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  1. import Control.Monad
  2. import Control.Monad.Writer
  3. import Data.Monoid
  4.  
  5. merge :: [Int]  -> [Int]  -> Writer (Sum Int) [Int]
  6. merge [] ys = return ys
  7. merge xs [] = return xs
  8. merge (x:xs) (y:ys)
  9.       | x <= y = do { temp <- merge xs (y:ys);  return (x:temp) }
  10.       | otherwise = do
  11.                      temp <- merge (x:xs) ys
  12.                      tell (Sum (length (x:xs)))
  13.                      return (y:temp)
  14.  
  15. mergeSort :: [Int] -> Writer (Sum Int) [Int]
  16. mergeSort [] = return []
  17. mergeSort [x] = return [x]
  18. mergeSort xs = do
  19.                 let (lx, rx) = splitAt (div (length xs) 2) xs
  20.                 slx <- mergeSort lx
  21.                 srx <- mergeSort rx
  22.                 merge slx srx
Compilation error #stdin compilation error #stdout 0s 0KB
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stdin
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Standard input is empty
compilation info 
prog.hs:2:8:
    Could not find module `Control.Monad.Writer'
    Perhaps you meant
      Control.Monad.Fix (from base)
      Control.Monad.Zip (from base)
    Use -v to see a list of the files searched for.
stdout
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Standard output is empty
https://ideone.com/YfP9cI
language:
Haskell (ghc 8.4.4)
created:
10 years ago
visibility:
public

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