[swift-users] Unsubscribe

[Jordan Rose]

Oops, thought the mailing list software was supposed to catch this. Unsubscribed you, Howard; sorry for the spurious message, everyone else.

-Jordan


> On Jul 14, 2017, at 10:03, Howard Perlman via swift-users <swift-users at swift.org> wrote:
> 
> Unsubscribe
> 
> 
> From: "swift-users-request at swift.org" <swift-users-request at swift.org>
> To: swift-users at swift.org 
> Sent: Friday, July 14, 2017 12:57 PM
> Subject: swift-users Digest, Vol 20, Issue 13
> 
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> Today's Topics:
> 
>   1. Swift 4 emulating Decoder behaviour (Joanna Carter)
> 
> 
> ----------------------------------------------------------------------
> 
> Message: 1
> Date: Thu, 13 Jul 2017 20:46:20 +0200
> From: Joanna Carter <joanna at carterconsulting.org.uk <mailto:joanna at carterconsulting.org.uk>>
> To: swift-users at swift.org <mailto:swift-users at swift.org>
> Subject: [swift-users] Swift 4 emulating Decoder behaviour
> Message-ID:
>     <A8DE64E2-5360-4B35-87D2-69F3F5B562E7 at carterconsulting.org.uk <mailto:A8DE64E2-5360-4B35-87D2-69F3F5B562E7 at carterconsulting.org.uk>>
> Content-Type: text/plain; charset=utf-8
> 
> Greetings
> 
> I notice that, with Swift 4, I can decode an object like this :
> 
>     • let retrievedSpot = try decoder.decode(ParkingSpot.self, from: retrievedData)  
> 
> And that will return a ParkingSpot, as expected, into retrievedSpot.
> 
> However, I thought I would try the same technique with one of my pet projects…
> 
> I have a protocol  and an implementing struct :
> 
>     • public protocol PropertyProtocol  
>     • {  
>     •  static var propertyType: Any.Type { get }  
>     •  
>     •  var untypedValue: Any? { get }  
>     • }  
>     •  
>     • public struct Property<valueT : DefaultValueProvider> : PropertyProtocol  
>     • {  
>     •  public static var propertyType: Any.Type  
>     •  {  
>     •    return valueT.self  
>     •  }  
>     •  
>     •  public var untypedValue: Any?  
>     •  {  
>     •    return value  
>     •  }  
>     •  
>     •  public var value = valueT()  
>     • }  
> 
> Now, I want to be able to use a "factory" method to create an instance of Property<T>, bound to its parameter type. So, I followed the same principal as Decoder :
> 
>     • struct PropertyFactory  
>     • {  
>     •  static func createProperty<T>(_ type: T.Type) -> PropertyProtocol where T : DefaultValueProvider  
>     •  {  
>     •    return type.createProperty()  
>     •  }  
>     • }  
> 
> DefaultValueProvider is defined as follows and String is extended to conform to it :
> 
>     • public protocol DefaultValueProvider  
>     • {  
>     •  init()  
>     • }  
>     •  
>     • extension String : DefaultValueProvider { }  
> 
> Now, this works fine if I pass a known type :
> 
>     • let pproperty = PropertyFactory.createProperty(String.self)  
> 
> But, if I hold the type to be passed in in an Any.Type or DefaultValueProvider.Type variable, then doing this :
> 
>     •    let type: Any.Type = String.self  
>     •  
>     •    if let propertyType = type as? DefaultValueProvider.Type  
>     •    {  
>     •      let p = PropertyFactory.createProperty(propertyType)  
>     •      
>     •      …  
> 
> Fails to compile with the message : Cannot invoke 'createProperty' with an argument list of type '(DefaultValueProvider.Type)'
> 
> Likewise Decoder.decode(…) will not accept storing the type in an Any.Type or Decodable.Type variable.
> 
> I find this odd and perplexing. Is this a known issue or has nobody yet realised that this could be useful ?
> 
> Joanna
> 
> --
> Joanna Carter
> Carter Consulting
> 
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> End of swift-users Digest, Vol 20, Issue 13
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