when loading the following code, hugs escaped and reported that ERROR "cal24.hs":10 - Instance of Integral (Ratio Integer) required for definition of Main.eval I don't know why and how to solve it, anyone help me? data ETree = Add ETree ETree | Sub ETree ETree | Mul ETree ETree | Div ETree ETree | Node Integer deriving Show eval :: ETree->Maybe Rational eval (Node x) = Just (fromInteger x) eval (Add t1 t2) = do { x<-eval t1; y<-eval t2; if x>=y then return (x+y) else Nothing } eval (Sub t1 t2) = do { x<-eval t1; y<-eval t2; return (x-y) } eval (Mul t1 t2) = do { x<-eval t1; y<-eval t2; if x>=y then return (x*y) else Nothing } eval (Div t1 t2) = do { x<-eval t1; y<-eval t2; if y/=0 then return (x `div` y) else Nothing }