Elm Sprockets

elm_sprockets.rb

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 = (`npm bin`).strip + "/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')