Question

这确保了互相排斥行动：

save(title, description)

这不是：

do
  lock <- newMVar ()
  let atomicPrint = withMVar lock . const . print
  mapM_ (forkIO . atomicPrint) [['1'..'8'],['a'..'h']]

你能解释一下原因吗？根据Hackage和equational推理中withMVar和bracket_的定义，我得出结论，两段代码应该做同样的事情。但是在GHCi中运行证明我错了。

Answer 1

你的订单错了;您正在获取锁定并且立即释放它;而the signature says：

bracket_ 
    :: IO a  -- computation to run first ("acquire resource")
    -> IO b  -- computation to run last ("release resource")
    -> IO c  -- computation to run in-between
    -> IO c  -- returns the value from the in-between computation

所以它应该是：

bracket_ (takeMVar lock) (putMVar lock ()) $ print x

withMVar和bracket_的行为不同

1 个答案: