Sorry that I couldn't respond to the previous thread.. I'm using Gmail and not an email client so I couldn't use the reply button. I'll try to resolve this issue soon ! Thanks Phillip for responding so quickly, I tried Data.Map first and ran into the issue you mentioned.. I've found a way to get around it, but it's ugly and verbose.. Instead, I'm wondering if there is a way to use the constructors for the "Val" types below, in place of the keys such as: data Val = Root Root | Oct Oct | Mode Mode deriving (Show, Eq, Ord) And matching against the constructors, instead of resorting to this: import IO import qualified Data.Map as Map type Root = String type Oct = Integer type Mode = Integer data Key = Root | Oct | Mode deriving (Show, Eq, Ord) data Val = Root_v Root | Oct_v Oct | Mode_v Mode deriving (Show, Eq, Ord) test = Map.insert Oct (Oct_v 3) . Map.insert Mode (Mode_v 0) . Map.insert Root (Root_v "2") $ Map.empty main = print test -- ----> fromList [(Root,Root_v "2"),(Oct,Oct_v 3),(Mode,Mode_v 0)] Many Thanks, Tom