[swift-evolution] TreeLiteralConvertible

[John McCall]

> On Apr 14, 2016, at 2:20 PM, Brent Royal-Gordon <brent at architechies.com> wrote:
>> No, you just need Tree to conform to both ArrayLiteralConvertible and IntegerLiteralConvertible, and it implements the latter by building a Value out of it.
> 
> That not only doesn't work if your type isn't a LiteralConvertible, it also doesn't work if you want to build a literal with variables:
> 
> 	let myTree: Tree = [1, [2, three]]
> 
> The real missing feature here is implicit lifting like Optional offers. With a LiftingConvertible protocol, you could say something like this:
> 
> 	enum Tree<Value> {
> 		case leaf(Value)
> 		case branches([Tree<Value>])
> 	}
> 	
> 	extension Tree: ArrayLiteralConvertible {
> 		init(arrayLiteral branches: Tree<Value>...) {
> 			self = .branches(branches)
> 		}
> 	}
> 	
> 	extension Tree: LiftingConvertible {
> 		init(lifting value: Value) {
> 			self = .leaf(value)
> 		}
> 	}

Another name for this feature is "user-defined implicit conversions".

John.