Evented Sass

Example 1 - Override.scss

/* === source: === */

@mixin some-mixin {
  overflow: hidden;
  width: 200px;

  @trigger "callback";                     /* Trigger an event */
}

.box {
  @include some-mixin;                     /* Call the mixin */
  @bind "callback" { overflow: visible; }  /* Use the triggered event to override something. */ 
}

/* === output: === */

.box {
  width: 200px;
  overflow: visible;                       /* No wasteful output. */
}

Example 2 - Extend.scss

/* === source: === */

@mixin some-mixin {
  overflow: hidden;
  width: 200px;

  @trigger "callback";                  /* Trigger an event */
}

.box {
  @include some-mixin;                  /* Call the mixin */
  @bind "callback" { margin: 0 auto; }  /* Use the triggered event to extend something. */ 
}

/* === output: === */

.box {
  overflow: hidden;
  width: 200px;
  margin: 0 auto;                       /* Extended output. */
}