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  1. data Result t
  2. 	= Ok t
  3. 	| Err String
  4. 	deriving (Show, Eq)
  5.  
  6. instance Functor Result where
  7. 	fmap f (Ok  x)   = Ok (f x)
  8. 	fmap _ (Err msg) = Err msg
  9.  
  10. instance Applicative Result where
  11. 	pure = Ok
  12. 	Ok  f   <*> Ok  x   = Ok (f x)
  13. 	Ok  f   <*> Err msg = Err msg
  14. 	Err msg <*> _       = Err msg
  15.  
  16. instance Monad Result where
  17.     (Ok v)    >>= k = k v
  18.     (Err msg) >>= k = Err msg
  19.     return v = Ok v
  20.     fail msg = Err msg
  21.  
  22.  
  23. data Item = Item {
  24. 	id    :: Int,
  25. 	name  :: Maybe String
  26. 	} deriving (Show, Eq)
  27.  
  28. getId :: Maybe Int -> Result Int
  29. getId  Nothing  = Err "No ID provided"
  30. getId (Just id) = Ok id
  31.  
  32. getItem :: Int -> Result Item
  33. getItem id
  34. 	| id `rem` 2 == 0  =  Ok  ( Item id  Nothing)
  35. 	| id `rem` 3 == 0  =  Ok  ( Item id (Just ("item-" ++ show id)))
  36. 	| otherwise        =  Err ("Item not found, ID = " ++ show id)
  37.  
  38. getName :: Item -> Result String
  39. getName (Item id Nothing   ) = Err ("Item has no name, ID = " ++ show id)
  40. getName (Item _ (Just name)) = Ok name
  41.  
  42. doSomethingClever :: Int -> String -> String
  43. doSomethingClever id name = show id ++ " => " ++ name
  44.  
  45. tryToDoSomethingClever :: Maybe Int -> Result String
  46. tryToDoSomethingClever idParam = do
  47. 	id   <- getId idParam
  48. 	item <- getItem id
  49. 	name <- getName item
  50. 	return (doSomethingClever id name)
  51.  
  52. demoInputs = [Nothing, Just 1, Just 2, Just 3]
  53.  
  54. main = mapM_ (print . tryToDoSomethingClever) demoInputs
  55.  
Success #stdin #stdout 0s 4192KB
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stdin
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Standard input is empty
stdout
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Err "No ID provided"
Err "Item not found, ID = 1"
Err "Item has no name, ID = 2"
Ok "3 => item-3"
https://ideone.com/MCfOc3
language:
Haskell (ghc 8.4.4)
created:
6 years ago
visibility:
public

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