> On Jun 8, 2017, at 8:27 AM, Gwendal Roué via swift-evolution
> <[email protected]> wrote:
>>
>> Le 8 juin 2017 à 16:51, James Froggatt via swift-evolution
>> <[email protected]> a écrit :
>>
>> Here, container is stored as a `let` value, and uses reference semantics,
>> while the proposal also clearly lists these `encode` methods as mutating.
>> With the current implementation of the proposal, the container must be
>> stored as a `var`, which leads to code like the following:
>>
>> var container = encoder.singleValueContainer()
>> try container.encode(data)
>
> Yes, practically speaking and with latest Swift 4, the container needs to be
> declared as `var`.
>
> I admit it's weird, and feels unnatural:
>
> public func encode(to encoder: Encoder) throws {
> // A mutated value that nobody consumes: so weird.
> var container = encoder.container(keyedBy: CodingKeys.self)
> try container.encode(latitude, forKey: .latitude)
> try container.encode(longitude, forKey: .longitude)
> }
>
>> This clearly wont work as expected if the container were to have value
>> semantics, and writing code like this feels plain wrong. Is SE-0166 really
>> intended to work with referrence-type encoders only?
>
> Actually, it can work with encoder/containers that have value semantics, and
> forward the mutations somewhere else (for example to a closure which fills a
> mutating container).
>
> But this is again bizarre, and contrieved:
> https://github.com/groue/GRDB.swift/blob/15bfe5f6cf76070cfb17216223bdebc6b158d654/GRDB/Record/Persistable%2BEncodable.swift
>
> <https://github.com/groue/GRDB.swift/blob/15bfe5f6cf76070cfb17216223bdebc6b158d654/GRDB/Record/Persistable%2BEncodable.swift>
>
> You make me think that those structs should swiftly be refactored into
> reference types.
I made precisely this comment during the review. The response was that the
container could potentially defined by a struct that contains some state which
it uses to write into a reference. For instance, imagine a type like this:
class MyEncoder: Encoder {
var buffer: Data
struct KeyedEncodingContainer<K: CodingKey>:
KeyedEncodingContainerProtocol {
let encoder: MyEncoder
var bufferOffset: Int
mutating func encode(_ value: String, forKey key: K)
throws {
let newData = try makeDataFrom(value, forKey:
key)
encoder.buffer.insert(newData, at: offset)
bufferOffset += newData.count
}
…
}
…
}
I argued that foreclosing designs like this would be acceptable—you could
either make `KeyedEncodingContainer` a class or move `offset` into `MyEncoder`
to get around it—but the proposers preferred to preserve this flexibility.
--
Brent Royal-Gordon
Architechies
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