@function is-int($str) {
  @for $i from 0 through 9 {
    @if '#{$i}' == $str {
      @return true;
    }
  }
  @return false;
}

@function to-int($str) {
  @for $i from 0 through 9 {
    @if '#{$i}' == $str {
      @return $i;
    }
  }
  @return $str;
}