> On 8. Jun 2017, at 04:58, Tony Allevato via swift-evolution
> <[email protected]> wrote:
>
> +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.)
>
>
When it comes to unbound method references, personally, I would love to see us
ditch currying in some future version of Swift and move to full-blown partial
application instead. We would need variadic generics if we wanted to expose
them as nicely-typed objects as we do with KeyPaths.
Anyway, I think what you want is something like this (where VirtualKeyPath is a
custom subclass of KeyPath which is lazily-evaluated using a closure).
extension KeyPath where Value: Collection {
func map<T>(_ descendent: KeyPath<Value, T>) -> VirtualKeyPath<Root, [T]> {
return VirtualKeyPath<Root, [T]> {
(obj: Root) -> [T] in obj[keypath: self].map { $0[keypath:
descendent] }
}
}
}
\Department.people.map(\.fullName).characters.count // type:
VirtualKeyPath<Department, [Int]>
Custom subclasses of KeyPath are not allowed, so you can’t actually do this. I
don’t know, maybe it wouldn’t be much overhead to add the one, closure-based
VirtualKeyPath — clearly the architecture is meant to be flexible. Maybe it’s
better to wait until Swift 5 for that, though.
- Karl
> On Wed, Jun 7, 2017 at 6:11 PM Xiaodi Wu via swift-evolution
> <[email protected] <mailto:[email protected]>> 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
> <[email protected] <mailto:[email protected]>> 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
> > <[email protected] <mailto:[email protected]>> wrote:
> >
> > 💯
> >
> >> On Jun 7, 2017, at 10:35 AM, Adam Sharp via swift-evolution
> >> <[email protected] <mailto:[email protected]>> 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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