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  1. {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, FunctionalDependencies, FlexibleContexts, UndecidableInstances, OverlappingInstances, IncoherentInstances #-}
  2.  
  3. class SubtypeOf a b | a -> b where
  4.   upcast :: a -> b
  5.   downcastSafe :: b -> Maybe a
  6.   downcast :: b -> a
  7.   downcast b = case downcastSafe b of
  8.     Nothing -> error $ "can not downcast the value"
  9.     Just a -> a
  10.  
  11. class (Ord a) => Rangable t a | t -> a where
  12.   lowLim :: t -> a
  13.   highLim :: t -> a
  14.  
  15. class Packable t a | t -> a where
  16.   pack :: a -> t
  17.   unpack :: t -> a
  18.  
  19. class MultipleTo t a | t -> a where
  20.   multiple :: t -> a
  21.  
  22. instance (Num a, Ord a, Rangable range a, Packable range a) => SubtypeOf range a where
  23.   upcast = unpack
  24.   downcastSafe b | b >= (lowLim $ pb) && b <= (highLim $ pb) = Just $ pb
  25.                  | otherwise = Nothing
  26.     where
  27.       pb = pack b
  28.  
  29. instance (Integral a, Packable range a, MultipleTo range a) => SubtypeOf range a where
  30.   upcast = unpack
  31.   downcastSafe b | b `mod` (multiple pb) == 0 = Just pb
  32.                  | otherwise = Nothing
  33.     where
  34.       pb = pack b
  35.  
  36. newtype Range1 a = Range1 {unRange1 :: a}
  37.                  deriving Show
  38.  
  39. instance (Num a, Ord a) => Rangable (Range1 a) a where
  40.   lowLim _ = 0
  41.   highLim _ = 10
  42.  
  43. instance (Num a, Ord a) => Packable (Range1 a) a where
  44.   pack = Range1
  45.   unpack = unRange1
  46.  
  47. newtype Range2 a = Range2 {unRange2 :: a}
  48.                    deriving Show
  49.  
  50. instance (Num a, Ord a) => Rangable (Range2 a) a where
  51.   lowLim _ = -10
  52.   highLim _ = 200
  53.  
  54. instance (Num a, Ord a) => Packable (Range2 a) a where
  55.   pack = Range2
  56.   unpack = unRange2
Compilation error #stdin compilation error #stdout 0s 0KB
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stdin
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Standard input is empty
compilation info 
[1 of 1] Compiling Main             ( prog.hs, prog.o )

prog.hs:22:0:
    Duplicate instance declarations:
      instance [incoherent] (Num a,
                             Ord a,
                             Rangable range a,
                             Packable range a) =>
                            SubtypeOf range a
        -- Defined at prog.hs:(22,0)-(27,16)
      instance [incoherent] (Integral a,
                             Packable range a,
                             MultipleTo range a) =>
                            SubtypeOf range a
        -- Defined at prog.hs:(29,0)-(34,16)
stdout
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Standard output is empty
https://ideone.com/v2CrAm
language:
Haskell (ghc 8.4.4)
created:
11 years ago
visibility:
public

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