[swift-evolution] [Review] SE-0166: Swift Archival & Serialization

[Rex Fenley]

Forgive me if I'm missing something, this is a very large proposal and a
lot to parse through. I like how everything is designed as far as I can
tell except I'm not sure from this proposal how one would compose multiple
different types of Decoders against the same output type. For example, with
Location we have.

// Continuing examples from before; below is automatically generated by the
compiler if no customization is needed.
public struct Location : Codable {
private enum CodingKeys : CodingKey {
        case latitude
        case longitude
    }

    public init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        latitude = try container.decode(Double.self, forKey: .latitude)
        longitude = try container.decode(Double.self, forKey: .longitude)
    }
}

However, this initializer seems strictly tied to the `CodingKeys` set of
coding keys. From what it appears, if this was used to decode from JSON the
format would have to always be:

{
"latitude" : 20.0
"longitude" : 20.0
}

I have a use case we're on my client we began the process of switching from
one version of an API to another. In one version we had a payload similar to

{
"user" : {
"uuid" : "uuid string..."
"can_send_message" : true
"can_delete_message" : false
}
}

this would result in the following Codable (from "user") from what I'm
following

public struct User : Codable {
private enum CodingKeys : CodingKey {
        case uuid
        case can_send_message
        case can_delete_message
    }

    public init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        uuid = try container.decode(Double.self, forKey: .uuid)
        canSendMessage = try container.decode(Bool.self, forKey:
.can_send_message)
        canDeleteMessage = try container.decode(Bool.self, forKey:
.can_delete_message)
    }
}

when we began switching over to the new api the payload was returned as

{
user {
"uuid" : "uuid string..."
"permissions" : {
"can_send_message" : true
"can_delete_message" : false
}
}
}

Here with "permissions" we have a new internal container with a separate
set of coding keys. Issue is in my use case I still need to maintain a way
to decode the old version simultaneously; we guard all new changes behind
feature flags in case something goes awry and we need to roll back our
changes.

How would one go about expressing this new Decoding and the previous
Decoding simultaneously on the same User type?


-- 

Rex Fenley  |  IOS DEVELOPER

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