Add @for.

lib/sass/constant/literal.rb

@@ -85,6 +85,10 @@ class Sass::Constant::Literal # :nodoc:
     true
   end
 
+  def to_i
+    raise SyntaxError.new("#{value.dump} is not an integer.")
+  end
+
   attr_reader :value
 
   protected

lib/sass/constant/number.rb

@@ -69,6 +69,11 @@ module Sass::Constant
       "#{value}#{@unit}"
     end
 
+    def to_i
+      super unless value % 1 == 0.0
+      return value
+    end
+
     protected
 
     def self.from_value(value, unit=nil)

lib/sass/engine.rb

@@ -92,6 +92,7 @@ module Sass
       begin
         render_to_tree.to_s
       rescue SyntaxError => err
+        err.sass_line = @line unless err.sass_line
         unless err.sass_filename
           err.add_backtrace_entry(@options[:filename])
         end
@@ -321,6 +322,8 @@ END
         import(value)
       elsif directive == "if"
         parse_if(line, root, value)
+      elsif directive == "for"
+        parse_for(line, root, value)
       else
         Tree::DirectiveNode.new(line.text, @options)
       end
@@ -334,6 +337,35 @@ END
       end
     end
 
+    def parse_for(line, root, text)
+      var, from_expr, to_name, to_expr = text.scan(/^([^\s]+)\s+from\s+(.+)\s+(to|through)\s+(.+)$/).first
+
+      if var.nil? # scan failed, try to figure out why for error message
+        if text !~ /^[^\s]+/
+          expected = "constant name"
+        elsif text !~ /^[^\s]+\s+from\s+.+/
+          expected = "'from <expr>'"
+        else
+          expected = "'to <expr>' or 'through <expr>'"
+        end
+        raise SyntaxError.new("Invalid for directive '@for #{text}': expected #{expected}.", @line)
+      end
+      raise SyntaxError.new("Invalid constant \"#{var}\".", @line) unless var =~ Constant::VALIDATE
+
+      from = Sass::Constant.parse(from_expr, @constants, @line).to_i
+      to = Sass::Constant.parse(to_expr, @constants, @line).to_i
+      range = Range.new(from, to, to_name == 'to')
+
+      tree = []
+      old_constants = @constants.dup
+      for i in range
+        @constants[var[1..-1]] = i.to_s
+        append_children(tree, line.children, root)
+      end
+      @constants = old_constants
+      tree
+    end
+
     def parse_mixin_definition(line)
       name, args = line.text.scan(/^=\s*([^(]+)(\([^)]*\))?$/).first
       raise SyntaxError.new("Invalid mixin \"#{line.text[1..-1]}\".", @line) if name.nil?

test/sass/engine_test.rb

@@ -454,6 +454,43 @@ a
 SASS
   end
 
+  def test_for
+    assert_equal(<<CSS, render(<<SASS))
+a-0 {
+  2i: 0; }
+
+a-1 {
+  2i: 2; }
+
+a-2 {
+  2i: 4; }
+
+a-3 {
+  2i: 6; }
+
+b-1 {
+  j-1: 0; }
+
+b-2 {
+  j-1: 1; }
+
+b-3 {
+  j-1: 2; }
+
+b-4 {
+  j-1: 3; }
+CSS
+!a = 3
+@for !i from 0 to !a + 1
+  a-\#{!i}
+    2i = 2 * !i
+
+@for !j from 1 through 4
+  b-\#{!j}
+    j-1 = !j - 1
+SASS
+  end
+
   def test_argument_error
     assert_raise(Sass::SyntaxError) { render("a\n  b = hsl(1)") }
   end