generator_example.php

<?php

describe(strict_types = 1);

/**
 * Example code for consuming data from a Store API (called Storeify) and chaining generators
 * together to filter, map, and eventually reduce all Order data into a
 * daily revenue total.
 */

function getOrdersFromLastDay(): iterable {
  $limit = 20;
  $requestParams = [
    'lastUpdatedAfter' => new DateTime('yesterday'),
    'limit' => $limit,
    'offset' => 0
  ];
  $orders = [];
  
  do {
    // Grab a batch of orders from Storeify and yield from that list.
    $orders = Storeify::getOrders($requestParams);
    yield from $orders;
    
    $requestParams['offset'] += $limit;
  } while (!empty($orders));
}

/**
 * Consumes a generator that produces lists of products
 * and produces a new generator that yields a flat list
 * of products.
 */
function flatten(iterable $itr): iterable {
  foreach ($itr as $products) {
    yield from $products;
  }
}

/**
 * Assume orders have a Shipping Country and a list of Products
 * [
 *    'ShipCountry' => 'US',
 *    'Products' => [
 *       [ 'name' => 'Pro Tech Toolkit', 'price' => 59.95, 'quantity' => 2],
 *       [ 'name' => 'iPhone 6 Battery', 'price' => 24.99, 'quantity' => 1]
 *    ]
 * ]
 */
$orders = getOrdersFromLastDay();

// Flatten list of orders into list of all products sold.
$allProducts = flatten(iterator_map(function($order) {
  return $order['Products'];
}, $orders));

// Only include 'Pro Tech Toolkit' purchases.
$toolkitProducts = iterator_filter(function($product) {
  return $product['name'] === 'Pro Tech Toolkit';
}, $allProducts);

// Up until this point, no work has actually been done.
// $toolkitProducts is a generator can be passed around to other functions
// as a lazy stream of pro tech toolkit products.

// Once iterator_reduce is called, it begins winding its way through
// the composed generators and actually pulling down resources from the Store API
// and mapping and filtering them.
$dailyToolkitRevenue = iterator_reduce(function($total, $toolkit) {
  return $total + ($toolkit['price'] * $toolkit['quantity']);
}, $toolkitProducts, 0);

Hello, thanks for the article on Medium about generators!
There is just a typo: describe(strict_types = 1); should be declare(strict_types = 1);.

Thanks,
Jaroslav