Building up specifications

CustomerSpecificationSet.php

<?php
// This relates to Matthias Verraes talk on unbreakable domain models 
// (which is very good).  Slides are http://verraes.net/2013/06/unbreakable-domain-models/
// and this is about the end of the talk (about three-quarters through the slide deck).
//
// I like the whole specification thing, but want to be able to combine them
// dynamically (in the language of the talk, the tenant can create their own
// specification of what a premium custom is), using building blocks we provide.

class CustomerSpecificationSet
{

    /** 
     * @return CustomerSpecificationSet 
     */    
    public function and (CustomerSpecification $specification) 
    {
    //...
    }

    /** 
     * @return CustomerSpecificationSet 
     */
    public function or (CustomerSpecification $specification) 
    {
    // ...
    }

    /** 
     * @return bool 
     */
    public function isSatisfiedBy (Customer $customer) 
    {
    // ...
    }
}

// "Domain Expert says... premium customers have three orders, at least
// one of which should be recent."
$premiumCustomers = new CustomerSpecificationSet;
$threeOrders = new CustomerIsPremiumWith3Orders($orderRepository);
$recentOrders = new CustomerIsPremiumWithRecentOrder($orderRepository);

$premiumCustomers->and($threeOrders)->and($recentOrders);

$premiumCustomers->isSatisfiedBy($aParticularCustomer);