> Not in a language with decent abstraction capabilities.

My question has nothing to do with the language. You are adding a relatively advanced construct which requires a lot of prior knowledge to understand: Free monad, monad, applicative functor, functor, monoid. Then you need to understand the laws that monads must obey, which are not enforced by the type system. Before you get there, a passing grasp with Haskell and higher kinded types will help, and some basics in category theory too.

Saying that adding the free monad doesn't have any cost in understanding is very naïve. You forgot how long it took you to reach that understanding.