directReports))
>> >>      let employeeNames = Set(allEmployees.map(\.name))
>> >>
>> >> This feels like a really natural way of working with key paths in a
>> functional style. It makes a lot of sense for collections, and possibly for
>> Optional too (although as far as I can see optional chaining is more or
>> less equivalent, and with more compact syntax).
>> >>
>> >> I’m hoping that this might be low-hanging fruit that could be
>> considered for the Swift 4 release. I’d be happy to have a go at writing a
>> proposal if there’s interest!
>> >>
>> >> –Adam
>> >>
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