Как поднять штангу (пособие для хаскелистов)

gistfile1.hs

data LegHealth = Healthy | Broken deriving (Show)
data HumanLegs a = Legs a deriving (Show)

instance Functor HumanLegs where
    fmap f (Legs st) = Legs $ f st


barbell :: LegHealth -> LegHealth
barbell _ = Broken -- playing with iron always ends very bad

useBarbell :: HumanLegs LegHealth -> HumanLegs LegHealth
useBarbell = fmap barbell -- yay, we lifted it!

main = do
    let leg = Legs Healthy
    case useBarbell leg of
        Legs Healthy -> putStrLn "I'm ok!"
        Legs Broken -> putStrLn "Oh noes :("