More On Json In Erlang

[ erlang json ]

Written on October 7, 2017

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.