Elixir needs at least 4 JSON parsers

Elixir has, at the time of this post, 3 JSON parsers hosted on expm.

Some time back I went looking for one and started exploring elixir-json, including the github repo.  I loved how the encoder used protocols to implement the encoding of different types.  It’s beautiful.  Kudos to @carloslage.

Then I looked at the decoder.  It’s full of elixir-isms… tuples, binary pattern matching, etc.  Over the last few nights I decided to implement my own JSON parser learning from Carlos were I ran aground.

I don’t like the use of HashDict for objects, but spaces rule out most options like Keyword lists.  I’m also not a huge fan of the nested case for dealing with the what’s next? of key/value pairs.

def parse_object( acc, << rest :: binary >> ) do
  { key, rest } = parse_content rest
  { value, rest } = lstrip(rest) |> parse_object_value 

  acc = [ { key, value } | acc ] 

  case lstrip(rest) do
    << ?}, rest :: binary >> -> { HashDict.new(acc), rest }
    << ?,, rest :: binary >> -> parse_object acc, lstrip(rest)
  end
end

Overall it feels very cohesive.  You can see clearly how the pieces compose after being dispatched from the parse_content.

def parse_content( << m, rest :: binary >> ) when m in ?0..?9, do: parse_number << m, rest :: binary >>
def parse_content( << ?", rest :: binary >> ), do: rest |> parse_string
def parse_content( << ?{, rest :: binary >> ), do: lstrip(rest) |> parse_object
def parse_content( << ?[, rest :: binary >> ), do: lstrip(rest) |> parse_array

I also do nothing to support invalid JSON content as with elixir-json.  Oh well, it was a learning tool.