[swift-evolution] [Idea] Set variables to "_" in two-stage init; remove IUO

[Karl Wagner]

Maybe there are several initialisers, with different ways of calculating 'y' depending on what you give it.
  

  
Karl
    
  

  
  
>   
> On Jun 15, 2016 at 2:26 PM,  <Charlie Monroe (mailto:charlie at charliemonroe.net)>  wrote:
>   
>   
>   
>  Is there a particular reason for not using lazy var here?  
>
> class MySuperClass {  
>  init() {}  
> }  
>
> class MyClass : MySuperClass {  
>
>  let x: Int  
>  lazy var y: String = self.someInstanceMethod()  
>
>  init(x: Int) {  
>  self.x = x  
>  super.init()  
>  }  
>
>  func someInstanceMethod() ->  String {  
>  return "Kaboom"  
>  }  
> }  
>
>
>
> >  On Jun 15, 2016, at 2:22 PM, Karl via swift-evolution  <swift-evolution at swift.org (mailto:swift-evolution at swift.org)>  wrote:  
> >   
> >  Say you have the following code. There is a property on MyClass (‘y’) which is derived from other data via an instance method; Two-stage initialisation.  
> >   
> >  class MySuperClass {  
> >  init() {}  
> >  }  
> >   
> >  class MyClass : MySuperClass {  
> >   
> >  let x : Int  
> >  var y : String  
> >   
> >  init(x: Int) {  
> >   
> >  self.x = x  
> >  super.init()  
> >  y = someInstanceMethod()  
> >  }  
> >  }  
> >   
> >  The code won’t compile because you call super.init before initialising all properties. The way to work-around this so far is to make the type of ‘y’ an implicitly-unwrapped optional. I don’t think it’s very elegant to have to change the type of ‘y’ in this case - it exposes implementation details and implies that the value may sometimes be nil, which is not the case.  
> >   
> >  What about if we allowed you to explicitly declare that it’s okay for ‘y’ not to be initialised before calling super.init? Perhaps by assigning it to the underscore:  
> >   
> >  self.x = x  
> >  y = _  
> >  super.init()  
> >  y = someInstanceMethod()  
> >   
> >  'y' would still effectively work as an implicitly-unwrapped optional - the value would be initialised to nil, and any attempt to use it before it is initialised would be a fatal runtime error as with an IUO. This also means that it couldn’t be a “let” value.  
> >   
> >  This change wouldn’t give you any additional safety when using two-stage initialisation; it would simply not require you to change the type of the property when doing so.  
> >   
> >  Thoughts?  
> >   
> >  Karl  
> >  _______________________________________________  
> >  swift-evolution mailing list  
> >   swift-evolution at swift.org (mailto:swift-evolution at swift.org)   
> >   https://lists.swift.org/mailman/listinfo/swift-evolution   
>
>          
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