We've already seen how simple types can be used to express domain constraints, as discussed in the Path-dependent types section. We implemented a lock that guaranteed at compile time that it is only possible to open and close it with the key created for this specific lock. We will conclude our study of type parameters and higher kinded types with two examples.

The first example will demonstrate an application of phantom types to create another version of the lock, which can guarantee the safety of state transitions at compile time without the use of inheritance.

The second example will show how self-recursive types can help to constrain possible subtyping.