///Build a list of values from 1 through `$n`.
///@ignore Documentation: http://sassylists.com/documentation.html#sl-range
///@param {Number} $n - maximum value
///@throws `$n` is not a number for `sl-range`.
///@throws `$n` is not unitless for `sl-range`.
///@throws `$n` is not greater than 0 for `sl-range`.
///@example
///   sl-range(5)
///   // 1 2 3 4 5
///@example
///   sl-range(1)
///   // 1
///@example
///   sl-range(-42)
///   // null
///@return {List | Number | Null}
 
@function sl-range($n) {
  @if type-of($n) != "number" {
    @warn "`#{inspect($n)}` is not a number for `sl-range`.";
    @return null;
  }

  @if not unitless($n) {
    @warn "`#{$n}` is not unitless for `sl-range`.";
    @return null;
  }

  @if $n < 1 {
    @warn "`#{$n}` is not greater than 0 for `sl-range`.";
    @return null;
  }

  @if $n == 1 { 
    @return $n;
  }

  $range: ();

  @for $i from 1 through $n {
    $range: append($range, $i);
  }

  @return $range;
}