Mastering Functional Programming

Generalizing the pattern

Obviously, the pattern we discussed previously is not specific to EitherT. It is something we are frequently going to encounter when dealing with effect types. It is not about Future and Either, but about combining two independent types.

Since this task repeats from effect type to effect type, it was generalized into a pattern. The pattern goes as follows. First, you pick up an effect type that you would like to be combinable with other arbitrary effect types. Then, you define the alternative combinable version of this type. This way, for Either, we have defined a combinable version of it, EitherT.

After that, you define all of the necessary type classes of the given effect type, optionally depending on whatever you ...