[英]return TVar after initialisation
是否可以在do块中返回新创建的TVar? 我尝试使用以下代码实现此目的:
type Buffer a = TVar [[(a,a)]]
newBuffer :: STM (Buffer a)
newBuffer = newTVar [[]]
launchGhosts :: [[(String,String)]] -> Buffer String
launchGhosts unblocked = do buff <- atomically newBuffer
atomically $ put buff unblocked
return buff
computeBlock :: Buffer String -> IO()
computeBlock buff = do i <- atomically $ get buff
putStrLn $ show i
put :: Buffer a -> [[(a,a)]] -> STM ()
put buff x = do writeTVar buff x
get :: Buffer a -> STM [[(a,a)]]
get buff = do x <- readTVar buff
return x
这应该允许我初始化共享内存,并在程序的其他位置使用它。 我想要分离内存初始化的主要原因是多次调用并发函数,而不必一次又一次地初始化内存。
类型检查器引发以下两个错误:
pacman.hs:65:29:
No instance for (Monad TVar)
arising from a do statement
Possible fix: add an instance declaration for (Monad TVar)
In a stmt of a 'do' block: buff <- atomically newBuffer
In the expression:
do { buff <- atomically newBuffer;
atomically $ put buff unblocked;
computeBlock buff;
return buff }
In an equation for `launchGhosts':
launchGhosts unblocked
= do { buff <- atomically newBuffer;
atomically $ put buff unblocked;
computeBlock buff;
.... }
pacman.hs:65:37:
Couldn't match expected type `TVar t0' with actual type `IO a0'
In the return type of a call of `atomically'
In a stmt of a 'do' block: buff <- atomically newBuffer
In the expression:
do { buff <- atomically newBuffer;
atomically $ put buff unblocked;
computeBlock buff;
return buff }
有谁知道问题出在哪里,或者是实现此代码背后想法的另一种方式?
更新:
launchGhosts :: [[(String,String)]] -> IO(Buffer String)
launchGhosts unblocked = do buff <- atomically newBuffer
atomically $ put buff unblocked
return buff
computeBlock :: IO(Buffer String) -> IO()
computeBlock buff = do i <- atomically $ get buff
putStrLn $ show i
更新:
pacman.hs:71:46:
Couldn't match expected type `Buffer a0'
with actual type `IO (Buffer String)'
In the first argument of `get', namely `buff'
In the second argument of `($)', namely `get buff'
In a stmt of a 'do' block: i <- atomically $ get buff
解决方案是将launchGhosts
声明为
launchGhosts :: [[(String,String)]] -> IO (Buffer String)
问题是您声明launchGhosts
返回了一个Buffer String
,它是TVar [[(String, String)]]
。 由于launchGhosts
使用do
块,因此其结果类型需要一个Monad
实例,根据您的签名,该实例为TVar
。 这就是第一个错误。
另一个问题是, atomically
类型为STM a -> IO a
atomically newBuffer
STM a -> IO a
,因此atomically newBuffer
是IO something
(实际类型)。 但是您在声明为Buffer
(即TVar
)类型的do
块中使用它,因此它也应该具有该类型(预期类型)。 这就是第二个错误所在。
编辑:
为什么要更改computeBlock
的类型签名? 我从没说过关于computeBlock
事情。
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