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

[Karl Wagner]

> On 8. Jun 2017, at 04:58, Tony Allevato via swift-evolution <swift-evolution at swift.org> 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 <swift-evolution at swift.org <mailto: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 <mailto: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 <mailto:swift-evolution at swift.org>> wrote:
> >
> > 💯
> >
> >> On Jun 7, 2017, at 10:35 AM, Adam Sharp via swift-evolution <swift-evolution at swift.org <mailto: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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