Sprockets producing Handlebars templates from Haml

config.rb

http_path = "/"
css_dir = "public/stylesheets"
sass_dir = "sass"
images_dir = "public/images"
http_images_dir = "images"

javascripts_dir = "public/javascripts"

fonts_dir = "public/fonts"
http_fonts_dir = "fonts"

line_comments = false

config.ru

$LOAD_PATH << File.expand_path('.')

require 'rubygems'
require 'bundler/setup'
require 'haml'
require 'sass'
require 'sass/plugin/rack'
require 'compass'
require 'tilt'
require 'sprockets'
require 'hamlbars'

Compass.add_configuration(File.expand_path('config.rb'))
Compass.configure_sass_plugin!
use Sass::Plugin::Rack

map '/javascripts' do
  sprockets = Sprockets::Environment.new
  sprockets.engines.register 'hamlbars', Tilt::HamlbarsTemplate
  sprockets.paths << 'apps'
  sprockets.paths << 'vendor/javascripts'
  sprockets.static_root = 'public/javascripts'
  run sprockets
end

map '/' do
  run Rack::Directory.new('public')
end

hamlbars.rb

require 'tilt/template'

module Tilt
  class HamlbarsTemplate < Template
    JS_ESCAPE_MAP = {
      "\r\n"  => '\n',
      "\n"    => '\n',
      "\r"    => '\n',
      '"'     => '\\"',
      "'"     => "\\'" }

    self.default_mime_type = 'application/javascript'

    def self.engine_initialized?
      defined? ::Haml::Engine
    end

    def initialize_engine
      require_template_library 'haml'
    end

    def prepare
      options = @options.merge(:filename => eval_file, :line => line)
      @engine = ::Haml::Engine.new(data, options)
    end

    def evaluate(scope, locals, &block)
      template = if @engine.respond_to?(:precompiled_method_return_value, true)
        super
      else
        @engine.render(scope, locals, &block)
      end
      if basename =~ /^_/
        "Handlebars.registerPartial('#{name}', '#{template.strip.gsub(/(\r\n|[\n\r"'])/) { JS_ESCAPE_MAP[$1] }}');\n"
      else
        "App.registerTemplate('#{name}', '#{template.strip.gsub(/(\r\n|[\n\r"'])/) { JS_ESCAPE_MAP[$1] }}');\n"
      end
    end

    # Precompiled Haml source. Taken from the precompiled_with_ambles
    # method in Haml::Precompiler:
    # http://github.com/nex3/haml/blob/master/lib/haml/precompiler.rb#L111-126
    def precompiled_template(locals)
      @engine.precompiled
    end

    def precompiled_preamble(locals)
      local_assigns = super
      @engine.instance_eval do
        <<-RUBY
          begin
            extend Haml::Helpers
            _hamlout = @haml_buffer = Haml::Buffer.new(@haml_buffer, #{options_for_buffer.inspect})
            _erbout = _hamlout.buffer
            __in_erb_template = true
            _haml_locals = locals
            #{local_assigns}
        RUBY
      end
    end

    def precompiled_postamble(locals)
      @engine.instance_eval do
        <<-RUBY
            #{precompiled_method_return_value}
          ensure
            @haml_buffer = @haml_buffer.upper
          end
        RUBY
      end
    end
  end
end

module Haml
  module Helpers

    module HamlbarsExtensions
      def iterate(name, &block)
        content = capture_haml(&block)
        "{{##{name}}}#{content.strip}{{/#{name}}}"
      end
    end

    include HamlbarsExtensions
  end
end

We are using backbone.js and wanted to have handlebars.js as our client-side template language. This means that you must compile the templates and register the partials with Handlebars so that they can be rendered from backbone view instances, but we never want to write that registration code. We have index.js.hamlbars files written in Haml, and concatenated by Sprockets as "Handlebars.compile('<p>haml to html</p>')" and "Handlebars.registerPartial" if the filename starts with an underscore. There are helpers for the Haml that allow us to use Haml within Handlebars lists.

The reason we load config.rb is that @imathis uses Guard and LiveReload to continuously compile his scss, so Sprockets isn't the only thing that cares about how Sass is configured. We plan to precompile all our assets for a non-Ruby server environment.

I just started working on making sprockets support sass better. It should land in the next few days: sstephenson/sprockets#23

That's great news. Thanks for the heads up and all your excellent work.