I have some lists of integers; e.g., [0,1,2,2,3,3,3,2,1,2,3,3,1] Think of each integer value as representing the indentation level in a hierarchical outline: e.g., 0 1 2 2 3 3 3 2 1 2 3 3 1 I want to convert the list into a structure that better represents the hierarchy. So, I first define a datatype to represent each node of the new structure: data Node = Nd Int [Node] That is, a node consists of an Int representing the value of the node, followed by a list of its immediate child nodes. (In principle, I can deduce the value of a node simply from the nesting level, of course, but in the real problem I'm trying to solve, each node contains other information that I need to preserve as well.) Next, I define some functions to perform the transformation isChild :: Int -> Node -> Bool isChild i (Nd j _) = (j > i) isChild _ _ = False prepend :: Int -> [Node] -> [Node] prepend i [] = [Nd i []] prepend i ns = (Nd i f):s where (f,s) = span (isChild i) ns unflatten :: [Int] -> [Node] unflatten ns = foldr prepend [] ns Finally, I add some code to display the result in an aesthetically pleasing way: showsNodeTail :: [Node] -> String -> String showsNodeTail [] = showChar '}' showsNodeTail (n:ns) = showChar ' '.shows n.showsNodeTail ns showsNodeList :: [Node] -> String -> String showsNodeList [] = showString "" showsNodeList (n:ns) = showChar '{'.shows n.showsNodeTail ns showsNode :: Node -> String -> String showsNode (Nd i ns) = shows i.showsNodeList ns instance Show Node where showsPrec n = showsNode This all works just fine, and when I enter unflatten [0,1,2,2,3,3,3,2,1,2,3,3,1] I get [0{1{2 2{3 3 3} 2} 1{2{3 3}} 1}] as expected. The reason I'm posting this here is that I have a gnawing suspicion that the unflatten/prepend/isChild functions, and possibly the Node data type as well, are not the most elegant way to go about solving the problem, and that I'm missing another more obvious way to do it. Any suggestions? Thanks, Steve Schafer Fenestra Technologies Corp. http://www.fenestra.com/