Swift 4.0のextension Optional : Decodable

extension Optional : Decodable /* where Wrapped : Decodable */ { @_inlineable // FIXME(sil-serialize-all) public init(from decoder: Decoder) throws { // Initialize self here so we can get type(of: self). self = .none assertTypeIsDecodable(Wrapped.self, in: type(of: self)) let container = try decoder.singleValueContainer() if !container.decodeNil() { let metaType = (Wrapped.self as! Decodable.Type) let element = try metaType.init(from: container) self = .some(element as! Wrapped) } } }

(Swift 4.0のコードに変更) (edited)

1:59 AM

本当はDecoderの処理を優先して、こう書きたい。

 let container = try decoder.singleValueContainer() if !container.decodeNil() { - let metaType = (Wrapped.self as! Decodable.Type) - let element = try metaType.init(__from: container) - self = .some(element as! Wrapped) + self = .some(try container.decode(Wrapped.self)) } } }

(edited)

2:05 AM

Swift 4.0では、Standard Libraryで提供されるJSONEncoderとかのCoderで特別扱いされるURLの側で「JSONEncoderだったら辞書表現をやめる」みたいな力技で対応されていました。 https://github.com/apple/swift/pull/10766 (edited)

JSONEncoder conditional conformance workarounds by itaiferber · Pull Request #10766 · apple/swift

What's in this pull request? Addresses SR-5206 for JSONEncoder and JSONDecoder. One of the limitations of not having conditional conformance at the moment is that the implementation of init(from:) ...