[lmm]

That signature looks a little confused - you're saying "Monad" but you're talking about the specific case of Some and Maybe. The interface looks something like:

    interface Monad<F<?>> {
      fun flatMap<A, B>(argument: F<A>, function: A -> F<B>): F<B>
      // also need pure/point here but ignore that for now
    }

and then a specific implementation for e.g. Option looks like

    singleton OptionMonad implements Monad<Option>
      override fun flatMap<A, B>(argument: Option<A>, function: A -> Option<B>): Option<B> =
        when(argument) {
          is Some -> function(argument.value)
          is None -> None
        }

whereas you have implementations for other types in terms of those specific types too:

    singleton FutureMonad implements Monad<Future> {
      override fun flatMap<A, B>(argument: Future<A>, function: A -> Future<B>): Future<B> = ...
    }

The tricky part is that the type parameter F is itself a parameterised type, so it's a slightly higher level of abstraction than most interfaces (and one that most languages don't support).