Hi, I was surprised that I could not find a "classical" binary tree data structure on hackage, mainly for teaching purposes, like: data BinTree a = Nil | Node (BinTree a) a (BinTree a) with a couple of utility functions and instances (i.e. in-order traversal). Surely, one may argue, that such a tree can always be defined on the fly when needed, but nobody would argue so for lists or tuples. (Although I've rarely seen someone redefining lists, it is quite common to introduce user-defined products or enumerations.) There are rose trees in Data.Tree and many other variants of trees are conceivable, ie. data Boom a = Leaf a | Node (Boom a) (Boom a) or a kind of combination: data BinLeafTree a b = Leaf b | Node (BinLeafTree a b) a (BinLeafTree a b) I don't know, why I find the above BinTree most important. I'm not even sure if such a tree could be re-used for Search- or AVL-trees that need strict fields with redundant size or height counters for efficiency reasons. In any case I would find such a data type at least as useful as http://hackage.haskell.org/cgi-bin/hackage-scripts/package/OneTuple Who would supply and maintain such a package? Or did I simply not search hard enough? Cheers Christian P.S. I took the (non-empty) "Boom" from the Boom-Hierarchy described in "An Exploration of the Bird-Meertens Formalism" by Roland Backhouse 1988, Groningen