Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.3.7)
// Compass (v1.0.0.alpha.18)
// ----

$elementSeparator: '__';
$modifierSeparator: '--';
$listBullets: (
    '.list--bullets': (
      list-style: none,
      border-bottom: 1px solid gray
  )
);
$listLines: '.list--lines';
$listItems: '.list__items';

@mixin modifier($modifier) {
  //$selector: $modifier;
  //@if type-of($modifier) == map {
    $selector: map-key($modifier, 1);
  //}
  $separatorPosition: str-index($selector, $modifierSeparator);
  //$block: str-slice($selector, 1, ($separatorPosition - 1));
  @at-root {
    #{unquote($selector)} {
      //@extend #{unquote($block)} !optional;
      @if type-of($modifier) == map {
        //@each $property, $value in map-get($modifier, 1) {
        //  #{$property}: #{$value};
        //}
      }
      @content;
    } 
  }
}

@mixin element($element) {
  $separatorPosition: str-index($element, $elementSeparator);
  $block: str-slice($element, 1, ($separatorPosition - 1));
    #{unquote($element)} {
      @content;
    } 
}

@mixin prototype($prototype) {
    @each $attr in $prototype {
      #{map-get($attr, $prototype)}: #fff;
    }
}

.list {
  color: #fff;
}

@include modifier($listBullets) {
  background: gray;
}

@include modifier($listLines) {
  background: yellow;
}

@include element($listItems) {
  border-bottom: 1px solid #fff;
}

SassMeister-output.css

(".list--bullets": ((list-style: none, border-bottom: 1px solid gray))) isn't a valid CSS value.