@function add-two($v) {
  @return $v + 2;
}

$add-two-fn: get-function(add-two);

@function add-two($v) {
  @error "This should not have been called.";
}

$lighten-fn: get-function(lighten) !default;

@function lighten($color, $amount: null) {
  $amount: 10% !default;
  @return call($lighten-fn, $color, $amount);
}

.result {
  captured-sass-fn: call($add-two-fn, 10);
  captured-ruby-fn: lighten(red);
}