The infamous rocket example for phantom types. With support for real oxygen/fuel data

Rocket.hs

data NoFuel
data Fueled
data NoO2
data HasO2

data Rocket fuel o2 = Rocket { fuel:: Int
                             , o2  :: Int }

createRocket :: Rocket NoFuel NoO2
createRocket = Rocket 0 0

addFuel :: Int -> Rocket NoFuel o2 -> Rocket Fueled o2
addFuel f r = Rocket f $ o2 r

addO2 :: Int -> Rocket fuel NoO2 -> Rocket fuel HasO2
addO2 o r = Rocket (fuel r) o

launch :: Rocket Fueled HasO2 -> String
launch x =  "we are gonna last " ++ (show $ daysToTravel x) ++ " days"

daysToTravel :: Rocket fueled hasO2 -> Int
daysToTravel r = min (fuel r) (o2 r)

a |> b = b a

main = do print $ createRocket |> addFuel 500 |> addO2 300 |> launch