// dump array key -> value without noise
function pprint($array) {
    echo "<pre style=\"background-color: #ddd;\">";
    print_r($array);
    echo "</pre>";
}

function randomString($length) {
    if (gettype($length) !== 'integer') {
        $length = 30;
    }
    $string = base64_encode(openssl_random_pseudo_bytes($length));
    return $string;
}

// [a] -> (a -> [b]) -> [b]
function _flatMap($arr, callable $fn) {
    return array_merge([], ...array_map($fn, $arr));
}
function _flatMapK($arr, callable $fn) {
    return array_merge([], ...array_map($fn, array_keys($arr), $arr));
}

// re-arranging parameters on FP-style built-in functions

// [a] -> (a -> b) -> [b]
function _map($arr, callable $callback) {
    return array_map($callback, $arr);
}
// [v] -> (k -> v -> w) -> [w]
function _mapK($arr, callable $callback) {
    return array_map($callback, array_keys($arr), $arr);
}

// [a] -> (a -> bool) -> [a]
function _filter($arr, callable $callback) {
    return array_filter($arr, $callback);
}
// [v] -> (k -> v -> bool) -> [v]
function _filterK($arr, callable $callback) {
    return array_filter($arr, $callback, ARRAY_FILTER_USE_BOTH);
}

// [a] -> b -> (b -> a -> b) -> [b]
function _reduce($arr, $init, callable $callback) {
    return array_reduce($arr, $callback, $init);
}