[swift-evolution] Will these two features be included in Swift 3?

[krzysztof@siejkowski.net]

True. DoubleProtocol introduces a layer of indirection. 

This might be a disadvantage, because it requires unnecessary verbosity of the casting to original type. It also might be useful, because it allows you to pick and choose only the subset of functionalities and ensure the method of Array extension won’t have an access to all the abilities the type provides. It’s basically a mask for the interface of original type.

protocol DoubleType {
    var isFinite: Bool { get }
}

extension Double : DoubleType {
    var isFinite: Bool { get {
        return self.isFinite
    }}
}

extension Array where Element : DoubleType {
    func onlyFiniteAllowed() -> [Element] {
        return self.filter { $0.isFinite } // I know nothing about $0 but if it’s finite
    }
}

Anyway, it looks like plain’n’simple extension Array Element == Double is already on its way, thanks to Simon and Jakob :)



-----Original Message-----
From: Alex Lew <alexl.mail+swift at gmail.com>
Reply: Alex Lew <alexl.mail+swift at gmail.com>
Date: December 8, 2015 at 11:19:41 PM
To: tuuranton at tutanota.de <tuuranton at tutanota.de>
CC: Krzysztof Siejkowski <krzysztof at siejkowski.net>, Swift Evolution <swift-evolution at swift.org>
Subject:  Re: [swift-evolution] Will these two features be included in Swift 3?

> I think the pattern here is
>  
> protocol DoubleProtocol {
> var doubleValue: Double { get }
> }
> extension Double: DoubleProtocol {
> var doubleValue: Double { return self }
> }
> extension Array where Element: DoubleProtocol {
> // in these methods, use the element's .doubleValue property
> // to get the actual double, on which you can then call Double's
> methods
> }
>  
> That way, if another type wants to conform to DoubleProtocol, it needs to
> be able to represent itself as a Double.
>  
> -Alex
>  
> On Tue, Dec 8, 2015 at 4:26 PM, Tuur Anton via swift-evolution <
> swift-evolution at swift.org> wrote:
>  
> > Actually, I just realized that your solution isn't at all what I'm after.
> >
> > Doing "extension Array" is not the same thing and wouldn't be just
> > a "syntactic sugar" for the code you came up with.
> >
> > Why? Because in your code, the extension method only knows that the
> > array's Element is a DoubleProtocol. It doesn't know it's a *Double* (which
> > is what I want it to be). As such, it won't have access to Double's methods
> > and such.
> >
> > This is probably okay for Doubles, but for other types, especially my own
> > custom types with their own methods, the difference is huge. "extension
> > Array" is what I want, but your solution won't help me if the
> > extension method needs access to MyStructType's methods.
> >
> > See what I mean, or am I missing something here?
> >
> > --
> > Securely sent with Tutanota. Claim your encrypted mailbox today!
> > https://tutanota.com
> >
> > 7. Dec 2015 13:14 by krzysztof at siejkowski.net:
> >
> > Concerning extension constraining, it’s already doable with:
> >
> > ```
> > protocol DoubleProtocol {}
> >
> > extension Double : DoubleProtocol {}
> >
> > extension Array where Element : DoubleProtocol {
> > func onlyForDoubles() -> String {
> > return "hello doubles!"
> > }
> > }
> >
> > [1.2].onlyForDoubles() // „hello doubles!”
> > ["a"].onlyForDoubles() // error: type of expression is ambiguous without
> > more context
> > ```
> >
> > However, I personally like the idea of making a syntactic sugar for that
> > case.
> >
> > All the best,
> > Krzysztof
> >
> >
> > On 7 December 2015 at 13:01:11, Krzysztof Siejkowski (
> > krzysztof at siejkowski.net) wrote:
> >
> > Concerning generic typealiases, the topic is already being discussed in
> > „Generic `typealias`s” thread:
> > https://lists.swift.org/pipermail/swift-evolution/2015-December/000132.html.  
> > The core Swift team approves:
> >
> > > Yes, this is definitely something that I (at least) would like to see.
> > Patches welcome :-)
> > > Chris (Lattner)
> >
> > All the best,
> > Krzysztof
> >
> > On 7 December 2015 at 12:41:05, Tuur Anton via swift-evolution (
> > swift-evolution at swift.org) wrote:
> >
> > Can you please add these features in Swift 3?
> >
> > 1. The ability to do this:
> > extension Array {
> > //extend arrays of doubles
> > }
> >
> > 2. Generic typealiases:
> > struct Foo {
> > let t: T
> > let v: V
> > }
> > typealias IntFoo = Foo //Error in Swift 2.1
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