SCSS Functions to manipulate border radius values

border-radius-input.scss

// ----
// libsass (v3.2.5)
// ----

$br: 1px 2px 3px;

@function blowup-border-radius($br) {
  $long-br: $br;
  @if length($br) == 1 {
    $long-br: ($br $br $br $br);
  }
  @else if length($br) == 2 {
      $long-br: join($br, $br);
  }
  @else if length($br) == 3 {
     $long-br: append($br, nth($br, 1));
  }
  @return $long-br;
}

@function adjust-radius-list($list, $delta) {
  $result: ();
  @each $item in $list {
    $val: $item + $delta;
    @if $val < 0 {$val: 0px;}
    $result: append($result, $val);
  }
  @return $result;
}

@mixin spread-border-radius($br){
  $br: blowup-border-radius($br);
  border: {
    top-radius: nth($br, 1);
    right-radius: nth($br, 2); 
    bottom-radius: nth($br, 3);
    left-radius: nth($br, 4);
  };
}

#a {
  /*Add a number and keep as list*/
  border-radius: adjust-radius-list(blowup-border-radius($br), 2px);
  /*Add a number and expand*/
  @include spread-border-radius(adjust-radius-list($br, 2))
}

border-radius-output.css

#a {
  /*Add a number and keep as list*/
  border-radius: 3px 4px 5px 3px;
  /*Add a number and expand*/
  border-top-radius: 3px;
  border-right-radius: 4px;
  border-bottom-radius: 5px;
  border-left-radius: 3px;
}