I have been trying simple Monad Transformers where I have for comprehensions involving M[F[A]] where M and F are monads. How can I make M[F[A]] and M[S[A]] work together in a for comp if S is a different monad?
For example:
val a: Future[List[Int]] = ...
val b: Future[Option[Int]] = ...
a requires a ListT[Future, Int] and b requires an OptionT[Future, Int] but these do not compose, do I need to use another transformer? Would this depend on the order I use them in the for comp?
Monad Transformers help you in composing two values of type F[G[X]].
In other terms, monad transformers work with F[G[X]] because they leverage the fact that you know how to compose two G[X] if Monad[G] exists.
Now, in case of F[G[X] and F[H[X]], even if you state that G and H have Monad instances, you still don't have a general way of composing them.
I'm afraid composing F[G[X]] and F[H[X]] has no general solution with monad transformers.