/**
* This method returns an option containing the value paired with the lookup key x.
*
* @access public
* @static
* @param ILinkedList\Type $xss the left operand
* @param Core\Equality\Type $x the key being looked up
* @return IOption\Type an option containing the associated
* value
* @throws Throwable\UnexpectedValue\Exception indicates that the list is not
* associative
*/
public static function lookup(ILinkedList\Type $xss, Core\Equality\Type $x) : IOption\Type
{
if ($xss->__isEmpty()) {
return IOption\Type::none();
}
$xs = $xss->head();
if (!ITuple\Module::isPair($xs)->unbox()) {
throw new Throwable\UnexpectedValue\Exception('Unable to process tuple. Expected a length of "2", but got a length of ":length".', array(':length' => $xs->__length()));
}
if ($x->__eq(ITuple\Module::first($xs))) {
return IOption\Type::some(ITuple\Module::second($xs));
}
return ILinkedList\Module::lookup($xss->tail(), $x);
}