-- Slide 64

data Expr a = Val a | Inc (Expr a) | Dec (Expr a) | Inv (Expr a) | Neg (Expr a)

evaluate::(Fractional a)=>Expr a -> a
evaluate (Val x) = x
evaluate (Inc e) = 1 + evaluate e
evaluate (Dec e) = (evaluate e) - 1
evaluate (Inv e) = 1 / (evaluate e)
evaluate (Neg e) = negate (evaluate e)