More palatable JSON serialization
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JSON Tagged Literals

This project is inspired by EDN, which provides much richer data structures than JSON. However if you're stuck with JSON for performance or interoperability, use JTL to make the best of it. JTL was originally written to support The Weathertron mobile weather app.


If you're OG, just grab jtl.min.js and throw it on your page. You now have the JTL object in your global scope.

If you're using ClojureScript, add

[com.keminglabs/jtl "0.1.0"]

to your project.clj dependencies and then

(:require [com.keminglabs.jtl :as jtl])

in your namespace form. Note that this is a plain JavaScript library, so if you want to pass options make sure you first coerce them to a JavaScript object:

(jtl/deserialize my-js-obj
                (clj->js {"tag_table" {"inst" (fn [d] ...)}}))


This library is two pure functions, serialize and deserialize, which move values between the rich semantics of your application and the impoverished semantics of JSON. To serialize a non-JSONable value (i.e., anything beyond a string, number, true/false/null, or array or string-key'd map thereof) you provide a tag and a JSONable representation of the value. To deserialize, you provide a function that takes the JSONable representation and returns the richer type.

Consider the classic problem of serializing dates. JTL comes with a built-in reader for the "inst" tag with the rfc-3339 string representations of dates:

JTL.deserialize({"#inst": "2013-04-05T00:00:00.000Z"})
  .getFullYear() //=> 2013

Note that the argument to deserialize is a data structure, not a string; use JSON.parse to convert your JSON string into data and then JTL.deserialize to lift that data into your application domain.

Tagged literals can occur at any (potentially nested) position in JSON data:

JTL.deserialize([0, 1, {"#inst": "2013-04-05T00:00:00.000Z"}])
  .pop().getFullYear() //=> 2013

Additional tags can be specified with a map of tags to reader functions:

opts = {tag_table: {inst: function(x){return new Date(Date.parse(x)).getFullYear();}}}
JTL.deserialize([0, 1, {"#inst": "2013-04-05T00:00:00.000Z"}], opts)
  //=> [0, 1, 2013]

and tags can be nested within each other:

var Order = function Order() {}

var opts = {
  tag_table: {
    order: function(x) {
      var o = new Order(); =;
      o.placed_on = x.placed_on;
      return o;}}};

var d = {"#order": {"id": 123, "placed_on": {"#inst": "2013-04-05T00:00:00.000Z"}}};
var res = JTL.deserialize(d, opts);
res.constructor == Order //=> true
res.placed_on.getFullYear() == 2013 //=> true

Note here that the tagged literal value #order was serialized as a JSON object its body, not just a string. There's no need to compact several values together in a string and try to regex them back out---tagged literal values are just JSON, so you should use the appropriate data shapes (ordered or associative collections, scalars) to serialize your values, using JTL to lift them into application domain objects.

By default, an error is thrown when an unknown tag is encountered, but that behavior can be changed by providing a default reader function:

JTL.deserialize({"#trouble": true})
  //=> Raises an error!
JTL.deserialize({"#trouble": true},
                          {default_reader: function(x){return 42;}})
  //=> 42

To convert your application values into JSONable values, you can use the serialize function. By default, only the inst tag is implemented:

JTL.serialize({"aDate": new Date()})
  //=> {aDate: {#inst: "2013-04-16T20:32:20.807Z"}}

but as with the deserialize function you can pass a second argument options map. The interesting key in this options map is constructor_table, which should map to an instance of goog.structs.Map with function constructor keys and serialization function values. A serialization function should take your higher level type and return an array of the form [string_tag, JSONable_value]. Your serialization functions will also be passed the options map given to JTL.serialize as the second argument; you can use this to recursively serialize composite types.

Using goog.structs.Map is going to be painful for anyone not using the Google Closure Library, so if you have suggestions for a nicer way to implement an open polymorphic dispatch system for serialization, please let me know.

For more usage examples, see the tests.

Serverside implementation

Since everything is just JSON, it's fairly easy to teach your server to emit tagged literals Here's an example of serializing JodaTime dates with the Cheshire JSON library in Clojure:

(ns ptron.server.util
  (:require [cheshire
             [core :as json]
             [generate :refer [add-encoder encode-map]]]
             [core :refer [now]]
             [format :refer [formatters unparse]]]))

(add-encoder org.joda.time.DateTime
             (fn [x jg]
               (encode-map {"#inst" (unparse (formatters :date-time)

(json/encode (now))
    ;;=> "{\"#inst\":\"2013-05-07T04:32:49.376Z\"}"

@sbecker shows us how it's done in Rails:

# Monkeypatch Rails time class to output json differently
class ActiveSupport::TimeWithZone
  def to_json
    {"#inst" => self}.to_json

# => {"#inst":"2013-05-06T14:27:55-07:00"}


If you think your application and its serverside endpoints might become popular, you should namespace your tags. I.e., use tags like "com.keminglabs.precipitron5000/forecast". Now Gzip is even more your friend.


You'll need Ruby and bundler to build the project. Use RVM to grab Ruby 1.9.3, install the bundler rubygem, then

bundle install
git submodule update --init

to get all of the dependencies. Then run

bundle exec rake minify 

to run CoffeeScript and minify its output with the Google Closure compiler.


Specs are written in CoffeeScript under spec/coffeescripts.

Start the test server with

bundle exec rake jasmine

and then open up localhost:8888 in your browser to run specs. If you are editing specs, make sure to run

bundle exec guard

so that the specs will automatically compile from CoffeeScript to JavaScript.


Thanks to @ninjascience for design discussions. Thanks to @fogus for feedback on README examples and raising questions about unknown type handling.