@function __sum($collection, $iteratee: '__identity', $this-arg: null, $args...) {
    @if $this-arg and __is-iteratee-call($collection, $iteratee, $this-arg) {
        $iteratee: '__identity';
        $this-arg: null;
    }

    $collection: __to-list($collection);

    $iteratee: __get-callback($iteratee, $this-arg, 3);
    $length: length($collection);
    $result: 0;

    @while $length > 0 {
        $value: __parse-float(__exec($iteratee, nth($collection, $length)));

        $value: __either($value, 0);

        $result: $result + $value;

        $length: $length - 1;
    }

    @return $result;
}


///
/// Gets the sum of the values in `$collection`.
///
/// @access public
/// @group Math
/// @param {Array|Object|string} $collection The collection to iterate over.
/// @returns {number} Returns the sum.
/// @example scss
/// $foo: _sum(4 6 2);
/// // => 12
/// 
/// $foo: _sum(('a': 4, 'b': 6, 'c': 2));
/// // => 12

@function _sum($args...) {
    @return call(get-function('__sum'), $args...);
}