[swift-evolution] [Idea] Allowing definition of custom behaviors for `as` casting

[Tony Allevato]

What benefit would this provide that writing them as initializers already
doesn't? Writing the first two lines this way seems much clearer from a
call-site point of view:

let user1 = User(json: ["firstname": "Yaman", "lastname": "JAIOUCH"])
let user2 = User(string: "Yaman JAIOUCH")

That aside, placing the "as" methods on the destination type makes it look
to me like you're allowing casts *from* User *to* AnyObject/String and not
the other way around.

On Tue, Jun 21, 2016 at 8:27 AM Yaman JAIOUCH via swift-evolution <
swift-evolution at swift.org> wrote:

> What you think about having the possibility to define custom behavior
> while casting with `as` keyword ?
> It could be quite handy to use while keeping a good level of readability.
>
> Something like this :
>
> struct User {
>     let firstname: String
>     let lastname: String
>
>     as(json: AnyObject) -> User? {
>         guard let json = json as? [String: AnyObject], let firstname =
> json["firstname"] as? String, let lastname = json["lastname"] else {
>             return nil
>         }
>
>         return User(firstname: firstname, lastname: lastname)
>     }
>
>     as(string: String) -> User? {
>         let components = string.componentsSeparatedByString(" ")
>         guard components.count == 2 else { return nil }
>
>         return User(firstname: components[0], lastname: components[1])
>     }
> }
>
> let user1 = ["firstname": "Yaman", "lastname": "JAIOUCH"] as? User //
> returns valid user
> let user2 = "Yaman JAIOUCH" as? User // returns valid user
> let user3 = 24 as? User // returns nil
> let user4 = NSDate() as! User // crash as usual
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