Skip to main content

More on JSON in Erlang

I've been digging deeper into my experiments with Erlang and have improved the JSON parser I discussed in a previous blog. The code lives at https://github.com/snorristurluson/erl-simple-json in case you want to take a look.
The project is now set up to take advantage of rebar3, the Erlang build system, and tests are run automatically on Travis CI. Thanks, Lou Xun!
The parser now does proper tokenizing, rather than the simple string split operations I did in my first pass. The parser itself is surprisingly simple when it's working with tokens - here it is in its entirety:
parse_json(Input) ->
    {Value, <<>>} = parse_json_value(Input),
    Value.

parse_json_value(Input) ->
    {Token, Rest} = tokens:tokenize(Input),
    case Token of
        open_brace ->
            populate_object(dict:new(), Rest);
        open_square_bracket ->
            populate_list([], Rest);
        {_, Value} ->
            {Value, Rest};
        Other ->
            {Other, Rest}
    end.

populate_object(Obj, Input) ->
    {Token, Rest} = tokens:tokenize(Input),
    case Token of
        close_brace ->
            {Obj, Rest};
        {qouted_string, Field} ->
            {colon, Rest2} = tokens:tokenize(Rest),
            {Value, Rest3} = parse_json_value(Rest2),
            Obj2 = dict:store(Field, Value, Obj),
            populate_object(Obj2, Rest3);
        comma ->
            populate_object(Obj, Rest)
    end.

populate_list(List, Input) ->
    {Token, Rest} = tokens:tokenize(Input),
    case Token of
        close_square_bracket ->
            {List, Rest};
        comma ->
            populate_list(List, Rest);
        open_brace ->
            {Obj, Rest2} = populate_object(dict:new(), Rest),
            populate_list(lists:append(List, [Obj]), Rest2);
        {_, Value} ->
            populate_list(lists:append(List, [Value]), Rest);
        Other ->
            populate_list(lists:append(List, [Other]), Rest)

    end.
This code handles the JSON snippets I've thrown at it so far, but I need to find a good source of JSON files to ensure it works on any valid JSON.
This project has proven to be a good way to gain some Erlang experience. It also fits really well for a TDD approach, which is always fun. I'm still struggling with how to apply TDD on a daily basis, working in a large (and old) codebase. Starting with a clean slate, on a small project, TDD is a no-brainer.

Comments

Popular posts from this blog

Large scale ambitions

Learning new things is important for every developer. I've mentioned  this before, and in the spirit of doing just that, I've started a somewhat ambitious project. I want to do a large-scale simulation, using  Elixir  and Go , coupled with a physics simulation in C++. I've never done anything in Elixir before, and only played a little bit with Go, but I figure,  how hard can it be ? Exsim I've dubbed this project exsim - it's a simulation done in Elixir. Someday I'll think about a more catchy name - for now I'm just focusing on the technical bits. Here's an overview of the system as I see it today: exsim  sits at the heart of it - this is the main server, implemented in Elixir. exsim-physics  is the physics simulation. It is implemented in C++, using the Bullet physics library. exsim-physics-viewer  is a simple viewer for the state of the physics simulation, written in Go. exsim-bot  is a bot for testing exsim, written in Go.

Working with Xmpp in Python

Xmpp is an open standard for messaging and presence, used for instant messaging systems. It is also used for chat systems in several games, most notably League of Legends made by Riot Games. Xmpp is an xml based protocol. Normally you work with xml documents - with Xmpp you work with a stream of xml elements, or stanzas - see https://tools.ietf.org/html/rfc3920 for the full definitions of these concepts. This has some implications on how best to work with the xml. To experiment with Xmpp, let's start by installing a chat server based on Xmpp and start interacting with it. For my purposes I've chosen Prosody - it's nice and simple to install, especially on macOS with Homebrew : brew tap prosody/prosody brew install prosody Start the server with prosodyctl - you may need to edit the configuration file (/usr/local/etc/prosody/prosody.cfg.lua on the Mac), adding entries for prosody_user and pidfile. Once the server is up and running we can start poking at it

Mnesia queries

I've added search and trim to my  expiring records  module in Erlang. This started out as an  in-memory  key/value store, that I then migrated over to  using Mnesia  and eventually to a  replicated Mnesia  table. The  fetch/1  function is already doing a simple query, with  match_object . Result = mnesia : match_object ( expiring_records , # record { key = Key , value = '_' , expires_at = '_' }, read ) The three parameters there are the name of the table -  expiring_records , the matching pattern and the lock type (read lock). The  fetch/1  function looks up the key as it was added to the table with  store/3 . If the key is a tuple, we can also do a partial match: Result = mnesia : match_object ( expiring_records , # record { key = { '_' , " bongo " }, value = '_' , expires_at = '_' }, read ) I've added a  search/1  function the module that takes in a matching pattern and returns a list of items wh