[Haskell-cafe] How to escape from typecheck error: Duplicate instance declarations ?

25 Jan 2013

      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.

[Haskell-cafe] How to escape from typecheck error: Duplicate instance declarations ?

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