generic-sample.php

<?php

interface Hash {
  public function hash(Hasher $hasher): int;
}

interface ConcurrentHashMap<K: Hash, V> {
  // ...

  public function flatten<S>(): ConcurrentHashMap<K, S>
    where
     V: iterable<S>
  ;
  
  public function mapConcurrently<R>((fn(K, V): R) $f): ConcurrentHashMap<K, R>;

  public function awaitAll<O>(): ConcurrentHashMap<K, O>
    where
      V: Future<O>
  ;
}

class AmphpConcurrentHashMap<K: Hash, V> implements ConcurrentHashMap<K, V> {
  // ...

  public function mapConcurrently<R>((fn(K, V): R) $f): AmphpConcurrentHashMap<K, R> {
    // ..
  }

  public function awaitAll<O>(): AmphpConcurrentHashMap<K, O>
    where:
      V: Future<O>
  {
     // ...
  }
}

function get_user_async(Id $id): Future<User> {
  // ...
}

function get_users(Id ...$ids): ConcurrentHashMap<Id, User> {
  $hm = new AmphpConcurrentHashMap::<Id, Future<User>>();
  foreach ($ids as $id) {
    $hm->insert($id, get_user_async($id));
  }

  return $hm->awaitAll::<User>();
}

$users = get_users($id1, $id2);
$comments = $users
  ->map::<vec<Comment>>(fn($id, $user): vec<Comment> => $user->getComments())
  ->flatten::<Comment>();

// $comments -> vec<Comment>