Hi, I'm trying to translate Example 2.3.3 (simple symbolic differentation) from Structure and Interpretation of Computer Programs into Haskell. Here is code that works (as far as see): --- data Term b = Var String | Const b | Sum (Term b) (Term b) | Prod (Term b) (Term b) newSum (Const a) (Const b) = Const (a+b) newSum (Const 0) t@_ = t newSum t@_ (Const 0) = t newSum a b = Sum a b newProd (Const a) (Const b) = Const (a*b) newProd (Const 1) t@_ = t newProd t@_ (Const 1) = t newProd (Const 0) t@_ = Const 0 newProd t@_ (Const 0) = Const 0 newProd a b = Prod a b deriv (Var x) (Const c) = Const 0 deriv (Var x) (Var y) | x == y = Const 1 | otherwise = Const 0 deriv x@(Var _) (Sum u v) = newSum (deriv x u) (deriv x v) deriv x@(Var _) (Prod u v) = newSum (newProd u (deriv x v)) (newProd (deriv x u) v) --instance Show (Term b) where show = showTerm showTerm (Var x) = x showTerm (Const c) = show c showTerm (Sum a b) = "(" ++ showTerm a ++ "+" ++ showTerm b ++ ")" showTerm (Prod a b) = "(" ++ showTerm a ++ showTerm b ++ ")" --- Where should I put type constraint (Show b) to be able to define Term b as an instance of Show class? Actually, I would like to say that Term b is an instance of Show iff b is and not to put constraint on b.