@function __either($this, $that: null, $predicate: '__is-truthy') {
    $this-bool: __exec(__base-callback($predicate), $this);

    @return if($this-bool, $this, $that);
}

/// Returns initial `$this` value unless the `$predicate` applied to 
/// `$this` is falsey; in which case, `$that` is returned.
/// 
/// The `$predicate` argument can be any type of supported Sassdash
/// callback.
/// 
/// @access public
/// @group Utility
/// @param {*} $this - The target preferred value
/// @param {*} $that [null] - The fallback value, in case `$this` is falsey.
/// @param {Function} $predicate ['is-truthy'] - The predicate to 
/// check `$this` against 
/// @returns {*} `$this` or `$that`
/// @example scss
/// $button-bg-color: false;
/// 
/// $foo: _either($button-bg-color, #C0FFEE);
/// // => #C0FFEE
/// 
/// $foo: _either($button-bg-color, #C0FFEE, _is-color);
/// // => #C0FFEE
/// 
/// $button-bg-color: green;
/// 
/// $foo: _either($button-bg-color, #C0FFEE);
/// // => green
/// 
/// // Usage with "shifted argument" callback
/// $foo: _either('abcde', 'xyz', str-index 'abc');
/// // => 'abcde'
/// 
/// $foo: _either('hijkl', 'xyz', str-index 'abc');
/// // => 'xyz'
@function _either($args...) {
    @return call(get-function('__either'), $args...);
}