only define the compass url helpers if they are not already defined.

lib/compass/sass_extensions/functions/urls.rb

@@ -1,8 +1,27 @@
 module Compass::SassExtensions::Functions::Urls
 
+
+  def self.has?(base, instance_method)
+    Sass::Util.has?(:instance_method, base, instance_method)
+  end
+
+  def self.included(base)
+    base.send(:include, StylesheetUrl) unless has?(base, :stylesheet_url)
+    base.send(:include, FontUrl) unless has?(base, :font_url)
+    base.send(:include, ImageUrl) unless has?(base, :image_url)
+    base.send(:include, GeneratedImageUrl) unless has?(base, :generated_image_url)
+  end
+
+  module StylesheetUrl
+    def self.included(base)
+      if base.respond_to?(:declare)
+        base.declare :stylesheet_url, [:path]
+        base.declare :stylesheet_url, [:path, :only_path]
+      end
+    end
     def stylesheet_url(path, only_path = Sass::Script::Bool.new(false))
       # Compute the path to the stylesheet, either root relative or stylesheet relative
-    # or nil if the http_stylesheets_path is not set in the configuration.
+      # or nil if the http_images_path is not set in the configuration.
       http_stylesheets_path = if relative?
         compute_relative_path(Compass.configuration.css_path)
       elsif Compass.configuration.http_stylesheets_path
@@ -18,9 +37,15 @@ module Compass::SassExtensions::Functions::Urls
         clean_url(path)
       end
     end
-  Sass::Script::Functions.declare :stylesheet_url, [:path]
-  Sass::Script::Functions.declare :stylesheet_url, [:path, :only_path]
+  end
 
+  module FontUrl
+    def self.included(base)
+      if base.respond_to?(:declare)
+        base.declare :font_url,       [:path]
+        base.declare :font_url,       [:path, :only_path]
+      end
+    end
     def font_url(path, only_path = Sass::Script::Bool.new(false))
       path = path.value # get to the string value of the literal.
 
@@ -45,9 +70,16 @@ module Compass::SassExtensions::Functions::Urls
         clean_url(path)
       end
     end
-  Sass::Script::Functions.declare :font_url, [:path]
-  Sass::Script::Functions.declare :font_url, [:path, :only_path]
+  end
 
+  module ImageUrl
+    def self.included(base)
+      if base.respond_to?(:declare)
+        base.declare :image_url,      [:path]
+        base.declare :image_url,      [:path, :only_path]
+        base.declare :image_url,      [:path, :only_path, :cache_buster]
+      end
+    end
     def image_url(path, only_path = Sass::Script::Bool.new(false), cache_buster = Sass::Script::Bool.new(true))
       path = path.value # get to the string value of the literal.
 
@@ -104,10 +136,15 @@ module Compass::SassExtensions::Functions::Urls
         clean_url(path)
       end
     end
-  Sass::Script::Functions.declare :image_url, [:path]
-  Sass::Script::Functions.declare :image_url, [:path, :only_path]
-  Sass::Script::Functions.declare :image_url, [:path, :only_path, :cache_buster]
+  end
 
+  module GeneratedImageUrl
+    def self.included(base)
+      if base.respond_to?(:declare)
+        base.declare :generated_image_url, [:path]
+        base.declare :generated_image_url, [:path, :cache_buster]
+      end
+    end
     def generated_image_url(path, cache_buster = Sass::Script::Bool.new(false))
       path = path.value # get to the string value of the literal.
 
@@ -160,8 +197,7 @@ module Compass::SassExtensions::Functions::Urls
 
       clean_url(path)
     end
-  Sass::Script::Functions.declare :generated_image_url, [:path]
-  Sass::Script::Functions.declare :generated_image_url, [:path, :cache_buster]
+  end
 
   private
 

test/units/sass_extensions_test.rb

@@ -98,6 +98,20 @@ class SassExtensionsTest < Test::Unit::TestCase
     assert_equal "true", evaluate("prefixed(-css2, css2-fallback(css3, css2))")
   end
 
+  %w(stylesheet_url font_url image_url generated_image_url).each do |helper|
+    class_eval %Q{
+      def test_#{helper}_helper_defers_to_existing_helper
+        c = Class.new do
+          def #{helper}(*args)
+            :original
+          end
+        end
+        c.send(:include, Compass::SassExtensions::Functions::Urls)
+        assert_equal :original, c.new.#{helper}("logo.png")
+      end
+    }
+  end
+
 protected
   def evaluate(value)
     Sass::Script::Parser.parse(value, 0, 0).perform(Sass::Environment.new).to_s