fork(1) download 
  1. import Data.Foldable (foldMap)
  2. import Data.Monoid (First(First, getFirst))
  3. import Data.Maybe (fromMaybe)
  4.  
  5. data Fruit = Fruit { color :: String, weight :: Int }
  6.  
  7. theFruitIsRed :: Fruit -> Maybe String
  8. theFruitIsRed (Fruit "red" _) = Nothing
  9. theFruitIsRed _               = Just "Fruit not red"
  10.  
  11. theFruitIsNotTooHeavy :: Fruit -> Maybe String
  12. theFruitIsNotTooHeavy (Fruit _ w)
  13.     | w < 500   = Nothing
  14.     | otherwise = Just "Too heavy"
  15.  
  16. checkRules :: Fruit -> Maybe String
  17. checkRules = getFirst . foldMap (First .)
  18.     [ theFruitIsRed
  19.     , theFruitIsNotTooHeavy
  20.     ]
  21.  
  22. main = mapM (putStrLn . fromMaybe "No problems!" . checkRules)
  23. 	[ Fruit "red" 1000
  24. 	, Fruit "red" 10
  25. 	, Fruit "blue" 600
  26. 	, Fruit "blue" 100
  27. 	]
Success #stdin #stdout 0s 8388607KB
comments (?)
stdin 
Standard input is empty
stdout 
Too heavy
No problems!
Fruit not red
Fruit not red
https://ideone.com/4Gk5Rh
language:
Haskell (ghc 8.4.4)
created:
6 years ago
visibility:
public

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