Allow _helpers.rb in blogs to provide custom Haml helpers. Document t…

…he provided ones. DRY up Haml rendering. Don't use OpenStruct for Haml context since it won't complain about methods that don't exist.

README.textile

@@ -177,6 +177,18 @@ converted. Layouts don't need to.
 
 Haml content is intentionally not filtered, so you can use any Ruby code.
 
+If you want to define methods you can call from your Haml files, create
+a _helpers.rb file in the root of your blog and put the methods there,
+inside a module named Helpers.
+
+Jekyll provides some helpers out of the box:
+
+  h(string)
+    HTML entity-escapes the input string.
+
+  link_to(text, url)
+    Creates a link to the URL with the linked text (or markup).
+
 h3. Local Server
 
 When previewing complex sites locally, simply opening the site in a web 

lib/jekyll/convertible.rb

@@ -59,6 +59,18 @@ def content_type
       end
       return 'unknown'
     end
+
+    # Sets up a context for Haml and renders in it. The context has accessors
+    # matching the passed-in hash, e.g. "site", "page" and "content", and has
+    # helper modules mixed in.
+    #
+    # Returns String.
+    def render_haml_in_context(haml_engine, params={})
+      context = ClosedStruct.new(params)
+      context.extend(HamlHelpers)
+      context.extend(::Helpers) if defined?(::Helpers)
+      haml_engine.render(context)
+    end
 
     # Add any necessary layouts to this convertible document
     #   +layouts+ is a Hash of {"name" => "layout"}
@@ -72,11 +84,10 @@ def do_layout(payload, layouts)
       payload["content_type"] = self.content_type
 
       if self.content_type == "haml"
-        context = OpenStruct.new(:site => self.site, :page => OpenStruct.new(payload["page"]))
-        context.extend(HamlHelpers)
-
         self.transform
-        self.content = self.content.render(context)
+        self.content = render_haml_in_context(self.content,
+          :site => self.site,
+          :page => ClosedStruct.new(payload["page"]))
       else
         self.content = Liquid::Template.parse(self.content).render(payload, info)
         self.transform
@@ -91,12 +102,10 @@ def do_layout(payload, layouts)
         payload = payload.deep_merge({"content" => self.output, "page" => layout.data})
 
         if site.config['haml'] && layout.content.is_a?(Haml::Engine)
-          context = OpenStruct.new(
-            :page => OpenStruct.new(payload["page"]),
-            :site => OpenStruct.new(payload["site"]),
+          self.output = render_haml_in_context(layout.content, 
+            :site => ClosedStruct.new(payload["site"]),
+            :page => ClosedStruct.new(payload["page"]),
             :content => payload["content"])
-          context.extend(HamlHelpers)
-          self.output = layout.content.render(context)
         else
           self.output = Liquid::Template.parse(layout.content).render(payload, info)
         end

lib/jekyll/core_ext.rb

@@ -19,4 +19,11 @@ def deep_merge(hash)
 
     target
   end
-end
+end
+
+require 'ostruct'
+class ClosedStruct < OpenStruct
+  def method_missing(symbol, *args)
+    @table.has_key?(symbol) ? super : raise(NoMethodError, "undefined method `#{symbol}' for #{self}")
+  end
+end

lib/jekyll/site.rb

@@ -50,8 +50,9 @@ def setup
       if self.config['haml']
         begin
           require 'haml'
-          require 'ostruct'
           require 'jekyll/haml_helpers'
+          helpers = File.join(source, '_helpers.rb')
+          require helpers if File.exist?(helpers)
           puts 'Enabled Haml'
         rescue LoadError
           puts 'You must have the haml gem installed first'