> Le 8 juin 2017 à 16:51, James Froggatt via swift-evolution
> <[email protected]> a écrit :
>
> I've just been trying out the new Coding protocol, and was rather surprised
> when trying to implement the `encode(to encoder: Encoder)` method.
>
> The Swift evolution proposal provides the following example code:
>
> public func encode(to encoder: Encoder) throws {
> // Generic keyed encoder gives type-safe key access: cannot encode
> with keys of the wrong type.
> let container = encoder.container(keyedBy: CodingKeys.self)
>
> // The encoder is generic on the key -- free key autocompletion here.
> try container.encode(latitude, forKey: .latitude)
> try container.encode(longitude, forKey: .longitude)
> }
>
>
> 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
You make me think that those structs should swiftly be refactored into
reference types.
Gwendal
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