Convert a JSON string into a valid JSON object

JSONStringTransformer.swift

// https://twitter.com/watura/status/708164552882425856

import Himotoki

struct Foo: Decodable {
    let JSON: [String: AnyJSON]
    
    static func decode(e: Extractor) throws -> Foo {
        let dictFromJSONStringTransformer = Transformer<String, [String: AnyJSON]> { string in
            guard let data = string.dataUsingEncoding(NSUTF8StringEncoding) else {
                throw customError("Failed to convert a string to a data: \(string)")
            }

            let JSON = try NSJSONSerialization.JSONObjectWithData(data, options: [])
            guard let JSONDict = JSON as? [String: AnyJSON] else {
                throw typeMismatch("\([String: AnyJSON].self)", actual: JSON, keyPath: nil)
            }

            return JSONDict
        }

        return try self.init(JSON: dictFromJSONStringTransformer.apply(e <| "json_string"))
    } 
}