## How to escape from typecheck error: Duplicate instance declarations ?

2013-01-25 20:18:02 GMT

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.

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