Actually that's not true. Now that we have Phantheck, all QuickCheck tests (which are a very pleasant superclass of unit tests) can just be lifted to the type l

Actually that's not true. Now that we have Phantheck, all QuickCheck tests (which are a very pleasant superclass of unit tests) can just be lifted to the type level with TH magic. This makes them dramatically more useful. This is because you cannot traditionally write a function that requires that its arguments pass some test, but you can write a function that requires they have a certain type. But now you can require arbitrary QuickCheck-able properties via types. Note that you can still capture any property you want via non-metaprogramming types (though you may require a trusted kernel to keep things performant) but that can be trickier to work with than a test for people who are much more accustomed to tests than types. Also note that writing a function that should work, but won't typecheck can also reveal a flaw in the model (or in what you thought the model should be, or in what kind of operations you thought should work)