Carlo Vivari wrote:
data AlgExp a = AlgExp { litI :: Int -> a, litB :: Bool -> a, add :: a -> a -> a, and :: a -> a -> a, ifte :: a -> a -> a -> a}
You're confusing sum and product types. That is, you're using a product type, but you probably need a sum type, like this: data Exp1 = LitI Int | LitB Bool | Add Exp1 Exp1 | And Exp1 Exp1 | IfThenElse Exp1 Exp1 Exp1 But in this case, using GADTs (beware: not Haskell 98, but a very popular extension) makes for a more elegant solution. Note the strong types, disallowing e. g. the addition of a number to a boolean value: data Exp2 a where LitI :: Int -> Exp2 Int LitB :: Bool -> Exp2 Bool Add :: Exp2 Int -> Exp2 Int -> Exp2 Int And :: Exp2 Bool -> Exp2 Bool -> Exp2 Bool IfThenElse :: Exp2 Bool -> Exp2 a -> Exp2 a -> Exp2 a Kalman ---------------------------------------------------------------------- Get a free email address with REAL anti-spam protection. http://www.bluebottle.com/tag/1