[Haskell-cafe] Is it safe to create global variables using unsafePerformIO?

[Ryan Yates]

I believe you are safe to do this for now with GHC.  There is some relevant
discussion here [1].

[1]: http://www.haskell.org/haskellwiki/Top_level_mutable_state

Note that doing:

    {-# NOINLINE counter #-}
    counter :: TVar Int
    counter = unsafePerformIO $ atomically $ newTVar 0

Is not safe [2].

[2]:
http://hackage.haskell.org/package/stm-2.4.2/docs/Control-Concurrent-STM-TVar.html#v:newTVarIO

Ryan




On Wed, Jan 29, 2014 at 2:32 AM, Alexander Alexeev <mail at eax.me> wrote:

> Hello!
>
> Lets consider the following code:
>
> import Control.Concurrent
> import Control.Concurrent.STM
> import System.IO.Unsafe (unsafePerformIO)
>
> {-# NOINLINE counter #-}
> counter :: TVar Int
> counter = unsafePerformIO $ newTVarIO 0
>
> incCounter :: IO Int
> incCounter = do
>   r <- atomically $ do
>     t <- readTVar counter
>     let t' = t + 1
>     writeTVar counter t'
>     return t'
>   return r
>
> main :: IO ()
> main = do
>   n1 <- incCounter
>   print n1
>   n2 <- incCounter
>   print n2
>   n3 <- incCounter
>   print n3
>
> This program prints:
>
> 1
> 2
> 3
>
> So we have a "global variable". Do I right understand that newTVarIO
> creates TVar and RTS memoizes it since 'counter' function is pure? If
> it's true, could it happen that under some circumstances memoized value
> will be deleted from memory? Or Haskell keeps all memoized values
> forever?
>
> Another issue which I'm afraid of --- would the given code be safe in
> multithread application? For example, is it possible to encounter a
> race condition if two threads will try to create a new counter in the
> same time?
>
> Is there any other problems which should be taken in account?
>
> --
> Best regards,
> Alexander Alexeev
> http://eax.me/
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