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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. data Item = Item {
  23.     id    :: Int,
  24.     name  :: Maybe String
  25.     } deriving (Show, Eq)
  26.  
  27. getId :: Maybe Int -> Result Int
  28. getId  Nothing  = Err "No ID provided"
  29. getId (Just id) = Ok id
  30.  
  31. getItem :: Int -> Result Item
  32. getItem id
  33.     | id `rem` 2 == 0  =  Ok  ( Item id  Nothing)
  34.     | id `rem` 3 == 0  =  Ok  ( Item id (Just ("item-" ++ show id)))
  35.     | otherwise        =  Err ("Item not found, ID = " ++ show id)
  36.  
  37. getName :: Item -> Result String
  38. getName (Item id Nothing   ) = Err ("Item has no name, ID = " ++ show id)
  39. getName (Item _ (Just name)) = Ok name
  40.  
  41. doSomethingClever :: Int -> String -> String
  42. doSomethingClever id name = show id ++ " => " ++ name
  43.  
  44. {-
  45.  
  46. tryToDoSomethingClever :: Maybe Int -> Result String
  47. tryToDoSomethingClever idParam = do
  48. 	id   <- getId idParam
  49. 	item <- getItem id
  50. 	name <- getName item
  51. 	return (doSomethingClever id name)
  52.  
  53. tryToDoSomethingClever :: Maybe Int -> Result String
  54. tryToDoSomethingClever idParam =
  55.     getId idParam >>= (\id ->
  56.         getItem id >>= (\item ->
  57.             getName item >>= (\name ->
  58.                 return (doSomethingClever id name))))
  59.  
  60. -}
  61.  
  62. tryToDoSomethingClever :: Maybe Int -> Result String
  63. tryToDoSomethingClever idParam =
  64.     case getId idParam of
  65.         Err msg -> Err msg
  66.         Ok  id  -> case getItem id of
  67.             Err msg  -> Err msg
  68.             Ok  item -> case getName item of
  69.                 Err msg  -> Err msg
  70.                 Ok  name -> Ok (doSomethingClever id name)
  71.  
  72. demoInputs = [Nothing, Just 1, Just 2, Just 3]
  73.  
  74. main = mapM_ (print . tryToDoSomethingClever) demoInputs
  75.  
Success #stdin #stdout 0s 4272KB
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Standard input is empty
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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/f4Bzen
language:
Haskell (ghc 8.4.4)
created:
6 years ago
visibility:
public

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