elm-sprockets.rb

# Elm Sprockets Integration for Rails 3
#
# I'm working on turning this into a Gem - see https://github.com/NoRedInk/sprockets-elm - but
# I know a lot more about Elm than I do Rails, and haven't gotten that version working yet.
# If you know how to help Gemify this, by all means please hit me up! https://twitter.com/rtfeldman
# I could definitely use the help.
#
# Anyway, in the meantime, this is what we're using at NoRedInk to integrate Elm into our asset
# pipeline, and it works like a charm. Just copy this into config/initializers/elm_sprockets.rb
#
# For this to work, make sure you have elm-make on your PATH. You can get it with:
# 
# npm install -g elm 
#
# Now you should be able to //=require any *.js.elm file (must end with .js.elm and not just .elm!)
# from any JS file that Sprockets knows about, e.g.
#
# //= require MyElmModule.js.elm

require 'open3'
require 'tempfile'

class ElmCompiler < Sprockets::Processor
  def self.default_mime_type
    'text/x-elm'
  end

  def evaluate(context, _locals)
    pathname = context.pathname.to_s

    if pathname =~ /\.js.*\.elm$/
      add_elm_dependencies pathname, context

      ElmCompiler.compile pathname
    else
      ""
    end
  end

  private

  # Add all Elm modules imported in the target file as dependencies, then
  # recursively do the same for each of those dependent modules.
  def add_elm_dependencies(filepath, context)
    # Turn e.g. ~/NoRedInk/app/assets/javascripts/Quiz/QuestionStoreAPI.js.elm
    # into just ~/NoRedInk/app/assets/javascripts/
    dirname = context.pathname.to_s.gsub Regexp.new(context.logical_path + ".+$"), ""

    File.read(filepath).each_line do |line|
      # e.g. `import Quiz.QuestionStore exposing (..)`
      match = line.match(/^import\s+([^\s]+)/)

      unless match.nil?
        # e.g. Quiz.QuestionStore
        module_name = match.captures[0]

        # e.g. Quiz/QuestionStore
        dependency_logical_name = module_name.gsub(".", "/")

        # e.g. ~/NoRedInk/app/assets/javascripts/Quiz/QuestionStore.elm
        dependency_filepath = dirname + dependency_logical_name + ".elm"

        # If we don't find the dependency in our filesystem, assume it's because
        # it comes in through a third-party package rather than our sources.
        if File.file? dependency_filepath
          context.depend_on dependency_logical_name

          add_elm_dependencies dependency_filepath, context
        end
      end
    end
  end

  def self.compile(pathname)
    temp_file = Tempfile.new 'compiled_elm_output.js'
    cmd       = `node -e "process.stdout.write(require('elm').getPathTo('elm-make'));"`

    begin
      # need to specify LANG or else build will fail on jenkins
      # with error "elm-make: elm-stuff/build-artifacts/NoRedInk/NoRedInk/1.0.0/Quiz-QuestionStore.elmo: hGetContents: invalid argument (invalid byte sequence)"
      Open3.popen3({'LANG' => 'en_US.UTF-8'}, cmd, pathname.to_s, "--yes", "--output", temp_file.path, chdir: Rails.root) do |_stdin, stdout, stderr, wait_thr|
        compiler_output = stdout.gets(nil)
        stdout.close

        compiler_err = stderr.gets(nil)
        stderr.close

        process_status = wait_thr.value

        if process_status.exitstatus != 0
          raise compiler_err
        end
      end

      temp_file.read
    ensure
      temp_file.unlink
    end
  end
end

Rails.application.assets.register_engine('.elm', ElmCompiler)
Rails.application.assets.register_mime_type('text/x-elm', '.elm')