An example of using flatMap to extract fields from a [String: AnyObject] dictionary into a concrete type

Deserialize.swift

import Foundation

func toString(obj: AnyObject) -> String? {
    return obj as? String
}

func toJSON(obj: AnyObject) -> [String: AnyObject]? {
    return obj as? [String: AnyObject]
}

func toInt(obj: AnyObject) -> Int? {
    return obj as Int
}

func toJSONArray(obj: AnyObject) -> [[String: AnyObject]]? {
    return obj as? [[String: AnyObject]]
}

struct Person {
    struct Name {
        let first: String
        let last: String
    }
    
    let id: Int
    let name: Name
}

func toName(obj: [String: AnyObject]) throws -> Name {
    guard let first = obj["first"]?.flatMap(toString) else {
        throw "unsupported structure"
    }
    
    guard let last = obj["last"]?.flatMap(toString) else {
        throw "unsupported structure"
    }
    
    return Name(first: first, last: last)
}

let jsonStr = "{\"people\":[{\"id\":0, \"name\":{\"first\":\"taco\",\"last\":\"dog\"}}]}"

let JSON: [String: AnyObject]
do {
    JSON = try NSJSONSerialization.JSONObjectWithData(jsonStr.dataUsingEncoding(NSUTF8StringEncoding), options: nil)
} catch(let error) {
    print(error)
    JSON = [:]
}

let people = JSON["people"].flatMap(toJSONArray).flatMap { JSON -> [People] in
    do {
        let name = try JSON["name"].flatMap(toName)
        return [Person(id: JSON["id"].flatMap(toInt) ?? -1, name: name)]
    } catch (let error) {
        print(error)
        return []
    }
} ?? []

print(people)