[swift-users] Making functions generic on optionals

[Kenny Leung]

Hi Jordan.

Thanks for the response.

> On Feb 5, 2016, at 2:34 PM, Jordan Rose <jordan_rose at apple.com> wrote:
> 
> The smallest way to write this is at the call site:
> 
> sortStringOpt.map { NSSortDescriptor.sortDescriptorsFromString($0) }

Sorry, but I don’t understand this solution. What type is sortStringOpt?

> But really I'm a little curious about why you're thinking about "every time [you] want to do this". Does this really come up that often? In this particular case, why would you ever not have a sort string?

In particular, sortString might come from user input, so it may be nil.

In general,  “every time I want to do this” was referring to providing a piece of API that could take optional or non-optional inputs without either burdening the provider with writing a stub method or the client with having to always deal with an optional return from the method.

-Kenny


> Jordan
> 
> 
>> On Feb 5, 2016, at 13:53, Kenny Leung via swift-users <swift-users at swift.org> wrote:
>> 
>> Hi All.
>> 
>> In the code below, I have the method
>> 
>> public class func sortDescriptorsFromString(sortString :String) throws -> [NSSortDescriptor]
>> 
>> In order to make life easier, I would like to make sortString optional, but then I would have to make the return type optional to to be able to return nil if the argument is nil. To get around this, I’ve added a stub method
>> 
>> public class func sortDescriptorsFromString(sortString :String?) throws -> [NSSortDescriptor]?
>> 
>> that declares the argument to be optional and the return type to be optional.
>> 
>> I get the feeling that there is a way to not have to write a stub method every time I want to do this. I also get the feeling that I should be able to accomplish it through generics. But I do not know how to write the declaration for such a method. Can anyone help?
>> 
>> Thanks!
>> 
>> -Kenny
>> 
>> 
>> extension NSSortDescriptor {
>> 
>>     public class func sortDescriptorsFromString(sortString :String) throws -> [NSSortDescriptor] {
>>         var descriptors = [NSSortDescriptor]()
>>         let components = sortString.split("[, ]+")
>> 
>>         for i in 0.stride(to: components.count, by: 2) {
>>             let key = components[i]
>>             let direction = components[i + 1]
>>             var descriptor :NSSortDescriptor?
>> 
>>             if SORT_STRINGS_ASCENDING.contains(direction) {
>>                 descriptor = NSSortDescriptor(key: key, ascending: true)
>>             } else if SORT_STRINGS_DESCENDING.contains(direction) {
>>                 descriptor = NSSortDescriptor(key: key, ascending: false)
>>             } else if SORT_STRINGS_CASEINSENSITIVE_ASCENDING.contains(direction) {
>>                 descriptor = NSSortDescriptor(key: key, ascending: true, selector: "caseInsensitiveCompare:");
>>             } else if SORT_STRINGS_CASEINSENSITIVE_DESCENDING.contains(direction) {
>>                 descriptor = NSSortDescriptor(key: key, ascending: false, selector: "caseInsensitiveCompare:");
>>             } else if SORT_DATE_ASCENDING.contains(direction) {
>>                 descriptor = NSSortDescriptor(key: key, ascending: true, selector: "compare:");
>>             } else if SORT_DATE_DESCENDING.contains(direction) {
>>                 descriptor = NSSortDescriptor(key: key, ascending: false, selector: "compare:");
>>             }
>> 
>>             if let nnDescriptor = descriptor {
>>                 descriptors.append(nnDescriptor)
>>             } else {
>>                 throw NSSortDescriptorError.UnsupportedSortDirection
>>             }
>>         }
>> 
>>         return descriptors
>>     }
>> 
>>     public class func sortDescriptorsFromString(sortString :String?) throws -> [NSSortDescriptor]? {
>>         guard let sortString = sortString else {return nil}
>>         return try self.sortDescriptorsFromString(sortString)
>>     }
>> 
>> }
>> 
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