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  1. {-# LANGUAGE FlexibleInstances #-}
  2.  
  3. unOp :: Read a => Show b => (a -> b) -> String -> String
  4. unOp f x = show . f $ read x
  5.  
  6. binOp :: Read a => Read b => Show c => (a -> b -> c) -> String -> String -> String
  7. binOp f x y = show $ read x `f` read y
  8.  
  9. instance Num String where
  10.   (+) = binOp (+)
  11.   (*) = binOp (*)
  12.   abs = unOp abs
  13.   signum = unOp signum
  14.   fromInteger = show
  15.   negate = unOp negate
  16.  
  17. main = print $ 2 + 2 == "4"
Success #stdin #stdout 0s 8388607KB
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https://ideone.com/CJifV3
language:
Haskell (ghc 8.4.4)
created:
6 years ago
visibility:
public

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