swift - How to have an optional trailing closure? -

i'm trying write function can call trailing closure syntax so:

func hello( message: string, closure: (( msg: string ) -> void)?) {     println( "called hello with: \(message)" );     closure?( msg: message );  } 

i expect able call function closure:

hello( "abc" ) { msg in      let = "def";      println("called callback with: \(msg) , i've got: \(a)");  }; 

and without closure, since it's optional:

hello( "abc" ) 

the latter doesn't work. says can't call hello argument list of (string).

i'm using xcode 6.3.2 , tested code within playground.

i'm not sure you've got definition of optional entirely correct. doesn't mean don't need supply value closure argument; means closure can either have value (of closure in case) or nil. therefore, if wanted call hello without providing closure write:

hello("abc", nil) 

however, can achieve you're after using default parameter values (i'd recommend have @ the swift programming guide: functions). function be:

// note `= nil`: func hello(message: string, closure: ((msg: string ) -> void)? = nil) {     println("called hello with: \(message)")      closure?(msg: message) }   // example usage: hello("abc") hello("abc") { println($0) }