[gpderetta]

With (unchecked) exceptions the custom validator can abort the whole stack non locally and transport the error to the original call stack, the validation library being none the wiser.

With stackful coroutines, the custom validator can transport the error to the original call site, handle it and resume back into the validation library if the error was recovered, or abandon the validation (as with exceptions) if unrecoverable.

With HKTs, the validation library can be agnostic on the specific return type of the custom validator.