[swift-evolution] Pitch: Import Objective-C Constants as Enums

[Jordan Rose]

> On Jan 17, 2016, at 22:10 , Douglas Gregor via swift-evolution <swift-evolution at swift.org> wrote:
> 
>> 
>> On Jan 17, 2016, at 7:13 PM, Jeff Kelley via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> A lot of Cocoa APIs have long lists of constant values, typically NSStrings. I’d like to pitch a way to import them as enums with associated types. I can write up a full proposal if people think this is a good idea, but here’s my thinking:
>> 
>> Let’s take the error domains in NSError.h for a quick example. These entries in the header:
>> 
>>> FOUNDATION_EXPORT NSString *const NSCocoaErrorDomain;
>>> FOUNDATION_EXPORT NSString *const NSPOSIXErrorDomain;
>>> FOUNDATION_EXPORT NSString *const NSOSStatusErrorDomain;
>>> FOUNDATION_EXPORT NSString *const NSMachErrorDomain;
>> 
>> 
>> turn into this in the Swift interface:
>> 
>>> public let NSCocoaErrorDomain: String
>>> public let NSPOSIXErrorDomain: String
>>> public let NSOSStatusErrorDomain: String
>>> public let NSMachErrorDomain: String
>> 
>> What I’m proposing is a way to import those as an enum instead. Similar to how we mark sections of Objective-C code with NS_ASSUME_NONNULL_BEGIN, we could mark it with something like NS_CASE_LIST_BEGIN. Then, this code:
>> 
>>> NS_CASE_LIST_BEGIN;
>>> 
>>> FOUNDATION_EXPORT NSString *const NSCocoaErrorDomain;
>>> FOUNDATION_EXPORT NSString *const NSPOSIXErrorDomain;
>>> FOUNDATION_EXPORT NSString *const NSOSStatusErrorDomain;
>>> FOUNDATION_EXPORT NSString *const NSMachErrorDomain;
>>> 
>>> NS_CASE_LIST_END;
>> 
>> would be imported as follows:
>> 
>>> enum ErrorDomain : String {
>>>     case Cocoa
>>>     case POSIX
>>>     case OSStatus
>>>     case Mach
>>> }
>> 
>> I can think of a lot of areas in Cocoa where these APIs could make things much more type-safe in Swift. Is this a good idea? Would people use this?
> 
> FWIW, this has come up a number of times in discussions among Swift developers (although not, IIRC, on swift-evolution). Our current favored way to write this in (Objective-)C would be with a new typedef of NSString * that has some special attribute on it, e.g.,
> 
>   typedef NSString * NSErrorDomain __attribute__((enum(string)));
> 
>   FOUNDATION_EXPORT NSErrorDomain const NSCocoaErrorDomain;
>   FOUNDATION_EXPORT NSErrorDomain const NSPOSIXErrorDomain;
>   FOUNDATION_EXPORT NSErrorDomain const NSOSStatusErrorDomain;
>   FOUNDATION_EXPORT NSErrorDomain const NSMachErrorDomain;
> 
> The typedef would import as a String-backed enum and all of the string constants declared with that typedef within the same module as the typedef would become cases of that enum. String constants declared with that typedef in a *different* module would become “static lets” within extensions of the String-backed enum.
> 
> Call that a +1 from me on your idea :)

I still have reservations about this:

- Most of these strings are not things you switch on, making the enum-ness not particularly interesting. I'd be happier with a RawRepresentable struct.
- Our current prefix-stripping logic relies on being able to see all the cases. You can do this with an enum because they're all declared in one block, but string constants they may cross multiple files. This isn't impossible to deal with, but I think it's counterintuitive. (An alternative would be to only use the type name for stripping each value individually.)
- I'm not sure where the line is between "an open set of choices represented as strings" and "strings with some defaults defined". (Very few of these are truly closed sets; if we add a new UIFont attribute next year, it should prefix-strip like everything else.)
- As a nitpick, I think a typedef for 'NSString' rather than 'NSString *' would be preferred, so that the '*' still shows up in the declaration in Objective-C. This is just a feeling though, and maybe it's just clinging to the way things are done now.

Jordan
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