Value of protocol type 'Encodable' cannot conform to 'Encodable'; only struct/enum/class types can conform to protocols

Question

I have the following Swift code

func doStuff<T: Encodable>(payload: [String: T]) {
    let jsonData = try! JSONEncoder().encode(payload)
    // Write to file
}

var things: [String: Encodable] = [
    "Hello": "World!",
    "answer": 42,
]

doStuff(payload: things)

results in the error

Value of protocol type 'Encodable' cannot conform to 'Encodable'; only struct/enum/class types can conform to protocols

How to fix? I guess I need to change the type of things, but I don't know what to.

Additional info:

If I change doStuff to not be generic, I simply get the same problem in that function

func doStuff(payload: [String: Encodable]) {
    let jsonData = try! JSONEncoder().encode(payload) // Problem is now here
    // Write to file
}

Answer 1

Encodable cannot be used as an annotated type. It can be only used as a generic constraint. And JSONEncoder can encode only concrete types.

The function

func doStuff<T: Encodable>(payload: [String: T]) {

is correct but you cannot call the function with [String: Encodable] because a protocol cannot conform to itself. That's exactly what the error message says.

---

The main problem is that the real type of things is [String:Any] and Any cannot be encoded.

You have to serialize things with JSONSerialization or create a helper struct.

Answer 2

You can use the where keyword combined with Value type, like this:

func doStuff<Value>(payload: Value) where Value : Encodable {
    ...
}