[swift-users] Cast CFString to String

[Dennis Weissmann]

I found a related bug here: https://bugs.swift.org/browse/SR-6204

> On Dec 5, 2017, at 10:10 AM, Dennis Weissmann via swift-users <swift-users at swift.org> wrote:
> 
> Hi swift-users,
> 
> I have found another weird behavior (IMO) and wanted to ask for the right way to handle this:
> 
> Imagine I want to switch over a Swift string which contains a UTI to map that UTI to an enum.
> 
> (A playground is attached for you to easily reproduce, this is tested with Xcode 9.1's included toolchain, also happens in projects)
> 
> I would expect the following to work:
> 
> import MobileCoreServices
> 
> enum MimeType {
> 
>     case text
>     case unknown
> 
>     public init(uti: String) {
>         // Source: https://developer.apple.com/library/content/documentation/Miscellaneous/Reference/UTIRef/Articles/System-DeclaredUniformTypeIdentifiers.html <https://developer.apple.com/library/content/documentation/Miscellaneous/Reference/UTIRef/Articles/System-DeclaredUniformTypeIdentifiers.html>
>         switch uti.lowercased() {
>         case kUTTypeText as String:
>             self = .text
>         default:
>             self = .unknown
>         }
>     }
> 
> }
> 
> The error I get here is
> 
> warning: UTITest.playground:8:26: warning: 'as' test is always true
>         case kUTTypeText as String:
>                          ^
> 
> error: UTITest.playground:8:14: error: expression pattern of type 'CFString' cannot match values of type 'String'
>         case kUTTypeText as String:
>              ^~~~~~~~~~~
>              ^~~~~~~~~~~
> The only way I found to resolve this is to also import Foundation (which makes sense but is not really obvious).
> 
> Alright, that gives me this:
> 
> import MobileCoreServices
> import Foundation
> 
> enum MimeType {
> 
>     case text
>     case unknown
> 
>     public init(uti: String) {
>         // Source: https://developer.apple.com/library/content/documentation/Miscellaneous/Reference/UTIRef/Articles/System-DeclaredUniformTypeIdentifiers.html <https://developer.apple.com/library/content/documentation/Miscellaneous/Reference/UTIRef/Articles/System-DeclaredUniformTypeIdentifiers.html>
>         switch uti.lowercased() {
>         case kUTTypeText as String:
>             self = .text
>         default:
>             self = .unknown
>         }
>     }
> 
> }
> 
> warning: UTITest.playground:8:26: warning: 'as' test is always true
>         case kUTTypeText as String:
>                          ^
> 
> error: UTITest.playground:8:14: error: 'CFString' is not implicitly convertible to 'String'; did you mean to use 'as' to explicitly convert?
>         case kUTTypeText as String:
>              ^
>                          as String
> 
> 
> Uhm, okay? So let's do that:
> 
> enum MimeType {
> 
>     case text
>     case unknown
> 
>     public init(uti: String) {
>         // Source: https://developer.apple.com/library/content/documentation/Miscellaneous/Reference/UTIRef/Articles/System-DeclaredUniformTypeIdentifiers.html <https://developer.apple.com/library/content/documentation/Miscellaneous/Reference/UTIRef/Articles/System-DeclaredUniformTypeIdentifiers.html>
>         switch uti.lowercased() {
>         case kUTTypeText as String as String:
>             self = .text
>         default:
>             self = .unknown
>         }
>     }
> 
> }
> 
> As weird as it looks, it works ... My question is: Is this behavior intended?
> 
> Thanks!
> 
> - Dennis
> <UTITest.playground>
> _______________________________________________
> swift-users mailing list
> swift-users at swift.org
> https://lists.swift.org/mailman/listinfo/swift-users

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