[swift-evolution] Group "typedef" language feature

Jaden Geller jaden.geller at gmail.com
Mon Mar 13 19:19:41 CDT 2017

Swift already supports equivalent semantics to what you’re looking for, though conditional conformances are required for this to work with dictionaries and arrays too.

protocol JSONValueType { }
extension String: JSONValueType { }
extension Double: JSONValueType { }
extension Int: JSONValueType { }
extension Dictionary: JSONValueType where Key == String, Value: JSONValueType { }
extension Array: JSONValueType where Element: JSONValueType { }
extension Bool: JSONValueType { }

let data = [String : JSONValueType]()
data[key] = "value"

let b: String? = nil
let a: JSONValueType = b // error: value of optional type 'Optional<string>' not unwrapped

Jaden Geller

> On Mar 12, 2017, at 7:52 PM, Elijah Johnson via swift-evolution <swift-evolution at swift.org> wrote:
> I don’t know if this feature has yet existed in any languague, but its pretty simple.
> Let say you are preparing data for a JSON encoder. Right now, you would add to an Dictionary or Array like this one:
> let data = [String:Any]();
> but what one is really saying is that “Any” is from a number of distinct types:
> group typedef JSONValueType : String, Double, Int, [String:JSONValueType], [JSONValueType], Bool
> So from the compiler’s point of view, this group typedef would be just like the “Any” type, only it would refuse to compile unless the proper type was passed. So the result would be, as an example, that there could be created an (almost) fail-safe JSON encoder:
> let data = [String:JSONValueType]();
> data[“key] = “value”;
> I’m sure this has plenty of uses outside of JSON encoding and would be pretty simple to do. One problem that it would solve it the fact that in Swift, everything is included by Any, including optionals, ex:
> let b : String? = nil;
> let a : Any = b; // result is warning: “Expression implicitly coerced from String? to Any"
> and so there is no way to include value types and objects without also including optionals, though this “Any” (like Object and void* in Java/C) syntax makes for poorly documented code when the type isn’t really “Any”, but a collection of known types.
> There already exist protocols, but they are not useful for types that one has not defined (like the basic system types), or for mixing particular values, structs, and classes.
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