Add simple partials implementation

Add a `render` method to Haml::Helpers to provide a simple
implementation of partials.

Author: mattwildig

lib/haml/partials.rb

@@ -0,0 +1,40 @@
+require 'haml'
+
+module Haml::Helpers
+
+  # A simple implementation of partials.
+  #
+  # Renders the named Haml file and returns the generated HTML.
+  # 
+  # If the named file is not found, the suffixes +.haml+ and +.html.haml+ are
+  # added and looked for, and then the same names but with the prefix +_+. The
+  # first file found will be used.
+  #
+  # +self+ is used as the context, so instance variables can be used in the
+  # partial and will have the same value as the parent template.
+  #
+  # This method makes no effort to cache the generated Ruby code, it simply
+  # uses +Haml::Engine.new().render+ each time.
+  #
+  # @param partial [#to_s] the filename of the partial to render
+  # @param locals [Hash] a hash of local variables to use in the partial
+  # @return [String] the HTML generated from the partial
+  # @raise [StandardError] if the partial file cannot be found
+
+  def render(partial, locals = {})
+    Haml::Engine.new(File.read(find_file_for_partial(partial))).render(self, locals)
+  end
+    
+  private
+  def find_file_for_partial(file)
+    ['', '_'].each do |prefix|
+      ['', '.haml', '.html.haml'].each do |suffix|
+        candidate = "#{prefix}#{file}#{suffix}"
+        return candidate if File.exist? candidate
+      end
+    end
+
+    raise "Partial #{file} not found"
+  end
+
+end