Here's the setup: I have a series of problems that use various logical connectives. The problem is that they're not all the same. So instead of creating one giant datatype (or duplicating much code), I'd like to assemble them like toy blocks. I've boiled down an example here: data LogicalConnective a = Not a | And [a] | Or [a] data BasicGoal a = Atomic String [Term] | Empty | Logical (LogicalConnective a) deriving (Show, Eq) data PreferenceGoal1 = Basic1 PreferenceGoal1 | Prefer1 PreferenceGoal1 This works OK, but PreferenceGoal1 is a dead end. I can't combine it with other connectives. So I try: data PreferenceGoal2 a = Basic2 (PreferenceGoal2 a) | Prefer2 (PreferenceGoal2 a) And this works fine, but seems impossible to explicitly type (ie, there is nothing to substitute for 'a' in a type declaration). Or am I wrong? Also, it could be that this is just an ugly way to represent things (it does require a huge number of constructors). Any suggestions? -Ron