Sattar_Fix

store.hs

import Control.Monad.Trans.State

data Cycle = Cycle { brand :: String, id :: Int, price :: Int } deriving Show
data Store = Store { cycles :: [Cycle], rentals :: [Rental]} deriving Show
data Rental = Rental { cycle :: Cycle, rentedOn :: Int, returnedOn :: Maybe Int } deriving Show

cycles' :: [Cycle]
cycles' = [ Cycle "BSA" 1 10
         , Cycle "BSA 2" 2 100
         , Cycle "BSA 3" 3 200 ]

store :: Store
store = Store { cycles = cycles'
              , rentals = []
              }

rent :: Store -> Int -> Int -> Store
rent store rentedId rentedOn' = Store { cycles = newCycles, rentals = newRentals }
  where 
        cycle = head . filter (\(Cycle _ id' _) -> id' == rentedId) $ (cycles(store))
        newCycles = filter (\(Cycle _ id' _) -> id' /= rentedId) (cycles(store))
        newRentals = Rental cycle rentedOn' Nothing : rentals(store)


rentS :: Int -> Int -> State Store
rentS rentedId rentedOn' = do
  oldState <- get
  put (rent oldState rentedId rentedOn')

main = do
  putStrLn $ evalState (rentS 1 1) store