Dynamic Mixins in SCSS

dynamicmixin.css

.span4 {
  width: 400px; }

.span3 {
  width: 300px; }

.span2 {
  width: 200px; }

.span1 {
  width: 100px; }

dynamicmixin.scss

@mixin spanx($index) {
  @if (typeof($index) == 'number') {
    @while $index > 0 {
      .span#{$index} {
        width: $index * 100px; }
      $index: $index - 1; } }
  @else {
    @warn "spanx mixin was called with a #{typeof($index)} argument"; } }

@include spanx(4);