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On Ivo Jansch's blog today, he takes the other side of things on the issue of fluent interfaces.
Ironically, Martin's original example already demonstrates the problem. (In the example) newOrder could be implemented in two ways: Create an order and return that new order, and Add an order to this customer and return the customer.
This can lead to confusion; the 'with()' method employs method b. If you think that newOrder returns an Order, and hence 'with()' is a method of the Order class, look at 'skippable'. Martin states that order lines are skippable. So what's in front of skippable() is an OrderItem. That means that with() must have returned an OrderItem. If that is true, then with() is a method of OrderLine too!
After Martin, Mike and Paul have demonstrated the usefulness of Fluent Interfaces, I'd like to take a look at the downside.
Confused yet? Well, be sure to check out his examples - they help to clear up at least the items mentioned above. His point is that, while the fluent interface does make it more readable, it can also make it more confusing - taking things down to such a simple level can sometimes eliminate some needed complexity to make things work properly (or as the user percieves "properly" to be)...