In the meantime, we'll entertain ourselves with the following ideas:
- Monad transformers require an instance of a monad for the outer layer
- A monad transformer itself has a monad
- Will something bad happen if we use a monad transformer as an instance of a monad for another monad transformer?
Let's try it out. We've already implemented two monad transformers so let's bring them together. To start, we'll define the type of stack. It will be EitherT wrapped in OptionT. This will give us an unwrapped type of the following code:
Future[Either[String, Option[Fish]]]
This can be interpreted as an operation which takes time and might return an error in the case of nontechnical failure and needs to have an explanation (technical failures are denoted by failed Futures). An Option represents an operation which can return no result in a...