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  1. {-# LANGUAGE GADTs, MultiParamTypeClasses #-}
  2. import Data.Word
  3.  
  4. data Expr a where
  5.   Value :: a -> Expr a
  6.   Cast :: (Castable a b) => Expr a -> Expr a
  7.  
  8. class Castable a b where
  9.   cast :: a -> b
  10.  
  11. instance Castable Word64 Word32 where
  12.   cast = fromIntegral
  13.  
  14. instance (Show a) => Show (Expr a) where
  15.   show (Cast e) = "Cast " ++ show e -- ERROR
  16.  
  17. main = print ""
Success #stdin #stdout 0s 6232KB
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Standard input is empty
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""
https://ideone.com/BWtyIR
language:
Haskell (ghc 8.4.4)
created:
10 years ago
visibility:
public

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