Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.3.5)
// Compass (v1.0.0.alpha.18)
// ----

@function option($map, $name, $default) {
  @return if(map-has-key($map, $name), map-get($map, $name), $default);
}

@mixin foo($opts...) {
 $opts: keywords($opts);
 border-color: option($opts, border, purple); 
 background-color: option($opts, bg, blue); 
 color: option($opts, fg, white);
 @if map-has-key($opts, border-width) {
  border-width: map-get($opts, border-width);
 }
}

.bar{
  // dont define border color or border width
  // but border color gets default value
  // border-width is not applied
  @include foo($bg:red, $fg:blue);
}

SassMeister-output.css

.bar {
  border-color: purple;
  background-color: red;
  color: blue;
}