[AdieuToLogic]

  > What if you want to write a library that
  works with either blocking calls or
  asynchronous promises?

  You don't need higher-kinded types for that.
  Just always return a promise, except the
  blocking version always return a promise that
  is already fulfilled.

While strictly speaking this approach will work, a benefit of employing higher-kinded types (HTK's) to express a solution is being able to optimize without having to alter the solution.

For this example, the Scalaz Id[0] type provides this adaptation. Since it conforms to what is expected of a container, yet does not require the overhead of fabricating one just to satisfy expectations, it affords HKT-based logic to be useful while automatically selecting the optimal implementation.

In short, working in an environment which supports HKT's often allows implementations to reduce needless overhead, simplify implementations (by only having to address "happy path" logic), and promotes stability in a code base due to formally expressing the expectations of collaborators.

0 - http://eed3si9n.com/learning-scalaz/Id.html