@function __get-callback($function: __identity, $this-arg: null, $arg-count: null) {
    $result: if($arg-count, __exec('__base-callback', $function, $this-arg, $arg-count), '__base-callback');

    @return $result;
}


@function __bind-callback-1($value) {
    $function: __this('function');
    $this-arg: __this('this-arg');

    @return __call($function, $this-arg, $value);
}


@function __bind-callback-3($value, $index, $collection) {
    $function: __this('function');
    $this-arg: __this('this-arg');

    @return __call($function, $this-arg, $value, $index, $collection);
}


@function __bind-callback-4($accumulator, $value, $index, $collection) {
    $function: __this('function');
    $this-arg: __this('this-arg');

    @return __call($function, $this-arg, $accumulator, $value, $index, $collection);
}


@function __bind-callback-5($value, $other, $key, $map, $source) {
    $function: __this('function');
    $this-arg: __this('this-arg');

    @return __call($function, $this-arg, $value, $other, $key, $map, $source);
}


@function __bind-callback-any($arguments...) {
    $function: __this('function');
    $this-arg: __this('this-arg');

    @return __call($function, $this-arg, $arguments...);
}


@function __bind-callback($function, $this-arg: null, $arg-count: null) {
    @if (not __function-exists($function)) {
        @return '__identity';
    }

    @if ($this-arg == null) {
        @return $function;
    }

    $_: __scope((
        'function': $function,
        'this-arg': $this-arg
    ));

    $arg-count-case: (
        1: '__bind-callback-1',
        3: '__bind-callback-3',
        4: '__bind-callback-4',
        5: '__bind-callback-5'
    );

    @if ($arg-count > 5) {
        @return '__bind-callback-any';
    }

    $result: __bind(__get($arg-count-case, $arg-count), $this-arg);
    $_: __scope(false);

    @return $result;
}


@function __base-callback-shifted($value, $args...) {
    $function: __this('function');
    $this-arg: __this('this-arg');
    $shifted-arguments: __this('shifted-arguments');

    $arguments: set-nth($shifted-arguments, 1, $value);

    @return __call($function, $this-arg, $arguments...);
}


@function __base-callback($function: __identity, $this-arg: $__undefined__, $arg-count: null) {
    $type: if(__function-exists($function), 'function', type-of($function));

    @if ($function == null) {
        @return '__identity';
    }

    @if $type == 'list' {
        $shifted-arguments: set-nth($function, 1, null);
        $function: nth($function, 1);
        $_: __scope((
            'function': $function,
            'this-arg': $this-arg,
            'shifted-arguments': $shifted-arguments
        ));
        $base-callback-shifted: __bind('__base-callback-shifted');
        $_: __scope(false);

        @return $base-callback-shifted;
    }

    @if __is-native($function) {
        @return __ary($function, 1);
    }

    @if $type == 'function' {
        @return if(not __is-undefined($this-arg) and __is-function($function),
            __bind-callback($function, $this-arg, $arg-count),
            $function);
    }

    @return if($type == 'map',
        __base-matches($function, __is-falsey($arg-count)),
        __base-property($function));
}


@function __callback($function: __identity, $this-arg: null, $guard: null) {
    @if ($guard and __is-iteratee-call($function, $this-arg, $guard)) {
        $this-arg: null;
    }

    @return __base-callback($function, $this-arg);
}


/// Creates a function that invokes `$function` with the `_this` binding of `$this-arg`
/// and arguments of the created function.
/// 
/// If `$function` is a property name the
/// created callback returns the property value for a given element.
/// 
/// If `$function`
/// is a map the created callback returns `true` for elements that contain
/// the equivalent map properties, otherwise it returns `false`.
/// 
/// If `$function` is a list the created callback returns the result of
/// the function (first item of the list) right-partially applied with the
/// rest of the list items, called with the value as the first argument (see example).
///
/// @access public
/// @group Utility
/// @param {*} $func [_identity] - The value to convert to a callback.
/// @param {*} $this-arg [null] - The `_this` binding of `$function`.
/// @returns {Function} Returns the callback.
/// @example scss
///     $fully-lighten: _callback(lighten 100%);
///     $lightened-color: _exec($fully-lighten, black);
///     // => white

@function _callback($args...) {
    @return call(get-function('__callback'), $args...);
}

@function _iteratee($args...) {
    @return call(get-function('__callback'), $args...);
}