Question

我正在从Marlow的书Parallel and Concurrent Programming in Haskell中学习forkIO和Mvar的简单用法。我在 MVar作为共享状态容器部分中公开了以下代码：

-- Creates a new Map within an MVar
new :: IO PhoneBookState
new = do
  m <- newMVar Map.empty
  return (PhoneBookState m)

-- Inserts a Key, Value in the Map, locking the MVar
insert :: PhoneBookState -> Name -> PhoneNumber -> IO ()
insert (PhoneBookState m) name number = do
  book <- takeMVar m
  putMVar m (Map.insert name number book)

-- Retrieves the Map, locking the MVar briefly
lookup :: PhoneBookState -> Name -> IO (Maybe PhoneNumber)
lookup (PhoneBookState m) name = do
  book <- takeMVar m
  putMVar m book
  return (Map.lookup name book)

PhoneBookState持有MVar (Map Name PhoneNumber)的位置。我的问题出在main函数

main = do
  s <- new
  sequence_ [ insert s ("name" ++ show n) (show n) | n <- [1..10000] ]
  lookup s "name999" >>= print
  lookup s "unknown" >>= print

这个想法是两个查询同时发生，但是在这种情况下，它不应该是

main = do
  s <- new
  sequence_ [ insert s ("name" ++ show n) (show n) | n <- [1..10000] ]
  forkIO(lookup s "name999" >>= print)
  lookup s "unknown" >>= print

我想念什么？

为什么这个Haskell程序实际上是并发的？

0 个答案: