It is because you use 'div' instead of '/'. div :: Integral a => a -> a -> a (/) :: Fractional a => a -> a -> a Rationals are instance of the class Fractional, but not of Integral Prelude> :i Fractional class Num a => Fractional a where (/) :: a -> a -> a recip :: a -> a fromRational :: Rational -> a fromDouble :: Double -> a -- instances: instance Fractional Float instance Fractional Double instance Integral a => Fractional (Ratio a) Hope this helps, Arthur On 13-apr-04, at 20:44, Meihui Fan wrote:
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 }
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