Hi, I wrote this program to work with formulas in propositional logic: data Formula = | Variable Char | Not Formula | And Formula Formula | Or Formula Formula | Imply Formula Formula | Equivalent Formula Formula deriving (Eq) instance Show Formula where show (Variable v) = [v] show (Not f) = "~" ++ show f show (And f g) = "(" ++ show f ++ " & " ++ show g ++ ")" show (Or f g) = "(" ++ show f ++ " v " ++ show g ++ ")" show (Imply f g ) = "(" ++ show f ++ " -> " ++ show g ++ ")" show (Equivalent f g ) = "(" ++ show f ++ " <-> " ++ show g ++ ")" To make a formula you can do something like: ghci> let f = Or (Variable 'C') (And (Variable 'A') (Not (Variable 'C'))) ghci> f (C v (A & ~C)) So, my questions are: 1. Do you think this is an elegant solution or would you define custom operators (or, and, ->, <->) instead? Or something completely different? 2. If I pack this code into a module how would add a new logic connective like xor or nand to Formula? Meaning, can I add another type constructor to it? 3. Is there a way to "nest" data definitions, e.g. I would like to replace the 'Variable Char' type constructor by 'Literal Char', whereas a Literal is either a Variable or a negated Variable. Literals can be negated, too. Thanks a lot. Alex