This is SASS function which adds up all the values in a list.

sum-function.scss

@function sum($list) {
    $total: 0;
    // Check it's the first item in the list and set it as the total
    @each $item in $list {
        @if $item == nth($list, 1) {
            $total: $item;
        }
        // For each item in the list, add it to the total
        $total: $total + $item;
    }
    @return $total;
}