iterable

• Iterable is a pseudo-type introduced in PHP 7.1.
• It accepts any array or object implementing the Traversable interface.
• Can be used as a parameter type to indicate that a function requires a set of values, but does not care about the form of the value set since it will be used with foreach.
  If a value is not an array or instance of Traversable, a TypeError will be thrown.

  <?php
  
  function foo(iterable $iterable) {
      foreach ($iterable as $value) {
          // ...
      } 
  }
  
  ?>

  Parameters declared as iterable may use NULL or an array as a default value.

  <?php
  
  function foo(iterable $iterable = []) {
      // ...
  }
  
  ?>

• Can also be used as a return type to indicate a function will return an iterable value.
  If the returned value is not an array or instance of Traversable, a TypeError will be thrown.

  <?php
  
  function bar(): iterable {
      return [1, 2, 3];
  }
  
  ?>

• Functions declaring iterable as a return type may also be generators.

  <?php
  
  function gen(): iterable {
      yield 1;
      yield 2;
      yield 3;
  }
  
  ?>
  

<?php

interface Example {
    public function method(array $array): iterable;
}

class ExampleImplementation implements Example {
    public function method(iterable $iterable): array {
        // Parameter broadened and return type narrowed.
    }
}

?>

Classes extending/implementing may broaden methods using array or Traversable as parameter types to iterable or narrow return types from iterable to array or Traversable.