Add an nth function.

lib/sass/script/functions.rb

@@ -969,6 +969,29 @@ def length(list)
     end
     declare :length, [:list]
 
+    # Gets the nth item in a list.
+    #
+    # Note that unlike some languages, the first item in a Sass list is number 1,
+    # the second number 2, and so forth.
+    #
+    # @param list [Literal] The list
+    # @param n [Number] The index into the list
+    # @return [Literal] The nth item in the list
+    # @raise [ArgumentError] If `n` isn't an integer between 1 and the list's length.
+    def nth(list, n)
+      assert_type n, :Number
+      if !n.int?
+        raise ArgumentError.new("List index #{n} must be an integer")
+      elsif n.to_i < 1
+        raise ArgumentError.new("List index #{n} must be greater than or equal to 1")
+      elsif n.to_i > (size = list.to_a.size)
+        raise ArgumentError.new("List index is #{n} but list is only #{size} item#{'s' if size != 1} long")
+      end
+
+      list.to_a[n.to_i - 1]
+    end
+    declare :nth, [:list, :n]
+
     private
 
     # This method implements the pattern of transforming a numeric value into

test/sass/functions_test.rb

@@ -607,6 +607,19 @@ def test_length
     assert_equal("1", evaluate("length(#f00)"))
   end
 
+  def test_nth
+    assert_equal("1", evaluate("nth(1 2 3, 1)"))
+    assert_equal("2", evaluate("nth(1 2 3, 2)"))
+    assert_equal("3", evaluate("nth((1, 2, 3), 3)"))
+    assert_equal("foo", evaluate("nth(foo, 1)"))
+    assert_equal("bar baz", evaluate("nth(foo (bar baz) bang, 2)"))
+    assert_error_message("List index 0 must be greater than or equal to 1 for `nth'", "nth(foo, 0)")
+    assert_error_message("List index -10 must be greater than or equal to 1 for `nth'", "nth(foo, -10)")
+    assert_error_message("List index 1.5 must be an integer for `nth'", "nth(foo, 1.5)")
+    assert_error_message("List index is 5 but list is only 4 items long for `nth'", "nth(1 2 3 4, 5)")
+    assert_error_message("List index is 2 but list is only 1 item long for `nth'", "nth(foo, 2)")
+  end
+
   def test_keyword_args_rgb
     assert_equal(%Q{white}, evaluate("rgb($red: 255, $green: 255, $blue: 255)"))
   end