{-# LANGUAGE ExistentialQuantification #-}

sick_animals.hs

{-# LANGUAGE ExistentialQuantification #-}

--
-- An animal that weighs < 5kg are deemed sick. Do we have any sick animals on our farm?
--

data Cow   = Cow { weightC :: Integer, color :: String } deriving (Show)
data Sheep = Sheep { weightS :: Integer, fluffinessFactor :: Double } deriving (Show)
data Dog   = Dog { weightD :: Integer, barks :: Bool } deriving (Show)

class Weighable a where
  weight :: a -> Integer

instance Weighable Cow where
  weight = weightC

instance Weighable Sheep where
  weight = weightS

instance Weighable Dog where
  weight = weightD

cowses   = [Cow { weightC = 11, color = "blue" }, Cow { weightC = 21, color = "yellow" }]
sheepses = [Sheep { weightS = 4, fluffinessFactor = 0.1 }, Sheep { weightS = 15, fluffinessFactor = 2.3 }]
dogses   = [Dog { weightD = 12, barks = False }]

--
-- 1st solution
--

containsSick :: Weighable a => [a] -> Bool
containsSick = any ((<5) . weight)

solution1 :: Bool
solution1 = containsSick cowses || containsSick sheepses || containsSick dogses

--
-- 2nd solution, why?
--

data Weighables = forall a. Weighable a => WS a

instance Weighable Weighables where
  weight (WS a) = weight a

animals :: [Weighables]
animals = map WS cowses ++ map WS sheepses ++ map WS dogses

solution2 :: Bool
solution2 = containsSick animals