Hello, haskellers. I am trying to write some generic subtyping issue. Here upcast is always safe operation because of subtype is always behaves like the parrent type. downcast is not the safe becase of not every parrent type value can be converted to children type. Rangeable here is the typeclass of values in some range, so downcasting to Rang1 or Range2 or any other type, having instance for Rangeable can be done by checking if value is in proper range. The same for MultipleTo, downcasting can be done with checking if value is multiple to some value. {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, FunctionalDependencies, FlexibleContexts, UndecidableInstances, OverlappingInstances, IncoherentInstances #-} class SubtypeOf a b | a -> b where upcast :: a -> b downcastSafe :: b -> Maybe a downcast :: b -> a downcast b = case downcastSafe b of Nothing -> error $ "can not downcast the value" Just a -> a class (Ord a) => Rangable t a | t -> a where lowLim :: t -> a highLim :: t -> a class Packable t a | t -> a where pack :: a -> t unpack :: t -> a class MultipleTo t a | t -> a where multiple :: t -> a instance (Num a, Ord a, Rangable range a, Packable range a) => SubtypeOf range a where upcast = unpack downcastSafe b | b >= (lowLim $ pb) && b <= (highLim $ pb) = Just $ pb | otherwise = Nothing where pb = pack b instance (Integral a, Packable range a, MultipleTo range a) => SubtypeOf range a where upcast = unpack downcastSafe b | b `mod` (multiple pb) == 0 = Just pb | otherwise = Nothing where pb = pack b newtype Range1 a = Range1 {unRange1 :: a} deriving Show instance (Num a, Ord a) => Rangable (Range1 a) a where lowLim _ = 0 highLim _ = 10 instance (Num a, Ord a) => Packable (Range1 a) a where pack = Range1 unpack = unRange1 newtype Range2 a = Range2 {unRange2 :: a} deriving Show instance (Num a, Ord a) => Rangable (Range2 a) a where lowLim _ = -10 highLim _ = 200 instance (Num a, Ord a) => Packable (Range2 a) a where pack = Range2 unpack = unRange2 but there is compilation error: Duplicate instance declarations: instance [incoherent] (Num a, Ord a, Rangable range a, Packable range a) => SubtypeOf range a -- Defined at ...:22:10 instance [incoherent] (Integral a, Packable range a, MultipleTo range a) => SubtypeOf range a -- Defined at ...:29:10 Failed, modules loaded: none. If I remove one of instances of SubtypeOf the program is compiling. How to write this instances properly, or to write proper type casting ? Thanks PS. My english is not very good, but I hope this is understandable.