JSON Parsing in Swift

June 20, 2014

This past week there was a post criticizing Swift’s static typing as limiting when it comes to “parsing” JSON. They weren’t really parsing JSON, rather just extracting values from a specific point in the structure, but the point remains: Swift’s static typing makes it difficult to extract values from a dynamic structure. The redditors didn’t have much to say, except for two voices of reason, who weren’t understood. Most of the redditors seemed to insist that JSON is fully schema-less and thus should have AnyObject as its representation, to keep things fully dynamic.

But JSON isn’t really as dynamic as they are claiming. Since they used AnyObject to represent a JSON object, this caused their frustration with downcasting. Instead, let’s make a library using static typing to help us out, by building an algebraic data type representing all possible JSON objects. According to, we have values which can be objects, arrays, strings, numbers, booleans, or null. Let’s convert the syntax diagrams into an ADT as follows:

enum JSONValue {
    case JSONObject(Dictionary<String,JSONValue>)
    case JSONArray(List<JSONValue>)
    case JSONString(String)
    case JSONNumber(Double)
    case JSONBoolean(Bool)
    case JSONNull

extension JSONValue: Collection {
    // Provide lookup methods for JSONObject and JSONArray.

extension JSONValue: Printable {
    // Prints as JSON.
    // This might not be the best way to do it:
    // See Haskell's `Pretty` and `Show`.
    let description: String {
    get {
        switch self {
        case let .JSONString(s): return "\"" + s "\""

And that’s it!1

This makes the operation they implemented as follows:

for item in json["blogs"]?["blog"] {
    let id = blog[“id”]
    let name = blog[“name”]
    let needspassword = blog[“needspassword”]
    let url = blog[“url”]

    println(“Blog ID: \(id)”)
    println(“Blog Name: \(name)”)
    println(“Blog Needs Password: \(needspassword)”)
    println(“Blog URL: \(url)”)

That’s a lot easier, once a proper JSON library is implemented, anyway. Swiftz has the start of one, similar in style to what I’m proposing here.

  1. Or close to it anyway. I’m simplifying here. See Swiftz for a more complete JSON library.