It is seductive to imagine that effect handlers in an algebraic effect system are not part of the program itself but metalanguage-level folds over the program tree. And in traditional free-like formulations, this is in fact the case. The Eff monad represents the program tree, which has only two cases:
data Eff effs a where
Pure :: a -> Eff effs a
Op :: Op effs a -> (a -> Eff effs b) -> Eff effs b
data Op effs a where