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  1. {-# LANGUAGE  FlexibleInstances, RankNTypes #-} -- (0)
  2.  
  3. class VarArgs a where prc :: String -> a -- (1)
  4.  
  5. instance VarArgs String where prc = id -- (2)
  6.  
  7. instance  (Show a, VarArgs r) => VarArgs (a -> r) where -- (3)
  8.     prc acc = \x -> prc $ acc ++ "/" ++ show x 
  9.  
  10. magic :: VarArgs a => a
  11. magic = (prc "seed:") :: (VarArgs a => a)
  12.  
  13. shmagic _ = (prc "shmagic:")
  14.  
  15. useMagic :: (forall a. VarArgs a => a) -> IO () -- (4)
  16. useMagic f = do
  17.     putStrLn $ f 1 
  18.     putStrLn $ f 1 "qwe"
  19.     putStrLn $ f 1 "qwe" [1, 2, 3]
  20.  
  21. main :: IO ()
  22. main = do 
  23.     putStrLn $ magic 1 2 "qwe" [1,2,3] 123.456
  24.     putStrLn $ magic 100500
  25.     putStrLn $ magic
  26.     useMagic magic  -- (5)
  27.  
  28.  
Success #stdin #stdout 0s 8388607KB
comments (?)
stdin
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Standard input is empty
stdout
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seed:/1/2/"qwe"/[1,2,3]/123.456
seed:/100500
seed:
seed:/1
seed:/1/"qwe"
seed:/1/"qwe"/[1,2,3]
https://ideone.com/DLTa60
language:
Haskell (ghc 8.4.4)
created:
7 years ago
visibility:
public

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