Added a Sass Function called nest that performs a nesting operation on (possibly) comma delimited selectors and emits the result as a string.

For example:

nest(".foo", "a, em, b")

would render:

.foo a, .foo em, .foo b

The nest function can take any number of arguments and each argument can have any number of comma-delimited selectors.

Rakefile

@@ -4,12 +4,12 @@ require 'lib/compass'
 
 # ----- Default: Testing ------
 
-task :default => :tests
+task :default => :run_tests
 
 require 'rake/testtask'
 require 'fileutils'
 
-Rake::TestTask.new :test do |t|
+Rake::TestTask.new :run_tests do |t|
   t.libs << 'lib'
   test_files = FileList['test/**/*_test.rb']
   test_files.exclude('test/rails/*', 'test/haml/*')

lib/compass.rb

@@ -1,3 +1,6 @@
+require 'sass'
+require File.join(File.dirname(__FILE__), 'sass_extensions')
+
 ['core_ext', 'version'].each do |file|
   require File.join(File.dirname(__FILE__), 'compass', file)
 end

lib/sass_extensions.rb

@@ -0,0 +1,13 @@
+require 'sass'
+
+module Sass::Script::Functions
+  COMMA_SEPARATOR = /\s*,\s*/
+  def nest(*arguments)
+    nested = arguments.map{|a| a.value}.inject do |memo,arg|
+      ancestors = memo.split(COMMA_SEPARATOR)
+      descendants = arg.split(COMMA_SEPARATOR)
+      ancestors.map{|a| descendants.map{|d| "#{a} #{d}"}.join(", ")}.join(", ")
+    end
+    Sass::Script::String.new(nested)
+  end
+end

test/sass_extensions_test.rb

@@ -0,0 +1,26 @@
+require  File.dirname(__FILE__)+'/test_helper'
+require 'compass'
+
+class SassExtensionsTest < Test::Unit::TestCase
+  def test_simple
+    assert_equal "a b", nest("a", "b")
+  end
+  def test_left_side_expansion
+    assert_equal "a c, b c", nest("a, b", "c")
+  end
+  def test_right_side_expansion
+    assert_equal "a b, a c", nest("a", "b, c")
+  end
+  def test_both_sides_expansion
+    assert_equal "a c, a d, b c, b d", nest("a, b", "c, d")
+  end
+  def test_three_selectors_expansion
+    assert_equal "a b, a c, a d", nest("a", "b, c, d")
+  end
+  def test_third_argument_expansion
+    assert_equal "a b e, a b f, a c e, a c f, a d e, a d f", nest("a", "b, c, d", "e, f")
+  end
+  def nest(*arguments)
+    Sass::Script::Functions.nest(*arguments.map{|a| Sass::Script::String.new(a)}).to_s
+  end
+end