Simple and strict, yet powerful object mapping made possible by Swift 2's 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: Any) throws -> Repository {
        return try Repository(
                    name: j => "nested" => "name", 
                    description: j => "description", 
                    stargazersCount: j => "stargazers_count", 
                    language: j => "language", 
                    sometimesMissingKey: j =>? "sometimesMissingKey",
                    owner: j => "owner", 
                    defaultBranch: Branch(name: j => "default_branch")

do {
    let json = try NSJSONSerialization.JSONObjectWithData(data, options: [])
    let repo = try [Repository].decode(json)
} catch {

How does it work?

A protocol

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

A parse-function

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

And shameless operator-overloading

The too-many generated overloads, all calling the parse-function, can be found in Overloads.swift. Return types include T?, [T?], [T?]?, Any and [String: T]?. When conditional protocol conformance is supported in Swift this won't be necessary, and automagic decoding of infinitly nested generic types (like [[[[[[[[[A???]]: B]]]?]]?]]) would work.

An overload may look like this:

public func => <T: Decodable>(json: Any, keyPath: KeyPath) throws -> T


Keypaths can be created from string and array literals as well as with explicit initializers. They can also be joined using the operators => and =>?. =>? is another operator that indicates that nil should be returned if the key to the right is missing.

  • When composing => and =>? operators in the same keypath, the strictness of => is still honoured.
  • Optional key paths (=>?) require an optional return type
let a: KeyPath = "a"
let b: KeyPath = ["a", "b"]
let c: KeyPath = "a" => "b" => "c"
let string: String? = json =>? "key1" => "key2" => "key3"`
                                ^^^^ allowed to be missing


Errors will be caught and rethrown in the decoding process to backpropagate metadata, like the JSON object that failed decoding, the key path to it, and the root JSON object.

From DecodingError.swift:

public enum DecodingError: ErrorProtocol, Equatable {
    /// Thrown when optional casting from `Any` fails.
    /// This can happen both when trying to access a key on a object
    /// that isn't a `NSDictionary`, and failing to cast a `Castable`
    /// primitive.
    case typeMismatch(expected: Any.Type, actual: Any.Type, Metadata)
    /// Thrown when a given, required, key was not found in a dictionary.
    case missingKey(String, Metadata)
    /// Thrown from the `RawRepresentable` extension when
    /// `init(rawValue:)` returned `nil`.
    case rawRepresentableInitializationError(rawValue: Any, Metadata)
    /// When an error is thrown that isn't `DecodingError`, it 
    /// will be wrapped in `DecodingError.other` in order to also provide
    /// metadata about where the error was thrown.
    case other(ErrorProtocol, Metadata)
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)


Int, Double,String, Bool, Date (ISO8601), NSArray, and NSDictionary types that conform to DynamicDecodable with the following declaration:

public protocol DynamicDecodable {
    associatedtype DecodedType
    static var decoder: (Any) throws -> DecodedType {get set}

This allows Decodable to implement default decoding closures while allowing you to override them as needed.

// Lets extend Bool.decoder so that it accepts certain strings:
Bool.decoder = { json in
    switch json {
    case let str as String where str == "true":
        return true
    case let str as String where str == "false":
        return false
        return try cast(json)

Note that when extending new types to conform to Decodable there is really no point in conforming to DynamicDecodable since you already control the implementation. Also note that the decoder properties are intended as "set once". If you need different behaviour on different occations, please create custom decode functions.

The default Date.decoder uses a ISO8601 date formatter. If you don't want to create your own decode closure there's a helper:

Date.decoder = Date.decoder(using: formatter)

When Decodable isn't enough

Don't be afraid of not conforming to Decodable.

let array = try NSArray.decode(json => "list").map {
    try Contribution(json: $0, repository: repo)


  • 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.


Swift version Compatible tag or branch
Swift 4.0 0.6.0
Swift 3.0 v0.5
Swift 2.3 v0.4.4
Swift 2.2 v0.4.3

Note on Swift 4.0 usage

Due to collisions with the standard library you will have to import ambiguous symbols specifically, in addition to Decodable as a whole.

This means you likely want the following

import Decodable
import protocol Decodable.Decodable

and you can import other symbols, e.g KeyPath, DecodingError, in a simlilar fashion (using import struct and import enum)