[swift-evolution] Implementing a protocol with default implementations

[Daniel Eggert]

Optional protocols are implemented with


  1> protocol P { 
  2.   func bar() 
  3. } 
  4. extension P { 
  5.   func bar() { 
  6.     // do nothing 
  7.   } 
  8. }

but all bets are off when I'm using this like so

  9> struct S {} 
 10. extension S : P { 
 11.   func baz() { 
 12.     print("Hello") 
 13.   } 
 14. } 
 15.  
 16. let s = S()
s: S = {}
 17> s.bar()
 18> 

This will not print anything, and the compiler will not tell me that I misspelled "bar" as "baz".

It would be extremely helpful to have a keyboard along the lines of "this is supposed to implement something in a protocol" -- which would give a warning (error?) if it doesn't.



I know there've been some discussions this, but particularly with complex protocol methods (take a look at NSURLSessionDelegate and friends), this is extremely tricky to get right, and the compiler does nothing to help me as a developer.

Take a look at this signature

func urlSession(session: NSURLSession, task: NSURLSessionTask, didReceive challenge: NSURLAuthenticationChallenge, completionHandler: (NSURLSessionAuthChallengeDisposition, NSURLCredential?) -> Void)

If I mistype that as

func urlSession(NSURLSession, task: NSURLSessionTask, didReceive challenge: NSURLAuthenticationChallenge, completionHandler: (NSURLSessionAuthChallengeDisposition, NSURLCredential?) -> Void)

or

func urlSession(session: NSURLSession, task: NSURLSessionTask, didReceiveChallenge challenge: NSURLAuthenticationChallenge, completionHandler: (NSURLSessionAuthChallengeDisposition, NSURLCredential?) -> Void)

it's very painful to track down why things don't work -> developers will be sad.

/Daniel