@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...);
}