haskell代写: Lab assignment 4

Lab assignment 4 16/4/18

In this lab your task is to experiment with STM in Haskell. I have given a stub of a Haskell program to help start your exploration.

import Control.Concurrent
import Control.Concurrent.STM
import Data.List
import Text.Printf

data Item = Water | Food deriving (Eq, Ord, Show)

data Player = Player
  { name :: String
  , gold :: TVar Int
  , inventory :: TVar [Item]
  }

createPlayer n g is = do
  g’ <- newTVar g
  is’ <- newTVar is
  return $ Player n g’ is’

main :: IO ()
main = do

  players <- atomically . sequence $
    zipWith3 createPlayer ["Alice", "Bob"] [5,0] [[Water], [Food]]

  let alice = players !! 0
      bob = players !! 1

  printCurrentState players
  putStrLn "-> Alice gives Water to Bob."
  atomically $ transferItemSTM Water alice bob
  printCurrentState players

printCurrentState ps = mapM_ printPlayer ps

printPlayer p = do
  g <- atomically $ readTVar (gold p)
  is <- atomically $ readTVar (inventory p)
  printf "%s has %d gold and the following items: %s\n" (name p) g (show is)

transferGoldSTM :: Int -> Player -> Player -> STM ()

transferGoldSTM amount source target = undefined
-- do

transferItemSTM :: Item -> Player -> Player -> STM ()
transferItemSTM item source target = do

  s <- readTVar (inventory source)
  t <- readTVar (inventory target)
  writeTVar (inventory source) (delete item s)
  writeTVar (inventory target) (item : t)

buyItemSTM :: Item -> Int -> Player -> Player -> STM ()
buyItemSTM item amount buyer seller = undefined
-- do

1. Implement transferGoldSTM, and extend the main method to show that it works as expected.
2. Implement buyItemSTM. It should check that the buyer has enough funds and that the seller has the item and otherwise block. Exemplify in main method again. [Hint: Use that STM actions are composable, and use retry.]
3. Exemplify in main how to use orElse to choose between buying items, where the first fails for either reason.