Functional utilities a la PHP

functional.php

<?php

function pipe(...$fns) {
  return function($x) use ($fns) {
    return array_reduce(
      $fns,
      function($acc, $fn) { return $fn($acc); },
      $x
    );
  };
}

function flip($fn) {
  return function($a, $b, ...$args) use ($fn) {
    return $fn($b, $a, ...$args);
  };
}