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@@ -379,13 +379,20 @@ END
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     def parse_mixin_definition(line)
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       name, args = line.text.scan(/^=\s*([^(]+)(\([^)]*\))?$/).first
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       raise SyntaxError.new("Invalid mixin \"#{line.text[1..-1]}\".", @line) if name.nil?
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+      default_arg_found = false
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+      required_arg_count = 0
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       args = (args || "()")[1...-1].split(",", -1).map {|a| a.strip}.map do |arg|
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         raise SyntaxError.new("Mixin arguments can't be empty.", @line) if arg.empty? || arg == "!"
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         unless arg[0] == Constant::CONSTANT_CHAR
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           raise SyntaxError.new("Mixin argument \"#{arg}\" must begin with an exclamation point (!).", @line)
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         end
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+        arg, default = arg.split(/\s*=\s*/, 2)
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+        required_arg_count += 1 unless default
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+        default_arg_found ||= default
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         raise SyntaxError.new("Invalid constant \"#{arg}\".", @line) unless arg =~ Constant::VALIDATE
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-        arg[1..-1]
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+        raise SyntaxError.new("Required arguments must not follow optional arguments \"#{arg}\".", @line) if default_arg_found && !default
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+        default = Sass::Constant.resolve(default, @constants, @line) if default
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+        { :name => arg[1..-1], :default_value => default }
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       end
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       mixin = @mixins[name] = Mixin.new(args, line.children)
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       :mixin
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@@ -399,14 +406,19 @@ END
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       args = (args || "()")[1...-1].split(",", -1).map {|a| a.strip}
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       args.each {|a| raise SyntaxError.new("Mixin arguments can't be empty.", @line) if a.empty?}
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-      raise SyntaxError.new(<<END.gsub("\n", "")) unless args.size == mixin.args.size
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-Mixin #{name} takes #{mixin.args.size} argument#{'s' if mixin.args != 1},
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-but #{args.size} #{args.size == 1 ? 'was' : 'were'} passed.
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+      raise SyntaxError.new(<<END.gsub("\n", "")) if mixin.args.size < args.size
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+Mixin #{name} takes #{mixin.args.size} argument#{'s' if mixin.args.size != 1}
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+ but #{args.size} #{args.size == 1 ? 'was' : 'were'} passed.
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 END
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       old_constants = @constants.dup
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-      mixin.args.zip(args).inject(@constants) do |constants, (name, value)|
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-        constants[name] = Sass::Constant.resolve(value, old_constants, @line)
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+      mixin.args.zip(args).inject(@constants) do |constants, (arg, value)|
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+        constants[arg[:name]] = if value
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+          Sass::Constant.resolve(value, old_constants, @line)
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+        else
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+          arg[:default_value]
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+        end
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+        raise SyntaxError.new("Mixin #{name} is missing parameter ##{mixin.args.index(arg)+1} (#{arg[:name]}).") unless constants[arg[:name]]
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         constants
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       end
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@@ -68,6 +68,9 @@ class SassEngineTest < Test::Unit::TestCase
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     "a-\#{!b\n  c: d" => ["Unbalanced brackets.", 1],
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     "!a = 1 & 2" => "SassScript doesn't support a single-& operator.",
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     "!a = 1 | 2" => "SassScript doesn't support a single-| operator.",
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+    "=a(!b = 1, !c)" => "Required arguments must not follow optional arguments \"!c\".",
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+    "=a(!b = 1)\n  :a= !b\ndiv\n  +a(1,2)" => "Mixin a takes 1 argument but 2 were passed.",
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+    "=a(!b)\n  :a= !b\ndiv\n  +a" => "Mixin a is missing parameter #1 (b).",
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     # Regression tests
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     "a\n  b:\n    c\n    d" => ["Illegal nesting: Only attributes may be nested beneath attributes.", 3],
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@@ -361,6 +364,45 @@ blat
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 SASS
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   end
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+  def test_default_values_for_mixin_arguments
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+    assert_equal("white {\n  color: #ffffff; }\n\nblack {\n  color: #000000; }\n", render(<<SASS))
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+=foo(!a = #FFF)
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+  :color= !a
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+white
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+  +foo
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+black
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+  +foo(#000)
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+SASS
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+    assert_equal(<<CSS, render(<<SASS))
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+one {
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+  color: #ffffff;
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+  padding: 1px;
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+  margin: 8px; }
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+
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+two {
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+  color: #ffffff;
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+  padding: 2px;
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+  margin: 8px; }
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+
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+three {
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+  color: #ffffff;
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+  padding: 2px;
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+  margin: 3px; }
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+CSS
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+!a = 5px
0
+=foo(!a, !b = 1px, !c = 3px + !a)
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+  :color= !a
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+  :padding= !b
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+  :margin= !c
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+one
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+  +foo(#fff)
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+two
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+  +foo(#fff, 2px)
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+three
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+  +foo(#fff, 2px, 3px)
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+SASS
0
+  end
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+
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   def test_interpolation
0
     assert_equal("a-1 {\n  b-2: c-3; }\n", render(<<SASS))
0
 !a = 1