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  1. {-# LANGUAGE MultiParamTypeClasses #-}
  2.  
  3. import Data.Char (toUpper, toLower)
  4.  
  5. data Lower = Lower String deriving Show
  6. data Mixed = Mixed String deriving Show
  7. data Upper = Upper String deriving Show
  8.  
  9. toUpperStr = map toUpper
  10. toLowerStr = map toLower
  11.  
  12. class Make a where
  13.   make :: String -> a
  14.  
  15. instance Make Lower where
  16.   make s = Lower (toLowerStr s)
  17.  
  18. instance Make Mixed where
  19.   make s = Mixed s
  20.  
  21. instance Make Upper where
  22.   make s = Upper (toUpperStr s)
  23.  
  24. class Convert a b where
  25.   convert :: a -> b
  26.  
  27. instance Convert Lower Lower where
  28.   convert = id
  29.  
  30. instance Convert Lower Mixed where
  31.   convert (Lower s) = Mixed s
  32.  
  33. instance Convert Lower Upper where
  34.   convert (Lower s) = Upper (toUpperStr s)
  35.  
  36. instance Convert Mixed Lower where
  37.   convert (Mixed s) = Lower (toLowerStr s)
  38.  
  39. instance Convert Mixed Mixed where
  40.   convert = id
  41.  
  42. instance Convert Mixed Upper where
  43.   convert (Mixed s) = Upper (toUpperStr s)
  44.  
  45. instance Convert Upper Lower where
  46.   convert (Upper s) = Lower (toLowerStr s)
  47.  
  48. instance Convert Upper Mixed where
  49.   convert (Upper s) = Mixed s
  50.  
  51. instance Convert Upper Upper where
  52.   convert = id
  53.  
  54. main = print (convert ((make "Hello World") :: Lower) :: Upper)
  55.  
Success #stdin #stdout 0s 6232KB
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Standard input is empty
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Upper "HELLO WORLD"
https://ideone.com/fl07c2
language:
Haskell (ghc 8.4.4)
created:
11 years ago
visibility:
secret

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