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  1. {-# LANGUAGE TypeFamilies #-}
  2. {-# LANGUAGE MultiParamTypeClasses #-}
  3. {-# LANGUAGE FlexibleInstances #-}
  4. {-# LANGUAGE IncoherentInstances #-}
  5.  
  6. main = do
  7.   print (g (undefined :: Int))
  8.   print (g (undefined :: Bool))
  9.   print (g (undefined :: Char))
  10.   print (g (undefined :: Float))
  11.   print (g (undefined :: Double))
  12.  
  13. data True
  14.  
  15. class CA t where
  16.   type A t
  17.   type A t = True
  18.   fa :: t -> String
  19.  
  20. instance CA Int where
  21.   fa _ = "Int"
  22.  
  23. class CB t where
  24.   type B t
  25.   type B t = True
  26.   fb :: t -> String
  27.  
  28. instance CB Bool where
  29.   fb _ = "Bool"
  30.  
  31. class CC t where
  32.   type C t
  33.   type C t = True
  34.   fc :: t -> String
  35.  
  36. instance CC Char where
  37.   fc _ = "Char"
  38.  
  39. class CD t where
  40.   type D t
  41.   type D t = True
  42.   fd :: t -> String
  43.  
  44. instance CD Float where
  45.   fd _ = "Float"
  46.  
  47. class CE t where
  48.   type E t
  49.   type E t = True
  50.   fe :: t -> String
  51.  
  52. instance CE Double where
  53.   fe _ = "Double"
  54.  
  55. class CAll t t1 t2 t3 t4 t5 where
  56.   g :: (t1 ~ A t, t2 ~ B t, t3 ~ C t, t4 ~ D t, t5 ~ E t) => t -> String
  57.  
  58. instance (CA t) => CAll t True t2 t3 t4 t5 where
  59.   g = fa
  60.  
  61. instance (CB t) => CAll t t1 True t3 t4 t5 where
  62.   g = fb
  63.  
  64. instance (CC t) => CAll t t1 t2 True t4 t5 where
  65.   g = fc
  66.  
  67. instance (CD t) => CAll t t1 t2 t3 True t5 where
  68.   g = fd
  69.  
  70. instance (CE t) => CAll t t1 t2 t3 t4 True where
  71.   g = fe
  72.  
  73.  
Success #stdin #stdout 0s 4696KB
comments ()
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Standard input is empty
stdout
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"Int"
"Bool"
"Char"
"Float"
"Double"
https://ideone.com/kFFlGT
language:
Haskell (ghc 8.4.4)
created:
9 years ago
visibility:
public

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