[swift-evolution] Proposal: Protocol methods should accept default arguments

[Ben]

Thank you Felix and Sean for the advice. I am happy to cautiously withdraw my suggestion, since this is a mostly acceptable workaround. The only hiccup appears to be with where inline documentation should be placed to have it appear in Xcode (in the extension, not the original protocol). I will think further and perhaps file a radar for Xcode later.

Incidentally, having explored the mailing lists further it seems that stuff like my original post should start in the swift-users list then works it's way over here. Would that be correct? I don't want to be extra noise in the wrong place.

- Ben




> On 4 Dec 2015, at 16:37, Felix Gabel <felix.gabel at me.com> wrote:
> 
> Default parameters are in my opinion an implementation detail and should be always handled by the conforming type. What can be done here is only declare the 4th method in the protocol and create a protocol extension with default forwarding.
> 
> On 04 Dec 2015, at 17:31, Ben <ben_cocoa_dev_list at yahoo.co.uk <mailto:ben_cocoa_dev_list at yahoo.co.uk>> wrote:
> 
>> Hi list,
>> 
>> I propose that methods defined in protocols should accept default arguments. This could help remove the need for method families where only one parameter is essential, but others are often used.
>> 
>> For example:
>> 
>> protocol Datastore {
>>   func executeQuery(query: String)
>>   func executeQuery(query: String, usingParameters: [Any])
>>   func executeQuery(query: String, usingParameters: [Any], logType: LogType)
>>   func executeQuery(query: String, usingParameters: [Any], logType: LogType, completionHandler:ClosureOfSomeSort)
>> }
>> 
>> Where typically the implementing object funnels the first three methods through to the final, more verbose method.
>> 
>> This could be shortened to:
>> 
>> protocol Database {
>>   func executeQuery(query: String, usingParameters: [Any] = [], logType: LogType  = .NotLogged  completionHandler:ClosureOfSomeSort? = nil)
>> }
>> 
>> Advantages:
>> - Only one method to implement
>> - Less to document, less cluttered interface
>> - Prevent objects adopting the protocol from using different default values
>> - Prevent objects from implementing similar methods in the family differently - fewer code paths to inspect, less confusion over behaviour since no argument values are hidden
>> 
>> This fits in well with the parameters section of the Swift API design page: https://swift.org/documentation/api-design-guidelines.html#parameters <https://swift.org/documentation/api-design-guidelines.html#parameters>
>> 
>> 
>> What do you think?
>> 
>> Regards,
>> 
>> Ben Barnett
>> 
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