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  1. import Prelude hiding(Maybe(..), lookup)
  2.  
  3.  
  4. data Maybe a = Just a | Nothing deriving (Show)
  5. instance Monad Maybe where
  6.   return x = Just x
  7.   (Just x) >>= f = f x
  8.   Nothing >>= _  = Nothing
  9.  
  10. findSomething :: Maybe Int
  11. findSomething = return 3
  12.  
  13. findSomethingElse :: Int -> Maybe Int
  14. findSomethingElse x = return (x+5)
  15.  
  16. lookup :: Int -> Int -> Maybe Int
  17. lookup x y = return (x*y)
  18.  
  19. someFailure :: Maybe a
  20. someFailure = Nothing
  21.  
  22. findThemAll :: Maybe Int
  23. findThemAll = do
  24.   x <- findSomething
  25.   y <- findSomethingElse x
  26.   lookup x y
  27.  
  28. findThemAllFail :: Maybe Int
  29. findThemAllFail = do
  30.   x <- findSomething
  31.   someFailure
  32.   y <- findSomethingElse x
  33.   lookup x y
  34.  
  35. main = print findThemAll >> print findThemAllFail
Success #stdin #stdout 0s 6220KB
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stdin
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Standard input is empty
stdout
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Just 24
Nothing
https://ideone.com/A40Z5X
language:
Haskell (ghc 8.4.4)
created:
10 years ago
visibility:
secret

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