I'm pleased to announce fixpoint 0.1, a (for now) small generic programming library which allows data types to be manipulated as fixpoints of their underlying functors. The library is mostly based on "Functional Programming with Bananas, Lenses, Envelopes and Barbed Wire" by Erik Meijer, Maarten Fokkinga and Ross Paterson. The basic idea is to use associated data types, a recent GHC extension, to associate recursive types with their underlying functors. The core of the library is just one typeclass: class Functor (Pre t) => Fixpoint t where data Pre t :: * -> * -- | Projection from the data type to its underlying functor. project :: t -> Pre t t -- | Injection from the underlying functor into the data type. inject :: Pre t t -> t Here, Pre t is a functor such that its fixpoint is t. For instance, for lists we have: instance Fixpoint [a] where data Pre [a] s = Nil | Cons a s project [] = Nil project (x:xs) = Cons x xs inject Nil = [] inject (Cons x xs) = x : xs With this, we can easily define things like catamorphisms: cata :: Fixpoint t => (Pre t s -> s) -> t -> s cata f = f . fmap (cata f) . project which can then be used for generic programming: size :: (Fixpoint t, Foldable (Pre t)) => t -> Int size = cata (F.foldr (+) 1) At the moment, the package doesn't contain much more than the above but this will change soon(ish). To enjoy the minimalistic interface of fixpoint-0.1, grab it from Hackage or from my site: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/fixpoint-0.1 http://www.cse.unsw.edu.au/~rl/code/fixpoint.html Comments and suggestion are always welcome. Roman