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  1. mapMM :: Monad m => (a -> m b) -> [a] -> m [b]
  2. mapMM _f [] = pure []
  3. mapMM f (x:xs) = do
  4.   y <- f x
  5.   ys <- mapMM f xs
  6.   pure $ y:ys
  7.  
  8. p :: Int -> Maybe Int
  9. p 1488 = Nothing
  10. p x = Just x
  11.  
  12. main = do
  13.   print $ mapMM Just [1, 2, 1488, 3]
  14.   print $ mapM Just [1, 2, 1488, 3]
  15.   print $ mapMM p [1, 2, 1488, 3]
  16.   print $ mapM p [1, 2, 1488, 3]
  17.   print $ mapMM p []
  18.   print $ mapM p []
  19.  
Success #stdin #stdout 0s 4404KB
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stdin
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Standard input is empty
stdout
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Just [1,2,1488,3]
Just [1,2,1488,3]
Nothing
Nothing
Just []
Just []
https://ideone.com/PYIAdN
language:
Haskell (ghc 8.4.4)
created:
5 years ago
visibility:
public

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