Simple enum function for doing things like automatic z-indices.

enum.scss

@function enum($keys) {
  $index-map: ();
  $running-index: 1;
  @each $k in $keys {
    @if type-of($k) == list {
      // NOTE: We do _not_ check if this value is greater than prior values.
      // This is intentional.
      $running-index: nth($k, 2);
      $k: nth($k, 1);
    }
    $index-map: map-merge($index-map, ($k: $running-index));
    $running-index: $running-index + 1;
  }
  @return $index-map;
}

// Like typical enums, you can also change the value mid-way through,
// and it will carry on incrementing after that.
// This does mean you can end up with multiple keys pointing to the same value,
// but I don't recommend that.
$app-z-indices: enum(
  'bottom'
  'middle'
  ('top' 10)
  'more-top'
);

@function app-z-index($key) {
  @if map-has-key($app-z-indices, $key) {
    @return map-get($app-z-indices, $key);
  }

  // throw errors for non-existent keys, that way it's just as good as vars.
  @error "'#{$key}' is not a known z-index key.";
}

.foo {
  /* should be 1 = 'bottom' */
  z-index: app-z-index('bottom');
}

.bar {
  /* should be 2 = 'middle' */
  z-index: app-z-index('middle');
}

.baz {
  /* should be 10 = 'top' */
  z-index: app-z-index('top');
}