@function __base-assign($map, $source, $customizer: false) {
  @if not __is-map-like($map) {
    @return $map;
  }

  $props: __keys($source);

  @if not $customizer {
    @return __base-copy($source, $map, $props);
  }

  @each $key in $props {
    $value: if(map-has-key($map, $key), __get($map, $key), $__undefined__);
    $result: __exec(
      $customizer,
      $value,
      __get($source, $key),
      $key,
      $map,
      $source
    );

    @if ($result != $value) or ($value == null and not map-has-key($map, $key))
    {
      $map: __set($map, $key, $result);
    }
  }

  @return $map;
}

@function __create-assigner-function($arguments...) {
  $assigner: __this("assigner");
  $length: length($arguments);
  $map: if($length >= 1, nth($arguments, 1), null);
  $customizer: null;

  @if ($length < 2 or $map == null) {
    @return $map;
  }

  @if (
    $length >
      3 and
      __is-iteratee-call(
        nth($arguments, 2),
        nth($arguments, 3),
        nth($arguments, 4)
      )
  ) {
    $length: 2;
  }

  @if $length > 3 and __function-exists(nth($arguments, $length - 1)) {
    $length: $length - 1;
    $customizer: __bind-callback(
      nth($arguments, $length),
      nth($arguments, $length - 1),
      5
    );
  } @else if $length > 2 and __function-exists(nth($arguments, $length)) {
    $length: $length - 1;
    $customizer: nth($arguments, $length + 1);
  }

  @for $index from 1 through $length {
    $source: nth($arguments, $index);

    @if ($source) {
      $map: __exec($assigner, $map, $source, $customizer);
    }
  }

  @return $map;
}

@function __create-assigner($assigner) {
  $_: __scope(
    (
      "assigner": $assigner
    )
  );

  $scoped-function: __bind("__create-assigner-function");

  $_: __scope(false);

  @return $scoped-function;
}

@function __assign($arguments...) {
  $assigner: __create-assigner("__base-assign");

  @return __exec($assigner, $arguments...);
}

/// Assigns own enumerable properties of source map(s) to the destination
/// map. Subsequent sources overwrite property assignments of previous sources.
/// If `$customizer` is provided it is invoked to produce the assigned values.
/// The `$customizer` is bound to `$this-arg` and invoked with five arguments;
/// ($map-value, $map-value, $key, $map, $source).
///
///
/// @access public
/// @group Map
/// @param {Map} $map - The destination map.
/// @param {Map...} $sources... - The source maps.
/// @param {Function} $customizer [false] - The function to customize assigning values.
/// @param {*} $this-arg [null] - The `_this` binding of `$customizer`.
/// @returns {Map} Returns `map`.
///
/// @example scss
/// _assign(( 'user': 'barney' ), ( 'age': 40 ), ( 'user': 'fred' ));
/// // => ( 'user': 'fred', 'age': 40 )
///
/// // using a customizer callback
/// $defaults: _partial-right(_assign, _either);
/// _exec($defaults, (( 'user': 'barney' ), ( 'age': 36 ), ( 'user': 'fred' )));
/// // => ( 'user': 'barney', 'age': 36 )

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

/// @alias _assign

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