[swift-evolution] [Pitch] Detecting and working with Optionals from Any

[Joe Groff]

'as?' should already do this. If you have an Any that contains an Optional<T> and cast 'any as? T', you'll get the value inside the Optional if there is one, or the cast will fail if the optional is nil or the type doesn't match.

-Joe

> On Jun 20, 2016, at 11:00 PM, Charlie Monroe via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I've recently written a CoreData editor on iOS which automatically generates UI based on the model which is described using classes such as PrimitiveProperty, etc. Since it automatically sets the value on the entity, it needs to convert the value to AnyObject in order to pass it to setValue(_:forKey:), so it needs to be able to detect whether the value is Optional and in case it is, either transform the non-nil value to AnyObject (String -> NSString, Array -> NSArray, ...). Which is currently really hard to achieve: 
> 
> var obj: IndexPath? = IndexPath()
> let anyValue: Any = obj
> anyValue.dynamicType /// Optional<Foundation.IndexPath>.Type
> 
> /// Using only anyValue, determine if it's Optional and retrieve its value if 
> /// non-nil as AnyObject.
> func isOptional(anyValue: Any) -> Bool {
>     // Error: Cannot downcast from 'Any' (aka 'protocol<>') to a more 
>     // optional type 'Optional<_>'
>     return anyValue is Optional
>     return anyValue as? Optional != nil
>     ...
> }
> 
> Unless there are major reasons why it's not exposed, I'd propose introducing a new function isOptional(anyValue: Any) -> Bool, which would simply call Builtin.isOptional just like _isOptional does in Builtin.swift. (which pretty much is just taking the current _isOptional, removing underscore and marking it public).
> 
> However, this still doesn't help with the issue of retrieving the value of the Optional. You now know the value in `anyValue` is Optional, but there is no good way to cast it to e.g. Optional<AnyObject>. Here we're getting into a vicious cycle that Any can be an Optional which is Any.
> 
> My second part of the proposal introduces another function:
> 
> func asOptional<T>(anyValue: Any) -> Optional<T>?
> 
> Which will:
> - return nil if !isOptional(anyValue)
> - return a non-nil value only if `anyValue` contains in fact an Optional of type T.
> 
> Usage:
> 
> if let anyObjOptional: AnyObject? = asOptional(anyValue: anyValue) {
>     if let anyObj = anyObjOptional {
>         // anyObj is now the actual content of the optional.
>     }
> }
> 
> As a sidenote, this is my current workaround:
> 
> private protocol _XUOptional {
>     var objectValue: AnyObject? { get }
> }
> 
> extension Optional: _XUOptional {
>     var objectValue: AnyObject? {
>         switch self {
>         case .None:
>             return nil
>         case .Some(_):
>             return self! as? AnyObject
>         }
>     }
> }
> 
> if let optional = anyValue as? _XUOptional {
>     let object = optional.objectValue
>     /// ...
> }
> 
> 
> 
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution