Today I published a toy project, that I worked on in the last couple of weeks. It is an LR Parser generator. I normally use yecc in my projects for parsing. It works ok, but I am not happy with usability. Besides that it seems to have issues with special (utf-8) characters. I also tried other libraries (like xpet), but struggling to use them for my purpose (probably my fault understanding the concepts). I created an LR parser years ago in c# and always dreamed of redoing it in elixir.

The parser is defined using a DSL:

  use ExLR

  lr skip_whitespaces: true do
    S <- S + "+" + :integer = fn ([a, _, b]) -> a + b end
    S <- :integer           = fn ([a]) -> a end
  end

It provides a parse function that can be used like

# parse("3+4")
{:ok, 7}

It contains a scanner, but it can also be called with a list of symbols (using a different scanner). Each symbol is a tuple of 3 containing the symbol, the data and the position:

parse([{:integer, 3, {0, 0}}, {"+", nil, {0, 1}}, {:integer, 4, {0, 2}}, {:"$", nil, {0, 3}}])

The scanner is initialised by going through the rules. It accepts:

• Strings
• :integer
• :text
• :quoted_text
• :ws

Special terminals can also be defined:

  terminal :zip_code, chars: ?0..?9, min: 4, max: 5

  lr skip_whitespaces: true do
    Address <- Zip + City = fn [zip, city] -> %{zip: zip, city: city} end
    Zip <- :zip_code
    City <- :text
  end

You can find the repository here: GitHub - mlankenau/exlr
I put a couple of examples in the test folder. I think they are a good reference.

I hope it is adding some values for other devs and highly appreciate any feedback.

Happy easter