["Request"] Monad, Applicative and Functor instances for Trampoline about bow

[truizlop]

I think this is now ready to continue!

[ferranpujolcamins]

Maybe also Traverse?

[truizlop]

Sounds good to me. Actually, I've needed Traverse on Trampoline more than a couple of times. Are you taking this as well?

[ferranpujolcamins]

Yes! :)

[ferranpujolcamins]

@truizlop

In order to add any typeclass instance to Trampoline we first need to add support for the higher kinded types simulation. Since this forces Trampoline to be a final class, the current enum case as a subclass approach won't work. We need to change to a wrapped enum approach as done in Free.

For the cases Done and Defer, the change is straightforward. For FlatMap no so much, because the subclassing mechanism was used to hide its type parameter A.

My proposed solution is to use Coyoneda. If you have closer look you'll see that FlatMap has the same data than Coyoneda<ForTrampoline, Trampoline<B>>, but Coyoneda hides the A parameter for us.

Here (trampolineTypeclasses branch) I implemented this approach successfully. However, the branch is based on #623 since I prefer the way Coyoneda is handled there (with polymorphic functions instead of casts). If that PR is finally not approved in a short time, I will of course rewrite the branch to work with the current Coyoneda implementation, I'm not trying to put pressure on you to approve the other PR.

Until we decide on which approach to follow, I'll keep this on hold.

[truizlop]

Closed by #636