@function __base-merge-deep(
  $map,
  $source,
  $key,
  $merge-function,
  $customizer,
  $stack-a: (),
  $stack-b: ()
) {
  $length: length($stack-a);
  $src-value: __get($source, $key);

  @while $length > 0 {
    @if __is-equal(nth($stack-a, $length), $src-value) {
      $map: __set($map, $key, nth($stack-b, $length));

      @return $map;
    }

    $length: $length - 1;
  }

  $value: __get($map, $key);
  $result: if(
    $customizer,
    __exec($customizer, $value, $src-value, $key, $map, $source),
    $__undefined__
  );
  $is-common: __is-undefined($result);

  @if $is-common {
    $result: $src-value;

    @if __is-list($src-value) {
      $result: if(__is-list($value), $value, __either($value, ()));
    } @else if __is-plain-map($src-value) or __is-arglist($src-value) {
      $result: if(
        __is-arglist($value),
        __to-map($value),
        if(__is-plain-map($value), $value, ())
      );
    } @else {
      $is-common: false;
    }
  }

  $stack-a: append($stack-a, $src-value);
  $stack-b: append($stack-b, $result);

  @if $is-common {
    $_: __this(
      "map",
      __set(
        $map,
        $key,
        __exec(
          $merge-function,
          $result,
          $src-value,
          $customizer,
          $stack-a,
          $stack-b
        )
      )
    );
  } @else if not __is-equal($result, $value) {
    $_: __this("map", __set($map, $key, $result));
  }

  @return __this("map");
}

@function __base-merge-function($src-value, $key, $source) {
  $map: __this("map");
  $customizer: __this("customizer");
  $stack-a: __this("stack-a");
  $stack-b: __this("stack-b");
  $is-source-list: __this("is-source-list");

  @if __is-map-like($src-value) {
    $base-merge-deep: __bind("__base-merge-deep");

    @return __exec(
      $base-merge-deep,
      $map,
      $source,
      $key,
      __base-merge,
      $customizer,
      $stack-a,
      $stack-b
    );
  }

  $value: __get($map, $key);
  $result: if(
    $customizer,
    __exec($customizer, $value, $src-value, $key, $map, $source),
    $__undefined__
  );
  $is-common: __is-undefined($result);

  @if $is-common {
    $result: $src-value;
  }

  @if ($is-source-list or not __is-undefined($result)) and
    ($is-common or not __is-equal($result, $value))
  {
    $map: __this("map", __set($map, $key, $result));
  }

  @return true;
}

@function __base-merge(
  $map,
  $source,
  $customizer: null,
  $stack-a: (),
  $stack-b: ()
) {
  @if not __is-map-like($map) {
    @return $map;
  }

  $is-source-list: __is-list($source);
  $func: if($is-source-list, "__list-each", "__base-for-own");
  $_: __scope(
    (
      "map": $map,
      "customizer": $customizer,
      "stack-a": $stack-a,
      "stack-b": $stack-b,
      "is-source-list": $is-source-list
    )
  );
  $base-merge-function: __bind("__base-merge-function");
  $_: __exec($func, $source, $base-merge-function);
  $map: __this("map");
  $_: __scope(false);

  @return $map;
}

@function __list-merge($list, $source, $customizer: null) {
  // todo
}

@function __merge($arguments...) {
  $assigner: __create-assigner("__base-merge");

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

/// Recursively merges own enumerable properties of the source map(s), that
/// don't resolve to `undefined` into the destination map. Subsequent sources
/// overwrite property assignments of previous sources. If `$customizer` is
/// provided, it is invoked to produce the merged values of the destination and
/// source properties. If `$customizer` returns `undefined` merging is handled
/// by the method instead. The `$customizer` is bound to `$this-arg` and invoked
/// with five arguments; (source-value, other-value, key, map, source).
///
///
/// @access public
/// @group Map
/// @param {Map} $map The destination map.
/// @param {Map...} $source... The source maps.
/// @param {Function} $customizer [null] The function to customize merging properties.
/// @param {*} $this-arg [null] - The `_this` binding of `$customizer`.
/// @returns {Map} Returns `map`.
/// @example scss
/// $users: (
///   'data': (( 'user': 'barney' ), ( 'user': 'fred' ))
/// );
/// $ages: (
///   'data': (( 'age': 36 ), ( 'age': 40 ))
/// );
/// $foo: _merge($users, ages);
/// // => ( 'data': (( 'user': 'barney', 'age': 36 ), ( 'user': 'fred', 'age': 40 )) )
///
/// // using a customizer callback
/// // todo

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