I am pleased to announce the 5th version of the unfoldable package. (This is the first announcement, you didn't miss anything.) http://hackage.haskell.org/package/unfoldable-0.4.0 Just as there's a Foldable class, there should also be an Unfoldable class. This package provides one: class Unfoldable t where unfold :: Unfolder f => f a -> f (t a) Writing instances of Unfoldable is similar to writing Traversable instances. For example, given a data type data Tree a = Empty | Leaf a | Node (Tree a) a (Tree a) a suitable instance would be instance Unfoldable Tree where unfold fa = choose [ pure Empty , Leaf <$> fa , Node <$> unfold fa <*> fa <*> unfold fa ] The choose function comes from the Unfolder class: class Applicative f => Unfolder f where choose :: [f x] -> f x (If f is an Alternative instance, choose is simply Data.Foldable.asum.) Different unfolders provide different ways of generating values, for example: - Random values - Enumeration of all values (depth-first or breadth-first) - Convert from a list - An implementation of QuickCheck's arbitrary should also be possible (still working on that) Some examples can be found in the examples directory in the github repo: https://github.com/sjoerdvisscher/unfoldable Ideas and comments are welcome! greetings, Sjoerd