@function __assign-defaults($map-value, $source-value, $args...) {
  @return if(
    __is-undefined($map-value) or __is-null($map-value),
    $source-value,
    $map-value
  );
}

@function __defaults($map: null, $args...) {
  @if $map == null {
    @return $map;
  }

  $arguments: ($map);

  @each $arg in $args {
    $arguments: append($arguments, $arg);
  }

  $arguments: append($arguments, "__assign-defaults");

  @return __assign($arguments...);
}

/// Assigns own enumerable properties of source map(s) to the destination
/// map for all undefined destination properties. Once a
/// property is set, additional defaults of the same property are ignored.
///
///
/// @access public
/// @group Map
/// @param {Map} $map The destination map.
/// @param {Map...} $source... The source maps.
/// @returns {Map} Returns `$map` with defaults.
/// @example scss
/// $foo: _defaults(( 'user': 'barney' ), ( 'age': 36 ), ( 'user': 'fred' ));
/// // => ( 'user': 'barney', 'age': 36 )

@function _defaults($args...) {
  @return call(get-function("__defaults"), $args...);
}