trains.hs

data TicketCategory = Infant | Child | Adult

recurseTickets :: Int -> IO Double
recurseTickets 0 = pure 0.0
recurseTickets n = do
  putStrLn "How old is the passenger?"
  age <- fmap read getLine :: IO Int
  let priceMultiplier = 
        case age of
          x | x `elem` [0..4] -> 0
          x | x `elem` [5..15] -> 0.5
          otherwise -> 1.0
  let price = (fromIntegral age) * priceMultiplier
  fmap (+ price) $ recurseTickets (n - 1)

main :: IO ()
main = do
  putStrLn "How many tickets would you like to purchase? (Children under 5 travel free)"
  tickets <- fmap read getLine :: IO Int
  fmap show (recurseTickets tickets) >>= putStrLn