main.php

<?php

declare(strict_types=1);

use Psl\Async;
use Psl\IO;

require 'vendor/autoload.php';

function fetch(string $id): string {
  IO\write_line('Fetching resource with id #%s', $id);

  Async\sleep(1); // mocking an I/O operation
  
  return $id;
}

final class Service {
  private array $cache = [];
  
  public function get(string $id): string
  {
    if (isset($this->cache[$id])) {
      return $this->cache[$id];
    }
    
    $item = fetch($id);
    $this->cache[$id] = $item;
    
    return $item;
  }
}

Async\main(static function(): int {
  $service = new Service();
  [$one, $two] = Async\concurrently([
    static fn() => $service->get('foo'),
    static fn() => $service->get('foo'),
  ]);
  
  var_dump($one, $two);
  
  $sequence = new Async\Sequence($service->get(...));
  [$one, $two] = Async\concurrently([
    static fn() => $sequence->waitFor('bar'),
    static fn() => $sequence->waitFor('bar'),
  ]);
  var_dump($one, $two);  

  return 0;
});