Scott J. writes: : | Sill I want to make objects packed with their objects and | functions. Doesn't mean that I have to use existential data types? Sometimes you can avoid using existentials by making all your object-updating functions return the post-update object explicitly. For example: module FiniteMap where data FM k v = FM {set :: k -> v -> FM k v, get :: k -> Maybe v, toList :: [(k, v)]} module RedBlack(empty) where import FiniteMap empty :: Ord k => FM k v -- a finite map implemented as a red/black tree ... In this case, the red/black tree itself is effectively a private field of the object. The privacy is achieved by *not* making the tree a field of the FM record. Instead, it gets passed around as the parameter of an auxiliary function: -- module RedBlack, continued empty = fm emptyRedBlackMap fm rb = FM {set = \k v -> fm (extendRedBlackMap rb k v), get = \k -> ..., toList = redBlackMapToList rb} HTH, Tom