[swift-users] Why can't I declare subscript<T> ?

Jens Alfke jens at mooseyard.com
Mon Apr 11 16:04:15 CDT 2016

Why can't a custom subscript operator be made generic? That is, Swift allows this inside a class/struct:
	func get<T>(key: String) -> T? { … }
but it doesn’t allow
	subscript<T>(key: String) -> T? { … }		// syntax error at the “<"

This doesn’t make sense to me, since subscripts are just syntactic sugar; the subscript operator ought to support whatever a named function can support.

In this case I’m implementing a class that contains a JSON payload, and I want clients to be able to flexibly access properties of the JSON and assign them to values, with implicit type-casting, i.e.
	var name: String = revision[“name”]	// invoke subscript with T=String
	var age: Int = revision[“age”]		// invoke subscript with T=Int
(I got this idea from the Tailor library, although it doesn’t use subscripts, for reasons I now understand.)


PS: I’m using Xcode 7.3; I believe that’s Swift 2.2?

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