Swift 2 JSON parsing done (more) right



Simple yet powerful object mapping made possible by Swift 2's new error handling. Greatly inspired by Argo, but without a bizillion functional operators.


struct Repository {
    let name: String
    let description: String
    let stargazersCount: Int
    let language: String?
    let sometimesMissingKey: String?

    let owner: User // Struct conforming to Decodable
    let defaultBranch: Branch // Struct NOT conforming to Decodable

    var fullName: String { return "\(owner.login)/\(name)" }

extension Repository: Decodable {
    static func decode(j: AnyObject) throws -> Repository {
        return try Repository(
                    name:                   j => "name", 
                    description:            j => "description", 
                    stargazersCount:        j => "stargazers_count", 
                    language:               j => "language", 
                    sometimesMissingKey:    j =>? "sometimesMissingKey",
                    owner:                  j => "owner", 
                    defaultBranch:          Branch(name: j => "default_branch")

How does it work?

A protocol

public protocol Decodable {
    static func decode(json: AnyObject) throws -> Self

A parse-function

public func parse<T>(json: AnyObject, path: [String], decode: (AnyObject throws -> T)) throws -> T

And shameless operator-overloading

The overloads, all calling the parse-function, can be found in Operators.swift

An overload may look like this:

public func => <T: Decodable>(lhs: AnyObject, rhs: String) throws -> T

Then there are also overloads for returning T?, [T?], [T?]?, AnyObject, [String: T]? and more.


ErrorTypes conforming to DecodingError will be caught and rethrown in the decoding process to set metadata, like the JSON object that failed decoding, the key path to it, and the root JSON object. There are currently three error-structs conforming to it:

  • TypeMismachError
  • MissingKeyError
  • RawRepresentableInitializationError
let dict: NSDictionary = ["object": ["repo": ["owner": ["id" : 1, "login": "anviking"]]]]

do {
    let username: String = try dict => "object" => "repo" => "owner" => "name"
} catch let error {
// MissingKeyError at object.repo.owner: name in {
//    id = 1;
//    login = anviking;
// }

Handling Errors

Expressions like j => "key" will throw directly, and catch-statements can be used to create the most complex error handling behaviours. This also means that try? can be used to return nil if anything goes wrong instead of throwing.

For convenience there is an operator, =>?, that only returns nil on missing keys, for APIs that indicate null in that manner, and to aid working with different response formats.

Overload Null Behaviour Missing Key Behavior Type Mismatch Behaviour Errors in subobjects
=> -> T throws throws throws uncaught (throws)
=> -> T? nil throws throws uncaught (throws)
=>? -> T? nil nil throws uncaught (throws)
try? => -> T nil nil nil caught (nil)


  • You can use Decodable with classes. Just make sure to either call a required initializer on self (e.g self.init) and return Self, or make your class final. ( This might be a problem though)
  • The Decodable-protocol and the =>-operator should in no way make you committed to use them everywhere.

For example you could...

  • Skip adapting the Decodable protocol, and parse things differently depending on the context (like defaultBranch in the example code).
  • Make your own protocols!
  • Create your own throwing decode-functions, e.g for NSDate, or convenience-extensions with your own date-formatter.
public class func decode(json: AnyObject) throws -> Self {
        let string = try String.decode(json)

        guard let date = ISO8601DateFormatter.dateFromString(string) else {
            throw NSDateDecodingError.InvalidStringFormat

        return self.init(timeIntervalSince1970: date.timeIntervalSince1970)