Hello Dan What is /unflt/ ? I'm presuming it can't be unflatten [*] as I don't see how it would work. If I was making a flatten class, I wouldn't want Monad as a constraint on the result container. Monad doesn't really say anything about "containers" - there are some containers that are Monads, but there are monads that aren't containers and there are containers that aren't monads. The Foldable type class in Data.Foldable is quite like a class for flattenable things, however it folds (flattens) into a monoidal thing (often called a "summary" value). This isn't so good for flattening into a list. List has a monoid instance but it is very inefficient. If you want to flatten into a list-like thing rather than fold into a summary value, you would really need a better class something like "Cons" class Cons t where nil :: t a cons :: a -> t a -> t a By the way, there is no Monoid instance for Data.Tree at least at version containers-0.3.0.0. [*] If you are wanting unflatten as well, things get quite complicated...