JSON decoder that handles an absent value or a null value

KeyedDecodingContainer.swift

extension KeyedDecodingContainer {
    func decodeIfPresentAndNotNullString<T>(_ type: T.Type, forKey key: Key) throws -> T? where T: Decodable {
        do {
            return try decodeIfPresent(type, forKey: key)
        } catch {
            if let valueAsString = try decodeIfPresent(String.self, forKey: key), valueAsString == "<null>" {
                return nil
            } else {
                throw error
            }
        }
    }
}

Improvement

 struct Nullable<T: Decodable>: Decodable {
     let value: T?
 
     init(from decoder: Decoder) throws {
         let container = try decoder.singleValueContainer()
         do {
             value = try container.decode(T.self)
         } catch {
             switch error {
             case DecodingError.valueNotFound:
                 value = nil
             case DecodingError.typeMismatch where try container.decode(String.self) == "<null>":
                 value = nil
             default:
                 throw error
             }
         }
     }
 }
 
 struct Test: Decodable {
     let nullArray: Nullable<Array<Int>>
     let nullStringArray: Nullable<Array<Int>>
     let array: Nullable<Array<Int>>
 }

Via Melbourne Cocoaheads Slack messages https://melbournecocoa.slack.com/archives/C02JQ2XKS/p1577523506045700 & https://melbournecocoa.slack.com/archives/C02JQ2XKS/p1577530901047600?thread_ts=1577530901.047600&cid=C02JQ2XKS