In trying to understand KeyPath I've been experimenting with this example:

let names: [String] = ["Chris", "Claire", "Finny"]
let optionalNames: [String?] = ["Chris", nil, "Claire"]

let mapped1 = names.compactMap { $0 }
let mapped2 = optionalNames.compactMap { $0 }
// let mapped3 = optionalNames.compactMap(\.self)

print(mapped1)
print(mapped2)
// print(mapped3)

This behaves just like I expect. But when I uncomment the line with let mapped3 = optionalNames.compactMap(\.self) I get the following issues which I'm not sure how to interpret:

error: Playground.playground:6:15: error: generic parameter 'ElementOfResult' could not be inferred let mapped3 = optionalNames.compactMap(.self)

Swift.Sequence:2:28: note: in call to function 'compactMap'

@inlinable public func compactMap<ElementOfResult>(_ transform: (Self.Element) throws -> ElementOfResult?) rethrows -> [ElementOfResult]

error: Playground.playground:6:40: error: cannot convert value of type 'WritableKeyPath<, >' to expected argument type '(String?) throws -> ElementOfResult?'

let mapped3 = optionalNames.compactMap(.self)

error: Playground.playground:6:40: error: cannot infer key path type from context; consider explicitly specifying a root type

let mapped3 = optionalNames.compactMap(.self)

My (apparently flawed) understanding of KeyPaths would suggest that mapped2 and mapped3 should be equivalent but apparently they're not.

If I try this

struct Person {
    let name: String
 }

let persons = [
    Person(name: "Mark"),
    Person(name: "Emma"),
]

let mapped4 = persons.map { $0.name }
let mapped5 = persons.map(\.name)

print(mapped4)
print(mapped5)

I get, as expected, the following output:

["Mark", "Emma"]

["Mark", "Emma"]

But if I replace the mapped5 definition with let mapped5 = persons.map(\.self) I get the error:

error: Playground.playground:18:23: error: no exact matches in call to instance method 'map'

let mapped5 = persons.map(.self)

Playground.playground:18:23: note: found candidate with type '((Person) throws -> ) throws -> Array<>'

let mapped5 = persons.map(.self)

It seems I'm misunderstanding what a keypath to \.self does here but I don't know how exactly. Could somebody clarify what's going on here?

Aaah, thank you! I thought I was crazy. 😅 Cheers, that explains a lot!