Adding a new transformer typeclass
First thing is the set of operations, or the signature.
Tell[F, E] has the signature
def tell(e: E): F[Unit].
Once you’ve got this down, you need to come up with a set of laws and a base typeclass. These have to be worked out at the same time, because choosing base classes with more operations gives you richer potential for laws but restricts implementations severely.
Come up with a rough sketch of the laws in Scala as methods but assuming no type parameters need to be
provided and all of the operations needed are in scope; use
<-> for the equality assertion.
To be continued.
Adding a new transformer typeclass instance
The characteristic which transformer typeclasses have in common is that instances of them are
commonly lifted through transformer stacks. I.e., if you have an
Ask[E => ?], you can
Ask[EitherT[E => ?, String, ?]]; a so-called inductive instance of
because the capability is not provided by
EitherT itself but the “inner monad”.
Because of this, there’s an argument to be made that there’s only one transformer typeclass instance you should need to provide: that of the “innermost” monad, the one that is actually implementing the typeclass operations.