The future of serialization & deserialization APIs

Evolution Discussion
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13

I think there's one common use case which is not covered by the current Codable design: heterogeneous/dynamic decoding/encoding.

Many times in my developing, I wanted to decode part of a json into an intermediate representation, and later further decode that thing into a specific type.

struct Outer: Decodable {
    let value: Int
    let partialData: PartialJSONValue  // PartialJSONValue is a placeholder for the heterogeneous json value type I need
}

let fullObject = try SomeDecoder().decode(Outer.self, from: someData)
if someDynamicCondition {
    let a = SomeDecoder().decode(A.self, from: fullObject.partialData)
} else {
    let b = SomeDecoder().decode(B.self, from: fullObject.partialData)
}

Because the current Codable mechanism lacks ways to express PartialJSONValue, I often resorted to something like [String: Any], which has its own fatal flaws (such as implementation difficulties and performance problems).

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