Type classes: expose super class functions in sub classes. (WIP)

[jbgi]

This is an alternative approach to #1620 achieving the goal of ensuring inheritance-based implementations of type classes, here via seal of type class + Template.

This PR also expose super class functions in sub classes: this avoid going through no-op methods to switch to super classes in many cases.

WIP: Monoid/Semigroup and Functor->Monad only. For early review. Remaining type classes will follow.

[jdegoes]

@jbgi Looks good to me! /cc @edmundnoble

[TomasMikula]

👍 I like it. It even seems clearer to me than the previous approach.

[TomasMikula]

Something for future thought: Maybe we could save some Templates if we defined the actual typeclasses to be used as

type Monad[F[_]] = InstanceOf[Monad.Class[F]]

where InstanceOf would declared as

type InstanceOf[A] <: A

and (privately) implemented as

type InstanceOf[A] = A

Now implementations of Monad wouldn't have to also extend Bind, Applicative, Apply, Functor. They would just extend Monad.Class and be "tagged" as Monad by zero-cost wrapping in InstanceOf.

(Implicits for Monad, if any, would now have to go in the companion of Monad.Class.)

[jbgi]

Interesting, need to try!

[TomasMikula]

Travis says that some tuts need to be fixed.

[jbgi]

@TomasMikula quick update: I tested your suggested encoding this week-end. Works very well and avoid quite a bit of boilerplate classes. Currently preparing a replacement for this PR use it. (also I am thinking that we could also use it in 7.3.x ...)

[TomasMikula]

Good to hear! Though I think it's fine to finish this PR (by fixing the build) and do the refactoring in the next round.

[jbgi]

closing in favor of #1647.