CODEWITHSUNDEEP
swiftprotocols
Freek Sanders
Freek Sanders

Reputation: 1257

Nesting structs in Protocol Extension: Type '...' cannot be nested in generic function '...()'

I have a protocol, let's say Fruit (see below).

Within one of the methods I want to use a custom struct. This results in the following error:

Type 'Packet' cannot be nested in generic function 'saveObject()'

Why is this not allowed? 

protocol Fruit: Codable
{
    var vitamines: Int { get }
    var description: String { get }
}

extension Fruit
{
    func saveObject()
    {
        struct Packet
        {
            let val1, val2, val3: Int
        }
        let packet = Packet(val1: vitamines, val2: 0, val3: 0)
    }
}

Looking for a solution or viable alternatives.
I've tried using a tuple but I need to save the Packet to Data, which is not easily possible using a tuple (as far as I know).

Upvotes: 2

Views: 2501

Answers (1)

Rob Napier
Rob Napier

Reputation: 299345

You cannot nest new types inside of generic structures in extensions this way. Allowing that would likely become very complex, since it is not quite clear whether this type would be Fruit.saveObject.Packet or <ConformingType>.saveObject.Packet. For example, consider the following (legal) code for how these kinds of types can escape, and the system has to deal with them, including knowing how to dispatch methods to them, how much storage they require, etc.

protocol P {}

func x() -> P {
    struct T: P {}
    return T()
}

type(of: x())

If you change this to make x() generic, then it is no longer legal:

func x<Y>() -> P {
    struct T: P {} // error: type 'T' cannot be nested in generic function 'x()'
    return T()
}

That said, if you believe that the language should be changed to allow this, then Swift Evolution is the process to suggest it. You should first think though how you would like this to work if Fruit had associatedtypes, if saveObject() were itself generic, and if Packet included reference to type variable defined in either of those places. (I'm not saying these are insurmountable problems at all. This may be an excellent feature and it may be possible to design it very well. You just need to think through how it interacts with other features of the language.)

The solution is to move Packet to the top level, outside the extension and outside the protocol.

Upvotes: 2

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