<===> input.scss
$list: foo bar baz;
$list--comma: foo, bar, baz;
$single: foo;

div {
  _list-space: list-separator($list);
  _list-comma: list-separator($list--comma);
  _single-item: list-separator($single);
}
<===> output.css
div {
  _list-space: space;
  _list-comma: comma;
  _single-item: space;
}